Planetary Radio • Jul 19, 2023

Mars' Axial Tilt: A Key to Gully Formation

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James Dickson

Research Scientist in Image Processing at the California Institute of Technology and Director of Polar Geospatial Center at University of Minnesota

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

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Sarah Al-Ahmed

Planetary Radio Host and Producer for The Planetary Society

The gullies of Mars may appear similar to water-carved channels on Earth, but their formation is more complex than meets the eye. Caltech's Jay Dickson joins Planetary Radio to discuss the planet’s changing axial tilt and the consequences of Martian climate change. Then Bruce Betts shares the beautiful dance of planets in the upcoming night sky and the reflections of the oldest person to ever travel to space.

Martian gullies
Martian gullies These Martian gullies, which were imaged by NASA's Mars Reconnaissance Orbiter, may have been formed by liquid water. The channels range from roughly 1 to 10 meters (3 to 33 feet) wide.Image: NASA/JPL-Caltech/University of Arizona
Martian gullies
Martian gullies This Mars Global Surveyor Mars Orbiter Camera image shows a gullied crater wall in the Terra Sirenum region of Mars. The fluids that formed the gully channels and deposited debris in aprons at the base of the crater wall may have percolated through layers in the layered bedrock exposed in the crater wall. As fluid seeped out of the ground, it undermined overlying layers of rock, and caused the formation of several compex alcoves, higher in the crater wall. The formation of alcoves requires the undermining and collapse of layered material at a point where the channel begins; this is a key observation supporting the hypothesis that martian gullies require groundwater, not snowmelt or other exotic processes or fluids, to form.Image: NASA / JPL / MSSS
Frosted gullies on Mars
Frosted gullies on Mars This HiRISE RGB colour view shows frost within gully alcoves in the southern mid-latitudes of Mars in winter. Notice how the slopes outside of the gully alcoves are mostly frost-free.Image: NASA / JPL-Caltech / University of Arizona / Tanya Harrison

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Question from the July 5, 2023 space trivia contest:

Who is the oldest person to go to space?


William Shatner

Last week's question:

In Bruce Bett's Ph.D. thesis, he quoted the musical group Warrant at the beginning of one of his chapters, saying, "Dancing with my shadow and letting my shadow lead." What shadow was he referring to?


To be revealed in next week’s show.


Sarah Al-Ahmed: Mars is no longer a watery world. So how did all of those gullies form? We're digging into the mystery this week on Planetary Radio. I'm Sarah Al-Ahmed of The Planetary Society with more of the human adventure across our Solar System and beyond. The oceans of Mars dried up or froze a long time ago, but some of the more recent features on the planet's surface, like Martian gullies, suggest that they could have been formed by flowing liquid water. How is that even possible? Our guests this week, Jay Dickson and his colleagues, think that they might have an answer, and it has everything to do with the tilt of the red planet over time. Then the ever awesome Bruce Betts will join me to talk about what's up in the night sky this week. We have to start off by congratulating the Indian Space Research Organization or ISRO. Their newest moon mission, Chandrayaan-3, successfully launched on July 14th. Chandrayaan-3 consists of a lander and a rover that will attempt to land near the moon's south polar region on August 23rd. You may remember that the Chandrayaan-2 Vikram lander crashed in 2019, but ISRO learned a lot of lessons from that mission. They say that they've performed numerous tests to ensure that Chandrayaan-3 goes according to plan. And in other space news, data from the European Space Agency's Cheops mission have revealed the shiniest exoplanet ever found, located 262 light years from Earth, Planet LTT 9779 b is roughly the size of Neptune and reflects 80% of its host star's light. That's more than even Venus, which sky watchers know is super shiny. Researchers believe this exoplanet may be shrouded in metallic clouds that act as a mirror on incoming starlight. And the European Space Agency's Mars Express Mission marked its 20-year anniversary on June 2nd, 2023. To celebrate, the spacecraft's high resolution stereo camera team created a new Mars mosaic. Each constituent image in the mosaic is individually color matched using high altitude imagery models. The result is a richer color global view of Mars than has ever been created before. You can find that image and more information about all of these stories in the July 14th edition of our weekly newsletter, the Downlink. Read it or subscribe to have it sent to your inbox for free every Friday at One of the things I love about planetary science is that the more you explore the worlds around us, the more questions present themselves. An observation that seems simple at first can spin off into realms of complexity that you really did not see coming. Our topic today is a great example of this. The gullies of Mars seem very familiar at first glance. They appear similar to water carb channels on Earth, but as we all know, Mars is not the ocean-covered world that it used to be. Some researchers have suggested that the gullies on Mars could have been formed by frozen carbon dioxide, more popularly known as dry ice. But new research by our guests this week, Jay Dickson and his colleagues, proposes that the gullies might be caused by something way more complex. Dry eye still plays a role, but to get to the real story behind Martian gullies, we have to understand the changing obliquity or axial tilt of the planet over time. Jay is a research scientist in image processing at the California Institute of Technology with a career spanning over two decades. Jay is a pioneer in the study of planetary surfaces and remote sensing imaging. His work has taken him to some of the most extreme corners of our planet, including the wilderness of the McMurdo Dry Valleys in Antarctica. He studies our planet and then applies what he learned to the enigmatic landscapes of the moon and Mars. Jay manages the Caltech Geographic Information Systems Laboratory and the Bruce Murray Laboratory for analysis and visualization of planetary data. Fun fact, that's the same Bruce Murray that co-founded The Planetary Society. Jay's most recent work, which we're about to dive into, involves an exploration into the Amazonian climate of Mars. But he's also a co-investigator on the Lunar Trailblazer, a NASA mission aiming to unlock the mysteries of our moon. His team's new paper is called Gullies on Mars could have formed by melting of water ice during periods of high obliquity and was published in the Journal Science on June 29th, 2023. Hi Jay, thanks for joining us on Planetary Radio.

