Mars Opportunity for Life Must Tolerate Salty Acid
The first slew of scientific papers from the Mars Exploration Rover mission appeared in Science Dec. 3,1 with the focus of interest on Opportunity’s evidence for past water at Meridiani, because Spirit found only “volcanic rock rubble and inorganic soils” in the presumed lakebed at Gusev Crater. Jeffrey Kargel (U.S. Geological Survey) sums up the 11 reports this way:2
The analyzed rocks mainly consist of iron oxides and hydrated magnesium, calcium, and iron sulfates; they were deposited in or altered by salty, acidic water, perhaps a sea. Together with orbital observations, the reports for the first time document the geology and geochemistry of a martian hydrological event. The results indicate aqueous sedimentation or aqueous alteration and are consistent with models of a warmer, wetter martian past.
Some of the features, like polygonal cracks, could have a non-aqueous explanation, such as repetitive freezing and thawing of ice, he admits, and the laminae could be explained by “ultracold concentrated acid solutions.” The consensus view, however, involves cycles of wetting and drying, with minerals spending a good deal of time soaking in water. But water, water everywhere was surely none to drink: “The mineral jarosite detected at Meridiani Planum requires highly acidic conditions,” he says, and the other minerals are consistent with an acid brine environment: “The mineral assemblage and chemistry is typical of acid mine drainage systems affected by sulfide oxidation …. Does martian geochemistry resemble a global acid mine pollution site of ochre and sulfate mineralization?” he asks.
Now that scientists believe there was some water for some time, how does this bode for hopes life existed in the past, and perhaps survives to this day on the red planet? The ESA’s Mars Express found methane, he reminds us, which could be a biomarker (see 11/14/2004 headline). Kargel can’t rule it out, but it seems a stretch:
Could martian methane be formed by life? Might Meridiani Planum’s salts be linked through life and water to regional concentrations of methane? Life exists on Earth at acidities and salinities comparable to those inferred for Meridiani Planum. A cold acid-sulfate geochemical model of Meridiani Planum overlaps with some models of the ocean on Europa, one of Jupiter’s moons where life could exist. Such “extreme environments” are rare on Earth, but may be common elsewhere. Few terrestrial species tolerate conditions that are simultaneously supercold, salty, and acidic; none of those that can survive such extremes also generate methane, but maybe on Mars they do, or maybe martian methanogens live in more alkaline and reducing regions.
Opportunity’s ORV racetrack resembles no other place on earth, but it resembles somewhat the Rio Tinto acid mine drainage in Spain, which has specialized microbes. Nevertheless, “Mars may never have been very earth-like,” he sighs, and “Although Meridiani Planum provides a record of aqueous processes, it might be a poor astrobiological site.”
The paper by Squyres et al.3 on the evidence for water, hopeful as it begins, explains why. It says that, although “High acidity and salinity do not pose insurmountable challenges to microbial life on Earth,” the organisms that survive it are specially built: “Such organisms, however, belong to specialized populations that have evolved to survive in highly acidic or saline environments. It is less clear,” therefore, “that such conditions are suitable for the kinds of prebiotic chemical reactions commonly invoked to explain the origin of life” (see 09/17/2002 headline on problems with salt). Not only that, but there’s another challenge any incipient life would have faced. Rover Opportunity found wind-driven sand among the assumed water-deposited minerals. This suggests that “ water on Meridiani Planum may have been regionally extensive but temporally discontinuous, increasing the difficulty of biological persistence over long time intervals.”
What does the debate about Martian water and life mean to us at home, who live on a privileged planet, where life is found in every environment from acid mine drainages to the lush thickness of tropical rain forests? The remote plausibility of any life on Mars contrasts sharply with what we observe on Earth, where living things thrive in the sea, in the desert, in caves, in the mountains, in the air, and on bustling freeways filled with humans driving to work, including scientists heading to NASA centers and universities, eager to read the latest radio signals from their distant robotic emissaries. How will we interpret the answer to the question: is there, or was there ever, life on Mars? Kargel concludes his summary with the alternatives:
The possible future discovery of life (or fossil life) beyond Earth, anticipated for millennia, would complete the Galilean revolution that removed Earth and its life from the center of the universe. Alternatively, if we search martian aqueous deposits and find them barren, then Earth might be seen as the only land of the living for light-years around. Methane and salts may then provide humans with raw materials for building a new civilization on Mars and with an increased respect for life on our own planet.
1Linda Rowan, “Opportunity Runneth Over,” Science, Vol 306, Issue 5702, 1697, 3 December 2004, [DOI: 10.1126/science.306.5702.1697].
2Jeffrey S. Kargel, “Proof for Water, Hints of Life?” Science, Vol 306, Issue 5702, 1689-1691, 3 December 2004, [DOI: 10.1126/science.1105533].
Someone send Kargel a copy of The Privileged Planet video; he’s got Galileo and Copernicus and their views mixed up. They were creationists and put God at the supreme position in creation, not man; Earth was a “cosmic sump” where the dregs and filth descended, and Satan was at the center of the Earth. Moving Earth to the status of a planet was a promotion, not a demotion, explains Dennis Danielson (editor of The Book of the Cosmos) in the film.
So here we stand, after centuries of wishful speculation, and after years of eager anticipation and certainty by some scientists that the rovers were going to find a waterbed of life on Mars, and they find the equivalent of a salty acid mine drainage. Yuck; sic the EPA on this place. If we were microbes, we would look for better real estate. Just because some microbes manage to get along under those conditions on Earth doesn’t mean they originated there. That’s the point some astrobiologists tend to forget. Earth’s acid- and salt-loving bacteria have elaborate molecular machinery to help them cope with the poison. It’s the last place any knowledgeable astrochemist would choose to have life come into being.
The Mars of 2004, now in much clearer focus, is dramatically different from that of Percival Lowell, H.G. Wells and Carl Sagan. It’s a toxic, dry, freezing wasteland that only a wishful-thinking astrobiologist could hope would provide company for the cosmically lonely humans who seek fellowship with aliens rather than a Creator. They don’t have to wait for proof Mars is lifeless; with what we know already, it is profoundly a time for “increased respect for life on our own planet.” The next logical step should be increased reverence for the Creator of planets and life.