Wherever life flourishes, it leaves a calling card written in organic molecules—and researchers have spent decades hoping to uncover these telltale signatures on Mars.
NASA’s Mars rover Curiosity has now given those hopes a considerable boost after finding organic deposits trapped in exposed rocks that were formed roughly 3.5 billion years ago (Science 2018, DOI: 10.1126/science.aas9185). The rover’s discovery at Gale Crater shows that organic molecules were present when that part of the red planet hosted a potentially habitable lake. It also proves that these traces can survive through the ages, ready to be discovered by robot explorers.
“We started this search 40 years ago, and now we finally have a set of organic molecules that tells us this stuff is preserved near the surface,” says Jennifer L. Eigenbrode of NASA’s Goddard Space Flight Center, who led the study.
Curiosity gathered mudstone samples and gradually heated them to 860 ºC, using gas chromatography/mass spectrometry to study the gases produced. It identified a smorgasbord of molecules, including thiophene, methylthiophenes, and methanethiol, which are probably fragments from larger organic macromolecules in the sediment. These organic deposits may be something like kerogen, the fossilized organic matter found in sedimentary rocks on Earth that contains a jumble of waxy hydrocarbons and polycyclic aromatic hydrocarbons.
The organic compounds that were originally transformed into martian kerogen could have come from three possible sources—geological activity, meteorites, or living organisms—but Curiosity’s data offer no insight on that question. “The most plausible source of these organics is from outside the planet,” says Inge Loes ten Kate, an astrobiologist at Utrecht University, who was not involved in the research. She notes that roughly 100 to 300 metric tons of organic molecules arrive on Mars every year, hitching a ride on interplanetary dust particles. “Three billion years ago, it was much more hectic in the solar system,” ten Kate says, so there would have been much larger deliveries of organics via interplanetary travelers.
Curiosity had previously detected chlorocarbons in martian soil, which were probably generated by reactions with the abundant perchlorate found on the planet’s surface. In contrast, the mudstone samples have delivered “what we expect of natural organic matter,” Eigenbrode says.
Meanwhile, the rover’s infrared spectrometer has been tackling the long-standing puzzle of martian methane (Science 2018, DOI: 10.1126/science.aaq0131). Orbiting Mars probes, along with telescopes on Earth, have previously seen occasional plumes of methane in the planet’s atmosphere, raising speculation that the gas could have come from geological activity or even methane-producing organisms.
Curiosity has taken methane measurements over 55 Earth months, spanning three martian years, which now reveal that the atmospheric concentration of the gas varies seasonally between 0.24 and 0.65 parts per billion by volume. “This is the first time that Mars methane has shown any repeatability,” says Christopher R. Webster at NASA’s Jet Propulsion Laboratory, who led the work. “It always seemed kind of random before.”
The rover also saw brief spikes in methane concentration to about 7 ppbv, which is consistent with previous remote observations of plumes, says Michael J. Mumma of NASA’s Goddard Space Flight Center, who has been chasing martian methane for more than 15 years but was not involved in Curiosity’s latest findings. “The ground-based detection is very important because it confirms the methane is there,” he says.
The methane’s source is still an open question. But Webster’s team says that the seasonal cycle rules out one of the leading suggestions: that organic molecules, delivered to the surface by meteorites and space dust, were broken down by ultraviolet light to produce the gas.
Instead, the cyclical nature of the data suggests that methane could be stored deep underground in icy crystals called clathrates and slowly escape to the planet’s topsoils. Laboratory experiments suggest that the soil could temporarily hang on to the gas, releasing more of it in the warmer martian summer to produce the seasonal cycles.
Mars’s newest satellite, the European Space Agency’s Trace Gas Orbiter (TGO), could help confirm that idea. It began to survey the whole planet for methane in April. “We’re all waiting with bated breath to see what they find,” Webster says. TGO should also measure the carbon isotope ratios in the methane it detects, which may provide hints at a biological or geological origin. And in 2021, ESA expects to land a rover on Mars that could drill up to 2 meters below the surface, where there might be better-preserved organics compared with the ones collected at Gale Crater, Eigenbrode says.
These lines of evidence could eventually help resolve questions about our own origins. Mars and Earth were once quite similar places, ten Kate says, yet life apparently failed to gain a foothold on the red planet. “Was there really no life on Mars, or did it just not survive?” she says. The answer could shed light on the crucial conditions needed to nurture the first life-forms on our own world.