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Revisiting Mars: The Search For Liquid Water And Life On The Planet Next Door
A few days after the Phoenix Mars lander touched down in May 2008 it sent back a picture of the ground underneath it. The spacecraft, which had aimed for a spot where its backhoe arm could reach ice, looked to be standing on a patch of it. Tests would confirm this in the coming months and scientists living vicariously through Phoenix would touch and taste ice on another planet for the first time.
Credit: University of Michigan/NASA
At mission control in Tucson, lead atmospheric scientist Nilton Renno was ecstatic when he saw the photo. It corroborated his research group’s prediction that the craft’s thruster-cushioned landing would strip away the top layer of soil and expose ice. He barely had time to celebrate before something else about the image caught his eye: a strange pattern on the lander’s leg.
Maybe it was paint, a colleague suggested, sandblasted in the landing. But Phoenix’s legs had never been painted.
The maverick University of Michigan professor who flies airplanes and gliders in his spare time had another theory. And when the craft sent back clearer pictures as the weeks progressed, he was convinced: he was looking at drops of salty water—not ice, but liquid water, a necessary ingredient for life that no one had ever detected let alone seen anywhere but Earth.
“The spheroid shape and the fact that there was a large range of sizes and changes in brightness made me think that they were liquid,” said Renno, of the Department of Atmospheric, Oceanic and Space Sciences. “When the third photo came about a month into the mission, I could see that the droplets were growing. It was very clear it was a major finding.”
Not all scientists agree. A few still believe the globules are frost. But in the years since the mission, evidence has mounted that the droplets are in fact liquid, and that small amounts of liquid water are common across Mars today. The findings are fueling new hope that the planet next door could be home to very basic forms of life.
Antifreeze As the Answer
The median temperature at the Phoenix landing site was -70 degrees Fahrenheit—far too cold for liquid fresh water. Exotic salts were central to Renno’s hypothesis from the start. In preparing for the mission, he had read theories that certain salt compounds could be present in the soils there, acting as antifreeze as well as sponges. They could drastically lower the freezing point of water and also pull vapor from the air to form brines. None of these compounds had ever actually been found there, however. That’s one reason Renno’s ideas were controversial.
“I thought he was crazy at the beginning,” said Ray Arvidson, a professor at Washington University in St. Louis who was in charge of the Phoenix’s robotic arm. “But now I believe him.”
About two months into the mission, as Renno was still making impassioned if not popular presentations about brines, mission scientists stumbled on a smoking gun. In a scoop of soil, Phoenix found perchlorate, a salt made of chlorine and oxygen. It’s naturally found on Earth in arid places like the Atacama Desert in Chile, thought to be an ancient seabed. Perchlorates are indeed capable of reducing the freezing point to landing site temperatures and pulling water from the atmosphere through a process called deliquescence.
Renno says he jumped when he heard the news. “For me, this was the most exciting moment,” he said, “after arguing with many people on the science team who asserted that there was no evidence for these salts.”
The discovery has given rise to a new branch in an old line of research—and speculation.
Liquid Water Fiction and Fact
The possibility of water and life on Mars has captivated humans for more than a century. In the early 1900s, it was a popular notion that Mars was crisscrossed with canals. Astronomer Percival Lowell’s widely read books suggested resourceful Martians had built them to irrigate their drying world with melted water from the poles.
As late as 1971, the canals were prominent features on Mariner mission planning charts, writes mission scientist and author of Water on Mars Michael H. Carr.
“Basically we would have been using a blank sheet of paper otherwise,” Carr said. The Mariner missions found no trace of canals. They were chalked up to optical illusions, and as Mariner 9 photographed 80 percent of Mars’ surface in the 1970s, a new picture of the planet emerged. It was believed to be dusty and dry in the present time. But the spacecraft found the first real evidence that water did flow there in the distant past. Carr was on the imaging team.
“We started recognizing these huge canyons and we were astounded,” he said. “The experts put together a massive flood hypothesis. There was a lot of resistance because we knew how cold the planet was. Yet, here were these obvious waterworn features.”
In the decades since, spacecraft that orbited, landed on and roved around Mars have found river valleys, dry lake beds and elaborate deltas. They’ve detected evidence of erosion, glaciers and possibly even the shoreline of an evaporated ocean. The impossible flood theory is now widely accepted.
All that was millions or billions of years ago. Until about the past 10 years, the signs of Martian water were echoes from other eras. Scientists can’t yet model how Mars managed a climate warm enough to support so much liquid water, but they are confident that it did. Renno and others are as certain it holds some today—not seas of it, but maybe enough to support microbial life.