Jay Dickson: Hi, Sarah. Thanks for inviting me. Glad to be here.

Sarah Al-Ahmed: A few months ago, actually, we were having a conversation on the show about the biggest image of Mars ever create , the global CTX mosaic. You were the lead on that project, right?

Jay Dickson: I was, yes, for about five years.

Sarah Al-Ahmed: Gosh, that was so cool. Bruce and I had a great time talking about that and I legit, I think, spent an hour just zooming in and out of it. It was awesome.

Jay Dickson: That's been a phenomenal project that I've been fortunate to lead and it's the culmination of about 20 years of imaging data of Mars from the Mars Reconnaissance Orbiter and the context camera that images Mars at about five meters per pixel. So I spent about five years on it. The team, the mission team, spent about 20 years getting all that data. So it's phenomenal to see scientists and the public at large have fun.

Sarah Al-Ahmed: And how cool is that, that you gather all this data over decades and decades and you can finally put it all into one giant map. I wish we could have that data on every planet. Can you even imagine?

Jay Dickson: Indeed, we are spoiled on Mars, that's for sure.

Sarah Al-Ahmed: But it's a good planet to be spoiled on. There are so many mysteries and so many interesting comparisons you can draw between our planet and Mars, which is what brings us here today. We're trying to piece together the mystery of what formed gullies on Mars. So I guess we have to start at the beginning. What are gullies on Mars and how are these different from, say, rivers?

Jay Dickson: Gullies on Mars, they're named that because there are features on earth. Those of us in Southern California see them in the San Gabriel Mountains all the time. These are channels that are carved into hill slopes, so typically pretty steep slopes. On Mars, they're typically found on impact crater rims, so steep slopes, and they're sinuous or winding channels that go down these slopes. They were discovered in the year 2000 when we first started getting high resolution images of Mars and there's been a debate in the community for now, almost a quarter of a century as to what formed them, because when you look at them, if you saw them on earth, you'd say, "Oh, they were carved by liquid water, no problem." But as the listeners know, Mars is extremely cold, extremely dry, and it's very hard to get liquid water on the surface today. So to answer your question, they're different from rivers because rivers go across country, they go on much lower slope terrain. So these are shorter channels on steeper slopes and they've been a real enigma for the scientific community for a quarter-century.

Sarah Al-Ahmed: That is an enigma. Are we sure that they formed more recently? I'm guessing because they're on craters that we can probably date.

Jay Dickson: Yes. So that was one of the really provocative factors upon their discovery is that the scientists who discovered them, Mike Malin and Ken Edgett down in San Diego, they noticed that all evidence points to them being very recent. Now, for Mars, very recent, we're talking the last 10 million years. They also postulated that they could be active today. So when we think about Mars, we have very strong evidence that Mars was very wet early in its history about 4 billion years ago, three and a half or 4 billion years ago. But the general sense is that Mars has been a very, very cold, very, very dry place for the last 3 billion years. So the fact that they appear so recent, which ... We know that because there are very few features that are on top of them, so there are no impact craters on them, typically, and they just have a very fresh appearance. We've known that they're at least either forming today or in the very, very recent past, the past million years or so.

Sarah Al-Ahmed: And that's puzzling. I know there are several other types of these features that they look like they were created by water, but the only explanations we can come up with are either there's some sand moving or maybe there's some sublimation of carbon dioxide ice. What is it about these gullies that suggests it must be some fluid that created them and not some other mechanic?

Jay Dickson: Well, sublimation of carbon dioxide gas is one of the better hypotheses for actually what is forming these features. Our paper says it's probably something else, but if you look at these gullies, they are changing today, and that's a very, very active area of research and there've been phenomenal observations that show changes to these systems. And we're debating how big are these changes, what exactly is happening. But the timing is very consistent with carbon dioxide ice sublimating or vaporizing into a gas and that's triggering potentially dry flows that's moving material within these gullies. There are some scientists who strongly argue that that could be forming these gullies wholesale and you don't need liquid water at all. So that is one hypothesis. It's very challenging to test because there's no process like that on earth. So frozen carbon dioxide doesn't ... Dry ice isn't stable on the surface of the earth, so that's a very active area of research of this alien process of carbon dioxide. Something is causing those changes today. Our paper argues that the gullies weren't formed that way.

Sarah Al-Ahmed: It's just the most recent changes that are caused by this carbon dioxide. That's really interesting because that would be a little frightening. You're looking at it thinking, "This thing formed quite a long time ago and it's moving."

Jay Dickson: Yes. The scientists who have mapped these changes, just stunning work, typically with a high-rise camera submeter imagery of the surface. So you can see it at a really small scale and in some features, not all, but in some features, you do see fairly substantial changes to these channels.

Sarah Al-Ahmed: So that means that something else must have formed them. And this is why I had to bring you in here because what this paper is suggesting is that liquid water helped form these gullies, but it was specifically because the tilts of Mars had changed so drastically over its history that it allowed for this liquid water. Wow, right off the bat. But I guess we have to define some terms here. This paper says that it's because of Mars's obliquity that this happened. Can you define obliquity for everyone who's just new to planetary science?

Jay Dickson: Yeah, if you take any introductory astronomy or planetary science class, you have visual demonstrations. So I'm going to do the best I can without visuals here. Obliquity is the tilt of the rotational axis of a planet. What that means is ... So there's an imaginary pole that goes from 90 degrees north in the Arctic. So I'll talk about earth in this instance, from the Arctic down to the south pole, from the north pole to south pole. If you extend that pole through the entire planet, that pole is tilted relative to the planet's orbit around the sun. So all planets have an obliquity or anything in orbit of something else that rotates has an obliquity. The amount of tilt is really, really important. So the moon, for instance, barely has any tilt, so it doesn't change too much over the course of a year. The earth's obliquity, that tilt of the rotational axis, is about 23.4 degrees, and that's enough to give us seasons. So it points the north pole at the sun during summer when we're recording this and it points the south pole towards the sun during Northern Hemisphere winter. That's why they have a summer. So obliquity is simply the tilt of the axis. Now, the earth's obliquity doesn't change by more than a degree or two over tens of thousands of years. That change can cause ice ages. So even a small change can have traumatic climatological effects. Mars' is obliquity, we know with some assurance that it changes by 10 degrees or more over the last couple million years. So that's what prompted us to start this study. What were these gullies like at high obliquity?

