Richard A Lovett
In ancient Roman mythology, Jupiter, the king of the gods, had the ability to hide behind a veil of clouds. Only his wife, Juno, could see through. Today another Juno is peering through Jupiter’s veil: the NASA spacecraft, launched in 2011, has been orbiting the planet since 2016.
In the last year, JunoCam, mounted on Juno as an afterthought, has wowed earthlings with breathtaking close ups of swirling Jovian cloud formations such as the Great Red Spot and the wondrously filigreed patterns at the edges of Jupiter’s multi-coloured atmospheric bands.
But Juno’s primary mission is to use its state-of-the-art scientific instruments to probe the giant planet’s inner secrets. Already the answers are coming thick and fast.
Juno is scheduled to remain in orbit for several more months, though the mission is likely to be extended in NASA’s next budget cycle. But eventually, to avoid the risk of hitting a moon and contaminating it with earthly bacteria, it will make a final, fatal plunge into Jupiter’s deadly gravitational embrace.
It’s been a mission laden with drama. When Juno reached Jupiter, it was one of the fastest artificial objects in history, moving at 265,000 kilometres per hour. This was followed by a white-knuckled 35-minute engine burn in which it would have shot past Jupiter on an endless voyage to nowhere if something had gone wrong. It then settled into a cigar-shaped 53-day orbit that alternately plunged to as close as 4,300 kilometres from the cloud tops and swung away to more than 8.1 million kilometres.
Initially, the plan was to burn the engines a second time and shorten the orbit to 14 days. But Juno had its own ideas. A frightening crash and reboot of the spacecraft’s main computer led to some uncertain moments; then the rocket engine was diagnosed as having a sticky valve. NASA’s engineers decided to leave well enough alone.
Since then, the computer and the instruments have been functioning admirably. Remaining in the initial 53-day orbit has slowed the rate of data collection by a factor of about four but not much else has changed, says Scott Bolton, the mission’s principal investigator.
“All the science is enabled by the close passes, so if you’re in a longer orbit, it’s pretty much the same,” he says. “The worst-case scenario is I have to be patient.”
The purpose of Juno’s elongated orbit is to minimise its exposure to Jupiter’s dangerous radiation belts, created by the giant planet’s enormous magnetic field. Radiation in these belts, which loop outward from pole to pole, can damage instruments, potentially shortening the mission’s life.
The spacecraft dives beneath the radiation belts, sweeps low across the planet from pole to pole, then quickly zips out again. Overall it’s just a few hours of exposure per orbit. Thanks to Jupiter’s rotation, a different part of the planet lies beneath the spacecraft on each passage, allowing it to examine the entire surface in a series of north-south strips.
Originally, the mission was slated for 37 orbital flybys, with the last to be conducted in early 2018. But with the spacecraft stuck in its longer orbit, the mission had only completed 12 of these by July, when it is hoped that NASA and the US government will green-light the extension of the mission by at least a year.
However long Juno keeps going, it’s already proven well worth its $US1.1 billion price tag.
Here are a few of the major findings:
Images
Photography isn’t Juno’s primary mission since a camera can’t look beneath the clouds that cover Jupiter. JunoCam was added late in the mission’s development. Its primary purpose was to share the excitement of space exploration with the public, says Candice Hansen, of the Planetary Science Institute, Tucson, Arizona. But the spacecraft was passing over parts of Jupiter near its poles that had never been seen before.
“The team and I couldn’t imagine not seeing what they looked like,” Bolton says.
JunoCam is hardly state-of-the-art. At closest approach, its resolution is only 15 kilometres per pixel – poor by the standards of cameras on spacecraft now orbiting the Moon and Mars. But its 58° field of view makes for eye-pleasing panoramic shots. “If you were close to Jupiter, your eyes would take in a great view,” Bolton says. But if you used binoculars to zoom in, you would look at only one spot. “You would lose the context.”
