If there is life on Mars, it certainly hasn't jumped out and mugged for the Mars rovers' cameras like many people had hoped. And most scientists agree it probably won't. In fact, any critters that lurk on the red planet today would almost certainly be part of an underground organization that has defied long odds and the harsh realities of a very unfriendly world.
So why all the excitement last week over once soggy rocks at Meridiani Planum?
After all, scientists already knew Mars once held a lot of water. The evidence is written all over the planet as scars of river erosion. All that's really new is scientists now know of a specific location where water was abundant.
Yet for some biologists, it isn't just the signature of ancient water at Opportunity's landing site that is exciting. It's also the salt that was left behind.
Water and salt -- specifically lots of what scientists call sulfates -- together can make brine. And brine is great stuff if you are a certain type of microbe.
While neither water nor brine actually imply life, fresh shafts of optimism now shine on the possibility that an ancient soup of organic materials might have allowed the genesis of microscopic organisms, which could still dwell in the belly of the red planet.
Pass the salt, please
Road crews lay down salt because it lowers the freezing temperature of water. The same is true of brine. And that's handy on a planet where the average global temperature is minus 63 degrees Fahrenheit (-53 C).
Briney underground reservoirs might have existed for long periods of time, according to computer models. Significant pockets might remain today. Importantly, there are organisms on Earth that thrive in environments so salty they'd make a McDonald's French fry cringe. They're called halophiles (pronounced halo-files).
Rocco Mancinelli of the SETI Institute studies these salt-resistant organisms. They are hardy, and hardly rare.
In a telephone interview last week, Mancinelli explained that halophiles are represented in all three primary domains of life -- Bacteria, Eukarya and Archaea, but that each has developed different ways of dealing with high-salt environments. That suggests the trait "probably arose more than once," he says, and so it is likely something that originates and develops easily.
"Such a trait could easily have evolved in a Martian organism as well," Mancinelli said. That is, he quickly added, assuming there ever were any Martian organisms.
Halophiles are interesting to Mancinelli in part because if life ever did begin on Mars, an evolving ability to endure higher and higher concentrations of salt might have been needed to allow organisms to survive to the present.
Early in its history, Mars almost certainly had more water at or near the surface. There might have been lakes or seas, and probably rivers -- at least in brief episodes. If there was no standing surface water, then at least there was more underground water than today, as last week's rover discovery shows.
Where there is water, minerals dissolve in it. When the water on Mars evaporated into outer space or retreated underground -- nobody is sure where it all went -- what remained would gradually have developed a higher concentration of dissolved salts, Mancinelli explains.
"When the concentration gets high enough, most organisms would die," he said. Halophiles, in this scenario, would get the planet to themselves.
In one terrestrial example of this, scientists recently found a previously unknown form of life thriving in California's Mono Lake, which has been slowly receding for decades, leaving a high concentration of minerals and salt that other organisms can't take.
The element of time
Mancinelli thinks brine pockets probably remain deep beneath the surface of Mars today. Some might have lasted for hundreds of millions of years. "These brine pockets may be moving around. They may merge and separate." But their suspected endurance is important, as would have been the duration of any surface seas.
Like water, time may be a crucial aspect to life. Nobody knows exactly when or how life on Earth began, but the oldest record of it dates back roughly 3.5 billion years on a planet that's been around for 4.5 billion years. Mars was born about the same time, just after the Sun formed.
For how much of that time on early Earth were the ingredients of life present, and how long did it take Nature to make the jump from chemicals and minerals to living cells? Likewise, how long might it have taken for life on Mars to develop, if it ever did?
"I don't know," Mancinelli said, "because we really don't know how long it takes for life to originate and evolve."
There are suggestions, however.
In 1953, Stanley Miller conducted a landmark experiment in biology. Wondering what might have been the original spark for life, he combined methane, hydrogen and ammonia -- substances then thought to dominate the young planet -- with water, and sent flashes of electricity through it all. Overnight, things changed.
In a series of experiments, Miller eventually cooked up 13 of the 31 amino acids needed for life.