Sarah Al-Ahmed: So what we're talking about here is the obliquity of a planet, which is its axial tilt relative to the plane of the Solar System, but there's another effect other people may have heard of, which is the procession of a planet's axis. These two things are different but slightly connected. So can you explain the differences between those?

Jay Dickson: Yeah. So the obliquity, as you mentioned, is the tilt of the rotational axis relative to orbital plane. The procession is ... The example that we're always giving to students in an introductory course is that if you had a spinning top, you will notice that as it slows down a little bit, it starts to wobble left and right a little bit. That's the procession. So the earth does that, just over much longer timeframes. So it's combination of the obliquity, the axial tilt, but also this rotational wobble that happens over a different scale. Those two add up to determine where the rotational axis is actually pointing. So two separate phenomena, but, as you mentioned, extremely closely related.

Sarah Al-Ahmed: How do we know that the tilt has changed so drastically?

Jay Dickson: There are two different ways of going through it. One research that I don't do myself is numerical simulations of Solar System evolution. So you put all the planets in their proper place. So Mars is affected by the orbit of Jupiter and so on and so forth, and how they gravitationally work together. Within the last 10 million years or so, if you crunch the numbers, Mars must have had its axis tilted if you simulate the Solar System from a numerical perspective. The work that I do is focused on the inverse way. Is there evidence on the surface that Mars underwent some dramatic climate changes that are best explained by an obliquity change? And there is. We don't have time to go into it all here, but we see ice beneath the surface at latitudes where ice is not stable today in regions of Mars where you couldn't get ice on the surface today. So that's best explained by the tilt of Mars' axis changing. There are other factors that go into it besides obliquity, but that's the main one. So there are two methods that have converged. One is the numerical models with predictions and simulations and then one is the empirical and geological evidence, and they paint a pretty clear story that the climate of Mars has changed, and that's likely due to the obliquity changing many times over the last 10 million years.

Sarah Al-Ahmed: Yeah. Well, a little change can cause an ice age. Just imagine what a 10-degree change could do.

Jay Dickson: Exactly.

Sarah Al-Ahmed: But earth's tilt is pretty stable. We have things like the moon or even our oceans stabilize our axial tilt. And this brought up a question for me, which is knowing that Mars' axial tilt changes so much, why is that, and did the fact that its ocean disappear in any way make that wobble even worse?

Jay Dickson: Wow. So you hit on the key factor early is that the earth has a large moon and the gravitational pull of the moon helps to keep the earth from wobbling back and forth, and that's the main reason that Mars does wobble much more. I shouldn't say wobble. Wobble is a separate thing for what we do.

Sarah Al-Ahmed: That's true.

Jay Dickson: So the obliquity changes. Mars doesn't have a large moon to stabilize or to keep it at a relatively constant value like the earth does. So the ocean shouldn't have or the lack of an ocean shouldn't, to my knowledge, play much of a role in the obliquity changing, whether Mars had one in the past or not. My assumption is that that shouldn't impact it very much.

Sarah Al-Ahmed: Well, that's good to know, at least, because there's a big uncertainty on how long Mars had water on the surface. We can take that out of the equation. So we're looking at Mars and how its climate changes over time based on this obliquity. How do we actually model this? Because this was a big part of how you came to this conclusion with the gullies.

Jay Dickson: Right, right. So there are a couple of groups in the world who write these very sophisticated computer programs that import all of our knowledge of the physics equations of how planets work, solar energy from the sun if Mars is at this distance. The obliquity, you can make predictions, how does the temperature of the surface at one location change from 25 degrees obliquity where Mars is now to 35 degrees obliquity where it was 630,000 years ago. And then you put those equations into a model that includes the topography of Mars. So we know the shape of Mars at a global level quite well and then we know things about the atmosphere. So everything we've learned about how climate works on earth, we can apply that to Mars, and it's very hard. But there are some advantages to modeling the climate of Mars that we don't have on the earth. One is the lack of large ocean or any ocean at all. The ocean complicates things on Earth, to a large degree, and Mars doesn't have an ocean right now, and it hasn't over the last million years, this timescale of this study. Second, Mars, to our knowledge, doesn't have plate tectonics. So the shape of Mars doesn't change like the shape of the Earth does, where you have continents drifting all over the place. Mars has, at the global scale, stayed very stable for millions of years. So if you look at it that way, we actually can understand the climate of Mars using these computer simulations well going back quite a long time. So that's what we run and that makes predictions about what we see on the surface, and then we take those predictions and test those predictions with our mapping of the surface.

Sarah Al-Ahmed: How quickly does Mars change in obliquity?

Jay Dickson: Last time ... So right now, Mars is around 25 degrees obliquity, quite similar to the Earth's tilt right now. The last time, it was at 35 degrees obliquity was about 630,000 years ago in that ballpark. So it's slow by human standards, but it's been doing that for millions of years. We know that with pretty high confidence going back and forth between low obliquity and relatively high obliquity. So it's on the hundreds of thousands of years' timescale.

Sarah Al-Ahmed: If the extreme is 35 degrees on one end, how close can it get to zero on the other end?

Jay Dickson: I don't have the charts in front of me, but I believe it goes down to about 15 in the last 10 million years or so. And then there are consequences to that. That's not what we study in this paper, but that's my understanding. I'd have to look up the papers, but I believe it fluctuates between about 15 and 35 in the last few million years.

Sarah Al-Ahmed: When you were modeling this, did you check even worse extremes, what it would be like at 40 degrees just to see?

