The Solar System’s planets, moons, and other fascinating objects host an astonishing range of environments – from blazing hot Mercury and Venus, through the more temperate zone where Earth resides, all the way out to frigid Pluto and beyond.

 

Some of the most fascinating environments in the Solar System are cryospheres: worlds whose surfaces are composed entirely of ice of one sort or another. These so-called “ice worlds” have surface conditions far more extreme than Antarctica in winter, so it may come as a surprise that some of them are among the most likely candidates for harboring life beyond Earth. Let’s take a look at four of the most interesting ice worlds in the Solar System.

Jupiter’s moon Europa, as photographed in 1996 by the Galileo spacecraft. Image credit: Galileo Project, JPL, NASA

Europa, Moon of Jupiter

Water comprises eight percent of Europa’s mass, with rock and, most likely, a nickel/iron core making up the rest. Its surface, however, is coated entirely by water ice, laced by easily visible brownish stripes that appear to be produced by tidal flexing (caused by its gravitational relationship with Jupiter) and/or plate tectonics similar to Earth’s (only with ice).

 

Europa’s crust includes fissures and active cryogeysers (ice geysers) that are evidence of thermal jets originating in the liquid water underneath the surface ice. So, could life exist on, or in, Europa? As we have seen on Earth, areas around thermal vents are teeming with unusual forms of life – so, Europa would seem to be a promising place to look for it. Because of this, Europa will be among the first worlds to be observed by the James Webb Space Telescope after its launch in 2018. The Webb Telescope will seek to determine whether microscopic sea life is blasted to the surface of the moon by cryogeysers.

In 2015, the Cassini spacecraft captured this image of Enceladus’s craters coupled with a circuitous pattern of cracks and fractures. Image credit: NASA/JPL-Caltech/Space Science Institute

Enceladus, Moon of Saturn

Scientists believe that Enceladus, one of Saturn’s 62 moons, is a prime candidate for extraterrestrial life. Its icy crust is scarred by fissures and cracks through which more than 100 geysers have been observed shooting liquid water into space from a subsurface ocean.

“Enceladus has got warmth, water, and organic chemicals, some of the essential building blocks needed for life.” – Dennis Matson, Cassini project scientist, NASA’s Jet Propulsion Laboratory, Pasadena, California

In addition, Enceladus is known to be rich in organic chemicals, and something – tidal action, radioactivity, and/or chemicals such as ammonia – is maintaining the subsurface water in a liquid form. As with Europa, scientists speculate that it is possible that life may have gotten a start – or even thrived – near hydrothermal vents under the icy surface.

In 1989, Voyager 2 discovered that Triton shows evidence of ice volcanoes. Image credit: Voyager 2, NASA

Triton, Moon of Neptune

Farther out in the Solar System, Neptune’s moon Triton has a surface composed mostly of frozen nitrogen. Like Enceladus and Europa, it has tectonic regions, cryovolcanoes that release water and ammonia, and geysers that spout nitrogen gas. Triton’s surface includes areas of smooth ice and cratered regions that give it the appearance of cantaloupe skin produced by overturning of the crust.  Evidence from Voyager 2’s fly-by, in 1989, suggests the remarkable possibility that Triton’s surface, warmed by Neptune’s tidal forces, might have been liquid for as long as a billion years before freezing over.

Charon has a belt of fractures and canyons that separates its relatively smooth southern plains from its northern terrain, which is capped by a dark polar area. Image Credit: NASA, Johns Hopkins Univ./APL, Southwest Research Institute

Charon, Moon of Pluto

Charon, the largest of dwarf planet Pluto’s moons, was first imaged by the Hubble Space Telescope in the 1990s; more recently, the New Horizons spacecraft paid a visit and collected invaluable data about this frigid world. Its surface is composed of water ice, and the presence of water crystals and ammonia hydrates in patches on its surface suggest that cryovolcanoes and cryogeysers are likely active on Charon. Although Charon is quite small, it has canyons on its surface that are up to six miles deep.

Life on an Ice World?

Methane, ammonia, and nitrogen ices might not sound amenable to life as we know it. Roiling environments with active volcanoes and geysers shooting out frozen water and gases may not sound like the most reliable place to call home. And who knows if anything might swim in subsurface oceans that never feel the warming light of the Sun?

 

Yet, some of the Solar System’s ice worlds entice scientists with the possibility of finding life forms on them. Confirmation of life beyond Earth would fundamentally change our understanding of our place in the universe, and would open endless avenues of future study. Amazingly, with resources for more detailed observation coming on-line soon, it is very much within the realm of possibility that we could learn of life’s existence on an ice world within our lifetimes. So let’s keep looking.