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Emily LakdawallaOctober 24, 2007

Flash back to DPS: Extrasolar planets, NEOs, asteroids, Titan, Pluto, and KBOs

Since I only spent two days at the Division for Planetary Sciences meeting in Orlando, I missed three days' worth of presentations. Darin Ragozzine, a graduate student working with Mike Brown and part-time worker for the Society was there and just sent me these notes. Thanks Darin! -- ESL

Darin Ragozzineby Darin Ragozzine

I must apologize about this "old" news of talks at the Division for Planetary Sciences in Florida, October 10-12. Hopefully, Planetary Blog readers will still find the following highlights interesting. Do feel free to contact me with questions (through Emily). As you'll see, I started with the hottest of planets and will end with the coldest.

ednesday morning started with a session on extrasolar planets. One interesting discovery is a Spitzer space telescope measurement of the hottest planet (HD 149026b) which has been identified at a scalding 2,200 K or so (about 1,900 degrees Celsius or 35,00 degrees Fahrenheit). At this temperature, you can have titanium clouds forming, which really makes you wonder what these hot Jupiters would look like if you could see them up close. (Some simulations of that are shown off at the highly-recommended extra-solar planet blog Systemic.)

then jumped over to the session on asteroids. Spitzer has really opened our infrared eyes, detecting photons from extrasolar planets, measuring spectra of asteroid belts around other stars, and probing the early universe for galaxies and other cosmologically interesting phenomena. And sometimes you get more than what you're looking for. During one of these cosmology surveys, Spitzer would look at the same place in the sky several times, which is also a great way to find asteroids: they're the objects that move between successive images. (This on-the-sky motion is mostly due to motion of the Earth and the parallax effect; Earth moves faster than most asteroids.) It happens that, at their surface temperatures of roughly 200 Kelvin, asteroids glow in the near- and mid-infrared where Spitzer is looking, so Spitzer actually has the potential of seeing asteroids that are quite hard to see from the ground because they are so small and faint. Using this method, Bidushi Bhattacharya and her colleagues have identified dozens of asteroids from Spitzer; many of them are new. Most are in the main belt but some are Trojan asteroids (orbiting along with Jupiter), and many more are to come in the future. I think it's so cool when you can mine data taken for a totally different purpose (cosmology) and do something unique and interesting.

erek Richardson gave a talk on behalf of Patrick Michel on the most recent addition to asteroid collision simulations: now they are able to realistically model the shapes and spins of the hundreds of fragments created when two small rocky worlds collide. Before this, they were focusing on the dynamics and general outcomes of these impacts, but now they've added this new physics. I must say, that the movie that he showed was one of the coolest scientific animations I've ever seen. Derek is known for very professional looking animations (check out his webpage), but this animation was super-cool.

thers are working on asteroid mapping using Hubble data as well as Adaptive Optics imaging from large ground-based telescopes. For many medium-size (~200-kilometer) asteroids, we now have a pretty good handle on their shapes. For the largest asteroids, Ceres, Vesta, and Pallas, we not only know their shapes, but also are starting to detect albedo variations (bright/dark spots) on the surface. Presumably all asteroids have such albedo patchiness, but for smaller asteroids, you pretty much can't disentangle the effect of albedo from shape, so it will be a while before we know what they really look like.

ater in the afternoon, Robin Canup gave a talk with more details about the formation of the Moon. She's an expert in the field and has been doing new simulations using Smoothed Particle Hydrodynamics (which also yields nice movies). Up until now, Moon-forming impacts have been modeled by a collision between a nearly Earth-sized body and a Mars-sized body, both not rotating. In this most recent study, she's added the effect of spin and finds that you can only make the Moon and the Earth have the right sizes and orbits ( i.e. angular momentum) if you start with the Earth initially spinning backwards (called retrograde) or not at all. If it's starts out spinning the same way it's spinning now (called prograde), then the Moon comes out too small. This is an interesting extension of the work on the formation of the Moon and just goes to show you that even though we are pretty sure about the basics of moon formation, there's still plenty of work left to do in this field. Of course, this is true of nearly all fields in planetary science and science in general.

Cassini RADAR view of Titan's north pole, October 2007


Cassini RADAR view of Titan's north pole, October 2007
This mosaic is composed of all synthetic-aperture-radar maps of Titan's polar regions acquired by Cassini to date. It has been cropped and reduced in size by 50% from an even larger mosaic available on NASA's Planetary Photojournal. Approximately 60 percent of Titan's northern polar region (poleward of 60 degrees north latitude) has been mapped as of October 2007, and of this area, about 14% appears to be covered with hydrocarbon lakes. The radar images are grayscale; they have been colored here with a color map that applies blue colors to the materials that are darkest to the RADAR instrument, and yellow colors to the materials that are brightest. This color scheme highlights the apparent lakes, but also shows that many lake-like features are not as dark as other lakes, and that darker channels appear to run down the interiors of less dark lakes.