Jay Dickson: Oh, that's the next project I'm working on. I am working with some climate modelers to see what Mars might have been like at 45 degrees obliquity. We don't have those initial numerical simulations. We don't have unique solutions that tell us that Mars was quite likely at that. There's a decent probability that it was. So we're actually testing where would you get glaciation on Mars if the axis was tilted by 45 degrees? So we can make those predictions and then using that mosaic that we already talked about, we can go and map and see if those features are actually there. So that's to be determined whether Mars was actually ever at that high obliquity, 45-degree state.

Sarah Al-Ahmed: That would be intense. That's a huge tilt.

Jay Dickson: Yeah.

Sarah Al-Ahmed: So as we get from 15 down to 35, how does this affect the formation of gullies?

Jay Dickson: Yeah. So that's what this paper tried to address. And we're not the first people to come up with this idea of, what if gullies formed relatively long ago, like recent geologically, but 630,000 years ago. So there was a paper in 2002 that proposed this by Franz Wacotard saying that maybe the last time Mars was at high obliquity, gullies could have formed. So it's been argued for a while, but we have a lot more information now than we did back then. And what we know happens now is that when the axis tilts to 35 degrees, there is a reservoir of CO2 ice, that dry ice we were talking about, that right now is trapped in the south pole of Mars, but at high obliquity at 35 degrees obliquity, that gets sublimated or it turns into vapor and goes into the atmosphere. So the calculations show that that should double the atmospheric pressure of Mars. So the atmosphere of Mars right now is really, really, really thin and this would make it really, really thin. So it's not going to make it a really dense atmosphere, but it doubles it. And what that does is that when you double the density of the atmosphere, that increases the pressure at the surface. And pressure is one of the key ingredients that you need to melt ice on the surface. So to melt ice on the surface, you need three things. I'm oversimplifying this, but you need three things. You need ice on the surface in the first place, so we had to show that that's there. Then you need temperatures above the freezing point for pure ice, the melting point for pure ice. Then the third thing you need is enough atmospheric pressure, otherwise it would just vaporize into the air like CO2 does. So our study was a detailed investigation of how does the pressure at the surface change at these gully locations and it made predictions that we were able to see correlations with for gullies on Mars.

Sarah Al-Ahmed: There seemed to be quite a bit of difference in how much tilt was required for the Northern Hemisphere to form gullies versus the Southern Hemisphere. Why is there such a big difference there?

Jay Dickson: Yeah. So I think a lot of the listeners to the show will know that Mars is like two different planets in one. In the Northern Hemisphere, where there's hypothesized to have been an ocean, it's very low topographically. So that's where you would get an ocean is in the lowlands. The Southern Hemisphere is at a higher elevation. And the way atmospheres work is that the lower your elevation, the more air you have above you, so the higher the pressure. So if you're in the Northern Hemisphere of Mars, you're at a low elevation, so you have more air above you, so you have higher pressure. And higher pressure means the higher likelihood that you could potentially get liquid water on the surface, that it would transition to a liquid instead of a gas. So you could theoretically have melting conditions on the surface of Mars today in the Northern Hemisphere. The problem is that it's hard to get ice to stay on the surface, so the ice would sublimate first before you reach the melting point. So we don't expect melting today because we only have two of the three components that you need. In the Southern Hemisphere, which is really the focus of our study, you're at a higher elevation so it's harder to get those high pressures that are the minimum required for melting of ice at the surface. But at high obliquity with this increased pressure, you do get above that pressure burial. We call it the triple point where liquid water could form. You are able to achieve that at exactly the same elevation that gullies have been mapped on the surface. And that's the key correlation is that the pressure increases just enough at these locations that you would predict potential melting. You need some other things to go right, but potential melting but not get melting at higher elevations. And that's 100% consistent with where gullies are mapped. So that's our argument is that this explains why you have gullies up to a certain elevation but not higher, and that is explained by liquid water at high obliquity, not today, but at high obliquity. So that's the main correlation that we try to talk about in the paper.

Sarah Al-Ahmed: Do we see any difference in the number of gully formations between the northern and Southern Hemisphere?

Jay Dickson: Yes, but for different reasons. There are many, many, many more gullies in the Southern Hemisphere. The reason for that's very clear is that you have more steep slopes. When we first started talking about this, I mentioned that these are relatively small channels that are formed into very steep slopes. Mars is strange in that the Southern Hemisphere has a lot of very steep slopes and the Northern Hemisphere has very few steep slopes. There are geological reasons for that that I won't go into, but you need steep slopes to form these features.

Sarah Al-Ahmed: So strange. I don't know how long it's going to take us to figure out why the southern part of Mars is so different from the northern part, but it has huge impacts on how we explore Mars. I mean, people will note we mostly land in the Northern Hemisphere because we need that atmosphere to cushion our spacecraft. We'll probably crash if we try it in the south.

Jay Dickson: That's exactly right. Some of the best places to send a rover are just too high and you don't have enough atmosphere to slow your rover down. So we had to eliminate those.

Sarah Al-Ahmed: Yeah, we'll have to get some really, really intense sky cranes to go check out the Southern Hemisphere.

Jay Dickson: Yeah.

Sarah Al-Ahmed: But in order to form these gullies, this paper proposes that it must be at least 35 degrees obliquity. That's intense.

Jay Dickson: Correct. We did some modeling at 30 degrees obliquity and at 25 degrees obliquity to make predictions about what we should see today. And there's only at 35 degrees and then only a few other properties of Mars orbit. I won't go into the details here, but basically we modeled the best possible case to maybe get liquid water in the Southern Hemisphere. And we didn't turn any knobs for this. We just said, "Here's what Mars was like 630,000 years ago. Could you have gotten liquid water at the minimum conditions for liquid water?" And it correlated very well with where we see these gullies.

