Planetary Radio • Mar 25, 2020

The Next 10 Years of Planetary Exploration

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On This Episode

Edgard rivera valentin

Edgard Rivera-Valentín

Staff Scientist for USRA Lunar and Planetary Institute

Jospeh orourke asu

Joe O’Rourke

Assistant Professor for ASU School of Earth and Space Exploration

Brett denevi

Brett Denevi

Planetary Geologist for Johns Hopkins University Applied Physics Lab

Bruce betts portrait hq library

Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

Kaplan mat headshot 0114a print

Mat Kaplan

Senior Communications Adviser and former Host of Planetary Radio for The Planetary Society

Preparations at NASA are underway for creation of the next planetary science decadal survey, a roadmap intended to guide exploration of our solar neighborhood from 2023 to 2032. Six scientists, each considering a different world or class of objects, will share their thoughts and hopes. The Planetary Society’s Emily Lakdawalla offers fun and fascinating science education suggestions for housebound families. Some lucky (?) listener will be getting a special message from Bruce and Mat if he or she wins the new What’s Up space trivia contest.

BepiColombo at Mercury
BepiColombo at Mercury Artist’s impression of the BepiColombo spacecraft at Mercury. The mission comprises ESA’s Mercury Planetary Orbiter (foreground) and JAXA’s Mercury Magnetospheric Orbiter (background). The image of Mercury was taken by NASA’s MESSENGER spacecraft.Image: Spacecraft: ESA / ATG medialab; Mercury: NASA / JHUAPL / Carnegie Institution of Washington
VERITAS The VERITAS mission would map Venus with radar and infrared spectroscopy.Image: NASA / JPL-Caltech
VIPER lunar rover
VIPER lunar rover Artist's concept of NASA’s Volatiles Investigating Polar Exploration Rover, or VIPER. VIPER is a mobile robot that will roam around the Moon’s south pole looking for water ice.Image: NASA Ames / Daniel Rutter

Trivia Contest

This week's prizes:

Bruce and Mat will record an outgoing message for your phone, if you dare.

This week's question:

Who was the first person to do a deep space EVA (extravehicular activity or “spacewalk”)? Deep space is defined as beyond low Earth orbit.

To submit your answer:

Complete the contest entry form at or write to us at [email protected] no later than Wednesday, April 1st at 8am Pacific Time. Be sure to include your name and mailing address.

Last week's question:

The Chandrasekhar limit is the maximum mass of a stable white dwarf star. In solar masses, what is the approximate value of the Chandrasekhar limit?


The winner will be revealed next week.

Question from the March 11 space trivia contest:

What is the second largest planetary moon in our solar system that orbits retrograde? (Neptune’s Triton is by far the largest.)


Our solar system’s second largest moon orbiting in retrograde is Phoebe at Saturn.


Mat Kaplan: [00:00:00] Planning the future through NASA's decadal survey, this week on Planetary Radio. Welcome. I'm Mat Kaplan of The Planetary Society with more of the human adventure across our solar system and beyond. They are intended to guide NASA's science and nearly the entire scientific community believes they are essential. As we approach formulation of the next decadal survey for planetary science, we'll survey the solar system through the eyes of six scientists. Like the rest of us, Emily Lakdawalla is learning to deal with the challenges of these strange times. My colleague is uniquely qualified to recommend ways to keep our minds and the minds of our children wandering the universe, even as we stay within our homes. She'll join us right after The Downlink headlines.

We'll close as always with Bruce Betts and What's Up. You'll get the chance to win a [00:01:00] personalized message from Bruce and me, if you want it. Here's this week's sample of the mission updates collected by Planetary Society editorial director Jason Davis for The Downlink. Like the rest of the world, the space community is being affected by COVID-19. All NASA employees except mission essential personnel are working remotely. Europe has taken similar measures, suspending its launches, even Buzz Aldrin is quarantined at home. It's unclear how severely the pandemic will affect upcoming mission schedules, but NASA officials say at least one is still on schedule for now, the Perseverance Mars Rover. Sadly, work on the James Webb Space Telescope has been halted by the pandemic.

There are signs that NASA's latest efforts to save the heat flow probe aboard the Mars InSight Lander may be working. The self-hammering instrument known as the mole has unsuccessfully been [00:02:00] trying to bury itself since March of last year. Engineers are now using InSight's robotic scoop to press down on top of the mole while it hammers. And when NASA astronauts make their first flight to the lunar surface as part of the agency's Artemis program, currently scheduled for 2024, they won't be making a pit stop at the Gateway, a small yet to be built lunar space station. NASA officials say they are still committed to building the Gateway later, but that it is no longer in the so-called critical path for the first moon landing, not counting Apollo of course.

More news and other great features are waiting for you online at You can also sign up to get The Downlink delivered to your inbox each week for free. Here's our solar system specialist, Emily.

Emily, thanks for joining me, uh, and this is such a critical time to be doing this. Of course, we are both doing it from home. [00:03:00] I know you're there with your daughters, uh, attempting to keep them busy and stimulated. My wife just left to, uh, go take care of our grandson while our, uh, our daughter works from home, and we're all looking at the same challenge. Those of us who are working with children and many of us, uh, [laughs] we're trying to keep our own minds busy. This is something you've given a lot of thought to I know over the years and it now seems more important than ever.

Emily Lakdawalla: Yes. Uh, it's certainly an interesting challenge for a lot of us to try to maintain our jobs and maintain our children's education, and maintain all of our sanity while we're stuck at home here.

Mat Kaplan: [laughs]

Emily Lakdawalla: Uh, we're actually doing pretty well. My daughters are older now. They're 13 and 10. The 13-year-old's, uh, schooling has transitioned seamlessly online. She's just going, uh, right along with all of her classes. The fifth graders, not so much, but she does have work to do every day. And then she is actually very good at keeping herself busy. But we all want to enrich our lives with science. We're all a little bored staring at the same four walls all day. So, let me give you a couple of ideas for [00:04:00] things you can do to keep the kids and yourself entertained, and then I can, uh, give you some suggestions for how to guide you in making your own activities up for your kids.

So first of all, uh, let's talk about just exploring museums from home. Most of the great museums around the world are really acting fast to put a lot of their exhibit materials online. Um, they've been doing this for years and years and now they're just foregrounding it all. And of course I'm gonna highlight the National Air and Space Museum whose two museums are now closed to the public, but they have something called Air and Space Anywhere where they have a, a single website that's a portal into all of their great online offerings. So you can go explore, uh, the United States' vast collections of space paraphernalia, aerospace, airplanes, spaceships, tours, uh, artifacts, all kinds of interesting things to look at and activities to do.

Another great activity, if you're interested in studying planets, is to study your own planet. And the Washington DC [00:05:00] Capital Weather Gang has something called Weather School for kids at home that they're operating off their Facebook page and they're encouraging children and their parents to go out and make observations of the world around them, of the changing weather, and those kinds of observations, they're science, it's the very first step into understanding how to make observations on other planets. So that's a really fantastic activity to do with your kids and it gets them outdoors as well. And observing how each day is different from the next day, which I think is really helpful right now when all the days seem to be blending together.

Mat Kaplan: [laughs]

Emily Lakdawalla: Um, the last ones, both, uh, more relaxed and I think really super fun, and that's something called Story Time From Space, where actual astronauts on the space station read books aloud while they are floating through the space station, and in different parts of the space station. The books that they read, a lot of them are picture books suitable for younger children, but they read middle school books as well. So, um, really kids of all ages and honestly even adults, uh, can really [laughs] enjoy [00:06:00] the astronauts, uh-

Mat Kaplan: [laughs]

Emily Lakdawalla: ... reading their books from station. Some of them are better reader than others, but it's all just wonderful. And periodically you'll see another astronaut floating around or, or hear pe-, hear cosmonauts talking in Russian in the background, and that's really fun.

