Mars might not be erupting, but it looks like Jupiter's moon Io sure is.

NASA's Juno spacecraft saw evidence of an eruption on what the space agency calls the "most volcanically active spot in the solar system" during a flyby in December.

NASA released an image on Monday showing Io half in shadow with a noticeable bright spot near its center.

NASA/SwRI/MSSS

The Juno team pointed four cameras at Io on Dec. 21. "No one expected we would get so lucky as to see an active volcanic plume shooting material off the moon's surface," Juno principal investigator Scott Bolton, who's with the Southwest Research Institute, said in a statement.

While Io is small compared with gigantic Jupiter, it's slightly bigger than Earth's moon in size.

NASA released another image from Juno's star camera. A closer look at Io inside the circle shows the glowing activity of several volcanoes. The smaller circle highlights a plume.

The images aren't hyper-detailed since Juno was about 190,000 miles (300,000 kilometers) from Io at the time.

The plume stands out thanks to its size.

"The ground is already in shadow, but the height of the plume allows it to reflect sunlight, much like the way mountaintops or clouds on the Earth continue to be lit after the sun has set," said Juno scientist Candice Hansen-Koharcheck.

Juno launched in 2011 and arrived at Jupiter in 2016. It is now halfway through its mission, with the aim of completing a map of the planet by the middle of 2021.

NASA says the new images will help scientists better understand how Jupiter's moons interact with the gas giant planet.