Sarah Al-Ahmed: It's very strange. Just trying to imagine Mars being tilted that far over. We'll be right back with the rest of my interview with Jay Dickson after this short break.

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Sarah Al-Ahmed: Other than the formation of gullies, are there any other things on Mars, other pools of water or things like this that we would see, or is it just not a huge amount of melting, just enough to form these gullies but not to form, say, lakes and stuff like that?

Jay Dickson: Yeah, I think it would be a much more ... I mean, we're pushing it to claim that liquid water carved these gullies. There are going to be some scientists who take our global study and look at it in more detail and really see if you can actually get melting at the [inaudible 00:28:34]. The global story points to that. But down to the level of details, we need to really see if this could actually happen. It would be a much larger stretch to suggest that there could have been standing bodies of water. It's one thing for a little bit of water to cause what we call debris flows, flows down a very steep slope like these gullies. But even under these generally more favorable conditions, if you put liquid water on the surface, it would evaporate right away. It's much, much harder to get standing bodies of water, unlike early Earth where the Perseverance rover is and where the Spirit rover was and Gusev crater or Perseverance rover and Jezero crater, those were very clearly standing bodies of water, but three and a half or 4 billion years ago. So we're not suggesting that there was anywhere close to that level of stability of water on Mars.

Sarah Al-Ahmed: Yeah, that's a whole different period of Mars's history. And I mentioned this a little bit earlier. This was specifically called the Amazonian Epoch on Mars. What is that?

Jay Dickson: The Amazonian, different scientists have different numbers attached to it, but broadly, it's about the last three to three and a half billion years of Mars' history. So most of Mars, but Mars is like the Earth about four and a half billion years old. So the Amazonian, this era of dry, hyper-arid, very cold conditions has lasted, we think, for about 3 billion years. But that doesn't mean nothing happened. We're claiming that we had small amounts of liquid water at these locations, but Mars has been dominated by wind and ice. So we see evidence for glaciers. Mars has polar caps that are really, really important. And then wind has been dominating erosion on the surface for a long, long time. But we also think that the Amazonian has been characterized by ice ages as you have these obliquity tilts that you could have glaciers down to maybe about 30 degrees latitude. There are a few areas with glaciers at the equator. So Mars has been still an interesting place in the Amazonian, just not as much water as early on in Mars' history.

Sarah Al-Ahmed: Yeah. Are there any other mysteries that this could solve for us on Mars? We're just thinking about gullies here, but what else could be impacted by this obliquity?

Jay Dickson: The big one is the history of ice on the surface, and that actually has pragmatic implications. If we do want to send humans to Mars, it would be nice if we had big reservoirs of ice waiting for them. So we think that the models have predicted that at these higher obliquity periods, you should have the accumulating ice close to the tropics of Mars, which we're going to have to send astronauts to somewhere where the sun is shining, so warmer areas of Mars. And if there's ice below the surface that was accumulated there during these high obliquity eras, then that's fantastic. So there's actual boots on the ground pragmatism at work here. There are some other features, other sinuous channels on much lower slopes that appear to date to this Amazonian period. They're fairly small, they're localized, but we see them in different places. They could be explained by higher obliquity, but that's not entirely clear. So there are some other smaller areas that could have seen small amounts of liquid water that obliquity changes could explain, but that's work to be done.

Sarah Al-Ahmed: This is cool because we've found evidence with previous rovers that there is, in fact, ice under the surface of Mars in places that we didn't really expect when we first went there. So this could solve a lot of interesting questions, at least for me, and I'm sure for other planetary scientists.

Jay Dickson: One of my favorite anecdotes is that the Viking 2 lander landed in not in the Martian Arctic, but at fairly high latitude, and it saw frost on the surface. But 35, 40 years later, we discovered that there was very likely ice right beneath the surface. It just didn't have the instrumentation to detect it. But we see from orbit at these locations that there was very likely ice not far beneath the Viking 2 rover that's still sitting there.

Sarah Al-Ahmed: Using these climate models, could we make predictions about how deep you'd have to dig to find the ice or how much ice is there?

Jay Dickson: Yes, we could make predictions. So there are a lot of soil scientists, regular scientists, we call them, on Mars. They model what's called a diffusion rate if you have some ice, some sublimation rates, and then how far would you have to dig to get to that ice. But we also have ways of getting direct evidence. One, through a process called gamma-ray spectroscopy that uses energy from the sun to measure ice within the top meter of the surface. That eventually led to what was the Phoenix mission that actually sampled some of that ice beneath the surface. And then a really, really cool technique that I love is there are scientists who find new impact craters on Mars. These are impact craters that have formed within the lifespan of these missions that have been there for 10 or 15 years. And some of those impact craters actually excavate ice. And that tells you, one, where is their ice beneath the surface, and you can use some physics to determine roughly how deep it is. So one thing I love about planetary science is that you need many different perspectives to solve problems. So the story of ice in the shallow subsurface of Mars is definitely one of those stories.

Sarah Al-Ahmed: And it could have deep impacts on our future ability to explore, especially if we want to send humans there. It'll be a while, but people have aspirations of putting humans on Mars by the late 2030s, 2040s. It's right around the corner. And the biggest problem is going to be getting them water.

Jay Dickson: Getting them water. If you don't have to bring all of the water you need with you, we call it ISR ... We're great for acronyms, ISRU, In-Situ Resource Utilization. If we can use the ice that's already there, which is to be determined, we haven't tasted it yet, then that would make the logistics of sending humans there much more achievable.

Sarah Al-Ahmed: Yeah. And a lot easier to access just water ice under the surface than it is, say, mining opals and Gale crater, which some other people have suggested.

Jay Dickson: Yes.

Sarah Al-Ahmed: How did you end up on this journey to try to figure out these gullies? Because you've adventured to places like Antarctica. You've done some cool stuff looking at the different ice caps on earth and watching them over time. How did that translate to studying Mars gullies?