The last couple of suggestions I have are back on The Planetary Society's website, A more passive, but really inspiring thing you can do is to just look through our vast space image library, We have so many gorgeous images from all over the solar system. If you look down at the bottom of each individual image page, there's keywords that you can click on and then you get a whole host of images that, uh, are tagged with that keyword. And so, um, there's so much to explore there. It's really fun. And then-

Mat Kaplan: It's a beautiful library. Yeah.

Emily Lakdawalla: Yeah. Um, I'm very proud of it. [laughs] So-

Mat Kaplan: You should be.

Emily Lakdawalla: Yeah. And then finally we actually have courses online that are suitable I think for both high school, uh, students and adults. I've created some space image processing tutorials where, uh, [00:07:00] I walk you through the very beginning steps of learning how to process space images. And of course, uh, uh, Dr. Bruce Betts has his own, um, uh, solar system, introduction to the solar system classes. You can get those at, and you can take a whole course on the solar system. And so, uh, all of those things I think would be great activities for kids of all ages.

Mat Kaplan: And I got one more to mention and that is the course on how to become a space advocate. Maybe you already are in your own mind, but if you want to make it happen in the real world, there's Casey Dreier's course, uh, for that as well. All three of these are terrific, and of course we got much more on the website. You might want to check out The Planetary Report. Uh, that new Equinox, Vernal Equinox edition is, uh, available right now. And, uh, that's, uh, something, Emily, that, uh, you had tremendous influence over up until just recently when now that you've moved on to, uh, other things. Listen, we still have some time, at least for our podcast listeners, there's so much content out there, not all of it at the [00:08:00] level of quality of the, uh, stuff that you've just described. How can parents and others figure out, uh, what's worth giving time to?

Emily Lakdawalla: Well, fortunately there's guidance in something called the science standards, and every single state has its own set of standards, but an awful lot of them are guided by something called the Next Generation Science Standards. They're sort of a-a guide to the kinds of topics that are suitable for children. And it's not just, uh, a list of topics like, "In first grade you study Earth." No, it's not that simple. It's not about the, the subject matter. It's about the kinds of scientific work that kids of different ages can be expected to do. So I went to the Next Generation Science S-standards website and I just pulled the standards for one particular topic, which is Earth's place in the universe. And so you can see how at different grade levels, uh, the standards ask kids to, um, be able to think about Earth's place in the universe in different ways.

For a first grader or a second grader, [00:09:00] you might expect children to be able to make observations of the Earth at different times of year and relate the amount of day light to the time of year. So you might ask kids to notice when the sun rises, when does it set? They're also learning at that age, how to read time on clocks. And so you can tie reading clocks with looking at when the sunrises and the sunsets. And that's the kind of activity that's appropriate for six-year-olds, five- and six-year-olds.

When you're looking at older kids like who are, you know, nine to 10, fifth grade in the United States, they're expected at that age to develop and use a model of the Earth, sun, moon system to describe the cyclical patterns of lunar phases, eclipses of the sun, and moon. You can see how as kids get older, they're expected to be able to, um, uh, tie their observations to mental models, to things, pictures that they can hold in their head about how Earth, and sun, and moon move with respect to each other. You can't expect a six-year-old to do that, but you can expect a 10-year-old to do that kind of thing. In middle school, [00:10:00] they're expected to understand the role of gravity in motions of the solar system. And you know, it goes on to be more sophisticated as you get kids older and older.

Mat Kaplan: This is terrific. I mean, it's not just learning science, it's learning how to think, uh, how to be rational and, and appreciate everything that's around us. I, I, I think this is just, uh, terrific. So how can people learn more about these standards?

Emily Lakdawalla: Well, you go to the Next Generation Science Standards website and they actually have a really easy form that you can use to plug in the age of the child and the topic area that you're interested in, whether it's Earth and the solar system, or biology, or some other topic. And then you can ask it to spit out, uh, the kinds of topics, the kinds of, uh, subjects and also provides you with a, um, a download of the parti-, of a much longer description of the standards for that particular age. I highly recommend that the parents who are doing science education for their own kids to go there and read. And it helps you understand the, the capability of your child at their [00:11:00] particular age. Um, what they're able to, um, hold in their heads and observe at the same time, and the kinds of reasoning you can expect them to be able to do given their age. It's really valuable.

Mat Kaplan: Great. Great suggestions, Emily. Thank you so much for all of these. There is one more thing that I'm going to mention. Uh, and I only just learned about it in time for us to record this segment. Some of you out there may be able to participate in it live. If not, my assumption is though, I'm not sure, I believe I, I, it's hard to believe that they would not make this available as a recording on demand after the fact. But I was contacted, uh, minutes ago by Danica Remy who is a co-founder of Asteroid Day, and Asteroid Day has gotten together with space agencies around the world, especially the European Space Agency, on Thursday, Thursday, um, evening for some of us, Thursday morning for others, they are going to put together a series of live webcasts. Uh, you can find out about it [00:12:00] at,

It's, uh, going to start at 3:00 PM GMT. That would be 8:00 AM Pacific Time. It's in five different languages, beginning with Dutch. The English broadcast will begin at 7:00 PM GMT. That's noon Pacific Time, Pacific Daylight Time, on Thursday, March 26th. Uh, the English portion will be hosted by physicist and science communicator, Brian Cox. So that alone would be worthwhile. But they are put, they have put together this tremendous list of celebrities, of scientists, and of astronauts. I mean, just in the English portion, uh, they've got Tim Peake from the UK, Tom Jones and Nicole Stott, both, uh, past guests on Planetary Radio. We don't have time to read all of these, but, uh, it is well worth checking out. Again, you can find out more at [00:13:00], us. It's not continuous over this period. There are four half hour programs in Dutch, German, Italian, and French, and then an hour of English. Again, that's at 7:00 PM GMT and noon PDT.

Emily, if nothing else, most of us can go outside, stand in the yard or in front of wherever we live, and look up at the night sky if we're lucky enough to have a clear one, or maybe out the window. Because as, uh, my wife said, uh, just before she left to take care of our four-year-old grandson, we can all keep looking up. Thanks very much, and, um, keep sheltering in place.

Emily Lakdawalla: [laughs] And I'll be putting some more stuff out on video as time goes on. So, stay tuned to for that.

Mat Kaplan: That's Emily Lakdawalla, our solar system specialists keeping our own minds and the minds of lots of children hopefully, uh, very busy during this unprecedented time around planet Earth. A new edition of The Planetary Report has been available to all for a [00:14:00] couple of weeks now. You'll find the digital version of the magazine at It offers a lot, including a main feature called The Next 10 Years, an introduction to the decadal survey. While there are surveys for each of the four science divisions of NASA, we're going to limit ourselves to planetary science.

The current survey's term ends in 2022. A new planning effort is just getting underway. It will lay out a recommended path for 2023 through 2032. It's remarkable how effective this process has been. With oversight by the National Academy of Sciences, it relies on scores of scientists for its formulation, with thousands more carefully following its progress and many attempting to influence it.

As the effort kicks off, The Planetary Society has invited six distinguished planetary scientists to give us an idea of what to expect. We'll hear from three of them [00:15:00] this weekend, and continue the conversations next week. We begin with Edgard Rivera-Valentin. Ed is a staff scientist with the USRA, the Universities Space Research Association, at the Lunar and Planetary Institute.

