1) Europa is an ice-covered moon of Jupiter

Europa is the fourth-largest of Jupiter's 67 moons and is roughly the size of our own moon. It was discovered by Galileo in 1610 and is named after a figure from Greek mythology.

It's believed that Europa has an iron core, covered by rock similar to the rock that makes up Earth's crust. Because of Europa's distance from the Sun (on average, 484 million miles, which is more than five times as far as the Earth is from the Sun), it is very, very cold. Its outer surface is made up of ultra-cold ice: -260°F at the equator, and -370°F at the poles.

Under that ice, though, most scientists believe there's a liquid ocean. That's what makes a mission to Europa so intriguing — because, as NASA planetary scientist Kevin Hand says, "if we've learned anything about life on Earth, it's that where you find the liquid water, you find life."

2) Europa likely has an ocean two times as big as Earth's

An illustration showing the calculated volume of water on Europa, compared to Earth. (Kevin Hand (JPL/Caltech), Jack Cook (Woods Hole Oceanographic Institution), Howard Perlman (USGS))

In the 1970s, calculations suggested that Jupiter's gravity might provide enough tidal energy (more on that below) to warm up Europa's interior and create a liquid ocean under its ice. When the Voyager spacecraft flew by Europa in 1979, it revealed a cracked, icy surface with very few craters — indicating that the ice was continually being recycled, a piece of evidence that supports the idea of liquid underneath.

Then, in 1997, when the Galileo probe flew by Europa, it found that the moon had a magnetic field, likely caused by the circulation of a saltwater ocean. Subsequent work has indicated that, in some places, this subsurface ocean is actually leaking through the ice, and calculations indicate the ocean could have two to three times as much total water as all of Earth's.

What's more, there's reason to believe that Europa could have some of the other ingredients necessary for life too. It's constantly bombarded by high-energy ions (from Jupiter's magnetic field) and sulfur (from volcanic activity on another of Jupiter's moons, Io). Certain salts and hydrogen peroxide are also believed to be on the surface. If all of these chemicals could make their way through the ice into a liquid ocean, they could lead to the sorts of chemical reactions in the water that would provide food for microbial life forms.

3) Europa's interior is warmed up by tidal squeezing
Although Europa is five times as far from the Sun as Earth and -260°F at the surface, scientists believe that it probably has a liquid ocean underneath the ice because of the way Jupiter's gravity squeezes the moon over time.

For example, when our moon orbits around Earth, its gravity tugs slightly on our oceans, causing tides. (And the Earth tugs on the moon, as well.) The same phenomenon happens on Europa, with Jupiter's gravity tugging on Europa. Except in Europa's case, the pull is much more powerful — because Jupiter is more than 25,000 times as massive as our moon.

How does that gravitational pull lead to heat? Europa's orbit around Jupiter is elliptical, so at times, it's a bit closer, and at other times, a bit farther. This means the strength of Jupiter's gravitational pull on Europa varies. As a result, "Europa kind of gets squished and squashed like a rubber ball," says Hand. "That results in friction, which generates heat, which is part of what we think helps maintain that liquid water ocean beneath the icy shell."

4) Life could exist in Europa's oceans

Apart from water, scientists believe that two other main ingredients are necessary for life: an energy source and organic compounds (that is, molecules built from carbon atoms). And there are two basic hypotheses for how energy and organics might make it to Europa's oceans.

One source might be Europa's surface. We know that there are high-energy ions, sulfur, and hydrogen peroxide there (which might provide energy), and it's possible that collisions with asteroids may have delivered organics — though it's not certain.

If the ice is thin enough to allow these substances to migrate through to the water, they could allow chemical reactions necessary for life. But the ice's thickness still isn't known. Models suggest it could be anywhere from one mile to 20 miles thick. There's indirect evidence that some water is leaking through, perhaps through cracks or plumes of vapor, but that water might come from smaller reservoirs, rather than the ocean itself.

Another possibility is that energy and organic compounds might come from Europa's interior. "Europa's seafloor could potentially look somewhat similar to our own seafloor, with hydrothermal vents and geological activity," Hand says. On Earth, scientists have discovered diverse ecosystems living off these seafloor vents and the chemicals they emit — so it's conceivable that the same phenomenon might occur on Europa.

5) NASA plans to send a probe to Europa in 2025
For years, scientists have advocated for a mission to Europa, and recently, it's attracted enough political support to make it seem likely. It received dedicated money for the first time in Obama's proposed 2016 NASA budget, and scientists are now hoping for a 2025 launch.

The mission might not definitively tell us if Europa has life, but it will answer a number of important questions about the odds of it. Among other things, the probe will seek to confirm the existence of the ocean, collect data on the thickness of the ice, look for evidence of organic molecules on its surface, and hopefully determine if chemicals are cycling back and forth between the interior and the surface.

If the Europa Clipper probe launches in 2025 as planned, it'll enter Jupiter's orbit sometime between 2027 and 2031 (depending on the type of rocket used). Over the course of three years or so, it'll fly by Europa an estimated 45 times — at distances ranging from 15 to 60 miles — exploring different parts of the moon. (The solar-powered craft will be orbiting Jupiter, rather than Europa, because Jupiter's much larger mass means that it's easier to fall into its orbit — allowing the probe to carry much less fuel.)

In addition, the craft will map Europa's surface in high resolution for a potential followup mission in the future. "Someday, we want to send a lander to the surface, but right now we don't have good enough data on it," Pappalardo says. If Europa continues to look like a promising place to find life, a lander might someday drill through the ice and directly sample what's beneath.

6) Life on Europa might look like life on Earth's ocean floor
The one thing we know is that if life exists on Europa, it's fueled by chemicals — not sunlight, like the majority of life on Earth. That's because so little sunlight reaches Europa that it'd be blocked out by the thick layer of ice, and it's far too cold for anything to live outside of it.

If life has evolved on Europa, the ecosystems that surround hydrothermal vents on Earth's ocean floor might give us the best idea of what it might look like. These vents, which emit heated water and dissolved chemicals, feed chemosynthetic bacteria, which in turn feed diverse groups of animals that are much different from those seen closer to the ocean's surface.

Still, there's obviously no guarantee that life on Europa looks anything like this. There might only be single-celled microbes. Or there might not be life at all.

Even if the basic ingredients for life end up being there, certain conditions might be wrong — the water might be too salty to allow for certain basic chemical reactions, for instance. Additionally, we have no idea how hard it is for life to evolve: we know it happened once, on Earth, but it took billions of years. So maybe the odds are against it happening under any conditions.