The last years have seen an increasing number of observations showing that Mars once had liquid water and that it has ice not far underground today. In the 36 years since the equivocal results of the Viking probes’ search for life space scientists have scaled back their experiments, looking instead for evidence that water once flowed on the red planet as well as evidence of ice at the poles and beneath the surface dust and sand. Most recently, just last week (December 8, 2011) the Mars Opportunity rover returned what NASA calls a “slam dunk” – a vein of gypsum, a mineral that incorporates water into its crystal structure. Absent water, gypsum cannot form.
Four decades ago the Mariner 9 spacecraft returned photos of what seemed almost certain to have once been the beds of ancient rivers and many more such features showed up in photos taken by later orbiting craft. Later, more sophisticated craft (the Mars Global Surveyor once returned spectrographic evidence of minerals on the Martian surface that, on Earth, are formed only in the presence of water. All of this evidence has grown only stronger with time, including photos suggesting that liquid water might still flow today at rare times of the year. But photos and information obtained from orbit are one thing – indirect evidence – having direct information from the surface is something else entirely.
More recent spacecraft have provided us with just such evidence – recent Mars landers have returned photos of stones that appear to have been tumbled in running water, as one example. But even more telling were the discoveries of minerals such as some clay minerals, and the very recent identification of gypsum. As a graduate student studying under the renowned geochemist Gunter Faure, I learned that minerals such as these include water molecules incorporated into their crystal structure and that, on Earth, they are not formed except in watery environments – their very presence tells us that water almost certainly existed on Mars at the time these minerals formed.
And there is even stronger evidence – in 2008 the Phoenix lander descended to the Martin surface atop the blast of landing rockets. These rockets blew away the topmost layers of soil, revealing bright patches that are now known to be water ice. All of this evidence – orbital photos, the presence of hydrous minerals, and the discovery of ice hiding just between the surface layer of dust and soil – points unambiguously to the presence of water ice on Mars today, and it raises the possibility that Mars might once have hosted (and perhaps still does host) living organisms. And liquid water might exist in more places in our Solar System than just Mars – there is ample evidence that oceans of water might exist in the outer Solar System as well, not to mention the recent discovery of an extra-Solar planet in its star’s “Goldilocks Zone” where temperatures should permit liquid water to exist on the planet’s surface.
All of this makes it seem likely that liquid water is not uncommon in the universe and, that being the case, it makes it even more likely that we will find an ever-increasing number of planets that might be able to host life as we know it. This is exciting, but it is also very limiting – very water-chauvinistic. Water is useful to terrestrial life because it is a great solvent and it does a great job of carrying in solution the ions and molecules that help to sustain life. But other liquids can serve this same purpose. In mid-2011 I was lucky enough to interview astrophysicist Neil deGrasse Tyson, Director of the Rose Center for Earth and Space at the American Museum of Natural History in New York City for a book I was writing. Tyson pointed out that liquid methane can serve as the foundation of life in a cryogenic environment, as can other liquid hydrocarbons (he also pointed out that, for all that we consider life’s diversity, all Earthly life is based on water and DNA, which might seem frightfully limited to organisms elsewhere in the universe). It might be that our paradigm of the requirements for life should be less restrictive than looking for water; rather, perhaps we should consider that any liquid solvent might serve as the basis for life. This not only broadens the environments in which we might search for life, but also makes things more complex – would we even recognize living organisms whose biochemistry was based on something other than water and DNA?
There is even more to it than this – much of what we consider to be vital to the evolution of life could well be simply our speculating about life when (as Tyson points out) we have a sample size of 1: Earth. University of Washington scientists Peter Ward and Donald Brownlee published a book (Rare Earth: Why Complex Life is Uncommon in the Universe) over a decade ago in which they made a compelling case that the rise of complex life on Earth required the Earth to be in the Solar System’s habitable zone, for the Solar System to be in the Galactic habitable zone, for the Earth to have an unusually large moon, and a number of other factors. Brownlee and Ward did a wonderful job of explaining why these factors might have contributed to the rise of complex life (including humanity), but their view – however well-reasoned – falls prey to the same degree of chauvinism that Tyson decried. We simply cannot assume that, just because the only planet known to have life has these characteristics that they are essential to life everywhere in the universe. After all, it was once thought that all earthly life depended directly or indirectly on the Sun, until the Alvin submersible discovered extensive ecosystems around deep-sea hydrothermal vents that are utterly divorced from the Sun as a source of energy.
Humanity has been considering the possibility of life elsewhere in the universe for millennia – not just science fiction writers but philosophers, scientists, and theologians as well. Think of the irony of finding – and looking right past – living organisms simply because they don’t conform to our expectations.
Dr Y is a certified health physicist, trained in nuclear power plant design and operations, with experience in nuclear power, environmental science, and planning for radiological and nuclear emergencies. He has 30 years of experience in the areas of nuclear and radiation safety (and, relevant to this piece) both BA and MS degrees in the Geological Sciences).