JOIN USLooking at Mars through Earth
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, South Africa, 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, USA, 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 Tullis have been on the hunt for life in the underworld, not just in South Africa but in mines in South Dakota, USA 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, fuelled by chemical reactions in the rocks and drips of water. “As long as you can get below the ice, no problems,” Tullis 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. 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?
Good to bad
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 Earth-like.
A fanciful but plausible notion is that life did originate on Mars, then travelled 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?
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.
Scientists led by Tullis 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 analysing the samples to understand this assemblage of life.
Results from an earlier trip to Beatrix befuddled Tullis. 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,” Tullis 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,” Tullis said.
Maggie Lau, a postdoctoral researcher in Tullis’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 neighbour, 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,” Tullis said.
The odds of Mars life, past or present, are just conjecture. 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, to be launched 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. Gilbert 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 labelled release experiment, developed by Gilbert 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.
Then other samples were heated to 320 degrees Fahrenheit to sterilise them. If microbes were generating the radioactive gases, then there should be no gas rising from the sterilised soil. “The response on Mars is well within the responses from terrestrial soils,” Gilbert 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.
Gilbert has proposed, again and again, sending another labelled release experiment to Mars, to no avail. NASA’s 2020 rover will be able to catalogue a wide variety of organic molecules, but carries nothing to look for life directly. Gilbert 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. Gilbert, the last surviving member of the Viking biology team, is 92. “All I have to do is last that long,” he said.