Nearly four billion years ago, when Earth was coming alive, Mars was gradually choking to death. The thick atmosphere that had warmed the red planet was leaking into space, and plummeting temperatures caused Martian lakes and rivers to freeze, turning the wet surface into a dry wasteland.
But it’s possible life took root in those early years. And very soon, a NASA robot will arrive at Mars with the goal of collecting rock samples that might contain ancient fossils, perhaps helping to answer one of humanity’s most fundamental questions: Are we alone in the universe?
First though, would-be Martian fossil hunters will have to decide where, exactly, to send that robot. (Also see "Inside the High-Risk, High-Stakes Race to the Red Planet.")
Currently known as Mars 2020, the next-generation rover will carry a sophisticated mobile geology lab designed to search for signs of tiny dead Martians—single-celled algae and bacteria that are the planet’s most likely ancient inhabitants.
Some of those extinct organisms might be microfossils entombed in rock. Or, much like the way an abandoned ant colony would leave behind anthills, long-dead microbes may have crafted rocky structures that are preserved in fossil form.
To narrow down the search, sensitive scientific instruments will sniff out biosignatures—minerals and molecules secreted by ancient life.
NASA already has a fairly good idea of the most promising places to look, including dried-up lake beds and extinct hydrothermal vents. But competition for the ultimate destination will be stiff, given the rover’s limited range.
“On Earth, you could have 50 graduate students walking all over western Australia looking for just the right place for ancient fossils,” says project scientist and Caltech geochemist Ken Farley.
“We're not going to get that luxury. We're going to land in one place and cruise around an area that might be over a 15-kilometer [9.3-mile] linear distance.”
While selecting the right spot for the Mars 2020 rover will be daunting, there is cause for optimism: Unlike seismically active Earth, Mars as a whole is an impeccably preserved fossil.
“On Earth, our ancient rock record has been through the washing machine and the ringer so many times that the fact that anything still retains any signature of its age is a miracle,” says Brown University’s Jack Mustard, one of the experts consulting with NASA on its fossil-finding mission.
“The rocks on Mars would not have been processed to the same extent, would not have been beat up as much, would not have been stretched and squished and heated and buried and exhumed,” he says.
As such, says Mustard, some 50 percent of the Martian surface contains intact rocks dating back to those crucial first billion years of the planet’s formation.
Still, NASA’s first and only encounter with a suspected Martian fossil underscores the challenges of finding evidence of extinct microbial life on another world.
Twenty years ago, a research team led by David McKay—a prominent NASA scientist who had trained the Apollo astronauts in geology—announced that they had found microscopic fossils. The primitive, bacteria-like organisms showed up inside a four-billion-year-old Martian meteorite that fell to Earth some 13,000 years ago.
Widely published imagery of the meteorite, called ALH84001, revealed wormlike shapes that captivated the public. But several geologists remained skeptical, and the meteorite soon became the most intensively studied four-pound rock in history.
Over the next two decades, researchers found other, more mundane explanations for the fossil-like formations, suggesting that they were the product of geology, not biology. For instance, the features are likely irregularities in the rock that were artificially enhanced when researchers were preparing the samples for the microscope.
The most convincing evidence that the meteorite contained fossils was the presence of magnetite crystals, which have a similar size and chemical composition as those of crystals created by certain bacteria on Earth.
But in recent years, geologists have discovered that shockwaves, like those the space rock must have been exposed to when it slammed into Earth, can also produce magnetite crystals. What’s more, magnetite crystals produced by bacteria have a distinctive string-of-pearls formation that was not found in the meteorite.
Clay, Chert, and Cauliflower
Geologists will face the same burden of proof when they get their hands on rock samples collected using the Mars 2020 rover. But unlike ALH84001—a random chunk of Mars hurled into space—these rocks will be selectively chosen based on qualities that make them strong candidates for preserving identifiable remnants of Martian life.