Neither Miller nor anyone else has figured out what actually triggered life, but the experiments suggested that with certain ingredients and perhaps a little lightning, some pretty magical stuff can happen.
The window for life
Miller believed that once the right raw materials were gathered, life might develop rather quickly. A century would be perhaps unreasonably brief, in his view, but if it didn't happen in a million years, he reasoned it probably never would.
Knowledge of the ingredients of Earth's early atmosphere has since changed, but still today, Harvard paleontologist Andrew Knoll takes a similar view of time frame for things to get going.
"I'd guess that the 10,000 to 1-million-year window is reasonable," Knoll, a member the Mars rover science team, told SPACE.com. "The other question, of course, is what it takes for life to persist on a planet. If water is present only intermittently, then any life that originates has little chance of surviving in the long run."
Other research has suggested that water might have flooded the surface of Mars in hellish bursts. Nobody can say if that was the case, or if so then how many centuries or millennia the bursts might have lasted. And so far, Opportunity has not determined how long its Meridiani Planum landing site was wet, nor when in the past the rocks were drenched. Further observations from the twin rover mission could provide some clues to this crucial puzzle, however.
"If water is present on the Martian surface for 100 years every 10 million years, that's not very interesting for biology," Knoll has said in the past. "If it's present for 10 million years, that's very interesting."
Even if biology got a foothold on ancient Mars, it is not clear if anything could have persevered long underground, with or without salty survival skills.
On Earth, organisms do thrive deep underground -- hundreds of feet below -- without a single ray of sunshine. They live off chemical energy instead, like methane or hydrogen produced in chemical interactions between water and rock.
Being a halophile, it should be noted, is not a condition for being an underground extremophile, as ultra-hardy microbes are collectively known. In fact one organism that might have done well on ancient Mars is desulfotomaculum, which uses sulfur as its energy source, said Benton Clark III, chief scientist of space exploration at Lockheed Martin and a member of the rover team.
"It can form spores as well, so it can hibernate over these interim times on Mars between the warmer spells," Clark said last week in discussing Opportunity's discovery.
Ultimately desulfotomaculum could not have endured the high salt concentrations that Mancinelli describes. Yet on ancient Mars, the door of life might have been open to creatures of various eating habits and with differing survival schemes. One thing most researchers agree on: the Sun was probably not a prime energy source.
Bruce Jakosky is a geologist at the University of Colorado, Boulder and director of its NASA-sponsored Center for Astrobiology. He helped pick the rover landing sites but has not been directly involved in the rover science explorations.
Jakosky says it might have made little difference to the question of biology whether the Opportunity landing site was once a sea or just a location of copious ground water. He says the type of organism one might imagine finding at Mars would likely use geochemical sources of energy, rather than sunlight.
"On Earth, we think that chemical sources of energy came first, followed by photosynthesis, which is a more complicated process," Jakosky said. "Chemical energy is available from day one."
What are the odds?
There could be a huge hitch in all this thinking. Scientists are currently debating whether underground terrestrial life is truly independent of sunlight, or if the chemistry of the surface -- including photosynthesis and its products and byproducts -- must filter down to make life possible below.
"Do those things that [an organism is] surviving on have to be coupled to the surface?" Mancinelli wonders. "The jury is still out."
Knoll, the paleontologist, says many underground bacteria "actually eat buried organic matter and so are inexorably tied to surface photosynthesis, even if indirectly."
He also cautions that life in a subsurface realm might need to routinely get from one oasis to another as a planet changes over time. "You only have to break this chain once for the experiment to end," Knoll says.
Terrestrial life has proven itself to be ubiquitous and resilient, able to essentially eat rocks if need be, or to eke out an existence under Arctic ice with only intermittently present films of liquid water. It can lay dormant for many thousands of years, awaiting the right environment to allow it to repair its cells and divide into new ones. But could the long-sought little green microbes have endured eons inside Mars?
"Underground life is a possibility for Mars' past and, with much longer odds, perhaps even its present," Knoll figures. "But in the absence of abundant surface life, I would assign a fairly low probability for the present day persistence of such ecosystems."
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