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Mars Exploration Rover illustration
The twin Mars rovers have helped scientists follow the water on Mars; is it now time to “follow the energy” in the search for past or present life? (credit: NASA/JPL)

New directions in the search for life in our solar system

For over a decade a major area of research in planetary science, particularly those efforts funded by NASA, has been to understand whether any world in our solar system beyond Earth did, or does now, harbor life of some fashion. While the focus has typically been in terms of habitability—whether conditions are or were right to support life—rather than directly searching for life, the topic of past or present life elsewhere in the solar system has been a driving force in much of the research and the selection of planetary missions.

The central question of these studies remains very much an open one, of course. Other than the disputed evidence of primitive past Martian life in meteorite ALH84001 (see “ALH84001 + 10”, The Space Review, August 7, 2006), there’s been no hard evidence that Mars or anywhere else in the solar system beyond the Earth has supported life. However, the research performed to date, particularly that on the two most promising worlds in the solar system for life—Mars and Jupiter’s moon Europa—have yielded many insights and changes in thought on the possibilities for life on these worlds and the ways to look for it.

From “follow the water” to “follow the energy”

NASA’s studies of Mars, including its selection of orbiter and lander missions, has been guided by a central theme, summarized in just three words: “follow the water”. Images of the planet’s surface have provided scientists with plenty of evidence that liquid water once existed on the surface, but few clues regarding how long, nor where the water went: did it escape into space, or was it locked up below the surface? Such questions were of undeniable interest to scientists seeking to understand the planet’s early history and evolution, not to mention to astrobiologists, given water’s essential role in life.

NASA’s studies of Mars, including its selection of orbiter and lander missions, has been guided by a central theme, summarized in just three words: “follow the water”.

This research has provided scientists with abundant evidence of past water on Mars, including the possibility that the water was on the surface for extended periods, forming lakes and even an ocean in the planet’s northern latitudes. More recently, the Mars Exploration Rover Opportunity turned up clear evidence that liquid water existed on the surface in the Meridiani Planum in the planet’s past. Prior to the landing “there had been a lot of skepticism expressed about the possibility that liquid water could be around n these early times” in Martian history, said Michael Carr, a planetary scientist with the US Geological Survey, during a session on Mars research during the annual conference of the American Association for the Advancement of Science (AAAS) in San Francisco last month. “But the Meridiani evidence is very convincing.”

What’s less clear, though, is what happened to that water. The leading explanation is that the vast majority of it is now hundreds of meters or even kilometers below the surface, but the three-dimensional distribution of it is a mystery, noted Stephen Clifford of the Lunar and Planetary Institute. Radar-sounding instruments on ESA’s Mars Express and NASA’s Mars Reconnaissance Orbiter should be able to shed light on and groundwater that is relatively close to the surface.

Some of that groundwater could be much closer to the surface. Spacecraft images of craters have revealed the existence of gullies that appear to have formed relatively recently by flowing water, perhaps from groundwater seeping out from the crater wall. Not everyone is convinced that the gullies are created by groundwater, though. “I am very skeptical that these are caused by groundwater seepage,” said Carr. “There’s a lot of geologic evidence against it.” Clifford noted that an alternative explanation could be the melting and sublimation of snow and ice deposits that formed in the craters during times when the planet’s obliquity, or axial tilt, was higher.

Regardless of the nature of the gullies, few doubt that water was prevalent on Mars early in its history, and that the water still exists today. For those interested in looking for evidence of life on Mars, that suggests a shift in direction is in order. Tori Hoelher of NASA’s Ames Research Center is advocating an approach he calls “follow the energy” that would try to identify areas of Mars that have or had enough energy to support life. “We always should be considering, as we move on in our search for life, is where we should go and what we should look for, and any additional information we can collect that helps us narrow down our targets, and narrow down how we should outfit our missions, is something we should carefully consider,” he said. “By considering the availability of energy we have the ability to quantify the prospects for life, the ability to potentially prioritize the targets.”

