Lots of folks have gone gaga about the discovery of salty water on the surface of Mars, and the excitement centers on one thing: the possibility that there could be life there. Well, we can’t rule out life yet, but the four Mars Rovers haven’t found any. What they have found are organic chemicals on the planet and sedimentary rocks with features resembling those formed by microbes on Earth, like stromatolites, stony structures built by cyanobacteria and—with some dated about 3.5-3.8 billion years old—the oldest definitive evidence of life on Earth. But what we see on Mars are only sort-of-similar rock structures; so far there has been nothing remotely resembling strong evidence of life (either past or present) on the Red Planet.
In a really nice post at the Planetary Society website, senior editor and “planetary evangelist” Emily Lackdawalla assesses the new evidence of briny water on Mars, and how it relates to the Big Question.
First, some photographic evidence for liquid water, with the caption:
Recurring slope lineae are narrow (0.5-5 m wide), relatively dark-toned features that form on steep (25-40˚), southern-hemisphere slopes, and that appear in early spring, grow longer in the downslope direction during spring and summer, and fade during autumn and winter.
The “linae” are the dark streaks that look like broomstraws; you can see them better in the black and white photos below.
Here are some of the seasonal changes that suggest moving water, as the “linea” (putative water channels) get longer over a season as they “flow” downhill toward the left (length of lines increases by the length of the white overlays). As noted below, these are probably growing channels of damp soil caused by moving water, rather than water rivulets themselves:
Here’s some of Lackdawalla’s summary, taken directly from her text (my emphasis):
- Recurring slope lineae are narrow streaks that appear seasonally on Martian slopes; they have been observed to form and fade in widespread locations on Mars. (Here’s an earlier planetary.org article on recurring slope lineae.)
- Past work on slope lineae, with the HiRISE camera on Mars Reconnaissance Orbiter, showed that they form in warm seasons when temperatures reach 250-300 kelvin [JAC: on Earth pure ice melts above 273 kelvin, or 0°C], which strongly suggested that a volatile species like water was responsible.
- The newly published work involves data from the CRISM spectrometer on Mars Reconnaissance Orbiter, and shows spectral evidence for hydrated salts (minerals containing molecular water in their structures) during the times when the slope lineae recur.
- The best mineral matches to the spectral data are magnesium perchlorate, magnesium chlorate, and sodium perchlorate.
- The presence of perchlorate salts could lower the melting temperature of water at Martian conditions by 40 kelvins, making it much easier for water to melt.
- This work is considered very strong evidence that at widespread locations on present-day Mars, conditions sometimes arise for brief flows of briny liquid water — probably not rivulets, just spreading wetness in the soil.
- The widely varying locations and geologic settings where slope lineae have been observed to form and recur make it difficult to identify a single mechanism for replenishing liquid water to drive the recurrent activity.
- The science team including Ojha and Alfred McEwen favor deliquescence as the source: perchlorate salts adsorb water vapor from the atmosphere until enough water is available to form a liquid and dissolve the salts.
That’s all pretty clear, but what does it mean for the possibility of life? Lackdawalla doesn’t see these seasonal rivulets as propitious, but perhaps there’s life elsewhere on Mars:
Personally, I don’t think extant life on Mars is any more likely because of today’s announcement than it was before. An incredibly salty, corrosive, transient water environment is not a very good place to look for life. I think a much more habitable environment is available in the thin films of water that Phoenix observed in the soil at its near-polar landing site. A less-accessible, but also less-radiation-fried and more-continuously-habitable place would be deep underground, where Mars’ internal heat could keep groundwater liquid for very long periods of time.
Lackdawalla points out one concern that I hadn’t absorbed, also discussed by Lee Billings in a post at Scientific American: how can we find out whether there’s life on Mars if there’s a chance that, by visiting the planet, we ourselves could infect it? As Billings notes, spacecraft are scrupulously sterilized before launch, but that doesn’t solve the problem:
Microbes that stubbornly refuse to die nonetheless turn up with regularity in NASA’s supposedly sterile clean rooms for preparing interplanetary spacecraft. Apollo astronauts even found bacteria on the moon that had survived an almost total vacuum inside the robotic Surveyor 3 lander that had touched down more than two and a half years earlier. If terrestrial microbes could live in places like that, why not in some of the more habitable parts of Mars?
The issue becomes worse if humans, who are of course ridden with microbes, are sent to the planet, something that Lackdawalla, but not I, see as inevitable.
I’m not sure how serious this problem is, for, after all, all life on Earth shares certain features implying descent from a common ancestor. That includes the similarity of the genetic code, the use of L-amino acids, and the similarity of gene sequences among diverse species. Presumably if we found Earth-derived microbes on Mars, their DNA—and again, Mars-evolved life probably wouldn’t even contain DNA as we know it—would tell us. I suppose a greater problem is if Earth-derived life were to extirpate or outcompete Martian life, rendering us unable to detect the latter, but that seems unlikely as well. Martian life would probably be competitively superior to that from Earth, though we don’t know for sure (think of all the island species on our planet wiped out by colonizers!).
To cover my tuchus, let me add that it’s not impossible that life on Earth could have descended from life that evolved on Mars, or vice versa. In that case we’d see fundamental similarities among the genomes of species upon each planet, but big disparities between the heredity material of organisms from the two planets, reflecting their independent evolution since the colonization event.
Finally, Google has honored the finding in today’s Google Doodle:
h/t: Matthew Cobb