19 March 2011

LPSC 2011: Day 2 – Cryospheres, Carbon, and Methane Skepticism

Posted by Ryan Anderson

Before I continue with my recap of LPSC, I should let you know that there is officially a spacecraft in orbit around Mercury! From everything I’ve heard, the MESSENGER orbital insertion went off without a hitch and the first image should be taken on March 29th and the science phase officially begins on Arpil 4th. So, congratulations to the MESSENGER team! We can all look forward to lots of new discoveries about Mercury in the coming years.

Now then, back to LPSC. Tuesday started with some more presentations on the Martian cryosphere. One of the first talks I saw was by Tim Haltigin about the statistics of polygonal terrain. Ice wedge polygons are common on Earth and Mars where there are freeze thaw cycles. Haltigin took a look at a claim that has been made for decades that as terrain “evolves” these polygons become more “regular”. By mapping out the polygons seen in terrains on Mars and using the width of the troughs as a proxy for the age of the features, Haltigan was able to show that, indeed, the polygons in older terrains were more regular in their size and spacing.

In the same session, Aaron Zent gave a talk about the formation of ice lenses in the Martian arctic. The difficulty here is that there is “excess” ice in the ground at places like the Phoenix landing site. Excess here means that there is so much ice that the ice itself is supporting the overlying material, it’s not just filling in the spaces between soil grains. The only way to get excess ice is to either have it snow out of the atmosphere and get buried, or to find some mechanism to concentrate it in the soil. Zent pointed out that even at very cold temperatures, there is a thin film of liquid water on ice, so you can actually have soil grains migrate through the ice without any true melting occurring. Zent said that this means that you can grow lenses of ice “indefinitely” on Mars.

A view underneath the Phoenix lander shows the ice exposed by the lander's rocked exhaust.

After the cryospheres session, the focus shifted to carbon on Mars. Steve Ruff gave an interesting talk about the possibility that the Comanche carbonates detected by the Spirit rover are evidence of a lake in Gusev crater. He joked as he began the talk that finally, this work “lets me use the L-word and Gusev in the same sentence”. Ruff suggested that, based on the fact that the spectrum of the Comanche outcrop is well-modeled by a mixture of the rock Algonquin and Mg/Fe-carbonates, along with the lack of hydrothermally-formed phyllosilicates and the distinctive texture and morphology of the Comanche outcrop all suggest that it may have been formed by an evaporating brine – maybe in a former Gusev lake.

Another particularly interesting, and potentially controversial, talk in the “Carbon on Mars” session was by Kevin Zahnle, examining the validity of the detections of methane on mars. He argued that most of the supposed detections so far are so weak or variable that they should not be trusted. The most reliable observation is the one by Mike Mumma that has been making news recently, but Zahnle pointed out that this one has some problems too. First of all, if the methane truly does come and go as suggested by the latest detections, then within a few thousand years, all those chemical reactions with methane would use up all of the available oxygen on Mars. Adsorption of the methane or the formation of clathrates wouldn’t work because other gases “out-compete” the methane for both of these processes. Zahnle said: “The variability of methane that they’re reporting is, as an atmospheric chemist, almost insulting.” The other problem is that the Mumma results are only seen when Mars is headed toward the earth and therefore its spectrum is blueshifted. This blueshift aligns the methane lines from mars with the lines from “heavy” methane on earth (that is, methane with some C13 instead of C12). So, if there is any mistake in the correction for the terrestrial atmospheric spectrum, it might be mistaken for methane on Mars. In Zahnle’s words: “None of the reported methane detections are at all convincing.” He also predicted that the instruments on Mars Science Laboratory are going to “crush” the idea of methane on Mars.