WITWATERSRAND BASIN, South Africa — A mile down in an unused mine tunnel, scientists guided by helmet lamps trudged through darkness and the muck of a flooded, uneven floor.
In the subterranean world of the Beatrix gold mine, they shed their backpacks, taking out tools and meticulously prepared test tubes to collect samples.
Leaning a ladder against the hard rock wall, Tullis C. Onstott, a geosciences professor at Princeton, climbed to open an old valve about a dozen feet up.
Out flowed water chock-full of microbes, organisms flourishing not from the warmth of the sun, but by heat generated from the interior of the planet below.
These tiny life-forms — bacteria and other microbes and even little worms — exist in places nearly impossible to reach, living in eternal darkness, in hard rock.
Scientists like Dr. Onstott have been on the hunt for life in the underworld, not just in South Africa but in mines in South Dakota and at the bottom of oceans.
What they learn could provide insights into where life could exist elsewhere in the solar system, including Mars.
Microbial Martians might well look like what lives in the rocks here at a deep underground mine.
The same conditions almost certainly exist on Mars. Drill a hole there, drop these organisms in, and they might happily multiply, fueled by chemical reactions in the rocks and drips of water.
“As long as you can get below the ice, no problems,” Dr. Onstott said. “They just need a little bit of water.”
Mars has long been a focus of space exploration and science fiction dreams. NASA has sent more robotic probes there than any other planet. But now there is renewed interest in sending people as well. NASA has been enthusiastically promoting its “Journey to Mars” goal to send astronauts there in the 2030s. Elon Musk, the billionaire founder of SpaceX, is promising that he will be able to get there a decade sooner and set up colonies.
Astronauts on Mars would be able to greatly accelerate the quest for answers to the most intriguing questions about the red planet. Was there ever life on Mars? Could there be life there today?
It was not that long ago that scientists had written off Mars as lifeless.
Forty years ago, NASA spent nearly $1 billion on its Viking mission, which revealed a cold, dry world seemingly devoid of organic molecules that are the building blocks of life.
But more recent missions have discovered compelling evidence that Mars was not always such an uninviting place. In its youth, more than three billion years ago, the planet was warmer and wetter, blanketed with a thick atmosphere — possibly almost Earthlike.
A fanciful but plausible notion is that life did originate on Mars, then traveled to Earth via meteorites, and we are all descendants of Martians.
Eventually, Mars did turn cold and dry. Radiation broke apart the water molecules, and the lighter hydrogen atoms escaped to space. The atmosphere thinned to wisps.
But if life did arise on Mars, might it have migrated to the underworld and persisted?
For a couple of decades, Dr. Onstott has been talking his way into South African gold mines, regaling the mine managers with the wonder of deep Earth life to overcome their wariness. In many ways, the mines provide easy access to the depths — a ride in a cagelike elevator, jammed against miners starting their shift, descending quickly as lights from the different levels zip past. Think of it as traveling through a 450-story skyscraper, going down.
Dr. Onstott and his colleagues had made repeated pilgrimages to this particular tunnel in this particular mine, Beatrix, 160 miles southwest of Johannesburg.
When miners carve out new tunnels, they poke holes through the rock to see what surprises might lie ahead. Sometimes the borehole taps into a section of fractured rock with water coursing through. Then the fracture is drained and plugged.
But this particular tunnel at Beatrix never entered production, so the borehole valve remains, allowing the scientists to return to draw samples from the same place.Continue reading the main story
At this level, almost a mile underground, the elevator gates open to a well-lit, concrete cavern with the unremarkable plainness of a parking garage. A minirailway system transports miners and ore back and forth. The side tunnel, though, is pitch black save for the helmet lamps, and the trek to the valve is a slosh through muck and over tangles of mangled electrical cabling.
Scientists led by Dr. Onstott made their most recent trip to South Africa in June last year. Over a couple of hours, they took their fill of the water and set up an apparatus that remains attached to the valve, trapping microbes, which were retrieved later in the summer. Since then, they have been analyzing the samples to understand this assemblage of life.