“In my view, the evidence for past liquid water pales in comparison to the evidence for liquid water now,” Renno said. “Now we can see the actual droplets that our calculations predict. We see them like you see water in a glass, rather than evidence that water was there in the glass a billion years ago.”
Credit: University of Michigan
The evidence has been trickling in since around 2000. Orbiting cameras captured gullies running down crater rims that had formed in just the previous decade. Spectrometers monitoring how sunlight reflects off Martian rocks at various wavelengths picked up liquid water’s tell-tale mark across much of the ground. More recent readings from other instruments show that the carbon dioxide in the atmosphere has recently interacted with liquid water on the surface and perhaps does so on an ongoing basis.
More Proof of Present-Day Water
The pace of new findings has quickened since Phoenix.
“One whole class of inquiry traces back to the discovery of perchlorate salts at the Phoenix landing site and the droplets on the lander’s strut,” said Christopher McKay, an astrobiologist at NASA’s Ames Research Center.
Initially, it seemed that the salts were evenly distributed through the shallow depths of the soil and some cited this as a reason why liquid water wasn’t likely to be present. But in 2010, a more thorough analysis revealed centimeter-sized patches of more concentrated salt—a sign that liquid water had been there, and recently. Water could redistribute the salts. This could have happened a few thousand years ago, or it could occur regularly as the seasons change, explained Arvidson, the lead robotic arm scientist who at first didn’t believe the droplets were liquid.
“There’s a growing recognition that there’s a modern hydrologic cycle on Mars that involves thin films of water,” Arvidson said, “perhaps enhanced by the presence of salts.” Renno, who with Spanish colleagues in 2009 recreated the strut droplets in a lab under Martian conditions, says these salts could be ubiquitous on Mars. He believes they could be responsible for keeping water wet in the gullies and thin films, as well as in other more recently found phenomena.
Dark streaks grow and fan from the tops of dunes in photos from the Mars Reconnaissance Orbiter analyzed by European scientists in 2009. The culprit, once again, is thought to be liquid water, or more specifically, saltwater. Diedrich Moehlmann, with the German Aerospace Center’s Institute of Planetary Research, calculated that salts would thicken the water and slow its flow, which is consistent with observations.
“These cryobrines give us quite a new view of Mars,” Moehlmann said.
McCay, the NASA astrobiologist, says the discovery of these particular salts on Mars is so paradigm-shifting that it reopens the results of a 1976 Viking spacecraft experiment thought to rule out the existence of organic molecules there.
In search of the building blocks of life, Viking landers heated soil samples and sniffed the chemicals they released. They found traces of two organic chlorine compounds, but at the time, scientists attributed them to contamination from Earth. Just last year, McKay and his colleagues repeated the experiment using perchlorate-spiked soil from the Atacama Desert, thought to be analogous to Mars. The results were strikingly similar. The same chlorine compounds showed up, and no additional organics. Perchlorate, though harmless in solid form, is poisonous as a vapor. It destroyed the organics in the Atacama soil and may have done the same on Mars.
“If we did the experiment now, we would conclude it was not contamination,” McKay said. “The perchlorates are leading us to reconsider and to conclude there are organics on Mars.”
The view that organics are not present informed scientists’ interpretations of other Viking tests designed to detect microbial life on Mars. Several experiments produced results that some scientists tentatively interpreted as positive at the time. But without the organics that would make up the microbes, most attributed the results to chemical reactions.
The Mars Science Laboratory, scheduled to launch in November, is designed to look again for organics—not life, but its components. Life needs liquid water to survive, but it doesn’t need much. From a microbe’s point of view, a droplet or thin film is an ocean. And like our oceans, the water can be salty, even very salty. “Extremophile” microorganisms that live in fringe environments on Earth have been found in places as cold and briny as the water on Mars is thought to be.
Renno, NASA astrobiologist McKay and their colleagues in Spain are involved in a project to test some of these Earthly extremophiles’ hardiness in mini Mars environments. While Michigan engineers are studying the formation and stability of brine pockets in Mars conditions, their colleagues at the Centro de Astrobiología in Madrid are seeding Mars chambers with salt-loving microbes from Antarctica and the Gulf of Mexico to see whether the microbes survive and reproduce.
It’s a thrilling project for Renno, an atmospheric scientist who is becoming an astrobiologist.
“I want to contribute,” Renno said, “to answering the question: Are we alone?”
Contacts and sources:
Nicole Casal Moore
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