Overall the combination of wide field and modest magnification has delivered stunning results.
Hansen says some images reveal unexpected features, such as ‘pop-up storms’ that produce scattered white cloud tops. “We initially called them thunderstorms, but there is no lightning. We need a different name,” she says.
Other images have revealed unexpectedly chaotic storm patterns near the north pole, close ups of the Great Red Spot and the swirling edges of Jupiter’s multi-coloured atmospheric bands, where streams of cloud race past each other at wildly differing speeds.
There is also a second camera, primarily intended for stellar navigation while the spacecraft was in flight. Called the star tracker, it’s also vital for keeping the spacecraft oriented so it can beam its data back to Earth. But when it’s not needed for other purposes, it can focus on Jupiter’s rings, which are fainter than Saturn’s but have been known since 1979.
Jack Connerney, an astrophysicist at NASA’s Goddard Space Flight Center, says it has already identified dozens of new objects, possibly including unknown tiny moons orbiting in Jupiter’s rings. It’s also possible, he notes, that some of the apparent sightings might be from flashes of material blown off Juno’s solar panels by micrometeorite impacts.
“We have to figure out if it’s a small thing close to the spacecraft, or a big thing far away,” he says. “It’s a big job because there’s so many of them.”
Radiation
While Juno’s orbit is designed to minimise its exposure to radiation, that doesn’t mean it encounters none. In fact, one of the spacecraft’s more important instruments is a particle detector, which measures radiation.
Already, scientists have found high-energy particles in unexpected locations. One is at the equator, just above Jupiter’s atmosphere. But they’re not the fast-moving electrons that make up the bulk of Jupiter’s radiation belts, according to Heidi Becker of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. Instead, Juno is encountering high-energy ions of hydrogen, oxygen and sulfur.
The most likely sources are gases escaping from the icy moon Europa and the sulfur-spewing volcanic moon Io, Becker explained at a meeting of the American Geophysical Union (AGU) in New Orleans last December. These fall toward Jupiter, hit the atmosphere and have their electrons stripped away, becoming the high-energy ions that for unknown reasons collect above the equator in the region where Juno detected them.
The other unexpected radiation zone was detected near the inner edge of the main radiation belts, which Juno grazes during each orbital pass. The particles in this zone are even more energetic than those near the equator – so energetic that the scientists aren’t even sure what type of radiation they are. Juno’s radiation detector wasn’t built to analyse such high-energy particles.
Their energy was estimated in a more brutal manner: they penetrated the hefty shielding of Juno’s star-tracker.
Because of its crucial role in navigating and orienting the spacecraft, the star-tracker is the most heavily shielded device on the entire spacecraft. Small amounts of radiation were expected to penetrate this shielding, showing up on the star-tracker sensors as random flashes of light.
But this radiation was hundreds of times more potent than anything they had expected. Becker is confident it has to be from some kind of heavy ion, but the rest is a grab bag of guesses. “The species, and where they might come from is something we’re still studying,” she says.
Great Red Spot
Jupiter’s Great Red Spot has a diameter the size of the Earth. The largest and most long-lived storm in the Solar System, with winds spinning at up to 430 kilometres per hour, has existed for at least 150 years and is one of Jupiter’s great mysteries. (The storm may be centuries older: the Italian astronomer Giovanni Cassini wrote of a “permanent spot” on Jupiter in 1665.)
Telescope images show us the surface, but the real mystery lies beneath. How far down do the roots of the Great Red Spot descend?{%recommended 5420%}
To find out, Juno takes the temperature of the storm at a range of depths. The ‘thermometer’ is a battery of microwave sensors that takes readings as the spacecraft flies above the surface, each sensitive to a different depth. The strength of these emissions is related to temperature, and their frequency determines how far they can travel upwards through the Jovian atmosphere to be detected by Juno.