Ed, welcome to Planetary Radio. I, I guess from reading about you, we could have talked to you about, just about anything in the solar system since your interests are, are pretty much in everything, at least out as far as the outer planet. But you, uh, got Mercury in this, uh, issue, the current issue of The Planetary Report. I'm glad that we can start with you there and we'll work our way out from the sun as we, uh, progress through talking, uh, to your colleagues, who also contributed to, uh, the magazine this time around. And let me just say again, welcome.

Edgard Rivera-Valentín: Thank you. Thank you. I'm happy to be talking to you.

Mat Kaplan: Mercury, fascinating little world. As you look over the last 10 or 20 years, we've learned a lot about this little world, haven't we?

Edgard Rivera-Valentín: We definitely have. Um, Mercury [00:16:00] is I'd say one of the more interesting ones. Um, and I was happy to write about it because we've gotten so much radar data on it. One of the first weird things that we found on Mercury was the discovery that its poles might have ice. So you wouldn't expect that when you're talking about the planet that's closest to the sun, right? Uh, you'd imagine a very hot world, there's no way you could have water or ice there. Uh, radar return from both the Arecibo Observatory in Puerto Rico and the Goldstone Solar System Radar showed that there was definitely something very bright right at the poles. And later, once we were able to send, uh, a spacecraft to Mercury, we were able to say, "Yep, there's definitely ice here," and there's still a lot of work going on trying to decipher what that ice is, how did it get there, and how is it forming or was it delivered? There's still a lot to learn about Mercury.

Mat Kaplan: So that's one of the things you'd like to learn more about.

Edgard Rivera-Valentín: Yeah.

Mat Kaplan: And i-i-is this ice, is it the same situation that we have on the moon where it's in these [00:17:00] permanently shaded areas that, uh, keep the sun from hitting it directly?

Edgard Rivera-Valentín: Correct. Yeah. So Mercury, it's still in its topography is in such a way that at the poles, some of the craters will have parts of them that will be permanently shadowed. They will never see the sun. And because of that, those areas actually can be really, really cold. Um, there you'll be able to store ice either right at the surface or right below the surface, covered by some regular. There are a little bit differences between the type of ice that we think we're seeing at Mercury versus the type of ice that we're seeing at the moon, because when you zap the moon with radar, the returns would tell you there's no such thing as ice there.

Mat Kaplan: Hmm.

Edgard Rivera-Valentín: Versus Mercury where it was immediate, there's definitely ice there. So we're thinking that the ice that's at the moon, it's, it's not a lot. It's port fi-, it's what we call port filling. So in the right width or the soil, there's some water ice that's filling in some of the holes inside the soil. While [00:18:00] on Mercury, it might be more like slabs of ice and ...

On Mercury it might be more like slabs of ice and-

Mat Kaplan: Huh.

Edgard Rivera-Valentín: ... soil mixed together. So there's a larger fraction of ice there compared to [inaudible 00:18:08].

Mat Kaplan: What else do we still want to know more about on mercury? I mean, after all, I mean, you mentioned other spacecraft. We had the Messenger spacecraft visit there, uh, and do terrific work up until recently. And, uh, this European spacecraft, BepiColombo, will be arriving before too long to, uh, tell us much more.

Edgard Rivera-Valentín: Mercury is more than just the ice. That's one of the things that I really like about it. Mercury is enigmatic from all the way from its formation. The type of data that we've gotten back from Messenger shows us that if you look at Mercury from a geophysics perspective it's mainly a core. About 85% of the volume of Mercury is its core. How did that even happen? Did you have... Did it form that way? Did it form by a bunch of objects that were just really metallic [00:19:00] and all of those metals ended up suddenly into a core or at one point or another they had a large impact combine strip away those outer layers leaving behind, uh, maybe just a mantle covering the core? We still don't know that part.

And also from a solar system formation perspective, uh, in a lot of these models that we use to try to understand how all the planets formed Mercury is really close to what's called one of those boundary conditions, the outer edge of those simulations. So we really can't quite get to making a Mercury. We can reproduce everybody else, but making a Mercury is a little bit more difficult in these types of models. We're getting some hints by looking at exoplanets, but we're still a little unsure how you even get a Mercury. Not only how do you reproduce the interior of it but how do you make it where it's at?

So there's a lot of information to learn about the interior of its body. From a geology perspective, it's covered in [00:20:00] just volcanic plains. There's pyroclastic deposits everywhere. So it was definitely a very active world at one point or another, even though we're seeing a quote unquote dead world today, but some of the data that Messenger brought back is showing us that it's actually still changing. It's contracting. So that's still changing its geology. ,

So BepiColombo when it gets there around... Let's see here. It launched in 2018. BepiColombo should get there on 2025. Um, it's still going to be elucidating a lot of these very important, very fundamental questions for Mercury. How did it form? How the heck do you get the interior, um, to be with something such a large core? And can we better understand the volatiles and the geochemistry that we're seeing on the surface?

Mat Kaplan: And there's one more factor which you mentioned in your TPR, uh, article, and that's the [00:21:00] magnetic field of Mercury, which is something that I... We've had conversations in the past with Sean Solomon about. Of course, he was the PI for the, the Messenger mission. It's still something that we need to learn more about?

Edgard Rivera-Valentín: Oh, for sure. The more you learn about magnetic fields the... in planetary science, the more you know that we don't understand them. [laughing] Um, that's the best way I could, uh, describe my mag- magnetism. Yeah, so there's still a lot to learn about how, uh, Mercury is actually generating its magnetic field precisely, better measurements, uh, across the planet at distances so we could better characterize it. BepiColombo will definitely be unraveling a lot from that perspective. I'm... That is one of the fields I am definitely not an expert in. It's just one of those, "Oh, that's really neat." [laughs]

Mat Kaplan: From everything that you've talked about, it sounds like just like with all the other bodies in the solar system, Mercury can teach us a [00:22:00] lot about everything in the solar system and maybe worlds outside the solar system as well. Am I, am I right about that?

Edgard Rivera-Valentín: Yeah, for sure. The more we understand the diversity of worlds that we have in our own solar system the better we can understand not only our place in our solar system but also put into context all this new data we're getting from exoplanets. Understanding each and every world, including the first world, Mercury, helps us in getting a better understanding of how even the entire galaxy works.

Mat Kaplan: I got one I got to ask you just because of my science fiction interests. Uh, it's gonna be a little bit out of left field. When I was growing up, when I was a kid, a long time ago, people thought that Mercury was tidally locked, that, uh, it always had one side facing the sun, one side facing away from. We know now of course that that's not the case, but it does rotate pretty slowly. Are you familiar with... There was a great book by a Kim Stanley [00:23:00] Robinson past guest on our show, 2312. And in it it's got a lot of highly speculative, marvelous stuff. He actually talks about a city on Mercury appropriately called Terminator, and this city crawls along tracks so that it can always stay in that twilight zone, the terminator zone. Uh, so that it's neither frozen nor roasted. I... A completely novel idea or have you ever heard of this?

Edgard Rivera-Valentín: No, I have not read this. Now I need to.

Mat Kaplan: I highly recommend it. There's a lot of other stuff in it like, like colonies on Io where I'm not sure I would ever want to even visit much less live there. It's an absolutely fascinating and... As is Mercury. I appreciate your taking a few minutes to, uh, introduce us to it. And I, I hope like with all of the, the articles by your colleagues, uh, that people will take a look at the digital version of the Planetary Report, which is available at [00:24:00] I've got just one more question for you. I know you're very involved with sharing science, uh, with the larger community and including young people, and there's one, one group in particular I'm curious about. Correct me if I get it wrong, but I think it's the Boricua Planeteers. Why... What's that about?

