High-tide: layering in a Mars rock photographed by Curiosity suggests the movement of long-ago water
High-tide: layering in a Mars rock photographed by Curiosity suggests the movement of long-ago water NASA/JPL

Odds For Life on Mars Tick Up—a Little

September of 2013 was a bad time for those who hope there's life on Mars. We've had evidence for decades that water flowed freely across the surface of the Red Planet billions of years ago, and that evidence has only gotten stronger and stronger the closer we look. Not only was there potentially life-giving water back then: Mars also had the right kind of geology to support mineral-eating microbes. And while all of that was in the distant past, the detection of methane in the Martian atmosphere by Earth-based telescopes and Mars orbiters raised hopes that bacteria might still be thriving below the surface—not unreasonable, both because all manner of Earthly critters do perfectly well below-ground and because the vast majority of methane in our own atmosphere results from biological activity. Mars's methane might come from a similar source.

But when the Curiosity rover sniffed the Martian air directly last year, it smelled...nothing. At most, there were just three parts per billion (ppb) of methane wafting around, and possibly much less than that. "We kind of thought we'd closed that chapter," says Christopher Webster of the Jet Propulsion Laboratory, lead scientist for the instrument that did the sniffing. "A lot of people were very disappointed."

Not any more, though. Just weeks after that dismal reading, Curiosity's Tunable Laser Spectrometer (TLS) picked up a whiff of methane at a concentration of 5.5 parts per billion. “It took us by surprise,” says Webster, and over the next two months, he says, “every time we looked there was methane. Indeed, the concentrations even rose, to an average of 7.2 ppb over that period, he and his colleagues report in a new paper in Science.

And then, six weeks later, the methane was gone, and hasn't been sniffed since. “It’s a fascinating episodic increase,” Webster says.

What he and his colleagues can’t say is where the methane is coming from. Because it’s transient, they think it’s probably from a fairly local source. But whether it’s biological or geological in origin, they don’t know. It's wise to be cautious, however, says Christopher Chyba, a professor of astrophysics and international affairs at Princeton. "Hopes for biology on Mars have had a way of disappearing once Martian chemistry has been better understood. But figuring out what's responsible for the methane is clearly a key astrobiological objective—whatever the answer turns out to be."

That's not the only important Mars-related paper in Science this week, either. Another, also based on Curiosity observations, concerns Mars's long-lost surface water, and one of the most important points is that there's a lot more of it left than most people realize—"enough," says Jet Propulsion Laboratory scientist Paul Mahaffy, lead author of the paper, "to cover the surface to a depth of 50 meters [about 165 ft]." That doesn't mean it's accessible: it's nearly all locked up in ice at the planet's poles, but some is also entrained in the clay Curiosity dug into when it was prowling the Yellowknife Bay area of Gale Crater.

PHOTOS: Martian Vistas: A Look at the Curiosity Rover’s Strange Home

The 4 kilometer (2.5 mile) diameter crater in this image appears relatively fresh, but not remarkably so.
The 4 kilometer diameter crater in this image appears relatively fresh, but with little erosion or underground upheaval on Mars and no evidence of an extremely recent meteor hit, "fresh" is a relative term.NASA/JPL-Caltech/University of Arizona
The 4 kilometer (2.5 mile) diameter crater in this image appears relatively fresh, but not remarkably so.
A towering dust devil casts a serpentine shadow over the Martian surface in this stunning, late springtime image of Amazonis Planitia. The length of the shadow indicates that the dust plume reaches more than 800 meters, or half a mile, in height.
Mars has extremely large temperature changes from winter to summer compared to the Earth. It gets cold enough to freeze carbon dioxide out of the atmosphere during the winter, but this ice is unstable when the warmer summer arrives and forces it to sublimate (transform directly back into a gas) away.
On Mars the seasonal polar caps are composed of dry ice (carbon dioxide). In the springtime as the sun shines on the ice, it turns from solid to gas and causes erosion of the surface.
A "fossa" is a cavity or depression..Floods of water and lava are thought to have emanated from the larger fossae nearby, perhaps forming the Athabasca channel and later filling it with lava.
Most of the dunes visible in this observation are barchan dunes. On barchan dunes, the steep slip face is between two "horns" that point downwind.
HiRISE acquired this color image of Santa Maria Crater, with the Opportunity rover perched on the southeast rim. Rover tracks are clearly visible to the east.Opportunity has been studying this relatively fresh 90 meter diameter crater to better understand how crater excavation occurred during the impact and how it has been modified by weathering and erosion since.
The dune gullies (at edge of the field and here in this subimage) appear active and are anomalous in their location (high latitude).
The dunes imaged here are similar to barchan dunes, commonly found in desert regions on Earth.
Sand dunes are among the most widespread aeolian features present on Mars. Their spatial distribution and morphology, sensitive to subtle shifts in wind circulation patterns and wind strengths, can relate to patterns of erosion and deposition, and give clues to the sedimentary history of the surrounding terrain.
This image is located within Aram Chaos near the outlet to Ares Valles. Aram Chaos is a 1300 kilometer (approximately 800 miles) diameter depression from which enormous cataclysmic releases of ground water are thought to have exploded onto the surface of Mars. The water then flowed northwards across the southern highlands, helping to carve the approximately 2000 kilometer (1200 miles) long Ares Valles outflow channel system.
Sand dunes are among the most widespread aeolian features present on Mars. Their spatial distribution and morphology are sensitive to subtle shifts in wind circulation patterns and wind strengths. These provide clues to the sedimentary history of the surrounding terrain.
The 4 kilometer diameter crater in this image appears relatively fresh, but with little erosion or underground upheaval
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NASA/JPL-Caltech/University of Arizona
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Some of that water, says Mahaffy, is tightly chemically bound to the clay and is not a big player in Mars's modern environment. Some is not quite so locked down and has been interacting with the tenuous Martian atmosphere for the past three billion years. The hydrogen in Martian water, as in Earthly water, may contain both a single proton and a single electron, or a proton and electron plus a neutron—so-called heavy hydrogen, or deuterium. As the Martian atmosphere has thinned over the eons, the ratio of hydrogen to deuterium in the water has gradually been dropping, as the lighter version escapes more easily into space. Since the modern water is twice as rich in deuterium as the water from billions of years ago, that suggests that there was about twice as much surface water in total at the earlier time, but its hydrogen residue has vanished.

"That's a fair bit of water," says Mahaffy, "but it's a lower limit. There could be much more beneath the surface today that we haven't seen. It was a really interesting time. There were a lot of aqueous processes going on, and a lot of flowing water."

Where there is (or was) water, there could be (or could have been) life. For Mars enthusiasts, that's why December of 2014 is a lot better than September of 2013.

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