Jay Dickson: I've gone from Mars to Antarctica and back to Mars. I'm actually a planetary scientist because of these gullies. When they were discovered in 2000, I was a college student and I was assigned this paper in a class and just thought this was so amazing, and it was both alien and familiar at the same time, and that just really motivated me. So off and on, I've been working on this gully problem since they were discovered, and I was a college student. The Antarctic work was that we saw some features in the McMurdo Dry Valleys of Antarctica that resembled these gullies on Mars. And Antarctica is special in that it's the most Mars-like place on Earth. There are significant differences specifically with the atmosphere, but it's a natural laboratory where there's no rainfall and it's a desert, so it's the closest we can get to Mars on Earth. So I spent about 10 years documenting and studying how those features change with time. And while we found some similarities, we also found major differences that were very informative that helped us to narrow down what could be happening on the surface of Mars. So then after 10 years of going to Antarctica, I came back, started a lab at Caltech and got back interested in this gully problem and used some guidance from our fieldwork in Antarctica to help us understand what's happening on the surface of Mars or what was happening on the surface of Mars, we think.

Sarah Al-Ahmed: This does bring up a question for me. Antarctica doesn't have crazy dust storms like Mars does. Do these gullies fill in with debris because of these dust storms over time? And does that, in any way, make it more difficult to study them?

Jay Dickson: Fantastic question. So, yes, our model is that, at 630,000 years ago, there was potentially liquid water at these locations that carved these channels. The implication of that is that we say that they've been sitting around there for half a million years, not eroding. But as you said, there's a lot of dust. You have these dust storms, you have wind, and then when you have a channel, that becomes a trap. So dust and sand can blow in and it can be hard for it to blow out. That brings us to the amazing changes that scientists are documenting on the surface today, where you see areas that didn't appear to have a channel before and now there's a small amount of erosion. And as I mentioned, some scientists have strongly argue that that's what's causing these wholesale. And I look at them and I think what you're talking about is that the dust and sand blows in there and that's relatively easy to remove. So the carbon dioxide related activity we're seeing today, we've argued, is those finer particles being mobilized within, but not eroding down into the channel itself. That's our hypothesis. There are very smart scientists who would strongly disagree with me. So what's great about science is that we can have active debates. Very impressive changes in these gullies. I think it's these sand and dust particles that are being mobilized. Other scientists think that it's the actual erosion of the channels.

Sarah Al-Ahmed: Are these gullies more likely to form carbon deoxidizes inside of them just because they're shielded from the sun a little more or anything like that?

Jay Dickson: You answered that question perfectly. Yes. The local topography, the local shape of the surface, really matters for this. So the main variable is how much sunlight you get on the surface. And if you are a little patch of CO2 frost or CO2 ice or water ice or water frost, if you want to survive, you better be in a shaded place. And these gully alcoves, which are the big amphitheaters at the head of the channels, those are fantastic, what we call, cold traps. So if CO2 ice or water ice ends up there, it's more likely to stay. It's more likely to accumulate. So that's what sets it apart from some region out on the plains where it's going to get sunlight for a large portion of the day, which increases the temperature, which vaporizes it, and then it goes away. So the cold trapping process is critical for concentrating what we call volatiles, in this case, CO2 and H2O, at these locations. So it's a feedback effect. If you can erode the channel and the alcove in the first place, it becomes a better place to trap these ices. And then if you can melt or sublimate those ices, then it erodes it more and you trap more. So it's a beautiful feedback effect. No matter what is going on, either it's CO2 or H2O, that feedback effect is definitely taking part in what we're seeing.

Sarah Al-Ahmed: I wish we could get a little helicopter out there to take pictures of these things up close. Just imagine what we could see just even in the layers of the rock that have been washed away or potentially blown away by sublimation.

Jay Dickson: Yeah. And a helicopter or something like that would be the way to do it because these are extremely steep slopes that it would be, right now, impossible to get a rover like Perseverance curiosity into one of these features. But I've been approached by some folks at JPL who ... Now with the success of ingenuity, maybe we can go to some of these higher risk locations, but send a helicopter instead of a rover and get some really close-up data. I'm all for that, as you might imagine.

Sarah Al-Ahmed: And we'll at least get two little helicopters to go along with more sample return. Fingers crossed. But it could be worth it to just drop off a bunch of little captors all over Mars. It would be hard to do something like Valles Marineris, but I would love to see it.

Jay Dickson: There are people in development stage planning those missions like that. And then I love the big tanks, the big rovers that we send there, but I would love to just send a hundred smaller either helicopters or rovers to a hundred different places and explore Mars that way.

Sarah Al-Ahmed: I know that when people think about Mars, something that always comes up for them is the potential for life. And at The Planetary Society, we're all about the search for life, and Mars does currently look like a dry dust ball that might not have anything on it, but how would this change in obliquity over time potentially impact life that could have existed on Mars during its watery past?

Jay Dickson: Sure. So disclaimer that I'm not a biologist, so take this with a grain of salt, but if our theory is right is that you had small amounts of water 630,000 years ago that carved these gullies, then that's a good thing for potential life on the surface of Mars, just we're constrained by what we know of life on earth. We don't know of life that can exist without liquid water. And we also don't know of places that have liquid water that's not extremely briny or salty that doesn't have life, but that doesn't mean we can just say on Mars that since there was water there, there must've been life. This is where some of our Antarctic experience can be informative, that there are a lot of extremophile scientists. These are people who study life in extreme environments and they've run experiments with algal mats and creatures like that in Antarctica that are able to survive in some freeze-dried states for decades, but they stay alive. And then when water comes back, they flourish again. On Mars, instead of decades, we're talking about hundreds of thousands of years if there was life at these locations. So I don't know if life can do that, but I'd like to go there and check and see for myself. That being said, we expect that if there was liquid water that carved these gullies, there wasn't much of it, it probably boiled really fast if it did melt, so it would've behaved very differently, which is a long way of saying that I think NASA's strategy to look for life on Mars from its early history is absolutely the right way to go.

