ANN ARBOR—After a five-year journey to Jupiter, the Juno spacecraft will enter orbit around the gas giant world on Monday. The mission will give insights into the earliest days of the solar system and how the heavy elements essential for Earth—and life—were distributed across our corner of the cosmos.

“Jupiter is actually quite important for our own existence,” said Sushil Atreya, professor of climate and space sciences at Michigan Engineering who has been involved in the mission since its inception more than a decade ago.

“It’s a big giant planet that formed first and took in bulk of the solar nebula material and allowed Earth, Venus and Mars to form the way they did, where they did relative to the sun, and with the composition they have.”

Atreya explained why studying Jupiter today can help scientists peer back to the origins of our solar system.

“When you look at Jupiter, you’re looking at the conditions 4.5 billion years ago when the solar system formed—material that was there in the primordial solar nebula,” he said. “Those conditions don’t exist on Earth or any of the other inner planets anymore because they’ve all changed. Jupiter is so big, and its gravity so strong that it has held onto its original material.”

Jupiter is 300 times as massive as Earth. Like the sun, it’s composed mostly of hydrogen. Roughly 45,000 miles beneath the striped skin that’s visible through ground-based telescopes, scientists believe the planet has a solid core that’s approximately 10-15 times Earth’s mass.

Juno’s measurements of Jupiter’s gravitational and magnetic fields will help clarify whether a core is there and, if so, how big it is, and how Jupiter’s internal dynamo works to generate the planet’s mammoth magnetic field. Understanding the size of the core, together with the composition of the atmosphere, will help scientists zero in on which of several Jupiter-formation theories is most likely.

Atreya subscribes to the idea that ice played a major role in Jupiter’s formation, and that Jupiter was probably born approximately where it is today about five times as far away from the sun as the Earth is, or farther out and then migrated in. Jupiter’s present orbit is past the point where water was in the form of ice in the solar nebula.

“Because the planet formed beyond the snow line, it incorporated large quantities of ice. That didn’t happen for Mars, Venus and Earth, which are much closer to the sun,” Atreya said. “Why is ice important? Planetesimals of ice and dust grains can trap other elements, and eventually, by collision and other processes, they get bigger and bigger and likely formed the core of Jupiter. Current hypotheses suggest more than half of the core mass should have been made up of water.”

Juno will be listening to Jupiter at radio frequencies for signs of water—as a proxy for oxygen—in the depths of Jupiter’s thick atmosphere. The Galileo spacecraft sent a probe down seeking similar information in 1995. Unfortunately for Earth-bound scientists, its death dive was in a dry spot.

“It was like the Sahara desert of Jupiter,” Atreya said. “So oxygen is a missing element in the puzzle of Jupiter’s formation scenarios. To understand whether Jupiter’s origin story is right or not, we need to know the abundance of water. That should be the main contributor to the core mass, but we could not measure its well-mixed abundance the last time.”

Juno will zip beneath the planet’s radiation belts to orbit the poles, with a period of once every 14 days. Because of Jupiter’s punishing radiation environment, the little craft only gets 37 trips around before its instruments will be affected. The mission is scheduled to end in February 2018.

Atreya’s role is to plan and design the spacecraft’s water and ammonia observations with an instrument called the Microwave Radiometer System. He’ll also assist in analyzing data to determine the distribution of water and combine results from the radiometer system with those of other Juno instruments for broader context. He will also interpret the Juno data using the results from the Galileo probe.

The veteran planetary scientist has been involved in the planning and execution of many of NASA’s milestone planetary missions from the Voyager on. He says this mission is also important for our understanding of how solar systems form in general.

“We’re getting to the point where we can begin to think about whether there’s the possibility of another Earth in an extrasolar planetary systems,” he said. “We’re still far from answering those questions, but with the information we’ll have from Juno, we will be one step closer.”

More information:

• Sushil Atreya
• Juno mission