NASA's Juno probe at Jupiter beamed back close-up photos of the planet's largest moon, Ganymede, for the first time in 2 decades

• NASA's Juno spacecraft zipped past Jupiter's largest moon, Ganymede, on Monday.
• Juno snapped an up-close portrait of the ice-encrusted moon for the first time in 20 years.
• NASA expects more Ganymede images soon, as well as data about its icy shell, which could hide an ocean.

Grey, heavily cratered, and peering out from the black of space, Ganymede looks a lot like our moon. But the icy rock is more than 400 million miles away - it's the largest moon in the solar system, and it circles Jupiter.

NASA's Juno spacecraft has been rocketing around Jupiter since 2016, but on Monday, it zipped past Ganymede, coming within 645 miles of the moon. No spacecraft had gotten that close in more than two decades - the last approach was NASA's Galileo spacecraft in 2000.

In just 25 minutes, Ganymede went from being a distant point of light from Juno's vantage point to a looming, round disk, then back to a point of light. It was just enough time for the probe to snap five photos.

NASA released the first two images on Tuesday; they're the most detailed snapshots ever captured of the gargantuan moon.

"This is the closest any spacecraft has come to this mammoth moon in a generation," Scott Bolton, who leads the Juno spacecraft team, said in NASA's press release. "We are going to take our time before we draw any scientific conclusions, but until then we can simply marvel at this celestial wonder - the only moon in our solar system bigger than the planet Mercury."

Scientists believe that Ganymede may host an ocean of salty water 500 miles beneath its icy shell - which would hold more water than Earth does. It's also the only moon in the solar system with its own magnetic field, which creates an aurora at its poles. Scientists hope the Juno flyby will help them learn more about both Ganymede's ice shell and its magnetic field.

The first Juno image, below, captures almost an entire side of the ice-encrusted moon. Each pixel covers about 0.6 miles (1 kilometer).

This image is just from the Juno camera's green-light filter. In the coming days, NASA expects to receive more images from the spacecraft, including those captured with its red- and blue-light filters. That will allow the agency to create a colorful portrait of Ganymede.

Juno's black-and-white navigation camera also snapped a photo, below, of Ganymede's dark side.

It's visible thanks to light scattered from Jupiter.