Peeling back Jupiter’s skin to find ammonia within

Jupiter’s ammonia-rich upper atmosphere has been laid bare, thanks to a radio telescope that can see through its cloudy shroud.

Astronomers from the US, Australia and the Netherlands saw signs of immense waves of ammonia billowing 100 kilometres beneath the gas giant’s flurry of clouds and mapped its distribution around the planet in 3-D. The work was published in Science.

Jupiter’s atmospheric make-up has had planetary scientists puzzled for decades. Its upper atmosphere, where the clouds form, has been hidden by those very clouds.

In the 1980s, radio telescopes such as the Karl G. Jansky Very Large Array, a system of 27 antennae constructed in New Mexico in the 1970s, found that while Jupiter’s blustery visible clouds are rife with ammonia, there is very little evidence of the stuff 100 kilometres below. 

But NASA’s Galileo probe, which plunged through Jupiter’s atmosphere in 2005, dropped into a region almost completely devoid of ammonia clouds. Data suggested the upper atmosphere didn’t, in fact, contain all that much ammonia, and that it was probably localised to a few billowy clouds here and there.

This new study, led by the University of California, Berkeley’s Imke de Pater, shows the opposite: a massive layer of ammonia wraps the planet and is likely replenished from deep inside.

They also used the Very Large Array, but only after it underwent a considerable facelift: a decade-long upgrade revamped the ageing 1970s technology. “It increased its capabilities by factors of 10 in a number of different dimensions,” says Robert Sault, an astronomer at the University of Melbourne in Australia and co-author of the paper.

This let them view the planet at the best spatial resolution achieved in a radio map: 1,300 kilometres. No small feat for an object around 600 million kilometres away that spins once every 10 hours.

Jupiter radiates heat into space. As radiation burbles through the gas giant, various molecules block certain wavelengths. 

By mapping radio wavelengths, the researchers saw “radio hotspots” – bright areas with little ammonia, such as the bright belt above the equator – interspersed with dark patches of thick plumes.

But they could only see to around 100 kilometres below the clouds – beyond that, the atmosphere was too opaque even for those longer wavelengths.

Radio maps of ammonia movement matched the storms you can see in visible light, such as the Great Red Spot in the image above.

They were even able to distinguish various ammonia compounds, such as ammonium hydrosulfide gas clouds and flurries of ammonia ice that form the upper atmosphere. 

The plumes of ammonia swelled in wave patterns. These “atmospheric waves” pointed to motion deep within the gas giant’s atmosphere. Tracing those waves will let astronomers better estimate the planet’s composition.

One interesting quirk observed by the team was that cyclonic storms – those that spin clockwise in the southern hemisphere and clockwise in the northern – were particularly radio-bright, signifying little ammonia in those areas.

Anticyclones, on the other hand, were radio-dark. Some were surrounded by a radio-bright ring, suggesting plumes of ammonia rise in their centre, spread and drop down like a cylindrical waterfall.

“It’s a bit puzzling,” Sault says. “The cloud tops are a zoo of features and we haven’t worked out everything about them all yet.”

But with NASA spacecraft Juno only a month from its destination, get ready for a slew of revelations. Instruments on board Juno will see several hundred kilometres into Jupiter’s atmosphere, but only in narrow strips as it performs its 32 orbits over the course of its 1.5-year mission.

So the Very Large Array will monitor Jupiter for at least some of Juno’s mission, Sault says: “We can look at the entire planet at the same time as it’s observing strips. Our technique is complementary to the mission.”