Was there ever life on Mars? Without a human geologist roaming the surface, hammer in hand, it's devilishly difficult to find out, and current robotic explorers aren't really built to answer that question. But in a paper published in the journal Astrobiology, a scientist describes patterns in Martian rock that she says look uncannily like the fossil signatures from primitive microbial life on Earth.
In the paper, geobiologist Nora Noffke of Old Dominion University in Virginia analyzed Curiosity's Mast Camera images of a spot in Gale Crater called Gillespie Lake Member, where the rocks could be almost 3.7 billion years old. While examining the images that the rover snapped in 2012, Noffke began to notice patterns in the rock that reminded her of structures she studied on Earth.
As Noffke pointed out, scientists think that Mars and Earth may have looked fairly similar in their early history. One NASA video even depicts the now-dry, dusty planet as having puffy clouds and blue water about 4 billion years ago.
"If we have a very similar evolution of the planet Mars and Earth — so both planets have a very comparable early evolution with respect to climate, with respect to environment — it makes sense to go take a look at images that were obtained by the rovers," Noffke said.
Noffke says she has spent two decades studying microbial mats -- primitive microbes on Earth that live in sheet-like layers, which can be billions of years old and can leave telltale signatures in the rock. After analyzing the Martian formation, she compared the patterns she saw to those she has studied in the microbial mat fossil remnants on Earth.
A number of features jumped out: There were triangular protrusions along plateau edges that, on our home planet, often mark areas where microbial mats once hung over the edge like little drapes and eventually turned to stone. There were also spots that looked rather like a flattened tube; on Earth, these were caused by what Noffke called "roll-ups," where the currents in moving water flipped the edge of a microbial mat over, as if it were a rug's edge.
The geobiologist mainly analyzed five images over several weeks, examining and sketching "every square inch of the rock surface," she said. Not only did she see erosional pockets and remnants as well as desiccation cracks that resembled the fossil signature of microbial mats on Earth, she also found that these various features seemed to occur together, not in isolation.
But would the scientist really bet that the Martian structures were left by long-gone microbes?
"I would not put down too much money," Noffke said. As she pointed out in her paper, this is just a hypothesis – and there are many non-biological processes that could also potentially explain the structures she examined in Curiosity's images.
Ashwin Vasavada, who recently took over as project scientist for Curiosity, said the rover's scientists analyzed the Gillespie Lake Member earlier in the mission and did not reach the same conclusion.
"We were surprised that such a bold claim was out there in the scientific literature," the Jet Propulsion Laboratory scientist said. "It's interesting.… It's just that we don't think the evidence is quite as strong as what is described in the paper."
That's not to say that scientists shouldn't look for such signs of past life in the rock, Vasavada added.
"The premise of looking for microbially induced sedimentary structures is not a bad one at all," he said.
In fact, he added, the Curiosity scientists are also poring over images taken by the rover, looking for similar signs of past microbial life.
But given that the Gillespie formation was sandstone, it probably formed in a more active stream environment, which would not be the most conducive for preserving organics, he said.
"In this particular case, that bell didn't go off for us when we examined these rocks in excruciating detail," Vasavada said. "So we will continue to look for these same structures when we think the context is correct to have our eye out for them."