Like water, though, Hoehler notes that “energy is everywhere” on the Martian surface; the question is what minimum threshold of energy is needed for habitability. While enough sunlight falls on the Martian surface to support photosynthesis, he said, other environmental factors will affect energy requirements. “Environmental conditions very much affect the requirements for energy,” he said, with more energy required for life to exist in harsh conditions. “We need a way to weigh and quantify these things.”

“By considering the availability of energy we have the ability to quantify the prospects for life, the ability to potentially prioritize the targets,” said Hoehler.

Jack Farmer, an astrobiologist at Arizona State University who has studied organisms in extreme conditions, like Yellowstone and Mono Lake, already has an idea of what would qualify as the best place to look for life on Mars: volcanic or other hydrothermal systems, where magma intrudes into subsurface water or ice deposits, causing the water to react with rock and bring minerals up to the surface. “I would submit that if there is biology on Mars, this is where you’re probably going to find it.”

Farmer also said that while, ideally, humans would be the best suited to search for evidence of past or present life on Mars, he realizes that robots will be the only tools available to astrobiologists for decades to come. “I think we can do a lot of the initial work robotically, and I think we should. I think that’s the efficient, economical way of doing it. Sending humans to Mars is a tough proposition, and I don’t think it’s going to happen for at least a few decades.”

“Life loves ice”

The other world in the solar system that has attracted the most attention from astrobiologists in the last decade has been Europa, the moon of Jupiter with the icy surface covering, perhaps, an ocean of liquid water. There has been no question here about the presence of water: Ron Greeley, a planetary scientist at Arizona State, noted that Europa has more water than the Earth. Instead, the questions have been about how much liquid water exists below the surface, and whether there is enough energy to combine with the water and known organic molecules to create life.

A cold, icy place like Europa would not seem to be the best habitat for life, but on Earth, at least, that’s not necessarily the case. “Life loves ice,” quipped Jere Lipps, a professor of integrative biology at the University of California Berkeley, during a session on Europa at the AAAS conference. In his research in the arctic and antarctic regions of the globe, he said, he’s seen many varieties of plant and animal life, including fish underneath Antarctica’s Ross Ice Shelf, living under hundreds of meters of ice and water.

The problem, though, as Lipps and others described, is how to find that life. Over the years scientists and engineers have proposed a number of mission concepts to search for life beneath the surface, from drills and penetrators to sophisticated submersibles. Lipps, though, is skeptical any of those missions will be flown for decades to come. “Drilling into the surface is way, way in the distance,” he said. “We don’t need to penetrate through the ice.”

Instead, he argues that there are ways for this life, incorporated into the ice, to make its way to the moon’s surface. Once on the surface, it might be possible to detect that life with an orbiter that carries an extremely high resolution camera—one with a resolution of a few centimeters per pixel—that could see layers and other features in ice created by biological activity. “With that resolution you can see a heck of a lot,” he said.

Lipps: “If we bring back a picture from Europa of life, they’ll pay.”

NASA’s Jupiter Icy Moons Orbiter (JIMO) could have potentially carried a camera like that, but the mission, originally planned earlier this decade as the cornerstone of NASA’s Project Prometheus space nuclear power and propulsion initiative, became a victim of shifting budgets as NASA turned its attention to the Vision for Space Exploration. While JIMO joins other, failed efforts to develop a Europa orbiter, scientists like Greeley remain committed to pushing for such a mission as NASA’s next once-per-decade “flagship” planetary mission. “What we hope to be able to do is to build a case to fly the next flagship mission to Europa,” he said, adding that the earliest they expect to be able to launch such a mission would be between 2015 and 2018.

Neither the orbiter—which would likely cost a billion dollars or more, even for a relatively simple mission—nor any more ambitious follow-on missions would be cheap. However, Lipps believes the ultimate customer, the taxpayers, would be willing to pay for it. “If we bring back a picture from Europa of life, they’ll pay.”

That sort of public support will be essential in the long run as astrobiologists seek evidence of life on other planets, and in the process learn more about life on Earth. “Science is data-driven, and the problem with understanding life at the fundamental level is that we only have one data point,” said Chris McKay of NASA Ames, at the end of the AAAS Mars session. “You need another data point.”