Scientists are gearing up to get the most detailed snapshot yet of Europa, one of the frozen water worlds in our Solar System potentially capable of hosting life.
NASA's Juno probe, which was originally designed to study Jupiter, will will zip over the planet's icy moon tonight at around 7:36pm AEST.
Europa may harbour a salty ocean underneath its frozen crust that locks up more water than on Earth.
Coming within 358 kilometres of the moon's red and white-striped surface, it will be the closest flyby of the tiny world in more than two decades.
"We're really excited that we're able to fly by Europa so close ... it wasn't in our original plan," said Scott Bolton of the Southwest Research Institute and head of NASA's Juno mission.
The last time a spacecraft buzzed this close to the icy world was in 2000, when Galileo passed within 350km.
But, Dr Bolton said, this one-off flyby will significantly add to what we know about Europa.
Screaming past at about 24km per second, Juno will take a new route and cover an area not seen in high resolution by Galileo, and investigate what lies underneath the moon's icy shell for the first time.
"Even though we only go by once, we go by with a very unique kind of instrumentation," Dr Bolton said.
Getting up close with Europa
Europa is one of Jupiter's 80 moons.
About 90 per cent of the size of our Moon, it is the smallest of four moons first discovered by Italian astronomer Galileo Galilei in 1610.
The first grainy images from a distance of 321,000km were captured by the Pioneer 10 spacecraft as it flew by in 1973.
Hints of a smooth surface crisscrossed by cracks resembling sea ice were revealed by Voyager 1 and 2 in 1979. These images provided the first evidence "water worlds" may exist beyond Earth.
In the late 1990s, the Galileo spacecraft captured Europa in even more detail in a series of flybys.
These images revealed a spectacular world unlike anything else in our Solar System, said planetary scientist Jonti Horner of the University of Southern Queensland, who is not involved with Juno.
"It looks totally alien compared to every other moon we have ever imaged," Professor Horner said.
Europa is covered in cracks that are more than 3000km long, but, unlike our Moon, there are surprisingly very few craters on its surface.
"That tells you Europa is an active world rather than something that is frozen solid, that is dead."
The smooth ice surface is thought to float above a salty ocean warmed by volcanic activity. When the crust cracks, water flows up and refreezes.
"The fact that those cracks are colourful tells you that's not pure water," Professor Horner said.
We don't know what causes the red colour, he said, but it may be dissolved minerals in the water.
Parallel ice ridges around the cracks resemble those seen in Greenland, suggesting the moon's icy shell could be riddled with pockets of water just under its surface.
Bends in Europa's magnetic field detected by the Galileo spacecraft, and more recent detection of hydrogen molecules in the moon's atmosphere by the Hubble Space Telescope, also hint there may be active geysers spraying plumes of water into the atmosphere.
The only way you get molecular hydrogen on an object like Europa is through chemical changes that occur when superheated water from volcanic vents reacts with rocks, Professor Horner said.
What Juno will do?
Europa is the second of Jupiter's moons to get the close-up treatment by the Juno spacecraft.
Since the spacecraft reached Jupiter in 2016, it has captured the most detailed and stunning images of the gas giant itself, as well as its largest moon, Ganymede.
On this flight, Juno will start collecting data when it is about 83,000km from its target, about an hour before the flyover on Thursday night.
As it comes in over the planet's night side, Juno's navigation camera will take a black-and-white photo of the moon illuminated by light reflected off Jupiter.
"We did this with Ganymede and it worked great."
Then it will cross over to the Europa's day side and take a series of colour images with a resolution of around 1km per pixel using JunoCam.
The spacecraft will also peer deep into the moon's icy shell for the first time using a tool known as a microwave radiometer.
This tool, which was designed to see through Jupiter's swirling clouds, measures temperatures at a particular wavelength coming off an object.
"We certainly discovered a lot of things [on Jupiter] we didn't anticipate with it," Dr Bolton said.
On Europa it will be able to measure the thickness of the frozen crust and detect changes in reflectivity that may indicate fractures or pockets of water near the surface.
"If we did see [pockets of sub-surface water], that would be a very important discovery because that may be the more likely place where liquid would make contact with the surface."
When water reaches surface through a new fissure it may spray out in plumes like a geyser, but despite hints, none have ever been directly detected.
"The jury's out a little bit on whether the plumes exist or not," Dr Bolton said.
And Juno would have to be extraordinarily lucky to detect any geysers spewing water into the atmosphere with a single flyby.
"You've got to be looking at the right time. Maybe they're not constant."
When will I be able to see new images of Europa?
The raw data will start coming back down to Earth via NASA's Deep Space Network after Juno completes a flyby of Jupiter, seven hours after it passes Europa.
"If everything goes as planned, I would say we'll have something within a day," Dr Bolton said.
And, if you are really keen, you could help process the raw data from JunoCam into images that everyone gets to see.
Once the data from the Juno mission is checked it is put onto a public website, where you can download it.
Many of the stunning images of Jupiter over the past six years have been created by citizen scientists from this data.
"The public actually make the pictures and often those are the ones we release."
Will the flyover tell us if there is life on Europa?
No, but it will come in handy for upcoming missions to the oddball frozen world.
NASA's Europa Clipper is set to launch in 2024, and the European Space Agency's Jupiter Icy Moons Explorer (aka JUICE) is slated for take off next year.
Both these missions, which should reach Jupiter at the end of this decade, will eventually study Europa in detail using a series of close flybys within 25km of its surface.
Along with using the microwave radiometer to look for anomalies in the ice sheet that might make good targets for future missions to explore, Juno will measure the radiation around Europa.
Radiation from Jupiter not only blasts the ice on the surface of Europa, it can be dangerous for spacecraft.
Juno will also measure how Europa interacts with Jupiter's magnetic field, which not only provides information about how radiation particles move around the moon, but may give some hints about what is happening under the ice sheet.
"[Magnetic field measurements are] the primary evidence for a liquid ocean," Dr Bolton said.
While Juno will not be able to confirm an ocean based on a single flyover, future missions like Clipper may.
But even if an ocean is eventually confirmed, detecting life on Europa will take more than flyovers.
Research suggests that the hunt for life would need to happen well below the surface of ice that has been churned up by asteroid impacts and exposed to radiation.
"We can't look for definitive evidence of life on Europa at the moment from Earth or from orbit around Europa because the ice sheet is in the way," Professor Horner said.
What you would need to do is send a spacecraft that could go through the ice, estimated to be about 10km thick, down to the bottom of any ocean near volcanic vents.
"Life may well not be right up at the surface because it will be down where the source of energy and nutrients is," he said.
"But that's a long-term goal. It's such an exciting world. I'm keen to see what we discover with this next set of images."
