Piecing together a Martian atmosphere

Curiosity is looking for a smoking gun of life — methane. Its first tests, however, have come back negative.

Signs of methane might point to methane-producing microbes in Mars soils. These microbes, along with other animals like termites and cows, account for about 90% of the methane in our own atmosphere. A simple carbon-containing gas, methane also often serves as an ingredient for more complex organic molecules that sustain life.

But NASA scientists say the Mars rover Curiosity has found virtually no methane at its landing site, Gale Crater. They currently place the upper limit on methane at five parts-per-billion, an amount that is expected to drop even further after statistical analyses.

Still, they have not come away empty-handed. Chemical analysis of these first atmospheric samples indicates that the Martian atmosphere was once much thicker, possibly explaining the past existence of liquid water on the planet’s surface.

Scientists determined this by looking at variants of the gases in Mars’ atmosphere. Though they are the same type of molecules, these variants, or isotopes, have different masses.

Ratios of these isotopes are much heavier than expected on Mars. This skew toward heavy gases suggests that the planet has been losing its atmosphere, a process that removes light isotopes and leaves behind heavy ones.

This explanation corresponds with evidence that water once flowed across the planet’s surface. A thicker atmosphere in the past would have likely been able to trap more heat and warm the planet enough to keep water liquid.

Continuing to study these isotopes will help researchers determine the processes that shape the Martian atmosphere.

As for methane, they have not called off the search just yet. Methane might be released both locally and seasonally, so scientists plan to keep sampling at different spatial and temporal scales.

P.S. Check out this endearing Curiosity piece from the New Yorker. It is old, but delightful.

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