On August 5, the world was abuzz about a rover named Curiosity landing on Mars. Now that feat of masterful engineering is over, the rover wakes up, starts its instruments and begins its business of chemistry.
More specifically, Curiosity is looking for elements and molecules that could be clues to past life on the planet. Elemental analysis is one of the most basic things chemists do, as it helps them figure out products formed in a reaction.
In graduate school, I worked in a lab that built complicated molecules that could be potential medicines. I mixed powders and liquids in a flask, waited for a reaction and then set about identifying what I made.
Most of the time I started a reaction knowing what the products should be. Sometimes I retrieved those products. Other times the reaction took an unexpected turn and I collected products with too few or too many atoms. That's when the chemical detective work began. Knowing what products the reaction created, I used my knowledge of chemistry, the reaction conditions, and the materials in the flask to guess how those side products might have formed.
Curiosity, the wandering robotic chemist, might find molecular products like methane or protein building blocks possibly formed by past life on Mars and now trapped in rocks and minerals. But scientists analyzing the significance of those clues have an extra challenge that I didn’t: They don’t know reaction conditions on the planet, like time, temperature and acidity, which could affect how those products formed.
But the minerals themselves, and the planet’s atmosphere, have information to help fill that knowledge gap. The rover vaporizes some rocks with a laser and identifies the elements in the rocks based on emitted light. If a rock contains salts like sodium and potassium, then perhaps it formed in a watery environment. Other elemental indicators tell the scientists if that water was too acidic to support life.
A recent study of Martian meteorites used a similar approach to conclude that carbon in the rocks came from volcanoes, not life. The scientists aimed a powerful laser at mineral grains in slices of the meteorite, identifying the elements in the minerals and structure of the carbon-containing compounds. The minerals likely formed in volcanoes; thus the graphite-like clusters inside the grains likely came from volcanoes too.
That mystery about the origin of carbon in Martian meteorites took 15 years to solve, because other experiments couldn’t rule out the possibility that the carbon formed while the meteorite sat on Earth.
If Curiosity finds molecules that might be evidence for the existence of life as we know it, scientists have to rule out similar contamination issues. Perhaps the molecules formed in the planet’s atmosphere and fell to the surface, or maybe compounds in the soil altered the measurements.
Elemental analysis of minerals will help scientists identify the environment on Mars about one to two billion years ago, during the middle of its known life. With those clues to the reaction conditions on the planet, scientists can then work backwards to figure out if the planet could have sustained life. I look forward to following how Curiosity helps scientists unravel complex questions about past habitability on Mars.
1. Science experiments on Curiosity, from NASA
2. Details of some chemistry instruments on Curiosity:
Sample Analysis at Mars, identifying carbon-containing compounds using instruments common to many labs here on Earth
3. ChemCam, studying the composition of rocks on a small scale and looking for interesting places to study with CheMin