26 June 2008

Does martian soil contain evidence for rain?

Posted by briony

There’s a news story floating around on the net about a new study on soil at the Viking, Pathfinder, and MER landing sites that’s coming out later this month. The study, by UC Berkeley prof Ronald Amundson and his team of terrestrial geologists, suggests that the chemical profiles in the soils at the landing sites may have been the result of precipitation (i.e. water rain).

Opportunity trench (Unmanned Spaceflight)Trench at Meridiani (Unmanned Spaceflight)

By comparing the elemental abundances in rocks and soils at the landing sites, the authors discovered that soils are depleted in many of the elements that are in the rocks, but have gained elements that can be transported and deposited by water. But this is only evidence for water, not for precipitation. The evidence for precipitation comes from the fact that the soils have more sulfates near the surface, and more chlorine underneath. Because sulfates tend to precipitate out of water before chloride salts, this implies that the water came from above. Voila, rain

Previously, it’s been assumed, with a good amount of hand waving, that the sulfur and chlorine to make the ubiquitous sulfates and chloride salts came from volcanic gases in the atmosphere, which is why we see them near the surface. However, to my knowledge, it hasn’t been suggested before that the sulfur and chlorine were delivered by rain. The general consensus among Mars scientists is that if it ever rained on Mars, it hasn’t happened in the past 3-4 billion years (since the Noachian). The soils at the landing sites are somewhat younger than that (don’t ask how much), so suggesting that they were affected significantly by rain is pretty contentious. This would certainly support the idea that climate change and short periods of extreme climate have had major effects on the surface on Mars, which is contentious in itself.

I read this paper, but I’m no expert on terrestrial or martian soils, and elemental geochemistry makes my head spin. So I really can’t comment on how this hypothesis will be received by the Mars community. It is worth noting that the the team that did the study is almost entirely made up of terrestrial (Earth) geologists, with no co-authors from any of the Mars instrument teams.

Atacama DesertThe hyper-arid Atacama Desert, Chile

It’s also worth remembering that the team is using our understanding of processes on Earth, at places like the Atacama Desert in Chile, to interpret data from Mars, and using Earth analogs always comes with an important caveat. When we say that places like the Atacama desert in Chile are arid like Mars, it’s important to keep in mind that anywhere on Earth is still orders of magnitude wetter than we think Mars has been for billions of years. So while the Atacama sees very, very little rainfall, rain (or even humidity!) will still have a measurable impact on soil formation and other geologic processes. So, because Mars is so incredibly arid, we do not have an analog on Earth for the type of soil formation that the current scientific consensus thinks happens on Mars.