For instance, the Mars 2020 fossil hunters will look for chert and silica—two types of minerals that, on Earth, are ideal for entombing and preserving biological material. Clay mineral deposits are also happy hunting grounds for geologists, since their presence implies there was once a lot of standing water that existed over a long period of time.
And of course geologists will pay special attention to distinctive rock formations. In 2008, the Spirit rover captured images of unusual, cauliflower-shaped silica formations inside Mars’s Gusev Crater that bear an uncanny resemblance to objects sculpted by bacteria living inside hot springs on Earth.
Stromatolites are another type of formation that are prime real estate for extant life. On Earth, these layered rocks are created in shallow water by microbial mats—wafer-thin colonies of tiny organisms held together by a mucus-like substance. The mats act like flypaper, trapping sediment. Layers gradually accumulate as the organisms repeatedly push their way upward through the silt.
This past August, a team of geologists announced that they had found a 3.7-billion-year-old stromatolite in Greenland, which would make it the oldest fossil ever found on Earth. But even with samples from our home planet, not all scientists are convinced that these ancient structures are fossils, since stromatolites can form without the assistance of microbial mats.
“When you're looking at stromatolites, it's not like you're looking at the morphology of a leaf,” says Farley. “It's a very subtle sort of thing.”
Still, he’s confident that the rover will be able to identify potential Martian stromatolites by examining the texture of the rock, the size of the individual grains, and the distribution of organic compounds.
“You can discern patterns that are precisely the type that you would expect a microbial mat to produce,” says Farley.
Where the Wild Things Were
While fossil-hunting scientists know what they’re looking for, a more complex question is where to start the search.
When seeking signs of life, NASA’s mantra is “follow the water.” The space agency has even designated “special regions” on the red planet that could possibly harbor modern microbial life, such as liquid saltwater that periodically flows downhill on certain steep crater walls.
But exploring these regions presents challenges for planetary protection—spacecraft are never entirely microbe-free, and there are risks that visiting special regions on Mars will contaminate them and hamper our hunt for alien life.
Luckily, the Mars 2020 rover will be exploring areas where water once existed, not where it is now.
The most promising sites hold rock formations that are 4.1 to 3.7 billion years old. During this era on Mars—which geologists have dubbed the Noachian Period—volcanic eruptions poured ash and gases into the atmosphere, trapping solar heat and warming the red planet. Liquid water likely existed both above and below the surface.
This period also coincided with the late heavy bombardment, a time when the rocky planets closest to the sun were relentlessly pummeled by comets and asteroids. This cosmic salvo, scientists believe, helped create conditions favorable for life. The powerful impacts brought more water to the surface and exhumed underground minerals with unique chemical properties.
In some instances, the bombardment created crevices and generated significant heat, forming thermal vents that many believe were natural laboratories for brewing up the organic compounds that led to life.
With those and other criteria in mind, NASA is currently considering eight candidate landing sites for the 2020 rover that might once have been inhabited.
Some researchers, including Mustard, are championing sites that are near the edge of Isidis Planitia—a 930-mile-wide plain that was formed when a massive object collided with the planet.
Mustard is bullish on this region for several reasons. For starters, surface features, such as an ancient crater lake and channels, show that water once flowed there. Also, he says, the geological activity created a clear timeline. The impact that formed Isidis occurred around 3.9 billion years ago, while nearby Syrtis Major is a volcanic province that formed some 3.6 billion years ago.
“That gives you a nice 300-million-year window in which a lot of activity occurred during an exciting time period in Martian history,” says Mustard. He sees compelling evidence for that activity in the region’s mineral deposits, notably olivine.
“It’s a beautiful mineral that makes great jewelry but also has some interesting characteristics,” says Mustard. “It's easily weathered and broken apart by chemical reactions and, in that process, it generates hydrogen gas.” And hydrogen, he says, is great feedstock for sustaining microbial communities.
