Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Monday, November 15, 2010

The grains of Mars

We know that something is afoot on Mars. Apart from the surprising and intriguing measurements of atmospheric methane plumes there is clearly something funky about the chemistry of the Martian atmosphere that cleanses it of this organic gas in very short order. Various ideas have been put forward, including the possibility that there is some important effect of the dust that is often lofted skywards in the great Martian duststorms.

Now a new work here on Earth suggests that both our planet and Mars may share something in dusty atmospheric chemistry. Shaheen et al. report in the Proceedings of the National Academy of Sciences that they have painstakingly identified a set of chemical reactions that occur between ozone, water, and carbon dioxide on an incredibly thin layer on the surface of atmospheric dust. The smoking gun is an enhancement of the oxygen 17 isotope in carbonate material on the dust grains compared to other natural carbonate sources. Isotopic fractionation points towards a different chemical history, involving the formation of hydrogen peroxide on the dust grain surfaces.

Remarkably, a similar isotopic excess was known to lurk in one of the famed Martian meteorites from the Allan Hills finds in Antarctica. While this doesn't necessarily prove a common chemical mechanism it is very suggestive that the type of grain surface chemistry that occurs in the Earth's atmosphere may happen on Mars too. Dust-grain surfaces offer a fast way for chemistry to occur by capturing reactants into a solid/liquid layer. As Shaheen et al. point out, the unusual carbonate deposits in this meteorite that have provoked so much intrigue as potential micro-fossils could possibly just be aerosol produced carbonates - we just didn't know about this mechanism for their formation.

Dust grain chemistry is not just confined to planetary atmospheres. Interstellar and interplanetary dust (admittedly far more microscopic than most terrestrial or Martian dust) plays a critical role in astrochemistry - accelerating reactions that might otherwise take an eternity, and engaging in all manner of cycles for molecular chemistry in the relatively warm and wet environment of protoplanetary disks. Given that it's perhaps not so surprising that it should play an important role in the soupy airways of rocky planets. Interesting questions remain - does this chemistry offer clues to Mars' ability to scrub itself clean of methane, and can it provide another window into paleo-climate both on Mars and the Earth?

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