The more we learn about Mars, the more we learn it's a deceptively active planet. Most recently, the Curiosity rover sniffed out a sharp spike in methane levels that dropped back down just as quickly, suggesting some yet-to-be-determined process is currently happening to trigger the aberration.
Top image: Hole drilled on sol 279 (May 19, 2013) to collect powdered Cumberland rock for analysis. The hole is about 1.6 centimeters in diameter, and 6.6 centimeters deep. Credit: NASA/JPL-Caltech/MSSS
The Tunable Laser Spectrometer (TLS) aboard SAM measures light absorption at specific wavelengths to determine methane, carbon dioxide, and water vapour concentrations. Image credit: NASA/JPL-Caltech
The Curiosity rover measured a methane spike ten times the average background levels in late 2013 and early 2014. The rover sniffs the atmosphere using the Sample Analysis at Mars (SAM) laboratory, taking 12 distinct measurements over a 20-month period. Four of those measurements within a two month period averaged 7 methane molecules per billion Martian atmosphere molecules. All the measurements before and after were under 1 part per billion, averaging only 0.7 parts per billion for normal Martian methane levels.
Methane measurements between August 2012 to September 2014 (sols 1-750). Image credit: NASA/JPL-Caltech
We've been excited about methane on Mars before, but this time it's a bit different. This time, Curiosity measured a spike that suggests current, active processes on Mars changing things right now. Fluctuations in the methane levels suggest both sources and sinks on Mars. Methane has one atom of carbon and four atoms of hydrogen: it can be combined, stored, and pulled apart in different ways.
This sharp spike is strange: Sushil Atreya, a member of the Curiosity rover science team, explains in a press release:
"This temporary increase in methane — sharply up and then back down — tells us there must be some relatively localized source. There are many possible sources, biological or non-biological, such as interaction of water and rock."
Sinks, sources, and transportation of methane around the Martian atmosphere. Image credit: NASA/JPL-Caltech/SAM-GSFC/Univ. of Michigan
The atoms can be combined through microbial processes, but molecule formation may also be triggered by geochemical reactions between water and olivine or pyroxene rock. Ultraviolet light can also trigger reactions generating methane from organic chemicals. The methane can be destroyed through photochemistry, when sunlight triggers reactions that break it down. This can produce formaldehyde and methanol, which in turn disintegrates into carbon dioxide, which makes up most of the Martian atmosphere.
In between creation and destruction, the methane can be stored underground a long time and be released slowly, or wind can redistribute methane to create local concentrations.
Along with the atmospheric readings, Curiosity has been sniffing out organic chemicals in the powdered sample drilled out of the Cumberland rock. Cumberland is a mudstone located near an eroding scarp, buried by overlaying debris protecting it from exposure to cosmic rays for most of the past three billion years.
Between cosmic rays breaking down organic compounds at the surface and ultraviolet light can trigger oxidation reactions, organic samples are difficult to find on Mars in Image credit: NASA/JPL-Caltech
It took months to be absolutely certain the organics were directly from Cumberland, and not something that hitchhiked from Earth inside the rover. Now scientists are certain the sample is genuinely Martian, they're giddy about the implications of what they found.
The Cumberland sample contains the first definitive organic molecules found on the planet — molecules that contain carbon and usually hydrogen — that could potentially (although not necessarily!) be building blocks for life.
Artist's concept of Curiosity munching on rocks. Image credit: NASA/JPL-Caltech
Alone, these molecules are a lot of almost-maybe potentially cool things: they may have formed on Mars, or have been delivered to the surface by meteorites; they could be formed by biological activity, or by geological processes that have nothing to do with life. What these molecules do indicate is that modern Mars is chemically active, and that in the ancient past it had favourable conditions for life.
Roger Summons, participating scientist for the Curiosity rover, glows about the potential wrapped up in this discovery:
"This first confirmation of organic carbon in a rock on Mars holds much promise. Organics are important because they can tell us about the chemical pathways by which they were formed and preserved. In turn, this is informative about Earth-Mars differences and whether or not particular environments represented by Gale Crater sedimentary rocks were more or less favorable for accumulation of organic materials. The challenge now is to find other rocks on Mount Sharp that might have different and more extensive inventories of organic compounds."
Chlorobenzene (an organic compound) levels in the Cumberland rock sample were far higher than in other samples or the background average. Image credit: NASA/JPL-Caltech
Exactly which specific organics are present in the Martian rocks remains a bit of a mystery: by heating the samples inside of SAM, perchlorates minerals alter the structures of the organic compounds just enough to complicate identification.
The Cumberland rock sample also gave Curiosity a taste of Martian water that was locked into lakebed minerals three billion years ago. When SAM heated up the sample, it released hydrogen isotopes locked in the rock for billions of years. Because lighter regular hydrogen isotopes escape from the upper atmosphere more easily than heavier deuterium, looking at the ratio of the hydrogen isotopes provides a signature of what water looked like at different times in Martian history. The Cumberland sample has a ratio about half of modern water vapour on the planet, suggesting Mars lost a lot of water in the time since Cumberland formed. But if Earth's oceans are any guide, the ratio is about three times higher than Mars' original water supply, indicating that Mars had already lost a whole lot of water before the rock was formed.
So, what now? Curiosity's project scientist John Grotzinger offers up yet ever-more questions:
"We will keep working on the puzzles these findings present. Can we learn more about the active chemistry causing such fluctuations in the amount of methane in the atmosphere? Can we choose rock targets where identifiable organics have been preserved?"
One last sample of the Cumberland rock remains. What exactly the project team will do with it is yet to be determined, but with a bit of luck, soon they'll be finding more unusual rocks around Mount Sharp to test. When they do, they'll hopefully confirm their discoveries, and maybe even extend the number of tantalizingly mysterious organic compounds expanding our understanding of Mars and its history.
Read more in this NASA/JPL press release on the findings discussed at the American Geophysical Union fall meeting this week, or this Science paper on the methane results. Keep up on the latest Curiosity research papers here.