Sol 8: Specks on the lander's legs could turn out to be briny water.NASA/JPL-CALTECH/UNIV. ARIZONA/MAX PLANCK INST."People always joked about finding oil on Mars," says Michael Hecht of the Jet Propulsion Laboratory in Pasadena, California. "What more could you ask for than to take a shovel-full of soil, rinse it with water, and have pure rocket fuel?" In terms of human exploration, says Hecht, "we actually found gold!".
In fact, Hecht and his colleagues on Phoenix's wet-chemistry experiment had found perchlorate ions. Perchlorate is a powerful oxidizing agent — that's why, in the form of ammonium perchlorate, it is used in rockets. And the idea that the Martian soil might contain oxidizers has been around since the days of the Viking landers as an explanation for the perplexing lack of organic molecules (see Nature 448, 742–744; 2007), with peroxides favoured for the task. When Hecht and his colleagues found perchlorate levels of a few per cent in the slurries of soil and terrestrial water they made in Phoenix's little chemistry beakers, they immediately saw they had come up with an interesting alternative.
But Hecht and his team got even more excited about some of the other properties of perchlorate salts. They have a great avidity for water and a low-temperature stability that makes them much less toxic than peroxides. Perchlorate salts can also, at high enough concentrations, make a pretty good antifreeze. These properties had implications for the possible habitability of the Martian subsurface — and also for its study. The absorptive tendencies might explain the stickiness that made the mission's soil samples so hard to handle.
The stability means that, unlike peroxides, perchlorates in the Martian soil would not necessarily be poisonous. They might even be food. On Earth, water contaminated with perchlorate is treated with bioremediation — that is, by getting microbes to eat it up. And the antifreeze effect could, conceivably, allow water to stay liquid in the comparatively warm conditions of a high-latitude Martian summer. If you want to fuel speculation, rather than rockets, the possibility of liquid water and microbe food on Mars is about as good as it gets.
Nilton Renno, a member of Phoenix's science team from the University of Michigan at Ann Arbor, became fascinated by that possibility. Early in the mission, Renno suggested that little specks on the lander's legs were splashes of salty mud kicked up by its retro-rockets during the landing. However, when the camera returned to check on the legs weeks later, the images suggested something extraordinary: the specks might be growing.
Renno now says that the specks — which he claims are not only growing but also merging and moving — had become briny water droplets. The lander legs, he says, are about 20 ºC warmer than the surface because of the spacecraft's heaters. This, combined with the effects of perchlorate, allowed droplets to form. If liquid water exists on the lander leg in just a few randomly spattered spots, he speculates, what about in the soil itself? On Earth, small layers of briny water exist within massive chunks of sea ice. Why couldn't the same be true in the boundaries between ice and soil on Mars where salts might be expected to accumulate? "I'm not saying that there's liquid water everywhere on Mars. I'm just saying that these results suggest that it is possible," says Renno.
The idea has been hotly debated within the team. Renno says he will submit a paper to the Journal of Geophysical Research that describes the droplets and asserts that briny water is "common". He says he has two dozen co-authors from the science team. But there are some notable absences on the list, including Hecht and Ray Arvidson, a senior mission scientist from Washington University in St Louis. "My impression is that Nilton is running with the most spectacular hypotheses," says Arvidson.
Principal investigator Peter Smith, who has not yet decided whether to stay on the paper as a co-author, says the 'barnacles' might have been created chemically by the lander's retro-rockets. "Maybe it's telling us more about thruster interaction with the surface than it's telling us about Mars."
Hecht agrees with Renno that the specks on the lander leg are probably made of H2O — but the frozen kind. He says that the lander legs, mostly in shadow, are likely to be colder, not warmer, and that the shifting of shadows can cause the temperature variations needed to explain the growth and movement of the ice. "We would all love to find liquid water dripping around Mars," says Hecht. "I think it's an extraordinary claim. I say, 'do you have the extraordinary proof?'".
The mission's last scoopful might have gone some way to providing that, with a sample from comparatively deep within a trough between two 'polygons' — a good spot for perchlorate to accumulate. Unfortunately the sample was so sticky it couldn't be dislodged into the chemistry beaker. Perchlorates giveth, perchlorates taketh away?
