A number of interplanetary destinations could harbor extraterrestrial life—finding it could be just a space mission away.
Last week, NASA announced one of its most exciting missions in recent memory: a plan to visit Europa, one of Jupiter's largest moons. Previous research has shown that the moon is covered with water ice, and may contain a liquid ocean underneath its surface—raising the tantalizing possibility that Europa could harbor life.
In recent years, the remarkable number of planets we've discovered orbiting distant stars (1780, at latest count) has shifted the focus of the search for extraterrestrial life to other solar systems. But these planets are far, far away, so it would likely take thousands of years to reach even the closest ones.
With the Europa announcement, it's worth remembering that there are a number of destinations here in our own solar system that we could visit (with unmanned probes) during our lifetimes and perhaps find life. Here's our rundown of the best bets:
A number of missions, including the 1995 flyby of the unmanned probe Galileo, have provided data on Europa that have led scientists to some interesting conclusions. Its surface is made of water ice, but is surprisingly smooth—it has a number of cracks, but very few craters—suggesting that the ice is likely of a relatively young age, and is continually reforming over time, erasing the effects of asteroid impacts.
Moreover, analysis of Europa's lineae (dark fractures that crisscross the ice's surface) shows that they're gradually moving, perhaps evidence of tectonic activity or volcanic eruptions underneath. If true, this activity could provide enough heat to generate a liquid ocean underneath the ice.
The hypothetical combination of volcanic activity and liquid water has prompted some scientists to speculate that Europa could harbor life, perhaps similar to the ecosystems on Earth that crop up around seafloor hydrothermal vents and flourish in the absence of sunlight.
Last year, data from the Hubble telescope indicated that in some spots, enormous jets of water are actually shooting out through small holes in Europa's icy surface. If NASA really does send a probe to the moon sometime during the 2020s—still a big if, due to the realities of government spending on space—it could fly through these jets and collect samples to search for extraterrestrial life.
Saturn's moon Enceladus is tiny: Its diameter is about four percent that of Earth's, about the width of Arizona. But in recent years, scientists have become convinced that the minute moon is about as likely to harbor life as Europa, for largely the same reason—it appears to contain a liquid water ocean under a cover of ice.
In 2008, NASA's Cassini-Huygens probe detected plumes of salty water vapor shooting out from the moon's south pole, and further analysis of the plumes confirmed the presence of organic molecules such as carbon, nitrogen and oxygen, thought to be necessary for life. Instead of a thick cap of ice, similar to the one found on Europa, Enceladus has a thinner coating of ice mixed with crust, and the speed at which these plumes were moving (upwards of 650 miles per hour) strongly suggest that they're being shot out from a liquid ocean present at the moon's southern pole.
The presence of liquid water—perhaps due to heating caused by the moon's natural radioactivity—along with rock, ice and vapor has led scientists to hypothesize the existence of a long-term water cycle, in which vapor is shot upward, settles back down to the planet's surface and condenses into a liquid, circulates deep into the moon's crust and then rises back to the surface over hundreds of thousands of years. This could hypothetically circulate the organic molecules over time, making the existence of microbial life on the tiny moon that much more likely.
The Cassini-Huygens probe is schedule to pass by the moon several times in 2015, but there are currently no plans to send a specialized probe that could land on its surface, or sample the water vapor plumes for evidence of life.
Because of its close proximity, we know more about Mars than any of the other destinations on this list, and much of what we've found is encouraging. Data from the Curiosity rover and other unmanned probes have provided evidence that the planet once featured flowing liquid water and freshwater lakes on its surface. The planet currently has permanent ice caps on each of its poles that are largely composed of water ice, and the soil contains about one to three percent water by mass, although it's bound to other minerals and thus inaccessible. There's also some evidence that the planet's crust might feature traces of organic compounds.
The one thing we haven't found, though, is indisputable evidence of life, either current or historical. Previous claims of microbial fossils found on meteorites that originated on Mars have been debunked, and all the soil and rock samples that our probes have analyzed have failed to provide a clear signature of any life form. Other aspects of Mars that seem to make current life unlikely are its extremely thin atmosphere (too thin to substantially protect against radiation from space) and its extreme cold (average surface temperature: -82ºF), which prohibits liquid water from forming at the surface.
Written By: Joseph Stromberg
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