The prospect of seasonal liquid water flows on the surface of Mars instantly revived discussions about whether the planet most like Earth in the solar system could host present day life. But it's not the only place where scientists are looking. At a congressional hearing this week, scientists listed their top four candidates for extraterrestrial life in the solar system. Other researchers are scanning radio and optical emissions from distant stars to hunt for technically advanced civilizations. In the future, scientists plan to look for chemical signs of life in the atmospheres of planets circling nearby stars. Here's a look at the most likely spots for life among Earth's neighbors.
NASA/JPL-Caltech/Univ of Arizona
Without hesitation, NASA's chief scientist Ellen Stofan told lawmakers that Mars is her top candidate for finding life beyond Earth. "We now know that Mars was once a water world, much like Earth, with clouds and a water cycle and indeed some running water currently on the surface. For hundreds of millions of years about half the northern hemisphere of Mars had an ocean possibly a mile deep in places," Stofan said. "Life as we know it requires liquid water that has been stable on the surface of a planet for a very long time. That's why Mars is our primary destination in our search for the life in the solar system," she added. NASA's next rover, scheduled to launch in 2020, will be outfitted with instruments to look for ancient microbial life, though Stofan, a geologist by training, believes it will take astronauts on Mars, cracking opening rocks and running experiments, to make the definitive discovery.
Photo: Scientists have found recent evidence of liquid water on the surface of Mars in the dark narrow streaks that cut into cliff walls all around the planet's equator.
The Jupiter moon Europa is roughly the size of Earth's moon, yet it hosts a salty ocean that has twice as much water as Earth's oceans. The Europa sea contacts a rocky core, which presents suitable conditions for life to brew. The moon also has abundant sources of energy. That leaves one big question in the search for life: Does it have organics? A mission targeted for launch in the 2020s will attempt to find organics that have welled up from the sea into cracks on the moon's icy surface. It also will search for a mysterious plume that may be behind a 2012 Hubble Space Telescope detection of water vapor above Europa's southern polar region. Scientists also want to know how deeply the ocean is buried beneath Europa's frozen crust. "That will be important for coming up with a strategy to search for life there," Cornell University planetary scientist Jonathan Lunine told the House Committee on Space, Science and Technology. "There's a lot of groundwork that has to be done on Europa ... if there are fresh organics in the cracks, that's a good place to go," he said.
Photo: Artist's illustration of a plume of water vapor shooting off the icy surface of Jupiter's ocean-bearing moon Europa.
NASA/JPL-Caltech/Space Science Institute
One of the biggest surprises from NASA's Cassini mission at Saturn was the discovery of plumes shooting into space from the moon Enceladus, now known to host a global subsurface ocean. "Make a list of the requirements for terrestrial-type life -- liquid water, organics, minerals, energy and chemical gradients and Cassini has found evidence for all of them in the plume," said Cornell University's Jonathan Lunine. "The most straightforward way to look for life is fly through the plume, which Cassini has done lots of time, with modern instruments that can detect signatures of life," he said.
Photo: Light reflecting off Saturn illuminates the surface of Enceladus and backlights the plume in this April 2013 image.
NASA/JPL-Caltech/Space Science Institute
Saturn's largest moon Titan presents intriguing prospects for life, though it likely would be very different than anything found on Earth. It is the only moon in the solar system with a thick, protective atmosphere. Cassini and its companion Huygens lander revealed a world with methane clouds, rain, gullies, river valleys and methane-ethane seas. "We cannot resist asking whether some biochemically novel form of life might have arisen in this exotic, frigid environment," Cornell University's Jonathan Lunine said. "Titan is a test for the universality of life as an outcome of cosmic evolution." To look for life, Lunine said a spacecraft would likely drop a capsule into a Titan sea so that can float across the surface and make measurements. "We don't know what we're looking for here, so a generalized search for patterns and molecular structures and abundances that indicate deviation from abiotic (non-biological) chemistry is appropriate," he said.
Photo: Scientists assembled this mosaic of near-infrared images of Titan taken by the Saturn-orbiting Cassini spacecraft.
So far, we only know life exists on one planet, Earth, but scientists don't know how it started or even if it had one or more false starts before ultimately taking hold. "Since Earth remains for now the only instance of an inhabited planet, the search for life also requires that we further develop our understanding of life on Earth," NASA's lead scientist Ellen Stofan said. "We know life is tough, tenacious, metabolically diverse and highly adaptable to local environmental conditions," she added. Scientists have discovered microbial life that consumes what would be considered toxic to others and life that can withstand radiation, cold, heat and other extreme conditions. "We do know that life evolved very rapidly here on Earth after conditions stabilized. That's a factor that makes us optimistic that there's life elsewhere in the solar system," Stofan said. Clues about how life started on Earth may be preserved on the moon, which holds the geologic record of the first billion years of Earth. "That's the time that life began on Earth. To understand what was happening geologically, we can do no better than turn to the moon," Cornell University's Jonathan Lunine said. "We really have no laboratory model for how life began on the Earth," he added. "One of the reasons for going out to environments in our solar system where the conditions for life are apparently there and possible is to see whether life actually began, to do the experiment in the field rather than in the laboratory." "It is remarkable that we have found four destinations in our solar system where life may actually exist, or have existed for quite some time in the past. Now is the time to actually go search," he said.
Photo: The far side of the moon, illuminated by the sun, crosses between the Deep Space Climate Observatory and Earth.