With all the recent excitement stirred up by the successful landing of Curiosity on Mars, it’s only natural that discoveries streaming home from the shiny new rover have overshadowed those of older missions still operating on the Red Planet. But Curiosity’s predecessor, the 9-year-old Opportunity rover, proved earlier this month that she isn’t ready to give up the limelight just yet. In an act that proclaimed the continuing importance of the aging spacecraft—looking tired and dusty but proud after weathering no less than five Martian winters—Opportunity beamed home a set of images that has scientists scratching their heads.
Opportunity captured the images with its microscopic imager in a rock outcrop on the western rim of Endeavor crater. To my novice eye—and at first glance—the images appear to show a fossilized colony of rugose horn corals—the same kind that dominated the Paleozoic seafloors of prehistoric Earth. But a little more research on my behalf revealed that the spherules depicted in the image are not horn-shaped but spherical, and that the largest specimen in the photo is a paltry 3 millimeters in diameter. So my premature alien coral hypothesis is out.
According to a NASA press release about the image, researchers are stumped by the appearance, and by preliminary chemical analyses of the spherules as well. “We have a wonderful geological puzzle in front of us,” said Steve Squyres, Opportunity’s principal investigator and professor of astronomy at Cornell University. Scientists examining the image have multiple working hypotheses of what the spherules might be, but no one is yet sold on an explanation.
At least one hypothesis has been ruled out: the spherules are not the same as those pesky but popular “blueberries” that litter the Martian surface near Opportunity’s 2004 landing site. In geological terms, the "blueberries"—so named because of their bluish hue in false-color images—are the result of minerals precipitating from liquid water inside sedimentary rocks. The resulting spherical regions of crystallized minerals in the rock are known as concretions, and when the rock is eroded away, the concretions are left scattered on the surface. This process is common in terrestrial rocks, and concretions rife with iron oxide from the Navajo Sandstone in the Southwestern U.S. are similar to hematite-rich Martian "blueberries." But what sets the latest find apart from the "blueberries" is their internal structure and chemical composition. “These objects are not rich in hematite,” said Squyres, “so we’re dealing with something new here.”
What I was really itching to know, after my zealous attempt to classify the structures as coral, was if paleontologists were joining in the fray—if they were fielding any ideas, and if there were any biological processes being considered to explain the spherules. Was there a chance that ancient Martian life, or some trace of it, had been discovered? The answer was as swift as it was disappointing: No. “There’s so many utterly straightforward ways of making spherical objects that look just like these,” said Squyres. “Nothing we’ve seen so far causes us to need to invoke any kind of biological activity.”
However, not everyone is so sure. “It’s true that there can be abiotic mechanisms that would form a sphere,” said Karrie Weber, a geomicrobiologist at the University of Nebraska, Lincoln. But she says that’s not the whole story. Weber recently authored a paper detailing the discovery of fossilized microbes that appear to have mediated the formation of concretions in the Navajo Sandstone. In the paper, published last month in the journal Geology, it’s shown that the Navajo Sandstone concretions initially formed in the absence of biological processes. “But there was a secondary transformation process that was biologically induced,” said Weber. “So even if they form, originally, abiotically—it doesn’t necessarily mean that life is not involved in that process.”
This is not so say, or even suggest, that the recently discovered spherules and well-known Martian "blueberries" are, in fact, biological in origin. Weber says that further experiments need to be executed, such as deeper analyses of chemical composition within the spherules, before any weighted claims can be made. One thing about all of these spherules and "blueberries" is, nevertheless, set in stone: Weber and most other scientists are pretty sure that they formed in the presence of liquid water. A wet Martian landscape is an exciting concept in itself, and is why so much exploration has been focused on our ruddy planetary neighbor.
So what would happen if one of our roving geology laboratories captured an image of something that defied simple explanation, of something that appeared lifelike?
“If Opportunity ever encountered something that suggested biology, I'd be in the hot seat to try to interpret it,” said Andy Knoll, a member of NASA’s Opportunity team and Earth Science professor at Harvard University. Knoll has spent decades trying to decode Earth’s history by studying sedimentary rocks—an endeavor that he says is exactly what the rovers are attempting on Mars. Knoll said that signals on Mars that would surely have exopaleontologists raising their eyebrows would include the detection of particular organic molecules, sediment patterns that depart from expectations, or atmospheric compositions that suggest anything other than chemical equilibrium. But he admits this hasn’t happened. “To date, nothing we’ve observed on Mars falls into [these] categories,” said Knoll. But do researchers have baseline expectations for Mars? A kind of red line that needs to be crossed in order to open discussions of biology for some observed phenomena? Sort of.
“A lot of these [Martian] interpretations involve comparisons to the nearest terrestrial analogs that we can come up with,” said Jack Farmer, a paleontologist and astrobiologist at Arizona State University who is currently working with NASA’s Curiosity rover team. The problem is that although ancient conditions on Earth and Mars might have shared environmental similarities, they are different planets with different potentials and properties. Another problem is that the scientific instruments for the direct study of Mars could fit into a small bedroom. “It’s going to be very difficult to get beyond the list of options,” said Farmer, referencing the spherule hypotheses. “I don’t see a strong basis for preferring any of the interpretations that have been offered.”
One reason that Farmer is reticent toward any particular hypothesis is because he’s been through a similar situation before, and learned a few valuable lessons. In 1984, a team of researchers picked up what turned out to be a meteorite from Mars in Alan Hills, Antarctica. The meteorite, dubbed ALH 84001, made headlines in 1996 when a sample was run through an electron microscope, and what appeared to be microbial fossils were observed. Debate ensued. “A lot of people early on didn’t really appreciate the challenge of separating out biological signatures from inorganic signatures,” said Farmer. “There’s a tremendous amount of overlap in the composition and structure [of biotic and abiotic material] at micro scales.”
The outcome of the debates surrounding ALH 84001 landed the apparent microfossils on the abiotic side of the spectrum. More importantly, the debates revolutionized the field of biosignature studies. “Now there’s a lot of higher expectations about what kind of data you need to make these kinds of statements,” said Farmer. “Caution is called for. It’s okay to offer alternative hypotheses, but they should be testable.”
Fair enough. I had rushed into the idea of fossilized Martian coral, but it quickly became apparent that the spherules are likely—but not positively—a typical, if not yet understood, artifact of Martian geology.
The ultimate prize for paleontologists interested in the question of life on Mars will be a sample return mission. For now, all we have are two mobile laboratories scouring the Martian surface for evidence of past geological processes, and for evidence of life. We only observe Mars through cameras, and touch her surface with cold robotic arms. “There’s some finite probability that there could be some biology involved [with the spherules],” said Farmer. “Can we get to that? Not with the payload we have on Mars right now.”
Even though the discovery of biology elsewhere in the Solar System would force a sea change in our understanding of the rarity of life—and of the universality of evolution—the fine tradition of science is guiding her adherents patiently and steadily toward the truth. “We are faced here with one of the most interesting scientific challenges of the entire mission and we’re having a wonderful time,” said Squyres. “It's going to take a while to work this out, so the thing to do now is keep an open mind and let the rocks do the talking.”