The image is a polar projection, with zero longitude (the sub-Saturnian hemisphere) toward the bottom. The leading hemisphere (centered at 90 degrees W) is to the left, and the trailing hemisphere (centered at 270 degrees W) is to the right. The largest lakes are clustered in an area on Titan's trailing hemisphere.

On Thursday, the morning started out with an interesting session on the geology of Titan. It's quite amazing, I would say, that between radar data, imaging, and the Huygens probe, we can now do geology on this distant moon-world. Quite a bit can be said, actually, and several different kinds of features (dunes, seas, lakes, mountains, suprisingly few craters, etc.) are seen on the planet. Alex Hayes of Caltech gave a great overview of his work on mapping the lakes and seas in the North polar region of Titan. There actually is a distinction between "lakes" and "seas" in this sense. On one side of the north pole there are large seas with drainage channels clearly indicating flow of liquid materials. On the other side of the north pole, there are lots of small lakes without connecting river networks that are probably created by a methane-filled subsurface. He used Earth-based terminology of "water tables" and "aquifers" to explain these lakes: the "methane table" rises and fills the lakes, sort of like pre-made wells. What we don't understand is why there's such a large difference between the two hemispheres. One interesting result is that there's a good chance that Cassini's extended mission will be able to see changes in the lakes due to evaporation or seepage back into the subsurface. This will clearly reveal a lot more information about the origin and evolution of these lakes. The south pole hasn't been covered much, but a recent radar swath does show lakes and the numbers are apparently consistent between the two poles. (Many expect the south pole to be even more "wet" since it's summer/autumn there.)

n the afternoon, I went to a session about our understanding of Kuiper belt object (KBO) surfaces. This is done by taking detailed spectra (usually in the near infrared) and trying to identify signatures of the expected ices: mostly water, methane, and ammonia (although we did hear about ethane on Quaoar and propane on Sedna). Though this sounds simple, it's hard to get good spectra of these faint objects and there are definitely ambiguities. Emily Schaller's talk pointed out that a feature previously thought to be due to ammonia on Quaoar is actually much better described by methane. Joshua Emery's talk indicated that Spitzer spectra might help, though, again, these objects are quite faint and sometimes not even seen. We also heard on Friday from Marla Moore about new better laboratory spectra of ammonia-related ices, so the work on this topic is clearly moving forward.

didn't go to the talk by Alan Harris on the NEO impact hazard, but I actually had already heard it at the Workshop on Binaries in the Solar System about a month ago. Alan Harris is known for doing calculations about the probability of death by asteroid impact. As mentioned in the link, he computes the highest average probability of death comes from large impactors (>~1 kilometer) that have global consequences. For example, even though globally catastrophic impacts only happen every million years, if a billion people die than the expected number of average deaths per year is approximately 1,000 people. This is the largest contributor to the asteroid risk; even though there are many smaller objects, the damage is "only" local so the cumulative amount of deaths per year is quite low. (Apophis falls in between a global and local catastrophe.)

t the conference last month, Al presented two new results. The first is that, from the most recent and best understanding of the observations (of which he is one of the world's experts), there is actually a dip in the number distribution of asteroids at sub-kilometer sizes. That is, previously a single power law has been assumed and extrapolated to the smallest objects to determine their abundance. Though there has been some inkling of this in past studies, but now it seems clear that the number of sub-kilometer asteroids is a few times less in some size ranges. So the previously used power law extrapolation overestimated the number of small near-Earth objects. Propagating this forward to the potential impact hazard, the number of deaths per year from these small impactors drops by a factor of three from previous estimates.

he second result was about the larger impactors, the ones that pose the most time-averaged risk. Based on the number of discoveries and orbits characterized, this risk has gone down by a factor of 10. Basically, Spaceguard (and NEAT, etc.) worked: we've identified the vast majority of NEOs larger than 1 km. We've also calculated their orbits and found that none of them will impact in the near future. So, the probability of an impact (in the next ~100 years) has gone down substantially since we've eliminated many of the unknowns. Our work isn't necessarily done, but the point was that the effort to find and characterize globally hazardous NEOs has been largely successful. With this knowledge, the probability of dying in an asteroid impact (of all kinds) in now down to about 1 in half million or 100 averaged deaths per year.

hope that the public reaction to this info will not be to completely ignore asteroids and their remaining potential threat (though small). Hopefully, they will recognize that we can feel more safe now is because of the research and survey work that the community and The Planetary Society have been pushing for the last decade or so.
Charon stellar occultation (1 of 3)

