Jupiter’s moon Ganymede will receive a visitor from Earth on Monday. At 1:35 p.m. ET, NASA’s Juno spacecraft will be zoomed in, just 645 miles above the surface of the solar system’s largest moon.
It’s the first time a probe has made a close visit to Ganymede since the Galileo mission flew in 2000.
Ganymede is an icy moon, and icy moons are attracting a lot of interest from planetary scientists these days. Ice is not unusual on moons, there have ever been some on Earth’s moon. But some large moons around exoplanets have large amounts of water, and some are believed to have liquid oceans beneath their icy surfaces.
Researchers have realized that these watery worlds could be home to some kind of life, even though they are far from the sun. The gravitational pull of the giant planets that orbit them may help explain how the ice melts.
Scientists used to believe that the heat must come from the star of a planet, in our case, the Sun. “I think we’ve realized in the last few decades that the notion of habitability has stretched,” says Scott Bolton of the Southwest Research Institute, who is the principal investigator for the Juno mission.
Ganymede has one feature that other icy moons lack.
“Ganymede is the only one that has its own magnetic field,” says Catherine de Clare, a planetary scientist at the California Institute of Technology.
Earth’s magnetic field protects us from charged particles ejected from the sun. Ganymede’s magnetic field does something similar. “Except for the solar wind, the stuff that Ganymede’s surface is shielded from is all this material that came out of Io,” says de Claire. Io and Ganymede are among the four largest moons of Jupiter. Io is the innermost moon and is volcanically active.
It is not clear whether this protection increases the chance of finding some kind of life in Ganymede.
“Jupiter’s environment is pretty fierce,” says Margaret Kevelson, UCLA Emeritus and Research Professor at the University of Michigan. “So I wouldn’t be too happy being the one to land Ganymede.”
Kevelson was a member of the science team on NASA’s Galileo mission to Jupiter, the last mission to closely visit Ganymede more than two decades ago.
But the Juno flyby should give scientists a better understanding of Ganymede’s magnetic field. It’s a complicated situation, “because it’s a tiny magnetosphere spinning inside Jupiter’s own massive magnetosphere,” says de Kler.
In addition to studying the magnetic field, Juno’s Microwave Radiometer (MWR) will provide information about Ganymede’s water ice crust. Other instruments will study the interaction between charged particles falling on Ganymede and the moon’s thin atmosphere, and the interaction that causes the aurora-like auroras known as the northern lights on Earth. Other instruments will study Aurora on Ganymede.
And of course there will be pictures.
“It’s going to be the pictures, I expect them to be amazing,” Bolton says. There will not be many pictures. Juno will be traveling at about 12 miles per second as its speed exceeds Ganymede.
Juno will fly over the places that Galileo took pictures of when it flew, as well as when it flew by NASA’s Voyager space probe in 1979.
“We’ll be able to look at those changes and detect possible changes that may have occurred in that region before, since Voyager or Galileo took a look,” Bolton says.
Juno, which has a primary mission to study Jupiter itself, will return for a closer look at its other moons. Next year, it will fly another of the Big Four, Europa, and later the Aeo.