Scientists are on the verge of confirming we are not alone in the universe, with two probes being sent to a mystery moon near Earth.
In just a few short years, we may know if we're not alone.
Two probes are being sent to a mysterious moon bursting with the ingredients of life. And expectations are high we'll find it.
Once it was thought life could not exist without the sun's warming rays.
We were wrong.
The equation for life (as we know it) is surprisingly simple: soluble water, an energy source, and organic compounds.
Jupiter's moon Europa appears to have all three.
That's why we're going there.
In August, NASA confirmed it would build a space probe – the Europa Clipper – to investigate this glistening gem of a world early in the 2030s. It followed the announcement in April by the European Space Agency to put the Jupiter Icy Moons Explorer (JUICE) in place by 2029.
It's a gamble.
But the odds of finding life is surprisingly high.
"Discovery now seems inevitable and possibly imminent," says University of Melbourne researcher Cathal O'COnnell.
And finding living creatures – even microbes – outside Earth may have huge social, religious and scientific implications.
Perhaps it is time to prepare.
It may not be far off at all.
"It seems inevitable other life is out there, especially considering that life appeared on Earth so soon after the planet was formed," O'Connell says. "And the definition of 'habitable' has proven to be a rather flexible concept too."
"A discovery, if it came, could turn the world of biology upside down," O'Connell says.
"Bacteria, fungus, cacti and cockroaches are all our cousins and we all share the same basic molecular machinery: DNA that makes RNA, and RNA that makes protein.
"A second sample of life, though, might represent a 'second genesis' – totally unrelated to us."
Biologists would be able to examine what parts of the machinery of life are fundamental. And they'd discover how much is the result of evolutionary accidents.
"A second independent 'tree of life' would mean that the rapid appearance of life on Earth was no fluke; life must abound in the universe.
"It would greatly increase the chances that, somewhere among those billions of habitable planets in our galaxy, there could be something we could talk to."
In some ways, however, discovering similarities would be even more radical.
It would mean the idea of panspermia – that formulas for life are seeded between worlds and even stars through comets and meteorites – has merit.
"As Mars was probably habitable before Earth, it is possible life originated there before hitchhiking on a space rock to here. Perhaps we're all Martians."
Either way, Europa will hold the key.
"The ancient question 'Are we alone?' has graduated from being a philosophical musing to a testable hypothesis. We should be prepared for an answer."
SALT OF THE EARTH
Jupiter's frozen moon Europa is a jewel of our solar system.
It's shiny and bright. That's because it's encased in a shell of water ice.
But when the Voyager 1 space probe flashed past in 1979, Europa's beauty proved more than skin deeper. It had shapely canyons, troughs and ridges. And there were very, very few craters.
Did this mean liquid water regularly welled up from beneath, remoulding and refreshing the surface?
It wasn't until the 1990s that the full extent of Europa's enigma was revealed. The Galileo probe found strong evidence there were oceans twice as big as Earth's beneath the ice. And that water seemed salty.
What's so significant about salt water?
It's a sure sign of active geological processes. The water must be interacting with rocks. It's leaching nutrients and minerals out of the moon's solid core.
"It may well be normal table salt (sodium chloride) – just like on Earth, says Lancaster University researcher Chris Arridge.
"This has important implications for the potential existence of life in Europa's hidden depths."
In fact, it makes Europa a potential microbial Garden of Eden.
FIRE BENEATH THE ICE
We have some idea what to expect.
Europa's slightly off-kilter orbit causes Jupiter's gravity to fluctuate. The moon's core is constantly being squeezed and released, generating friction – and a molten core.
We've seen how hydrothermal vents enrich the depths of our own planet's deepest, darkest seas. They support thriving communities of microbes converting the mineral-laden fluids into energy.
And the ingredients for life aren't exactly rare.
"Carbon, hydrogen, oxygen and so on are among the most abundant elements in the universe," Arridge says. "Complex organic chemistry is surprisingly common."
Unexpectedly common, in fact.
This shouldn't be surprising: Some 6500 light years away is a massive floating cloud of alcohol.
That's a bit further than the average drive-through. But, interstellar comets such as 2I/Borisov and Oumuamua may have done something just like that.
Does Europa have enough? Or the right mix?
That's what the Clipper and JUICE are being sent to find out.
And the odds are good.
In 2017, sea ice researchers from the University of Tasmania calculated that some microbes they had found in the Antarctic already had what it takes to thrive in Europa's oceans.
So why wouldn't something evolve there also?
SPIES IN THE SKY
Both the Europa Clipper and JUICE probes will carry a variety of sensors to peer beneath the ice.
They will measure the minute fluctuations in the moon's gravity. These are caused by changes in the density of whatever is beneath – such as a mountain range, or a mineral deposit.
Both also carry ground-penetrating radars.
These are expected to be highly effective: the colder ice gets, the more transparent to radar it becomes.
Europa's surface at the height of day is a frosty -170C.
Planetary scientists expect the ice to be somewhere between 15 and 25km thick. But it may be much thinner in some places.
The Hubble Space Telescope has captured fuzzy indications of plumes of water may be erupting from Europa's South Pole. The evidence isn't as strong as that for another ice moon, Saturn's Enceladus. But it's promising.
If so, deep fractures must be obvious in the icy crust – pointing to shallow lakes of liquid water.
This is a core component of the space probes' mission: to scout the ideal location for a potential lander mission. It would have to drill through the surface to see what lurks beneath.
The Europa Clipper and JUICE probes are well suited to finding the telltale traces of life. But they can't get up close and personal.
Planetary scientists around the world have been advocating for decades that a second mission must be prepared.
One that will touch down on Europa's icy surface. And dig deeper.
It's no easy task.
Europa has only a thin atmosphere. So parachutes won't work. Any lander must use heavy rocket motors to land. There's also the intense, relentless radiation from nearby Jupiter.
All this must be overcome before the granite-hard ice can be tackled.
Drills won't cut it.
So scientists are exploring the potential of lasers – or even an unshielded nuclear reactor – to melt its way through.
"One way or another, we will get there," says University of Birmingham space sciences researcher Gareth Dorrian.
"The final challenge might then be ensuring that the spacecraft or submarine, having finally reached the ocean, doesn't get eaten by something swimming around in the deep!"