James Elliot, Jay Pasachoff, and others

Charon stellar occultation (1 of 3)
On the night of July 10-11, the 6.5-meter Clay Telescope at Las Campanas Observatory in Chile spotted Pluto, Charon, and a faint star. This image was taken before the occultation event. Pluto (top) and the star (bottom) are visible; the star overwhelms the dim light from Charon.
Finally, on Friday, we've moved all the way to the edge of the solar system. In the session on Pluto, a few teams presented on their results of studying occultations of Pluto passing in front of a distant star. Pluto is currently crossing the galactic plane (as projected on the sky), which explains the multiple occultations per year. (Unlike other KBOs, which are hard to discover when they are in the star-filled galactic plane, and which haven't moved significantly in their sluggish centuries-long orbits around the Sun.) One of the goals of these occultations is to try to understand Pluto's atmosphere better. Since one of the occultations crossed over the southwest United States, where there are many excellent telescopes, these occultations are revealing quite detailed structures in Pluto's atmosphere, which appeared about the same as in 2002 and 2006. Since Pluto is currently drifting further away from the Sun (perihelion passage was in 1999), scientists anticipate that eventually it will get too cold and the whole atmosphere will just freeze into a surface frost (as appears to be the case on Eris right now, since it is at nearly its furthest from the Sun). But this will probably take several years and may be well after New Horizons passes by the Pluto system in July 2015.

nother goal of these occultation measurements would be to identify the radius of Pluto, which is actually not well known. It seems like this would be easy... when the light from the star totally cuts out, then that's the surface of Pluto. Charon's radius was recently measured this way. However, it turns out that you cannot measure the actual surface radius of Pluto using occultations essentially under any circumstances. Even if the occulted star is directly behind Pluto, light is refracted (bent) through the tenuous atmosphere in such a way that it obscures the measurement of Pluto's surface. These occultations do give information about the temperature structure of the middle and upper atmosphere. I always figured this could be used to estimate the radius of Pluto based on the surface pressure. But it turns out that the surface pressure of Pluto is completely controlled by the precise vapor pressure of the Nitrogen... which is extremely sensitive to temperature. If you get the temperature wrong by less than a fraction of a degree, you can change the estimation of Pluto's radius by several kilometers!

So occultations won't work directly, but Elliot Young spoke about how the radius of Pluto could be teased out of information from the mutual events with Charon. Back in the late 1980s, Pluto and Charon passed in front of one another as viewed from Earth and also cast shadows on each other. Since we learned the precise radius of Charon in 2006 (from an occultation; Charon has a negligible atmosphere), combined with some of what we know from Pluto's atmosphere, we can go back to this old data and try to see if the radius of Pluto can be determined. He admitted that this method is fraught with potential errors, but did give a radius of 1,150 +/- 7 km as his current best guess (which I notice is ~20 km different than the abstract). I'm sure the New Horizons team would like to know exactly where to point their cameras, so hopefully his analysis works out.

nother unknown target for New Horizons' cameras are Nix and Hydra, the two small moons of Pluto discovered in 2005. Marc Buie and Dave Tholen talked about their recent observations of these moons. The results are somewhat preliminary, but they do feel like they know the orbits of all three moons pretty well. This is no trivial calculation since (by Newton's third law of actions and reactions) all four bodies are constantly gravitationally tugging on each of the others. Nix and Hydra are pretty small, so they don't affect Charon too much, but there seems to be a perceptible change in the orbits due to the presence of all four bodies. It appears that we'll definitely have good pointing information when New Horizons flies by.

inally, in the last session of the conference, we ventured past Pluto into the orbital structure of the Kuiper belt. The talks highlighted our current understanding of the number of KBOs on different kinds of orbits and clues on the formation of the Kuiper belt are starting to peek out. For example, it has been known for a while that there seems to be a difference between KBOs that orbit near the plane of the planets (less than about four degrees inclination, called "classical") and the KBOs that have larger orbital inclinations. Keith Noll did a survey using Hubble to look for KBO binaries: objects with moons. He clearly finds that the low-inclination objects have a much higher fraction of binaries than the higher-inclination objects. It is this kind of information that gets theorists thinking: what could cause such a big difference in two populations that are basically in the same place?

ne new theory about the formation of the Kuiper belt that appears to explain its architecture pretty well is the possiblitiy of a "rogue" planet. This study was presented by Patryk Lykawka and makes a big prediction: we'll discover a new planet soon. This planet would be about the size of Mars and would currently be on an orbit where it would have been pretty hard to find, but future surveys (like PanSTARRS) are likely to find this planet, if it exists. The planet originally formed between the giant planets, but got thrown out when they got big. It stayed in the Kuiper belt region for a while, stirring things up (for instance, creating the high-inclination objects) and then got thrown out to a relatively distant orbit (somewhere between Eris and Sedna). So, keep your eyes open in the next few years for the discovery of another "planet". (Planet X? Planet IX? The Large Dwarf Planet?) Note that there are other theories of the formation of the outer solar system that do an excellent job of explaining the facts, but do not require another planet (such as the "Nice" Model). This planet would be extremely cold all the time, in a great contrast to the vaporizingly-hot HD 149026b that I mentioned at the beginning.

ll in all, it was a nice conference. It's always fun to get together with other planetary scientists and talk about our exciting field!

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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