Edgard Rivera-Valentín: Yes. So Boricua Planeteers is a group of Puerto Rican planetary scientists, including myself and a bunch of my friends from PR. We're spread across the US. The point of the group is to increase the visibility of latinx, specifically in this case, Puerto Rican scientists, but to also bring back planetary science to Puerto Rico. So PR we have the Arecibo Observatory, right? The best radar telescope, the second largest radio telescope. But education wise on the island astronomy hasn't been one of the major focuses. In fact out of the about a 100 universities that we have in Puerto Rico only three offer [00:25:00] bachelors degrees in even physics. And there's no astronomy degree granting program in PR yet.

So we thought of putting together this group to be able to increase the ability for students to get into planetary science, to give them those opportunities in Puerto Rico and across the US, and to let people know that there are such thing as latinx scientists doing really cool science.

Mat Kaplan: That's outstanding. Great outreach work and, and great science, uh, to compliment it. Thanks so much, Ed. I- I'm a very glad that, uh, you could join us to kick off this, uh, coverage of what's ahead the next 10 years for our solar system.

Edgard Rivera-Valentín: Thank you so much. It was a pleasure.

Mat Kaplan: Edgard Rivera-Valentin of the Lunar and Planetary Institute. We'll take up Venus next right after a brief break.

Deborah Fischer: Hi, I'm Yale astronomer, Deborah Fischer. I've spent the last 20 years of my professional life searching for other worlds. Now I've taken on the 100 Earths Project. We want to discover 100 earth sized [00:26:00] exoplanets circling nearby stars. It won't be easy. With your help, the Planetary Society will fund a key component of an exquisitely precise spectrometer. You can learn more and join the search at Thanks.

Mat Kaplan: Continuing our survey of the solar system, we move out one big rock from Mercury for a conversation with Joseph O'Rourke. Joe is an assistant professor in the school of earth and space exploration at Arizona State University. He serves on the steering committee of NASA's Venus exploration analysis group. Joe, welcome to Planetary radio as we, uh, continue our little tour of the solar system looking 10 years out. Glad to have you here.

Joseph O'Rourke: Thank you so much for having me. I'm excited to talk about Venus.

Mat Kaplan: You say that it is the most Earth-like planet that there is. You're not the first person I've heard say that, but it still sounds slightly outrageous. Can you make that case?

Joseph O'Rourke: Yes. If we were an alien astronomer looking at our [00:27:00] solar system using the same telescopes that we use to study exoplanets today Earth and Venus would be indistinguishable. They have the same mass, the same radius to within reasonable uncertainties. Venus is just a tiny bit smaller than earth and they're both compositions are similar. So if you were an alien astronomer looking at our solar system to first order you would think that Venus and Earth are similar planets.

Of course, when you look more closely Venus is different than Earth in terms of its habitability. Venus is a hellish wasteland, whereas Earth has been [inaudible 00:27:35] for billions of years. If we want to understand anything about rocky planets, we need to understand why Venus and Earth are so different on the surface, but so similar in almost every other respect.

Mat Kaplan: You take me back to when I was a little kid. I remember seeing artists' concepts of the surface of Venus and it looked like something from 65, 70 million years ago on earth. [00:28:00] Huge plants. It was hot. It was tropical, and, you know, something like dinosaurs wandering around and then we got this rude awakening, right, which partly came from people like one of our founders, Carl Sagan. It's kind of toasty down there, a lot more than tropical.

Joseph O'Rourke: Yes, exactly. Uh, science fiction books would show pictures of a jungle Venus because we thought we... We've known for a long time that Venus has clouds that cover the entire surface. Early astronomers thought those might be water clouds like on Earth, uh, in which case Venus would be a sort of swampy muggy world. But we now know that those clouds are sulfuric acid. The atmosphere is over 90% carbon dioxide, and the surface temperatures on Venus are hot enough to melt lead. So not a place you'd want to spend much time.

Mat Kaplan: That is the great cliche yet so true. Don't bring anything made out of lead to the surface of Venus on-

Joseph O'Rourke: Yes.

Mat Kaplan: When you visit there as a tourist. Okay. So then along comes Magellan, that [00:29:00] enormously successful spacecraft. It's hard to believe that it arrived at, uh, at Venus 30 years ago. I love that you pointed... point out that you were 10 days old when it happened. And we learned a little bit more about, uh, Venus because we were finally able to look through those clouds with, with some kind of accuracy, right?

Joseph O'Rourke: Exactly. You can't see the surface with visible light, but you can see the surface with radar and in a few spectral windows using infrared light. So the Magellan mission produced these amazing global maps of the surface with a resolution of just over 100 meters per pixel. And those geologic maps revolutionized our understanding of Venus basically by revealing that we have no understanding of Venus. [laughing] The surface geology, it's, uh, revealed that Venus is... has a young surface. It's an active world, but the surface geology is unlike any other planet in the solar system.

Mat Kaplan: The only other two spacecraft, Venus Express, it's done did... finished its work in [00:30:00] 2014, Akatsuki, that that plucky little spacecraft that had such trouble getting into orbit, but it's still there today doing some work. Have we learned much more from them and, and what about?

Joseph O'Rourke: Yes, we've learned tons from both of those fabulous missions. My own background is in geophysics and geology. And those two missions were designed to study, uh, atmospheric science, uh, but Venus Express in particular carried an infrared instrument that provided some constraints on the surface. It's provided these fascinating hints that terrain called tessera on the surface might have granite light compositions, which would mean that they are analogous to continents on Earth and signatures of abundant liquid water at some point in the past. Japanese mission has discovered, uh, an array of amazing meteorological features, uh, such as this huge, uh, stationary wave in the atmosphere. And it's produced some of the best maps of 3D wind speeds in the Venus [00:31:00] atmosphere.

Mat Kaplan: That has, uh, come up, uh, before on our show, a little bit of, uh, those results from Akatsuki. Uh, we all know that there still nevertheless has been this long drought in, uh, missions to Venus as you mentioned, but maybe it's going to come to an end. You must be pretty thrilled as a Venus guy to see that, uh, there are a couple of missions that are now being considered as finalists or semi-finalists anyway, no finalists I think, by NASA.

Joseph O'Rourke: I would call them finalists. Uh-

Mat Kaplan: Yeah.

Joseph O'Rourke: Yes, the VERITAS and the Da Vinci Plus missions. Um, NASA should pick both of them. [laughing] The science, uh, that both the missions would do does not really overlap. They aren't redundant with each other. And the Venus community has consistently said that the science goals of these missions are top priorities for the Venus community.

Mat Kaplan: Would either of these or maybe both be able to give us some more evidence about those strange structures that indicated [inaudible 00:31:56] you, you say in the article, it's possible that once upon a [00:32:00] time Venus was a very different place and maybe it did have oceans as we have today on Earth.

Joseph O'Rourke: Yes. The VERITAS mission is the natural successor to Magellan. It would use a updated radar instrument and a much better infrared camera, uh, to return data that are at least in order of magnitude, often many orders of magnitude, better than Magellan. So I really want to see the VERITAS mission fly in order to answer some of these questions that the community has debated answers to over the past three decades. I would say that Venus absolutely has volcanic activity and tectonic activity that have occurred in recent times. Uh, we have lots of powerful evidence for recent volcanism on Venus.

Mat Kaplan: Mm-hmm [affirmative].

Joseph O'Rourke: You see what looked like volcanic flows that are probably fairly young and there are chemical species in the atmosphere that would decay within a few million years if they weren't being constantly replenished by volcanoes, uh, in recent times, at least what counts to [00:33:00] geologists as recent times.

