Update: NASA revealed evidence of plumes of water vapor erupting from Europa's surface. Read about the announcement here. But for a primer on Europa and all the things we hope to find there one day, keep reading. 

On Monday, NASA is set to reveal evidence of "surprising activity" on Jupiter's moon Europa. The space agency teased the event last week, much to the chagrin of science journalists everywhere who are planning to cover the news, whatever it is. NASA has a bad habit of starting rumors about big discoveries, always leading to much online buzz and speculation, but the press conferences never turn out to be about aliens:

(I mean, really: If NASA officials were sitting on findings that huge, they would either keep that secret on lock while they confirmed all their data or shout it from the rooftops immediately. You don't tease about aliens.)

We'll have the new Europa news up on Speaking of Science ASAP after Monday's press conference, which is set to start at 2 p.m. Eastern. But in the meantime, we'll follow up that bitter "sorry, no aliens" shot with a delicious chaser: Here are all the fascinating things we already know about Europa that make it one of our favorite moons, and some ideas about what today's findings might reveal.

We already know that Europa has an ocean, which is as cool as it sounds.

Including Earth, there are at least five likely ocean worlds in the solar system. Only Earth and Titan are known to hold liquid on the surface. But Europa is a very respectable subsurface ocean world. Below its icy crust, an ocean covers the entire moon, kept liquid by friction from Jupiter's massive gravitational pull. It may hold more water than all of Earth's oceans combined.

That's why some scientists hold out hope for finding alien life on Europa: It's not like we expect to find life everywhere we find water, but it's an excellent place to start.

NASA's Europa mission, which aims to conduct a detailed survey of Jupiter's moon and investigate its habitability, is now entering the development phase. (NASA Jet Propulsion Laboratory)

We know it could maybe, possibly support life.

While scientists have yet to find any direct evidence of microbial life in Europa's watery depths — let alone signs of an intrepid space squid civilization — there are indications that the moon's ocean might be able to support some kind of Earth-like life.

Unlike Titan's fancy-pants surface ocean (which is made of super-cold liquid methane), Europa's underground sea is thought to contain actual water – liquid H2O. And some studies suggest that the ocean might be able to produce the right kinds of minerals to support life as we know it, even if the moon doesn't have any volcanic activity (more on that in a minute).

We know that strange microbial life can thrive in the frigid subglacial lakes of our own planet's polar regions, so there's reason to hope that some kind of microbes might have evolved on Europa as well. We can assume that NASA's assurance that Monday's news doesn't involve alien life extends to the microbial level, because astrobiologists don't discriminate against single-celled beasties. But it's possible that scientists have found evidence for some of the geological activities or molecules associated with life in deep, dark thermal ocean vents.

We hope there are some mysterious plumes at play. 

In 2012, the Hubble spotted water vapor above Europa's surface. Scientists determined the most likely culprit to be a plume of water vapor spurting out of Europa's south pole. The plume would have been big, shooting out 20 times as high as Mt. Everest. But no plumes have been spotted since.

That was a big disappointment, because the volcanic activity that could produce giant geyser plumes would also help support life in the ocean depths by creating warmth and powering certain chemical reactions. Plus plumes make it easier to study ocean chemistry. The moon Enceladus – an icy ocean world orbiting Saturn – releases smaller geyser plumes with aplomb. NASA's Cassini orbiter was recently able to go diving through those frequent plumes to analyze the moon's chemistry, looking for signs of habitability.

But for now, Europa's own plume activity remains unconfirmed. Was the water vapor produced by some other phenomenon we haven't thought of yet? Was it indeed a plume, but just a one-off spurt caused by an unseen impact? Or does Europa experience plumes that are more episodic than Enceladus's, perhaps dictated by the incredibly strong gravitational forces of its host planet Jupiter?

So even if Monday's news isn't a first — even if the big reveal is that scientists spotted a water plume again — geyser activity on Europa could still be a pretty exciting prospect.

We're going to visit very soon. 

NASA has set a 2022 launch date for a mission to Europa, so we'll be visiting the moon relatively soon even if Monday's announcement turns out to be a dud.

Europa is bathed in too much radiation from Jupiter's seething magnetic field for a spacecraft to orbit the icy moon. Instead, the mission will orbit Jupiter and occasionally swing around Europa for a quick flyby. The spacecraft will take pictures and use remote probing instruments to try to determine Europa's habitability.

If any areas of the moon show particular promise, a future mission could land on the surface and drill into the ice. But don't hold your breath for that subsurface dive. Scientists think Europa has about 60 miles of ice in its crust, compared to the half mile or so that we have to drill through to study subglacial lakes on Earth. It's going to take a lot of work to create a spacecraft that can survive Europa's radiation levels, land on an icy surface, operate at temperatures of -180 degrees Fahrenheit and tunnel down into 60 miles of solid ice before it even starts collecting data.

But if scientists have found something truly exciting in the new Hubble images being discussed on Monday, perhaps the agency will be motivated to attempt a landing there sometime soon.

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