Some researchers believe this chemical reaction was crucial in transforming Earth into a living planet. What’s more, the process of breaking down olivine produces carbonate minerals, which are also present in the Isidis area.
“It says that, wow, we've got the entire geochemical pathway from beginning to end that on Earth we know has a tightly coupled relationship to microbial communities,” says Mustard. “Could that have occurred on Mars? Those minerals are a natural and exciting reason to look for biosignatures that could be preserved in the rock record.”
Life in Them Thar Hills?
Mustard adds that he’s not a big fan of lake environments and prefers to focus the search at sites that were once thermal vents.
“You had to have photosynthesis and other evolutionary innovations to generate the biology that preserved life in lakes on Earth,” he says. “What if that never happened on Mars?” It’s far more likely, in his view, that we’d find life that was sustained by the energy produced at hydrothermal, mineral-rich hot spots.
Jim Rice at the Planetary Science Institute in Arizona is also eager to check out ancient Martian hot springs—more specifically, the ones discovered by NASA’s Spirit rover during its exploration of Gusev Crater. There, Spirit found deposits of a mineral called opaline silica at a site known as Home Plate.
“The way you get silica in nature is you have to have a lot of water, you have to have high temperatures,” says Rice, who was also the geology team leader for both the Spirit and Opportunity rovers. “This told us this area around Home Plate had lots of water and lots of heat. It's probably something like hot springs, maybe even geysers going off there.”
Like the Isidis Planitia region, Gusev Crater has deposits of carbonate rocks, which are extra-exciting because they don’t form in highly acidic water, says Rice.
“The carbonate rocks tell us the water was more a neutral pH,” he says. That suggests the water at Gusev was more friendly to life than ancient lake beds found by NASA’s Opportunity rover to be scattered over Meridiani Planum, where “the water chemistry was like battery acid,” Rice says.
Clay deposits offer further evidence that Gusev was once an active site. Clay minerals take many years to form in water and, like carbonates, they aren’t found in acidic environments. “The water had to be there for a significant period of time, geologically speaking,” says Rice. “It wasn't just spit out. The water didn't just gush one afternoon and then was gone.”
And earlier this year, two geologists from Arizona State University offered an additional, intriguing interpretation of Spirit’s discovery. The silica deposits have a distinctive shape—like cauliflowers poking up through the Martian soil—that bear an uncanny resemblance to silica formations shaped by bacteria living in geysers in the Chilean Atacama Desert, which has an environment that might have mimicked conditions in Gusev Crater billions of years ago.
As is the case with stromatolites, though, the silica cauliflowers could also be natural formations, and scientists would need to study them up close to know for sure.
Many of these deposits and formations are concentrated in an area of Gusev Crater called the Columbia Hills that’s around 0.6 miles in diameter. Rice argues that the multiple geological goodies concentrated in this relatively small area make it an especially attractive landing site. And, he notes, we’ve already surveyed the area.
“It seems natural to go back there,” he says. “I'm a field geologist, and in field geology you go back to your sites as much as possible.”
The instruments on board the Mars 2020 rover will allow it to identify rock samples that are strong candidates for containing fossils, but the rover itself won’t be able to verify their existence.
That will have to wait until the rocks are brought back to Earth, either by a future human expedition or a robotic sample return mission. Once geologists on Earth can get their hands on them, they’ll be able to search for fossils in labs using the most advanced equipment available.
As with all efforts to study ancient fossils, there will be a high burden of proof to demonstrate that the rock formations are biological, not geological, in origin. And the Mars researchers will have the unprecedented additional burden of examining rocks that might contains signs of life unlike anything we’ve found on Earth—something that may be truly in a word, alien.
“It would be so awesome,” says Mustard. “There's definitely a potential for life-forms that might have existed there that had a different biochemistry. If it were staring us in the face, hopefully we'd be able to recognize there’s some organizational aspect that would point to it not being a geologic process.”