Mat Kaplan: We all know that you, you geologists, your, your timescales are a little bit different from those of us who just deal with, uh, lifetimes of humans. Wha- what are we talking about? Millions of years ago or, or tens of thousands of years ago.

Joseph O'Rourke: Arguably tens... as early as tens of thousands. Uh, what would be really exciting with the mission, again, like VERITAS is you can use modern radar techniques to study active surface de- deformation at Venus. And so you could possibly see much stronger evidence for, um, active meaning today, not just geologically recent volcanism on Venus.

Mat Kaplan: So I take it that the radar we're talking about that we would be able to send now all these decades after Magellan would deliver far better performance than Magellan could.

Joseph O'Rourke: Absolutely. The maps of the surface of Venus we have now are comparable to what we had for Mars in the 1970s. And I think it's time that, uh, the most interesting planet in our solar system, uh, that we had comparable data from it, uh, that [00:34:00] we can achieve on any other planet.

Mat Kaplan: Let's go to a, a theme which I think is going to run through every one of these conversations, uh, with you contributors to the current issue of the Planetary Report, and that is how the study of a word like Venus can help us understand other worlds in our solar system and of increasing importance the worlds, we're discovering the thousands of them that we find circling other stars that we, we talk about a lot on this show.

Joseph O'Rourke: Yes. I think the exoplanet revolution is one of the most powerful motivations for further exploration of Venus. If we don't understand why Venus and Earth are different than we don't know in general how rocky planets evolve and what governs whether they're habitable or not. And in that case, if we can't understand Earth and Venus it's useless to speculate about the possible fates of, uh, rocky worlds around other stars. If we can't understand the exoplanet in our own backyard, [00:35:00] uh, how will we understand the exoplanets that we can't go out and touch, go out and observe at, at close range?

Mat Kaplan: Well, best of luck to you and all the other Venus scientists out there who have their fingers crossed that, uh, NASA picks at least one and to make, uh, you happy, both of those venous missions which are now being considered as discovery class missions and would, and would visit Venus for the first time in, well, quite a few years. Before I leave you though, I got to ask you, you, you warned me. In fact, we had to change the time of our conversation a little bit because you had to, uh, have a meeting with some folks from JPL. I'll say a remote meeting because of course you are observing social distancing like the rest of us. And you mentioned that it had something to do with a, a proposal that you guys have for a mission. What's this about?

Joseph O'Rourke: I am the principal investigator of a mission called Athena, which is a small sat about the size of a mini fridge before we extend the solar panels that would visit one of the largest [00:36:00] asteroids in the main asteroid belt, uh, to understand how [inaudible 00:36:04].

... -roids, and the main asteroid belt, uh, to understand how water-rich it is. To understand how water has influenced its, uh, formation and evolution, and thus to understand, um, how the, uh, planetesimals that were formed on the asteroid belt may have delivered water to the inner solar system in the earliest epoch of planet formation.

Mat Kaplan: Well, you probably know that, uh, we at the Planetary Society and our members, we care a lot about those, uh, those little rocks out there, as well. And next week we'll be, uh, talking, uh, with your, uh, colleague, who contributed the article about small bodies, uh, to the current issue of the Planetary Report, so good luck with that proposal as well, it, does sounds like it's still in fairly early stages, Joe. Um, maybe we can check back with you as, uh, we learn whether we're going to be headed back to Venus before too long.

Joseph O'Rourke: I would love to talk more. Hopefully, with good news.

Mat Kaplan: Thanks again, Joe.

Joseph O'Rourke: Thank you so much.

Mat Kaplan: Joe O'Rourke of Arizona State University. Our last preview of the coming Planetary Science [00:37:00] Decadal Survey takes us not to earth but its big natural satellite. Brett Denevi is a planetary geologist at the Johns Hopkins University, Applied Physics Lab in Maryland. She serves as deputy principal investigator for the Lunar Reconnaissance Orbiter Camera, the powerful imager that has revealed the entire surface of the moon in unprecedented detail. Brett, thanks so much for joining us here on Planetary Radio, the third of our three conversations today based on, uh, this, uh, next 10 years coverage in the Planetary Report. I'm so glad you could join us. Welcome.

Brett Denevi: Yeah. Thank you. My pleasure.

Mat Kaplan: I wanna start with the first line in your piece about the moon, uh, because I just love this. “For the moon, our partner in a billions year long dance, the 2010s were a decade of profound revelations.” Okay, so take us through it. What, what was so great about what we learned over the last 10 years?

Brett Denevi: Ah, yes. Starting in [00:38:00] um, 2009, we had the Lunar Reconnaissance Orbiter, uh, went into orbit around the moon. We've also had really an international flotilla of spacecraft at the moon, and they have given us just a-a-a whole new kind of global picture of the moon in terms of things like, uh, volatiles. We've learned about water ice at the moon's poles. We've learned that there may be a, a kind of volatile water cycle at the moon. We've learned about the history of volcanism on the moon. We've thought for a long time that volcanism ended approximately around two billion, one to two billion years ago-

Mat Kaplan: Hmm-

Brett Denevi: ... We've now found that really it actually may have extended to essentially the present day, geologically speaking, you know, 50 to a 100 million years ago, which really h-has us trying to understand how the moon evolved [00:39:00] thermally in order to support, you know, melting of the interior and eruptions of lava onto the surface. Uh, at this point in the moon's history. We've also found that, um, the moon isn't dead in terms of tectonic activity. We're seeing evidence that the moon's tectonism is, is still ongoing and is influenced by tidal stresses from the earth.

Mat Kaplan: [laughs] I've had reason to say this in each of these conversations today because I'm old, I can remember the great debate about whether the moon's craters were formed by impacts or volcanism and while the impacts have won this battle. It is just fascinating to hear that volcanic activity is, is still as you said in geologic terms on the timescales that you and other geologists deal with, that it's really pretty recent. I guess I missed that there is still tectonic activity. So the moon is more dynamic than we thought not many years ago?

Brett Denevi: Yeah, absolutely. And [00:40:00] then if you extend that back to, um, include impacts as well. You know we are able with a spacecraft that's been in orbit so long around the moon to actually take before and after pictures of new impact events that have formed on the moon in the intervening time. So we are also able to constrain the current present-day impact rate and see how even fairly small impact events that leave a crater that's around, you know, 10, 20 I think the biggest one we've seen is 70 meters across. Those are just having effects that are really reaching vast distances across the moon surface and churning up that upper-most surface. So in terms of a dynamic place, it's still slow [laughs]-

Mat Kaplan: [laughs].

Brett Denevi: ... but it-i-it's evolving.

Mat Kaplan: I'm glad you brought up the ability to do this before and after stuff 'cause it's, it's not entirely relevant to our conversation, but it sure is a great demonstration [00:41:00] of the power of this mission, The Lunar Reconnaissance Orbiter that you're so intimately involved with. I saw last December that LRO had actually found the, uh, I guess we can't call it the landing site, but the, the crash site for that courageous attempt by India to put a Lander on the moon, which, which of course didn't end that well, but you can actually see because of LRO's amazing resolution where this thing came down and, and what it did to the surface.

Brett Denevi: Yeah. We've found a couple, sadly a couple crash sites now, um, as well as for the Beresheet Lander from, um, Israel, but of course we've also imaged some successes too. There are the Chang'e 3 and Chang'e 4 Landers from China. Chang'e 4 has the Yutu-2 Rover still exploring the far side and we check in on it every month, seeing how much [00:42:00] progress the Rover has made of watching it explore from above. So that is a really fun thing that we can do. And hopefully in the coming couple years here, we will image, uh, more successful landings on the moon.