By peering at Jupiter in six different microwave frequencies, Juno is able to measure the atmosphere’s temperature at six depths and produce a heat map for each of them. Juno’s scientists can then combine data from all six frequencies to build a 3D heat map that extends down 350 kilometres into the atmosphere, the deepest to which the frequencies detectable by its microwave antenna can be seen.
“It’s helping us see the invisible,” says Bolton. “If you were flying by Jupiter and had X-ray vision [this is what] you might be able to see. This instrument is capable of peeling back layers as if Jupiter was an onion, [to] see what’s inside.”
So far, the instrument has revealed the Great Red Spot is both wide and deep, Andy Ingersoll of the California Institute of Technology, Pasadena, reported at the 2017 AGU meeting. The storm’s hot zone descends as far as the microwave sensors can trace it.
“How deep beyond that is still to be determined,” says Ingersoll. Knowing the depth of the Spot’s roots is an important clue to understanding its strength and longevity, he adds.
Ingersoll’s favourite theory is that the Spot’s durability is due to its ability to ‘swallow’ smaller storms, each of which contains energy. “It’s like a food chain,” he says. “A big fish eating little fish.”
Such storm mergers have been observed on Earth, albeit on a much smaller scale, but more knowledge about the Spot’s three-dimensional structure will be needed to determine if Ingersoll’s idea is correct.
Gravity field
To peer deeper, Juno is designed to make careful measurements of Jupiter’s gravity field – a delicate task done by monitoring minute Doppler shifts in the radio signals the spacecraft sends back to Earth as variations in Jupiter’s gravity field change its speed. These shifts are so minute, in fact, that it’s necessary to account for tiny factors such as the way the spacecraft’s speed could be affected by the absorption and re-radiation of sunlight.
Measurements of Jupiter’s gravity field may help scientists determine the ultimate depth of the Great Red Spot’s roots, by allowing them to determine how far density (and therefore gravity) differences extend into the atmosphere. But they have already produced another important find: Jupiter’s atmospheric bands aren’t just superficial; they extend surprisingly deep into the planet’s interior.
On Earth, tiny variations in gravity can be used to map groundwater and subterranean rock structures. On Jupiter, they can be used to map differences in the density of the atmosphere, which in turn reflect differences in its temperature, at depths far beyond the limit of the microwave sensors. {%recommended 7386%}
The gas in different atmospheric bands (and the Great Red Spot) has different densities, which affects the distribution of mass and therefore the gravitational field. These gravitational fluctuations make Juno speed up and slow down by tiny amounts as it passes overhead, which in turn changes the frequency of its transmissions via the Doppler effect.
With data from multiple orbits and some stunningly complex mathematics, scientists can use those Doppler shifts to reverse engineer the density patterns in the atmosphere.
Previously, the microwave instrument had traced the boundaries between Jupiter’s atmospheric bands to depths of at least 350 kilometres – in itself a surprise. But in a suite of other papers published on 8 March 2018 in Nature, scientists using Juno’s gravitational data concluded that Jupiter’s surface bands extend to depths of about 3,000 kilometres.
One implication is that there’s a heat source driving the winds that form these bands, even at depths where the pressure reaches 100,000 times that of Earth’s atmosphere. According to Jonathan Fortney, of the University of California, Santa Cruz, it also means that the atmosphere extends so deep it comprises a full 1% of Jupiter’s mass, compared to only 0.0001 per cent for the Earth. But that may just be the beginning. For example, future gravity-field measurements might be able to reveal tides in Jupiter’s atmosphere, created by the pull of its larger moons.
The microwave instrument has also revealed chemical variations, not only with depth, but latitude, including what JPL scientist Michael Janssen reported in late 2016 as a “giant plume” of ammonia rising from the depths near Jupiter’s equator.
What exactly such a plume means isn’t clear, he said. But in the big picture, it’s just another discovery from a spacecraft named after a goddess who could peer into the unseen. “Welcome to the new Jupiter,” Janssen says.
