The Curiosity rover has detected organic molecules in ancient rocks on Mars.
Planetary exploration can be a long, hard slog, full of discoveries that take time to weave together. Since landing on Mars in 2012, NASA’s Mars Science Laboratory rover Curiosity has uncovered clays, sedimentary rocks formed in streams, and evidence that a lake and life-friendly conditions existed some 3½ billion years ago in Gale Crater. The rover has also detected hints of a seasonal methane cycle.
But scientists have wanted something else from Curiosity, too: conclusive evidence of multiple types of organic compounds. Organics are those than contain carbon and hydrogen — which describes the vast majority of carbon-bearing molecules (including methane, the simplest organic). Organics appear all sorts of places in the solar system, from comets and meteorites to Saturn’s moon Titan. Despite the provocative name, they don’t have to be made by life; abiotic processes create them, too. But life does make and use some of them, including sugars and amino acids. So searches for extraterrestrial life, whether extant or extinct, often focus on organics.
In the June 8th Science, members of the Curiosity team report the conclusive detection of a seasonal methane cycle and the presence of several organic compounds in ancient mudstones. Neither of these means we’ve found Martian life, but they do make the question of whether it ever existed a more reasonable one to ask.
The studies used different instruments inside Curiosity’s Sample Analysis at Mars (SAM), a microwave-sized lab package that makes up a large chunk of the rover’s body. (You can find a detailed explanation of SAM in this 2012 blog by Emily Lakdawalla.) For the mudstone study, Jennifer Eigenbrode (NASA Goddard) and colleagues heated rock powder to immensely high temperatures — nearly 900°C, or 1,600°F. By going to such high temperatures, the team avoided the ambiguity of a 2015 result that found organics that had been produced by interacting with the nasty perchlorate salt on the Martian surface, instead of clearly identifying what was originally in the rock.
As the pulverized mudstone heats, it generates gases, either because molecules in the rock are undergoing chemical reactions or because they’re breaking down. These gases travel down a pipeline and are ionized, sniffed, and “weighed” to determine their molecular masses and specific components, revealing their chemical identities.
Based on these data, the researchers determined that the mudstones contained organic compounds made of carbon, hydrogen, and sulfur. The sulfur might explain why the organics in these samples, which come from the Murray formation at the base of Aeolis Mons, were less degraded than those in other rock formations tested earlier. Sulfur is more vulnerable to oxidation than carbon is, so it acts as a “sacrificial element,” Eigenbrode says, serving as a chemical victim in Mars’s oxidizing environment in place of the carbon. The same process happens on Earth, when the presence of sulfur protects coal and shale deposits, she adds.
If the organics Curiosity tested were from life, they’ve been altered an awful lot: They don’t show the diversity of molecules or the structural patterns usually created biologically.
A separate paper by Christopher Webster (NASA Jet Propulsion Laboratory) and colleagues details the methane cycle. Curiosity has detected methane before, including a startling spike back in 2013. The new result is based on detections via two methods that reveal a clear seasonal trend, with a peak in the northern hemisphere’s late summer. (Gale lies just south of the equator.)
In previous work, planetary scientists had suggested that the methane might be from the breakdown of meteorite debris by ultraviolet sunlight, which would be the most likely source of carbon. But this time the team says that scenario doesn’t match the levels they’re seeing. Instead, the researchers think the gas is released from the subsurface, perhaps via fissures in the rock. Some process that’s related to surface temperature must be involved in the release, but the researchers can’t say much more than that.
Jennifer L. Eigenbrode et al. “Organic Matter Preserved in 3-billion-year-old Mudstones at Gale Crater, Mars.” Science. June 8, 2018.
Christopher R. Webster et al. “Background Levels of Methane in Mars’ Atmosphere Show Strong Seasonal Variations.” Science. June 8, 2018.
Inge Loes ten Kate. “Organic Molecules on Mars.” Science. June 8, 2018. (Perspective piece.)
Emily Lakdawalla. “More Than You Probably Wanted to Know about Curiosity’s SAM Instrument.” The Planetary Society. November 30, 2012.