Mat Kaplan: How useful was the, uh, the LCROSS mission, which of course was sent with the intent of creating a crater that the LRO and, and, uh, even from earth, uh, we could keep an eye on. Did that teach us much about the moon?

Brett Denevi: Oh, yes, absolutely. In terms of viewing from the earth, I think that was of limited success. But the LCROSS had a shepherding spacecraft which followed along behind the impactor. The impactor itself made a crater, um, within the Cabeus impact crater on the moon, at the moon's South pole. And the Ejecta that was thrown above the surface was observed by that LCROSS shepherding spacecraft. [00:43:00] And as well as, uh, by the Lunar Reconnaissance Orbiter. And one of the really great discoveries from that mission was the confirmation of around five weight percent water ice-

Mat Kaplan: Hmm.

Brett Denevi: ... uh, in that crater. So that was really, you know, we've seen evidence for water ice in the form of radar measurements. In the form of neutron spectrometer detecting hydrogen, but this was, um, really confirmation of an abundance of ice that is pretty substantial, uh, near the surface in one of these permanently shadowed regions that is just incredibly cold and can keep water, um, in those shadows essentially for billions of years potentially.

Mat Kaplan: This is extremely exciting of course. And, and maybe it leads us to the real theme of, uh, of this piece, which is looking out toward the next 10 years. You've got three big questions you pose. [00:44:00] The last of these is water, but, but, uh, let's start with the first one. What the moon may be able to tell us about the, the history of our solar system and in a particular this age of bombardment, the late heavy bombardment that we've talked about a little bit on this show before.

Brett Denevi: Yeah. You mentioned that the moon is our partner in space. And so one of the really most valuable things that the moon can tell us about is the impact history of the earth-moon system. Earth has ongoing very dynamic geologic activity where most of the large impact craters have been erased and certainly those that were forming in the, uh, first 500, 600 million years of our history. So we have almost no record of that from earth, but the moon does preserve this record where we have dated large impact basins like the Imbrium basin on the moon's near [00:45:00] side. Um, it's over a thousand kilometers across and we've dated that to occur, have occurred around 3.9 billion years ago. What's interesting about the timing of that basin and other basins that also seem to have formed around the same time is that on earth we think that is right around the time when earth's life was first beginning to emerge.

Mat Kaplan: Hmm.

Brett Denevi: ... And these impact events that are recorded on the moon and thus, you know, they were surely happening as well on earth. They're huge[laughs]-

Mat Kaplan: [laughs]

Brett Denevi: ... hugely, um, influential in terms of what would have, what the surface environment would have been like on earth. If the Imbrium impact basin formed on earth instead, it would have vaporized the Earth's oceans. It would have sterilized the crust down to hundreds of meters depth-

Mat Kaplan: Oh,[crosstalk 00:45:54]

Brett Denevi: ... So these were not really conducive to life forming around the same time. [00:46:00] So understanding when these huge impact basins, when that, um, what we call the late heavy bombardment. When did that really die out? Was there a peak in the impact rate right around this 3.9 billion years ago? It's in-interesting for Earth's early history as well as, you know, the solar systems. If you think about, you know, a big impact event that requires an impact door. So where were these huge, uh, bodies coming from at that point in the solar system's history? Most of accretion and sweeping up of all of the little, uh, leftover materials for accretion should have completed by that time. So why would you have this huge stream of impactors coming relatively late, um, after the solar system first formed? That's also another, you know, really big question that affects our understanding of all the way out into the Kuiper belt, these dynamics that were going on.

Mat Kaplan: A great mystery. Um, [00:47:00] let's go on to your second question. I wish we had more time-

Brett Denevi: Mm-hmm [affirmative].

Mat Kaplan: ... to go into all of these and maybe another time we will. But your second question that you've posed for the next decade or this decade actually is about the structure and composition of the moon's interior. And it occurs to me that there are moons all over our solar system, small, smaller bodies like our own moon. And I assume that if we learn about our own moon, it may tell us more about those or at least some of them and, and maybe about some of the planets too.

Brett Denevi: Right. Well, learning about, you know, the moon's interior structure is something that can really teach us about how all, um, you know, rocky bodies, uh, in the solar system would have formed once they differentiated into a core, a mantel and a crust. And the moon is such a perfect example of that because we have this beautifully preserved original flotation crust. The moon was once largely to entirely molten, light [00:48:00] minerals floated to the surface. And that is what you see in the light areas of the moon surface. The plagioclase original ancient crust. Trying to understand that original crust, uh, how the interior structure was evolving in terms of that differentiation. Um, what the core looks like. Are there layers within the interior? And then trying to understand if the interior of the moon, how it evolved thermally and could have continued to produce some of these volcanic deposits that we think may have occurred relatively recently as we're discussing.

Mat Kaplan: Speaking of volcanic activity. Your last question touches on that. Magnetism is mentioned along with the water that you've already brought up and, and other volatiles at, uh, the moon's, uh, poles. So I-I guess even though we know the water is there and we know that this volcanic activity is [00:49:00] much more recent than we thought, we still have a lot to learn.

Brett Denevi: Yeah. And I kind of cheated and squished two separate questions-

Mat Kaplan: [laughs] You did.

Brett Denevi: ... together in there because [laughs] I couldn't leave, I couldn't, you know, I couldn't leave one out, but, um, they are separate questions they-re-

Mat Kaplan: You're , you're forgiven. [laughs]

Brett Denevi: ... the linkage there [laughs]. The linkage to there is that they could be explored in similar ways and the way that would be, you know, really wonderful to explore some of the moon's volcanic features or even, you know, some of the moon's polar, uh, deposits is through these lo-, a long duration Rover. Um, and then there again, you're kind of cheating. You don't have to go to one place and answer only one question. You can... we've have decades of questions built up now since our last, uh, lunar landing and so many places that we would love to explore that if you have a Rover that is capable of surviving the long 14 earth day [00:50:00] and very cold lunar night, you can really start to go in depth. And it's like, you know, our little robotic geologist friend who could-

Mat Kaplan: Hmm.

Brett Denevi: ... explore some of these volcanic deposits, understand how these rivers of lava formed, how did we get some of the weird compositions that we see on the moon? And then for the polar deposits really, you know, traverse around, look at how these water ice deposits may be distributed. Because the evidence that we have now is not that they're, you know, everywhere in permanently shadowed regions at the poles. It's that they're patchy and they may vary spatially. They may vary with depth beneath the surface. So we really need to kind of get in there and get our feet on the ground, whether it's our own or you know-

Mat Kaplan: [laughs].

Brett Denevi: ... Rover's wheels, uh, trudging around and be able to, to dig in and explore.

Mat Kaplan: The success of, of [00:51:00] Yutu-2 that, that great little-

Brett Denevi: Mm-hmm [affirmative].

Mat Kaplan: ... Chinese Rover on the, on the far side, it's been through a whole bunch of these lunar nights now. That must give you some hope.

Brett Denevi: Yeah, absolutely. And you know, even some of the surface missions from our last era, um, the Apollo era, but you know, there was the Lunokhod rovers, there were some of the Surveyor Landers that, um, did survive night back then. So it is not some huge technological challenge that we can't overcome. It's just something that we need to invest-

Mat Kaplan: Hmm.

Brett Denevi: ... in a little development and make sure we, we have the um, ability to have these sophisticated new instruments that can take advantage of a longer time on the surface and being able to explore greater distances.

Mat Kaplan: If you would, uh, just very briefly, uh, take us through these other missions or, or efforts that you, uh, mentioned in your article and that you're hoping to see in, in this [00:52:00] decade.

