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 Astrobiolog'ia 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 MooreThe University of Michigan College of Engineering
