NASA’s Juno mission will probe Jupiter’s atmosphere in search of clues to how the largest planet in the Solar System, and the Solar System itself, were formed from a primordial cloud of gas.
Jupiter is probably the oldest planet, but it keeps its secrets veiled beneath the clouds and massive storms we can see from Earth. By sending Juno beneath the planet’s radiation belts in a polar orbit that takes it just 3,000 mi. above the cloud tops, scientists hope to unravel some of those mysteries with sounding measurements that should reveal the planet’s composition and structure.
“The primary questions that we’re after have to do with the origin of Jupiter, the origin of the Solar System and how planets were made, and how and why the planets are a little bit different than the Sun, and particularly the history of the volatiles that eventually led to Earth and life itself,” says Southwest Research Institute scientist Scott Bolton, the Juno principal investigator. “In particular what we’re after is how much water or oxygen is inside of Jupiter.”
The nuclear-powered Galileo mission dropped a probe into Jupiter’s atmosphere in July 1995 that returned data for almost an hour. But while the probe obtained good measurements for nitrogen and sulfur, it stopped working—crushed by the mounting pressure 150 km below the cloud tops—before it could return a valid figure for water. Finding that figure will be one of the primary objectives of Juno.
“We don’t really know where the water came from on our Earth,” Bolton says. “We have these oceans and people say, ‘Well, maybe they came from comets.’ Some of the people like to think that it came from the heavy bombardment period. . . . Maybe Jupiter was bombarded, too, and that’s where its water came from. Maybe that’s where the heavy elements came from. We don’t really know. Those basic measurements are fundamental to tell us the history of Jupiter, to place where it formed, how long it took to form and the process that made it. It kind of gives us the traceability of these volatiles at that stage of the Solar System.”
Basically, Juno will use its microwave radiometer to measure the absorption of radio waves by different components of Jupiter’s atmosphere, including water and ammonia. The atmosphere “glows” in radio wavelengths, and the frequency tells scientists what is blocking the radio waves or letting them through.
“If I go down deeper with a longer wavelength, then the atmosphere can become more transparent to me at the longer wavelength, and I’ll see the glow from deeper down,” Bolton says. “The reason the atmosphere is transparent or opaque is because water and ammonia are doing the absorbing in the atmosphere at these particular wavelengths. . . . At the top of the atmosphere and the very high frequencies, I’m measuring ammonia. Deeper down I’m measuring water because that’s what’s doing most of the attenuation. So it’s sort of a trick. That’s our microwave experiment, and that’s largely the basis of Juno.”
A probe in polar orbit close to the planet also was attractive to scientists trying to figure out the planet’s internal structure by making very precise measurements of its gravity, using the Doppler shift in spacecraft communications back to Earth. Based on that data, researchers may be able to determine if there is a core of heavy elements at the center of Jupiter, something they don’t know today.
NASA was already considering a polar-orbiting mission to Jupiter just to measure the planet’s full magnetic field, so Juno filled the bill for that. And the polar orbit gave researchers a chance to use the aurora there to study the polar magnetosphere “as a freebie,” he says.
“All these groups formed together under me to create Juno,” says Bolton.
Once the data start flowing in 2016, Juno will be able to answer questions that go beyond the particulars of its target planet. Scientists believe the Solar System was formed when a cloud of hydrogen and a little helium coalesced into the Sun, with the leftovers going on to form Jupiter. But Jupiter has other elements, and accurately measuring them—starting with water—has long been a goal of space scientists.
“Something must have happened after the Sun was formed to allow Jupiter and the rest of the planets to be formed with a little bit different mix,” Bolton says, explaining that the cooling of the primordial planetary disk allowed different materials to separate out at different rates, beginning with the water ice. The goal of Juno is to fill in the blanks.
“We go to Jupiter to do this because it’s big,” he says. “It has most of the material other than the Sun. If I take everything else in the Solar System I don’t get Jupiter.”