One of NASA's missions is the search for life in the universe. It seems like we'll have to look outside our solar system to find any, with Mars probably dead, Venus too toxic, Mercury too hot, and the giant planets lacking a surface. So the focus has shifted to the Kepler space telescope, which has discovered Earth-like planets in other star systems.
But just because the planets in our system probably don't harbor life doesn't mean the moons are out of the question. Satellites of Jupiter, Saturn, and Neptune, along with the asteroid Ceres and dwarf planet Pluto, contain a layer of ice. Underneath, heated by the planet's core, could be liquid water, and thus... life. It's not impossible: There are bacteria on Earth that live in hydrothermal vents deep beneath the ocean's surface.
Do any moons have liquid oceans? Here are the leading contenders.
Jupiter's moon Europa has a thick crust of ice. The tidal tug of the giant planet almost certainly heats the moon enough to keep some of the ice, below the surface, in liquid form. The water would probably be salty, like Earth's oceans, which gave way to life.
The moon is so attractive that NASA has planned an un-crewed mission for the 2020s, which will "orbit the giant planet about every two weeks, providing many opportunities for close flybys of Europa. The mission plan includes 45 flybys, during which the spacecraft would image the moon's icy surface at high resolution and investigate its composition and the structure of its interior and icy shell," according to the agency. Why not send a lander and just drill into the ice? Because the layer is thought to be about 12 miles thick—for comparison, the ice sheet at the South Pole is only about two miles thick.
We know Enceladus has water; it spews 1,000 tons of it into space every hour! That ice becomes one of Saturn's outer rings. The situation here is the same as Europa; Saturn is so massive and so close that it churns the moon's interior, keeping it warm enough for liquid water to exist below a layer of ice.
Those strips in the image above are actually fissures, cracks caused by tectonic activity that allow geysers of water to burst through. They're young, some only 10 years old, which means Enceladus is still an active world. Unfortunately, farther from sun, the ice is even thicker here than on Europa—25 miles. Even if a probe could drop into a fissure, that's a long way down.
Instead of being heated by its parent planet Neptune, Triton may get its internal heat from the radioactive decay of its core. A liquid ocean may lie beneath the surface of frozen nitrogen and methane.
Triton has cryovolcanoes—volcanoes that blast liquid gasses instead of lava and molten rock (and which should be in the next Star Trek film because they sound so cool). The Voyager 2 probe witnessed eruptions of nitrogen, ammonia, and water. This activity actually gives Triton a thin nitrogen atmosphere. There's simply not much data on Triton because we've only had that one Voyager 2 flyby, and that was in 1989. But the geysers are real, so there's some water beneath the surface, if not enough for a global ocean.
Yes, even tiny, frigid Pluto—40 times farther from the sun than Earth is—might conceal liquid water. Astronomers involved in the New Horizons mission to Pluto believe the planet's smooth, heart-shaped feature, Sputnik Planum, was caused by a massive impact that allowed underground water up to the surface. The ocean could be 60 miles under Pluto's surface, but still there. If water could exist in that hostile an environment, that means the search for life could encompass most of the solar system.
There's one more candidate for an ocean, much closer to home. Ceres is a dwarf planet, the largest body in the Asteroid Belt. Last year, the Dawn spacecraft arrived and imaged it at high resolution for the first time. The planet is mostly rock and water ice. But Dawn found bright spots that look like outgassing, or possibly even cryovolcanoes like Triton has. Salty water is erupting from below Ceres's surface, so there may be an ocean of liquid water about half as close to us as Europa is. It's an exciting discovery that, along with New Horizons, shows just how much more we have to learn about our solar system.
NASA has already launched a probe to another asteroid, where it will gather a sample from the surface and return to Earth in 2023. Perhaps it will offer clues to Ceres's water as well.
What would life be like in these subsurface oceans? If they're bigger than microbes, they could be anything. There's very little light miles below the surface of a planet far from the sun, so the creatures would probably be blind, and sense their environment based on smell, sonar, or some other mechanism. They wouldn't be mammals like dolphins because there's no way to get to the surface for air. However, we know Earth octopuses can be very intelligent, so it's unclear just what might be waiting for us on in the waters of these frozen worlds.