Sarah Al-Ahmed: Yeah. As you know, that's a hard challenge right there. And can you imagine how much people would freak out if we got those samples back from our sample return and there was even the slightest clue of something fossilized? It would revolutionize everything we know.

Jay Dickson: It would be amazing. Yep.

Sarah Al-Ahmed: So you already hinted at this a little bit earlier that now you're using models to see what it might be like at even further obliquities, but what other plans do you have to keep studying this topic?

Jay Dickson: I talked about how I went from Mars to Antarctica and then back to Mars. I've accepted a new job running the Polar Geospatial Center at the University of Minnesota, and that's polar on the earth. So I am going back to polar science on the earth using what we've learned from exploring the Solar System to tackle some of the fundamental problems of polar geoscience on Earth, which has much more to do with climate change on Earth, the changes in the Arctic and the Antarctic. So studying features on Mars that do change over time, we're going to take a similar perspective and study features that we know are changing on the Earth as well. So I'm taking a detour for a while.

Sarah Al-Ahmed: I love that, though, because so frequently at this job, we have to justify to people why is it so important to study other worlds. Why is it important to invest in knowing more about space? And, frequently, this topic comes up, Earth is in this interesting place between not a hothouse like Venus, not a dry ball like Mars. So if we can understand those other worlds, it could really help us understand what's going on with our own planet.

Jay Dickson: Absolutely. And from a technical level, when we explore Venus or the Moon or Mars, we can't go there ourselves. We've sent some astronauts to the moon, but our Mars program is entirely robotic. So we have had to invent instrumentation, we have to test out techniques of traverse ability. All that ingenuity can now be applied back to the Earth with all of our remote sensing abilities from satellites and frequently as technology developed for the space program that we're applying back to the Earth. So that's what I'm about to do with my new job.

Sarah Al-Ahmed: Well, congratulations on your new job. I'm sure that's going to be really cool and very rewarding because understanding what's going on with our planet right now in particular is so important to the future of humanity.

Jay Dickson: Agreed.

Sarah Al-Ahmed: Well, hopefully we continue to learn more about these gullies. And I tell you, I didn't even know Mars could tilt that far before I read your paper. That blew my mind. So today, I learned.

Jay Dickson: Fantastic. Yeah, a lot of people have spent a lot of years trying to figure that out, but the evidence is all over Mars that this has actually happened. It's incredible.

Sarah Al-Ahmed: Well, thanks for joining me, Jay.

Jay Dickson: Thank you, Sarah.

Sarah Al-Ahmed: Now, let's check in with Bruce Betts, the chief scientist of The Planetary Society for What's Up. Hey, Bruce.

Bruce Betts: Hey, Sarah.

Sarah Al-Ahmed: How's your week been?

Bruce Betts: Frazzly. How about yours?

Sarah Al-Ahmed: Frazzly tambien.

Bruce Betts: I'm not going to try. I made up the word in English. I probably shouldn't make it up in Spanish, too. Okay. Should we talk ... Speaking of frazzly, the sky, the planets, they're moving and they're making some big moves right around now. So we've been hanging with Venus for many months now in the evening sky over in the west. Well, sad news, Venus dropping below the horizon in the next two to three weeks. So watch it getting lower and lower. You can still see it over in the west after sunset looking super bright. Same deal. Mars has been crossing the sky, although it's quite dim right now for Mars, but it will be going away in a few weeks. It's up above Venus in the evening west. On the 20th, the crescent moon is near Mars, but you can watch them go away. But, wait, I've ordered some replacements for those of you who look in the evening sky. And, by the way, Venus will be back someday soon. It'll be back, I promise. It always comes back. But in the evening sky, we're getting Saturns moving over to the evening. Not quite evening yet. It's rising in the middle of the night, but late evening over in the east looking yellowish and Jupiter coming up a couple hours later and both are high up in the pre-dawn. Jupiter always looking quite bright. And so those will have to satisfy you, but you can still check last minute, last time by now Venus and Mars. Don't miss it.

Sarah Al-Ahmed: It's funny, a few weeks ago, we were talking about Mars being in the sky and about that really detailed Mars image that you brought up where you zoom in really far and you could just see all the details on Mars.

Bruce Betts: This was from Mars Reconnaissance Orbiter, I think.

Sarah Al-Ahmed: Exactly.

Bruce Betts: And they put out a gazillion pixel. Okay. Not a gazillion pixels, but they sewed a lot of stuff together.

Sarah Al-Ahmed: It might as well have been a gazillion pixels.

Bruce Betts: Yes, it's cool. Now, tell me a story.

Sarah Al-Ahmed: Well, yeah, the story is that the person I interviewed on the show this week, Jay Dickson, was actually one of the leads on that project. I didn't do that on purpose, but that is so cool.

Bruce Betts: That is, that is. Mars is neat. Did you know that?

Sarah Al-Ahmed: It really is. I feel like I've been diving into Mars stuff for the last few weeks and no matter how much I learned about it, I learned that there's just so much I don't know.

Bruce Betts: People studying it still know there's a lot they don't know, but they're working on it.

Sarah Al-Ahmed: They're working on it.

Bruce Betts: Working on it. All right, we move on to this week in space history. It was the first time humans walked on another world. So all 11 this week in 1969. And onto random space fact. So Mercury, if you hang out on the surface of Mercury, which is not recommended, but if you did and you were watching the sky, you would eventually see the sun appear to move in the opposite direction that it usually moves for a few days or a few Earth days and then it would head back the other direction. And you can even imagine being on the surface of Mercury and a period where you would see the sun rise, it would go back down and then rise a second time. This is because Mercury, having the most elliptical orbit of the planets by quite a bit, actually, its speed increases near parhelion and it actually ends up changing, which is faster compared to the orbital angular speed, the rotational angular speed. So that's a groovy word thing to look for next time you are on Mercury. The day side of Mercury at that particular time. So have fun. Go for it, Sarah.