Brett Denevi: Oh, well, so tied into these, you know, questions that we've been discussing. One of the biggest thing is sample return. We would love to be able to get samples from some of these really key geologic terrains like these impact basins that we don't know the ages of. But we have, you know, our guesses [laughs]. We want to-

Mat Kaplan: Hmm.

Brett Denevi: ... confirm those to see was there really this huge catastrophic event in the moon's, you know, fairly early history, around 3.9 billion years ago. There is some hope that we could take advantage of advances in technologies, do some in-situ dating. So bring our, our laboratory instruments to the moon, but really samples, you know, as we've seen now with the reopening of or the, the opening of samples that haven't been, um, touched-

Mat Kaplan: Hmm.

Brett Denevi: ... since the Apollo mission that brought them back, you know, now we can explore in depth and samples are essentially the gift that [00:53:00] keeps on giving because technology advances and science advances, so you can interrogate those samples 50 years later. So with new questions and learn new things. So sample return is really key for a lot of these different questions that we wanna answer.

Mat Kaplan: This lunar geophysical network that you mentioned, this is a-

Brett Denevi: Mm-hmm [affirmative].

Mat Kaplan: ... very exciting concept.

Brett Denevi: Yes, absolutely. Uh, the geophysical network would be to look at the structure of the moon's interior. It's um, you know, placing seismometers and heat flow, uh, probe measurements. Um, and those would be, you know, something that you would want to do, not just in one location, but distributed across the moon's surface's network to really be able to pinpoint locations, uh, you know, seismic events and, um, their depths in the Lunar interior, so that we can learn about, you know, the structure and composition of the moon's [00:54:00] interior.

Mat Kaplan: So this would be, it sounds like a lot like the insight mission...

So this will be, it sounds like a lot like the InSight mission on Mars, hopefully, with the, you know, successful mole devices, uh, but- but you put a bunch of these, uh, a network of them, uh, on our much nearer neighbor.

Brett Denevi: Yeah, absolutely. And there were seismic m... investigations from the Apollo missions that lasted well beyond, um, you know, when humans were there. Uh, those gave us, with older technology and much noisier measurements, those gave us hints at, uh, the things we could learn now from a- a modern set of measurements and investigations.

Mat Kaplan: Mm. Brett, let's close with the renewed interest, um, by NASA and the United States in putting humans on the moon, the Artemis program, and, uh, maybe one of the side effects of that is this renewed interest in- in lunar science. Have you seen anything in this? And- and does it [00:55:00] also, uh, make you hopeful that, uh, maybe, uh, science is going to benefit by, uh, uh, men and women going up there to plant some more flags?

Brett Denevi: Oh, absolutely. I mean, anytime you can have science and exploration working together, they both benefit. There's a lot of interesting science going on now to understand the south polar terrains where astronauts will hopefully return. And then, I will just poke at your question a bit in terms of a renewed interest in lunar science because-

Mat Kaplan: [laughs].

Brett Denevi: ... the lunar science community has been [laughs], you know, going on all of these years, and the interest is always very high. One of the main areas that can really benefit science from the Artemis program is the ability to return, you know, large quantities of samples to answer some of these questions, to explore, you know, a new terrain. Um, you know, no samples have come back yet from the south [00:56:00] polar region, which is very different than a lot of the nearside, uh, volcanic deposits and it's in this, you know, true ancient highlands terrain.

And then, just the fact that we can also start, you know, having the astronauts bring along these long duration investigations that they could leave behind on the surface, and those could continue to send back, um, data for many years after the astronauts are there. And then, especially, if you have a situation where you have this longer term presence on the moon, such as a field station that is part of, you know, the phase two of the Artemis program, that is when you can really start doing some, you know, exceptional science that requires longer drives, more, uh, detailed geologic field work rather than kind of the s... the shorter stays that are envi- envisioned at the beginning of the Artemis program.

Mat Kaplan: [00:57:00] When I step outside this evening and, uh, look up at the moon, uh, maintaining social distancing of course, uh, I will be thinking about all of this. It- it sounds like at least a potential for a very exciting decade as we, uh, learn more about, uh, our closest neighbor in space, the one that, and I stole it from you, that the one that we've been doing [inaudible 00:57:21] with for billions of years. Thanks so much for, uh, giving us this- this review and preview.

Brett Denevi: Yeah, absolutely. My pleasure.

Mat Kaplan: Brett Denevi of the Johns Hopkins University Applied Physics Lab. Our survey of the next 10 years of solar system exploration will continue next week with Mars, the outer planets and the smaller bodies of our solar neighborhood.

Time for What's Up in the time of COVID-19 on Planetary Radio. We are joined by the Chief Scientist of The Planetary Society, that's Bruce Betts, and, uh, I- I hope you're doing well, holding up in these, uh, [00:58:00] strange times that we're all living through.

Bruce Betts: I have been. I've- I've been sick and- and I don't know what... with what, but not severe, so we're good.

Mat Kaplan: Yeah.

Bruce Betts: The nice thing is the night sky, uh, you can go out and see it without around you.

Mat Kaplan: You know, I actually finished a little segment with Emily saying exactly that today that, uh, you know, even if you can only look at a window, I hope you have clear skies, as Bruce said. So, uh, what should people look out of their window at?

Bruce Betts: Well, if they're looking in the evening, try to get a view to the southwest and you'll see Venus just super bright up high. And on the March 27th and 28th, it will be hanging out, uh, with the moon, the crescent moon, looking quite lovely. And it's got, to its upper left, kind of in a line is, uh, Aldebaran and Betelgeuse, so two bright stars. And then the pre-dawn, we still have a planet party right now in the pre-dawn east.

You've got Mars, red, getting brighter and brighter over the coming [00:59:00] months. Mars hanging out in between Jupiter to its upper right, the brightest object, and then Saturn to its lower left. And they'll be snuggling and, uh, Mars will snuggle Saturn on the 31st and then move, uh, down to the lower left of Saturn. And you've got a clear view to the horizon, you still can pick up Mercury to the lower left of Saturn much farther down. So there's all sorts of good stuff to look at.

We move on to this week in space history. It was 1974 that we got our first closeup look from a spacecraft at Mercury as Mariner 10 flew by Mercury for the first time.

Mat Kaplan: Uh, just a flyby mission, as you've said, right?

Bruce Betts: Yeah, just flyby, and it did three flyby's and then, uh, it just started barking.

Mat Kaplan: [laughs].

Bruce Betts: It was like, "Mercury, Mercury, Mercury."

Mat Kaplan: [laughs]. You expect that from Pluto, but not Mercury. [laughs].

Bruce Betts: [laughs]. That- that's the real [01:00:00] reason Mariner 10 didn't go into orbit around Mercury. By Messenger, they've worked out some, you know, ability to go to make noise even in space, so Messenger used the, and, uh-

Mat Kaplan: [laughs].

Bruce Betts: ... controlled the... stopped the barking. So [laughs][crosstalk 01:00:15]...

Mat Kaplan: You may continue, sorry. [laughs].

Bruce Betts: Not surprisingly, I'm working at home [laughs].

Mat Kaplan: [laughs].

Bruce Betts: There are large mammals. All right, uh, [laughs] we move on to Random Space Fact.

Mat Kaplan: Well played, sir. [laughs].

Bruce Betts: Okay. So, if the Earth and moon were, I don't know, let's say, six feet away from each other, 1.8 meters-

Mat Kaplan: [laughs].

Bruce Betts: ... then the sun would be a very safe, more than seven football fields away.

Mat Kaplan: Cosmic distancing. [laughs].

