3 January 2011

AGU 2010 – Days 3 and 4: Exoplanets, Impact Basins and Alteration

Posted by Ryan Anderson

Hello again folks! I’m back from a break spent almost entirely offline with friends and family in Michigan. But now that it’s a new year, I think it’s time we wrapped up business from way back in 2010. Namely: AGU recaps!

I’m going to merge my Wednesday and Thursday updates because I actually didn’t get to see that much stuff on Wednesday. My poster was Wednesday afternoon, so the whole afternoon was devoted to that, and I actually got caught in the morning while setting up and chatted with people for a while then too! I’m a little shocked that so many people wanted to hear about comparing multivariate calibration methods for quantitative laser-induced breakdown spectroscopy, but I guess I can’t complain.

Wednesday was also the Geoblogger lunch. It was really nice to meet some of my fellow AGU geobloggers as well as others I knew from their online presence but had never met in person. Plus, the food was good!

I did manage to see a few talks on Wednesday though, starting with a few on super-earths. Super-earths are exoplanets several times the mass of the earth, but which still may be rocky rather than icy or gaseous like the giant planets in our solar system. Diana Valencia put it well in her talk when she described planets like these as a new “lab” where theories developed based on the planets in our own solar system can be tested. She highlighted two potential super-earths and showed that even just knowing the radius of potential super-earths can put important constraints on their composition. In particular she took a look at potential super-earth GJ1214b and found that its radius is too large for it to be solid even if it were somehow composed of pure ice. It has to have at least some hydrogen or helium atmosphere.

Artist's rendition of a super-earth close to its host star.

Next up, David Bercovici took a look at mantle convection in super earths. There are some theories that suggest that planet tectonics plays a vital role in making the earth habitable by producing fresh rocks which react with the atmosphere and buffer the climate. The bottom line of Bercovici’s talk was that the size of a terrestrial planet doesn’t matter as much as the surface conditions (especially the surface temperature).

Joost van Summeren gave a talk about tectonics on synchronously rotating exoplanets, which for some reason never cease to fascinate me. These planets orbit their stars like the moon orbits the earth: with one side always facing inward and one side always facing out. That means that if there is no atmosphere, one side is much hotter than the other. van Summeren showed that this leads to some interesting effects on mantle convection, with a big upwelling forming on the hot side and a downwelling on the cold side. This leads to a very thick, stagnant crust over the night side, but a thin and possibly tectonically active zone over the day side.

On thursday I started off the day with Bill Bottke’s talk about the delivery of water to the moon by giant impacts. But it was actually about more than just water. See, the mantles of the moon and the earth have too much of the “siderophile” elements. Siderophiles are literally “iron-loving” elements that tend to separate out from the early magma ocean and follow iron down into the core. The fact that elements like iridium and platinum and gold are present in the mantle means that they were delivered late in the formation of the earth and moon. Bottke showed that the siderophiles were probably delivered by a few giant impacts rather than lots of tiny impactors, and that the giant impacts could also have delivered the 1-3 parts per million of water in the lunar mantle if the impactors contained even 0.1% water.

Giant impacts may have delivered water and gold (and other rare metals) to the moon's mantle, and have been invoked to explain many of the other quirks in the solar system, such as Uranus' strange tilt, Venus's slow backwards spin, and Mercury's huge core.

Another cool lunar impact talk was Mark Robinson’s presentation about impact melts as seen by LRO. He showed some beautiful images of strange “ponds” on the moon which appear to be far from any source of fluid. In some cases, he pointed to evidence that the fluid sloshed up the side of the depression before finally coming to rest. The hypothesis is that these ponds were formed by melted rock ejected from distant impacts, possibly hundreds or thousands of kilometers away! Robinson vividly described the view from the surface as this melt came raining down as an “incandescent cloud of giant hail”.

A big topic of discussion in the lunar science community was the South Pole Aitken basin (SPA), which I learned earlier in the week is not actually centered at the south pole of the moon. Rather, its rim stretches from the south pole to the crater Aitken at 17 degrees south. SPA is especially interesting because it was formed by a gigantic impact, which would have dug up material from deep inside the moon. SPA is interesting enough that a mission called Moonrise has been proposed to sent a robotic spacecraft to the basin and return samples to earth with the goal of determining the age of the basin and studying the chemistry of the rocks from the crust-mantle transition in the moon.

After a brief break during which I sat in on part of the blogging panel that AGU hosted, I came back to hear a few more SPA talks and then a set of talks about impact-induced hydrothermal activity on Mars. Paul Niles started the session off by discussing the importance of oxygen isotopes in interpreting the significance of hydrothermal activity on Mars. He said that with high temperature alteration, the water will contain more “heavy” oxygen. But based on studies of the oxygen in carbonates in the martian meteorites, the alteration happened at low temperatures, which might mean that impact hydrothermal activity was not important overall.

Next up, Susan Schwenzer disagreed and said that hydrothermal activity associated with impacts is important. She showed a detailed hydrothermal alteration model and suggested that we should see widespread but non-uniform alteration in the ancient martian crust due to impact hydrothermal alteration.

Joe Michalski gave a talk about the possibility of carbonates deep in the Martian crust and showed HiRISE images of Leighton Crater, where those carbonates may be exposed. He presented spectroscopic evidence along with textures that suggest the carbonates were injected into fractures in the rock. Michalski pointed out that if all the carbonates on Mars are buried deep in the crust, the age-old mystery of where all the CO2 in the atmosphere went is no longer a problem.

Next Bethany Ehlmann and John Carter both gave talks about impact craters and altered rock. Ehlmann showed that in most cases the altered rock that appears in and around impact craters includes minerals which indicate low-temperature alteration and that so far all signs point to excavation only rather than alteration caused by the impact. John Carter (whose name pretty much made it mandatory that he study mars) came to the same conclusion that the impacts were excavating rocks altered at low temperatures.

For the final talk on Thursday I ducked over to the Venus session to hear my collaborator Sam Clegg discuss the LIBS/Raman instrument that would go on the proposed SAGE mission to Venus. LIBS is laser-induced breakdown spectroscopy, in which the laser actually zaps a rock and turns it into a plasma, then collects the spectrum of the plasma to determine its composition. That’s what I’m working on for MSL. The instrument on SAGE goes a step further, with the capability to turn down the laser power and conduct Raman spectroscopy, in which the laser excited the molecules in the target but doesn’t break them apart. A combined Raman/LIBS instrument would give elemental and mineral compositions. It’s especially good for Venus because the analysis doesn’t take long, and every second counts when your spacecraft could succumb to the crushing pressures and hellish temperatures on the Venusian surface.

A cool graphic that I shamelessly stole from the SAGE website.