Sarah Al-Ahmed: I did not know that. But extra funny because everyone's all worried about when Mercury grows into retrograde, but what happens when the sun goes into retrograde? You're in danger.

Bruce Betts: Mercurians freak out. We try to convince them there's nothing to it, but they're a superstitious lot, those Mercurians. All right, shall we move on to the trivia contest?

Sarah Al-Ahmed: Yes.

Bruce Betts: All right. I asked you, who is the oldest person or who is and was at the time and still is the oldest person to have flown in space suborbital flights count long as they get above the von Karman line of a hundred kilometers [inaudible 00:49:50] we do? Sarah.

Sarah Al-Ahmed: Everyone got this one and they have to get this one right because the answer is William Shatner, our beloved captain of the original Star Trek enterprise. But I remember watching this when it happened and just this guy had the most intense reaction to going to space that I've seen upon returning. A lot of people say some beautiful stuff when they come back, but William Shatner was shook.

Bruce Betts: Yeah, no, it's impressive. And just to be clear, this was not just as flights on the enterprise. This was a flight on Blue Origin suborbital flight. October 2021.

Sarah Al-Ahmed: Yeah, he was 90 years old. Can you imagine living a whole life on Earth acting in this role that meant so much to so many people that love space only to get to finally go to space at age 90? That's beautiful. I hope that's true of us. I would love to go to space, but our winner this week is regular listener, Norman Casson. Finally get to give a prize to Norman. I know you've been riding in every week, so you're going to get a special grab bag of space prizes, including one of the last rubber asteroids. So I'm really excited to give that away.

Bruce Betts: Congratulations.

Sarah Al-Ahmed: That was a really sinister congratulations.

Bruce Betts: Congratulations. How was that?

Sarah Al-Ahmed: That's better.

Bruce Betts: That's better. Yeah. Okay. I just need a little coaching.

Sarah Al-Ahmed: But we got a lot of really great comments about Shatner and his experience. And Norman actually mentioned that Shatner wrote about this in his book about the experience going to space. And I remember he said this when he came down to the ground, he said, "When I looked in the opposite direction into space, there was no mystery, no majestic awe to behold. All I saw was death," which is one of the most intense descriptions of the overview effect I've ever heard in my life.

Bruce Betts: Yeah, he took a different twist on it. I think that's more the flying in space part of the overview effect, maybe not.

Sarah Al-Ahmed: Right? The other side of the coin, one side is Earth is beautiful and should be protected and the other side is, oh, no.

Bruce Betts: Don't go there, it's death.

Sarah Al-Ahmed: But this was really cool. We had someone write in, Henry Sanford-Crane from Elkton, Maryland, USA, who said, "I've been a Planetary Society member on and off since the '80s and the society has kept my interest in astrophysics alive all this time. And 45 years later, I'm now working on a PhD in astrophysics."

Bruce Betts: Wow. That is very cool.

Sarah Al-Ahmed: That is so cool. I love it. I had so many friends when I was going to school that said a very similar thing. They had loved space for a long time and finally decided to go and pursue either their first degree in astrophysics or getting a PhD at a later point in their lives. And I was so proud of all of them and anybody, really, who goes through the effort to get a degree in astrophysics. It's intense but beautiful. And Marc Dunning from Ormond Beach, Florida wrote in to say that he loves the show and loves the info digging. It inspires. A lot of people have told me that particularly your random space fact leads them on these rabbit holes of space discovery where you can't stop and you end up in that Wikipedia down the rabbit hole situation.

Bruce Betts: Yeah, it's pretty much how I create them half the time, at least. So I start in one part of the universe and end up in a different part. Anyway.

Sarah Al-Ahmed: Yeah, that was awesome. And Marc also said that our new member community gives him hope for the future of humanity. I know that's like a big claim, but I'm having that same experience being in there, just seeing people interact and have a fun time on social media in a non-toxic way talking about space. I love it.

Bruce Betts: Yeah, it's pretty cool.

Sarah Al-Ahmed: And, of course, we don't have a new space trivia contest question this week because next week is our last day announcing our space trivia winner. So we've already selected, but I cannot wait to read you some of these comments that people wrote us in for the last trivia contest question because it was about your PhD thesis and you thought maybe people wouldn't find it, but they found it, Bruce. They found it and they read it.

Bruce Betts: Oh my gosh.

Sarah Al-Ahmed: No, really, though. They-

Bruce Betts: Oh, no. How insulting were they? You should talk before we record the next show.

Sarah Al-Ahmed: No, it's all great stuff and it was really wonderful. And I love that people are bringing up the member community and so many of the messages that they're sending us. I know it's a big change to move our space trivia contest into our member community, but I think it's going to be a lot of fun in there. And I want to reassure people all over again, please send us your messages. We want to continue reading your cool poetry and all your thoughts about random space facts and anything else we talk about on the show. So please go ahead. You can continue to write us at our email, which is [email protected].

Bruce Betts: Dancing with my shadow and let my shadow lead.

Sarah Al-Ahmed: That was the tricky bit. People tried to look up that lyric and the band you mentioned and that did not get them to the right place. They literally had to find your paper.

Bruce Betts: Shockingly, Warren didn't link to my thesis or the paper I wrote that relates to that.

Sarah Al-Ahmed: We'll get into the comments.

Bruce Betts: I mean, not [inaudible 00:54:59]. Yeah, yeah. I'm scared and excited all at the same time. All right. Everybody, go out there, look up at the night sky and think about dancing with your shadow and let your shadow lead. Thank you. Good night.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week to share why the discovery of phosphorus on Saturn's moon, Enceladus, might make it an even better place to search for life off of Earth. Planetary Radio is produced by The Planetary Society in Pasadena, California, and is made possible by our wonderful members. You can join us as we team up to support missions like Mars Sample Return at Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. And until next week, Ad Astra.