Bruce Betts: #universaldistancing.

Mat Kaplan: [laughs].

Bruce Betts: I'm starting it right now.

Mat Kaplan: Also well played.

Bruce Betts: [laughs]. All right. We move on to the trivia contest, and, uh, I had pointed out, [01:01:00] Neptune's Triton is by far the largest solar system moon to orbit retrograde, and I asked you what is the second largest planet moon in the solar system to orbit retrograde or the opposite direction of the planet's rotation? How did we do, Mat?

Mat Kaplan: A terrific response this time. Uh, and I'm not surprised 'cause we had a great prize and- and of course, everybody's stuck at home, listening to Planetary Radio, one would hope. Um, here is...

Bruce Betts: What torture.

Mat Kaplan: [laughs]. Not our winner, but Joel Lechter in Quebec, this was his response, "I'm hoping this turns out to be Saturn's moon, Phoebe, discovered by William Pickering in 1899 from photographic plates taken a year earlier at, uh, an observatory in Peru." Is Joel and- and so many other of our entrants, are- are they correct?

Bruce Betts: They are correct, and I did not know the tip at about the Peruvian observation.

Mat Kaplan: Yeah, interesting, huh? He says, "Boyden Station of the Carmen Alto Observatory, uh, [01:02:00] in Peru, near Arequipa, Peru," actually. Yes, I'm glad to hear that and so is or will be, when he hears about this, Henry Sanford-Crane in Maryland, a first time winner, as far as I can tell. He indeed said, "Phoebe?" He has won himself a Planetary Society rubber asteroid and a hard cover copy of Ann Druyan's new book, Cosmos: Possible Worlds, the companion to the absolutely outstanding television series that we were talking to Ann about, uh, just a couple of weeks ago on this show.

Henry adds, "The original Cosmo series, when it came out on DVD, made me go out and upgrade from my VCR player." He still says, "Billions and billions," in Carl's accent, of course, uh, Carl- Carl's dialect, almost. Um...

Bruce Betts: [laughs].

Mat Kaplan: [laughs]. We love that, of course. Well, congratulations, Henry. We're- we're, um, we're happy to hear that you're enjoying the new Cosmos as well. A bunch of other good stuff. Rob [inaudible 01:03:00][01:03:00] in Massachusetts, "I was looking for something clever to say about the name Phoebe and was blown away with how many people, famous, real people, and mythological, ships, plants, animals, journals, astrological features and music are named Phoebe."

Bruce Betts: And, uh, one of, uh, The Planetary Society's employee's daughters is named Phoebe, that's probably the most important.

Mat Kaplan: Laurence Desind in New Jersey, "I remember memorizing in the 1950s that Saturn had nine moons." Hah.

Bruce Betts: [laughs]. It does have nine.

Mat Kaplan: [laughs].

Bruce Betts: [crosstalk 01:03:36] it has 73 additional moons, plus [crosstalk 01:03:38].

Mat Kaplan: [laughs]. Ah, God. I love living in the future. He said Phoebe was the last of the nine back in- back in those days, and get this, three, count 'em, three poems. Jean Lewan, Fairchild Air Force Space in, uh, Washington state, "Neptune tops the largest moon to orbit retrograde. Second, one of Saturn's orbs, though this [01:04:00] is not meant to throw shade, named for Phoebe, a Titaness in Greek mythology, which has craters named for Argonauts, who, with Jason, sailed the seas." And then, just to top it off, he added, "Phoebe's daughter, Leto, also gave us Apollo and Artemis, handy to know."

Bruce Betts: [laughs]. Mythological space fact.

Mat Kaplan: Dan Taylor in Minnesota, "Oh, Phoebe, you are not so bright as your name suggests. The scars reveal your troubled past. Cassini was right about how icy and cold you are, and I wonder, as you go against the grain, if this is just your independence you maintain."

Bruce Betts: Wow. People are so serious.

Mat Kaplan: A little bit lighter here, same theme though, from Marine Bends in Washington state, "Phoebe belongs to Saturn, but she chooses her own path. Retrograde, she prefers, Cassini did the math."

Bruce Betts: [laughs].

Mat Kaplan: [laughs]. I- I would be remiss [01:05:00] if I didn't mention all of the folks, and there must've been scores, who talked about The Expanse and Phoebe, because of course, everybody who've seen The Expanse knows that Phoebe actually originated far, far across the galaxy, and you do not wanna go there on vacation, not unless you're looking for a protomolecule hogey or something like that. [laughs].

Bruce Betts: [laughs]. Well, thank you for that important safety tip.

Mat Kaplan: [laughs]. That's really it. We can go on.

Bruce Betts: Maintain universal distancing.

Mat Kaplan: [laughs].

Bruce Betts: By the way, uh, don't know if it got mentioned there, but, uh, Phoebe's diameter, 213 kilometers compared to Triton's 2,710 kilometers. So, a wee bit smaller.

Mat Kaplan: We did have a couple of people who say it, it was like a 10th or less than a 10th the size of, uh-

Bruce Betts: Yeah.

Mat Kaplan: ... of that- that next, uh, big retrograde body up.

Bruce Betts: Retrograde. Okay. Moving on.

Mat Kaplan: [laughs]. [01:06:00] Was- was that a dog version, a canine version of the word retrograde?

Bruce Betts: Yes [laughs]. Max is trying to learn astronomical terms.

Mat Kaplan: [laughs].

Bruce Betts: So we move on to the next question, which is who was the first person to do a deep space EVA? So, extravehicular activity, and by deep space here, I'm defining it as outside of low Earth orbit. First person to do a deep space EVA. Go to

Mat Kaplan: You have until April 1st, no fooling, Wednesday, April 1st, at 8:00 AM Pacific time, to get us this answer. Gotta give you a special heads up about prizes. Because The Planetary Society is observing good social distancing and sheltering at home, there's nobody there to send out prizes and, uh, we wanna respect, uh, the other folks that we, uh, also have sending out prizes now and then, like Chop Shop, [01:07:00] although of course, you can still check out The Planetary Society store at Instead of sending out prizes for the duration, Bruce had this great idea, something we used to do a long time ago, haven't done in ages. Tell them about it.

Bruce Betts: Well, if- if you want this prize, which is, of course, amazing, uh, Mat and I will record a message for you, for example, to use on your voicemail, or just Mat if you don't want me.

Mat Kaplan: [laughs]. Just Bruce, for that matter, or neither of us. It's totally optional. It's not required [laughs]. [crosstalk 01:07:35]...

Bruce Betts: [laughs]. Your prizes that you can say no.

Mat Kaplan: Um, [laughs]. Anyway, that's the deal. If you're interested, I guess what we'll do is, you know, send you a little MP3, staying in touch, of course, in a virtual fashion, that's your prize. Plus, our- our undying admiration.

Bruce Betts: [laughs].

Mat Kaplan: And with that, we're done.

Bruce Betts: Always. All right, everybody. Go out there, look up on that sky and think [01:08:00] about staying healthy. Best wishes, everyone. Thank you, and good night.

Mat Kaplan: And the same to you, Bruce. He's the Chief Scientist of The Planetary Society who joins us every week here for What's Up. Planetary Radio is produced by The Planetary Society in Pasadena, California, and is made possible by its members who are helping to keep us in a golden age of solar system exploration. Join them by visiting

Mark Hilverda is our Associate Producer, Josh Doyle composed our theme, which is arranged and performed by Peter Schlosser. Hope you're staying safe, everybody, and keeping everyone around you safe as well. Want something else to help pass the time? Give us a review or a rating in Apple podcasts or some place else. Thanks, and ad astra. [01:09:00]