Emily Lakdawalla • May 07, 2006
OPAG, Day 2: Ground-based study of the small bodies in the outer solar system
After the political discussions of the morning, Mike Brown stood up to give the "highly subjective view of one ground-based astronomer," he said.
"Mostly I call them 'small bodies' rather than 'primitive bodies' because I see them as test particles of the planet formation process. The analogy that I like to use I got out of some EOS publication, where there was a freighter going across the northern Pacific and all these float toys fell off the freighter. These things float around and land in different places, and oceanographers used where they landed to figure out the currents and the winds. The Kuiper Belt objects are kind of like that.
"To do this, you need to know where these are. The big second-generation surveys are DES (NOAO), CHFT, and Palomar. The key result from the surveys is this plot." He showed a graph of all known objects, plotted by their semimajor axis versus their eccentricity, and talked about how it could be divided into many distinct populations. "13 years ago, before any of these objects had been found, I don't think anybody really believed that Neptune had migrated. So we've slowly been figuring out these populations.
"The major things that are still unknown: inside the classical Kuiper Belt, there are two entirely distinct populations. One is a low-inclination population, and one is a more extended-inclination population. These are in exactly the same place in space. It's very difficult to do that mechanically; it's like trying to heat up only half of a cup of coffee. The other big one is the radial distribution in the Kuiper Belt. You get a big peak right around 43 AU, then there is a big dropoff right around 50 AU.
"What you'd like to be able to do is take one of these plots like from Hal Levison, let it evolve over time, and you eventually plow outward through the Kuiper Belt, and finally Neptune migrates out and depending on the exact choice of the initial conditions you end up with a distribution of Kuiper Belt objects and compare them to the real one. But you can't. The reason you can't is because of the biases in the surveys. The initial surveys were focused on finding objects. Even the second-generation surveys were focused on finding objects, not on recovering them. The best one is CFHT. They realized that recovering objects takes 3, 4, even 5 times as much time as finding them in the first place. This is a problem that has yet to be solved. We really know that the Kuiper Belt doesn't extend out from 50 to 80 AU. But you can't see anything out beyond 100, because most surveys don't look for anything moving that slowly.
"One other population that I like to talk about, are objects that are really out beyond the Kuiper Belt. Sedna -- when we first found it we thought it would be circular or scattered. We were really hoping it would be circular. We were shocked when we figured out what it was, and it's THAT." His initial graph was a plot of the current locations of objects in the Kuiper Belt, projected on the plane of the solar system; Sedna sat just beyond the belt, but not far from it. Then he dropped in Sedna's orbit. The Kuiper Belt shrank to cover less than a tenth of the slide, and the orbit of Sedna swept way out in a gigantic ellipse that always remained far outside the Kuiper Belt, as far or farther than its current position beyond the Kuiper Belt. "It never comes close to an outer planet; there has to be something out there beyond the Kuiper Belt. This thing is probably ½ or ¾ the size of Pluto. If you have the same size distribution of objects out there as you have in the Kuiper Belt, then you have a substantially larger population." There was some quibbling about this; it's kind of poor practice to create statistical arguments from very small numbers.
"There have been now a dozen proposed models for how Sedna got there. Planet X; a single rogue star; or you form the Sun inside a dense cluster and what you're seeing is the Oort cloud that was formed at the outer part of the solar system. It's pretty easy to figure out which is happening once you find other bodies.
"Nothing that we can see from Earth is remotely primitive. Most of the objects that are big enough to study have had processing; even if they aren't, their outer bits have been processed. For a long time, spectroscopy has been a serious disappointment. Very few objects show spectra that look very interesting. In the past couple of years, larger objects have been found that help in two ways. They're brighter, so you can get more signal, and they have more interesting surfaces.
"For a long time Pluto remained unique; it was quite exciting to finally find something that was spectroscopically similar, 2003 UB313. The major difference is that UB313 is not red in the visible. It looks like UB313 is like Pluto but it doesn't have the dark spots. We don't see any photometric variation with rotation -- we don't know what the rotation is because we don't see any variation. It could be pole on, it could be not rotating, or it could be homogeneous. It does appear to be really high albedo, 85% albedo. It's a very bright icy surface.
"FY9 has incredibly saturated methane lines. The way to get saturated lines is to have the path length through the ice be very long. You have 1-centimeter grains of methane ice on this, which is absolutely crazy. The model requires some ethane, much smaller grains, in tiny dark patches. None of these stories make sense when you compare to Pluto and UB313. We have a nice class of objects to study.
"Something like 10% of these objects have largish satellites. But of the four largest objects, there are six satellites. If you look at the formation models for Kuiper Belt satellites -- Robin Canup essentially predicted outer, small, icy satellites, because when they form from an impact the outer satellites are made of the icy mantle. EL61's satellite is a big block of ice. There is no Kuiper Belt object that looks like these satellites of the largest Kuiper Belt objects.
"Outstanding future questions: the most important question for studies of the outer Kuiper Belt are the birth of the sun, and the evolution of the solar system. For large objects, all the same questions you can ask about Pluto. For all the other objects, it's figuring out what we're looking at. One of my biggest disappointments is Spitzer and its poor performance at 70 microns. The 70-micron channel is just not that good.
"There is no NASA program for this whatsoever. There are no thoughts about programs, telescopes, whatever. But that's OK, because we're doing fine.
"New survey capabilities: Pan-STARRS: 2007+. These claims of dates are never exactly right. If done correctly, you could actually do the Kuiper Belt job right. Discovery Channel Telescope in 2009. LSST in 2012. These are the things that can do the most important work out there. All seem to be going strong.
"New followup capabilities: laser guide star adaptive optics at Keck; Gemini and VLT adaptive optics are coming online. CARMA, ALMA, EVLT, JWST, and TMT (thirty meter telescopes). It's an odd conclusion: NASA really hasn't done much, but that's OK. What can NASA do? Invest a comparatively tiny amount in R&A. Otherwise, we can get a free ride on the backs of the astrophysics community. If you cut off the R&A, all of this just stops. Actually it doesn't stop because the VLT has really taken over the Kuiper Belt work. Having 4 telescopes and a kajillion instruments and a thousand people, they've taken over the field of physical characterization.
[I wrote in my notes here: it's nice to hear someone not complaining.]
"All these new surveys, very few people involved with them have much experience with the realities of ground-based surveys and the horrible consequences of weather. I am skeptical about how well these things are going to work. If you really want to do it right, you put a 2-meter telescope in space with a 1-degree field of view, which could cover 180 degrees per day to 25th magnitude. 3 years gives you all objects in the outer solar system to 25th magnitude. That's my pipe dream; sign me up for that.
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