Two ice worlds, Pluto and Europa, are closer to being explored by robotic spacecraft, and could be our best bets for finding extraterrestrial life in our solar system.

This week, the latest images of Pluto were released by the New Horizons spacecraft, which is destined to fly past the dwarf planet in July. Also this week, NASA announced that its new budget includes funds for the development of a mission to Jupiter's icy moon Europa, to be launched in 2020.

Both of these ice-covered worlds – along with several others in our solar system, such as Saturn’s moon Titan – are believed to harbour vast oceans beneath their icy surfaces. Europa’s ocean is thought to contain more water than all the oceans on Earth combined. These watery environments could harbour life akin to that found under ice and in the deep oceans on Earth.

As we report this week on Quirks & Quarks, scientists in Antarctica drilled down through a floating ice shelf and found fish thriving underneath the ice, in total darkness, 800 metres from the surface, and about 560 kilometres in from the open ocean. Somehow, the fish have found a way to survive in an extreme, almost desert-like environment.

Likewise, tube worms, crabs, clams and other animals thrive around hot water vents in the very deep ocean, under tonnes of pressure, totally cut off from the sun.

The key is chemosynthesis

Instead of using photosynthesis by plants as the base of their food chain, these deep ocean creatures use chemosynthesis, where chemicals in the hot water coming out of the vents are metabolized by bacteria that are then eaten by other creatures, and so on up their food chain. This self-sufficient system is independent of other life on the surface of the planet.

If life can exist in these extreme ocean environments on Earth, it’s reasonable to believe it could have evolved on ice-covered worlds in space. In fact, the odds may be better at finding life under ice in space than finding it on the surface of Mars.

The dozens of robots we’ve sent to the Red Planet have shown that Mars used to have lakes, rivers and possible oceans in the distant past, but now it is a cold, dry, desert world – quite hostile to life. Even though Mars has an atmosphere, there is no ozone layer to shield the surface from harmful UV radiation from the sun.

Life – including humans, if we go there one day – would find it difficult to survive without protection of some kind, or by living underground. Is it any wonder we haven’t found life on Mars? Perhaps it’s hiding in caverns that we haven’t discovered yet.

The protective properties of ice

Ice, on the other hand, is a very good protector against radiation. A layer a metre thick will stop just about everything the universe can throw at it (except neutrinos; they go through just about anything, including rock).

Ice on Jupiter's moon Europa, seen here in an artist's concept, could be hundreds of kilometres thick, providing a safe environment for life swimming or crawling around in the ocean below. (Image credit: NASA/ESA/K. Retherford/SWRI)

The ice on Europa could be hundreds of kilometres thick, providing a safe environment for life swimming or crawling around in the ocean below. Measuring the thickness of that ice will be one of the prime objectives of the Europa mission, including a search for cracks, where a later mission could insert a probe to check out the ocean directly.

Whether or not Pluto has an ocean under its icy surface should be determined this summer, when New Horizons makes its once-only fly-past. The probe is equipped with a suite of instruments that will map the surface, study the thin atmosphere and attempt to identify the inner structure of the dwarf planet.

The spacecraft will also check out the five moons that orbit Pluto, especially Charon, which is half the size of Pluto itself. It’s the presence of that large moon that scientists believe could provide the energy needed to keep an ocean in liquid form.

Tidal forces between the two bodies would produce heat in the core of the planet that could melt the ice from the bottom up, perhaps through hot water vents like we see on Earth.

Our search of life on other worlds has so far turned up nothing. But we have been looking for life that, like us, crawls around on the surface of a planet. Now, we are beginning to look at truly alien environments, where life could exist inside worlds rather than on them.

So, keep your eyes on the news this summer. There is lots more to come from the icy realm at the edge of the sun’s family.