A New Type of Life
The existence of what biologists now call the Earth’s deep biosphere was unknown to almost all biologists at the time of the Viking mission. Life lived at the surface, in the soil or in the oceans. At the bottom of the food chain, the so-called primary producers, were plants and microbes that used photosynthesis and sunlight to power the conversion of carbon dioxide into organic molecules. Other creatures ate the plants and microbes, and then larger creatures ate the smaller ones.
In someplace that was always dark, it seemed obvious there could be no primary producers and therefore no life at all.
Some scientists noticed close to a century ago this might not always be true. Edson S. Bastin, a geologist at the University of Chicago, wondered why some petroleum was “sour” — with high levels of sulfur that not only corroded pipes but also generated more pollution when burned.
Bastin realized bacteria could do that, in particular a type of bacteria that does not need oxygen and eats sulfur compounds known as sulfates and excretes hydrogen sulfide — the rotten egg smell — and bicarbonate, the unwanted chemicals in sour crude oil. He and a colleague, Frank E. Greer, successfully cultured such bacteria from groundwater from an oil field, and Bastin speculated these could be descendants of bacteria that had been trapped in ocean sediments more than 250 million years ago.
Other scientists contended that this must be a mistake, merely surface microbes on the drill that had contaminated the sample.
In the late 1980s, the Department of Energy started a drilling project, carefully pulling up pristine cores from a couple hundred yards down. Bacteria, fungi and other microbes abounded in the cores.Continue reading the main story
That spurred additional research and discoveries, including bacteria that lived in the heat of underground oil deposits and finally, a microbe that degraded oil as Bastin had predicted. Today, the deep biosphere is thought to account for 10 to 20 percent of mass of all life on Earth.
Life, biologists also discovered, perseveres in other environments once thought sterile — highly acidic water, highly alkaline water, highly salty water, boiling water around volcanic vents at the bottom of the ocean.
“The truth is it’s virtually everywhere we look,” Penny Boston, the director of the NASA Astrobiology Institute, said in July at a panel celebrating the 40th anniversary of Viking’s landing on Mars.
A Methane Mystery
Results from an earlier trip to Beatrix befuddled Dr. Onstott. He had expected the mine microbes to be feeding off organic matter dissolved in the water. In this picture, the ecosystem would be largely devoid of primary producers and instead subsist on leftovers, the detritus of long dead organisms washed down from above or deposited with the sediment 2.9 billion years ago.
“The only problem was that we didn’t have any indication they were eating the organic matter in the fracture water,” Dr. Onstott said.
They figured out that the carbon molecules in the microbes came from methane, a plausible answer. Microbes known as methanogens consume hydrogen and carbon dioxide and produce methane; other microbes known as methanotrophs eat methane. But the Beatrix water contained little of either.
“It didn’t make any sense at all,” Dr. Onstott said. “It made zero sense to us.”
Maggie Lau, a postdoctoral researcher in Dr. Onstott’s laboratory, started examining the genetic snippets for clues of how the Beatrix community of microbes worked.
With the newest data, it turned out there was a wider community of primary producer microbes, eating nitrogen and sulfur compounds.
In essence, the waste of one microbe helped feed its neighbor, and only a little bit of methane, an energy-rich molecule, was enough to power the entire community.
“Now, for the first time, we’re getting a true description of the ecosystem,” Dr. Onstott said. “We think it’s a fairly common phenomenon.”
After working at Beatrix, the scientists went north, to the Limpopo region close to South Africa’s border with Zimbabwe. There, they collected water from hot springs, whose source of water from far below should carry up underground microbes.
One sampling site was in the middle of a small village where houses do not have plumbing and the spring, the hottest in South Africa, is a pond that serves as a communal spot for cleaning laundry. Another is a pool at a resort, once catering to the country’s white minority.
Surprisingly, the spring waters contained almost no microbes — barely enough DNA to analyze. But water from two locations contained considerable amounts of methane, an encouraging sign.
The methane is also a possible connection to Mars. A dozen years ago, three teams of scientists, one using data from the European Space Agency’s Mars Express orbiter, the other two using observations from Earth, made the controversial claim of methane floating in the Martian atmosphere.
That was a surprise, because sunlight and chemical reactions destroy methane; any methane there would have had to have been released recently. There are two ways to produce methane. One is a geological process that requires heat and liquid water. The other is methanogens.
Perplexingly, the readings of methane later vanished.
As NASA’s Curiosity rover drove across Gale Crater a couple of years ago, it too detected a burp of methane that lasted a couple of months. But it has not detected any burps since.
Perhaps an underground population of methanogens and methanotrophs is creating, then destroying methane quickly, accounting for its sudden appearance and disappearance from the atmosphere. If Beatrix is a guide, the methane could be providing the energy for many other microbes.
Conventional wisdom is that Martian life, if it exists, would be limited to microbes. But that too is a guess. In the South African mine, the researchers also discovered a species of tiny worms eating the bacteria.
“It’s like Moby Dick in Lake Ontario,” Dr. Onstott said. “It was a big surprise to find something that big in a tiny fracture of a rock. The fact it would be down there in such a confined space slithering around is pretty amazing.”
An Uncertain Quest
The odds of Mars life, past or present, are just conjecture.
Even if life did arise on Mars four billion years ago and later migrated underground, could it have survived for four billion years?
There are reasons to be skeptical. When low on water and energy, microbes can slow their metabolism or enter a state of suspended animation, able to revive when conditions improve. But many biologists doubt that such a tenuous hold on life could extend for a few billion years.
Mars also lacks Earth’s plate tectonics, a continual recycling of the outer crust of the planets that fractures rocks and exposes new minerals for microbes to eat.
If life is deep underground, robotic spacecraft would not find them easily. NASA’s InSight spacecraft, scheduled to launch in 2018, will carry an instrument that can burrow 16 feet into the ground, but it is essentially just a thermometer to measure the flow of heat to the surface. NASA’s next rover, launching in 2020, is largely a clone of Curiosity with different experiments. It will drill rock samples to be returned to Earth by a later mission, but those samples will be from rocks at the surface.
All this new interest in possible life on Mars is a sort of vindication for Gilbert V. Levin, one of the scientists who worked on Viking. Dr. Levin is sure he discovered life on Mars 40 years ago, and everyone else has been drawing the wrong conclusions from the Viking data. If he is right, then perhaps rediscovering life on Mars may require just scratching the surface.
The two Vikings carried what was known as the labeled release experiment, developed by Dr. Levin and another investigator, Patricia A. Straat. Essentially, radioactive food made with unstable carbon-14 was added to samples of Martian soil. The idea was that if microbes digested the food, the carbon-14 would be released in a stream of radioactive carbon dioxide and other gases rising out of the soil.
That is exactly what happened.
Then other samples were heated to 320 degrees Fahrenheit to sterilize them. If microbes were generating the radioactive gases, then there should be no gas rising from the sterilized soil.
That, too, is what happened.
“The response on Mars is well within the responses from terrestrial soils,” Dr. Levin said, “most closely the Arctic and Alaska.”
But in the absence of organic molecules, other Viking scientists discounted the possibility of life. It was like claiming the existence of a city in a place lacking wood, steel, bricks or any other building materials.
Dr. Levin has proposed, again and again, sending another labeled release experiment to Mars, to no avail. NASA’s 2020 rover will be able to catalog a wide variety of organic molecules, but carries nothing to look for life directly.
Dr. Levin may finally get his wish with ExoMars, a European rover scheduled to launch in 2020. He is working with one of the teams building one of ExoMars’s instruments to see if it could be modified to incorporate the labeled release apparatus.
There is a bit of a race against time. Dr. Levin, the last surviving member of the Viking biology team, is 92. “All I have to do is last that long,” he said.Continue reading the main story