Planetary science
By Jove!

A mission to Jupiter is designed to investigate the giant planet’s history—and the histories of its cousins in other solar systems

Science & technologyJul 2nd 2016 edition

IN 1543 Nicolaus Copernicus proposed, in a mathematically rigorous way, that the Earth is not the centre of the universe, and thus that all things do not revolve around it. In fact, only the Moon does so. Seven decades later Galileo Galilei provided more direct proof of Earth’s lack of specialness. He looked at Jupiter through a primitive telescope and found that the planet had four moons of its own.

Four centuries after Galileo’s discovery, it remains impossible to understand the solar system without understanding Jupiter. The sun accounts for 99.8% of the solar system’s mass. But Jupiter, which is more than twice as massive as the other seven planets put together, makes up most of the rest. Its heft shapes the orbits of the other planets, the structure of the asteroid belt and the periods of many comets. And the four moons observed by Galileo (seen to the left-hand side of Jupiter in the picture above) have proved merely the biggest members of an entire solar system in miniature: at the moment Jupiter has 67 known satellites.

The picture was taken on June 21st by Juno, a probe belonging to NASA, America’s space agency, that is named after the Roman goddess who was both Jupiter’s wife and his sister. If all goes according to plan, Juno will become a 68th satellite of Jupiter on July 4th, arriving almost five years after it was launched. Though Jupiter has had other man-made visitors, all but one of them simply flew past it on their way elsewhere, taking a few photographs to send back home while they gathered energy from the Jovian gravitational field in a so-called slingshot manoeuvre, to speed their journeys up. Only Galileo, which arrived in 1995, has previously gone into orbit around the place.

Dancing with death

Doing so is a risky business. Juno, which is, at the moment, moving at around 250,000 kilometres an hour, is one of the fastest man-made objects ever built. When it arrives its guidance computer will have just over 30 minutes to slow the craft down and thread it into a series of long, looping orbits that will cause it to swoop to within 4,500km of the tops of Jupiter’s clouds and then zoom out again to a distance of more than 2.5m km. If anything goes wrong during this deceleration, the probe will have to fix the problem itself. Assistance from Earth will be impossible, for radio signals from mission control in California take nearly an hour to reach it.

Yet a fix may be needed. Jupiter is a hostile place. Its enormous magnetic field traps and accelerates high-energy particles (mostly protons and electrons) thrown off by the sun. That gives it the fiercest radiation belts of any planet in the solar system. Such radiation plays havoc with electronics. Galileo suffered more than 20 radiation-related glitches over the course of its eight-year mission. These included repeated resets of its main computer, glitches in its cameras and problems with its radio.

Juno’s electronics are protected by a 200kg titanium vault that has walls a centimetre thick. Its looping orbits are designed to minimise the time it spends in the most radioactive zones. Even so, the radiation will take its toll. NASA expects the craft’s visible-light camera and infra-red instruments to endure for eight orbits or so. Its microwave sensor is rated for 11. Then, in February 2018, when its circuits are on their last legs, it will fire its engine one final time, propel itself into the Jovian atmosphere and destroy itself—a fate already suffered by Galileo. NASA is required by law to ensure that there is no chance any hardy Earthling microbes could hitch a ride to the Jovian moons—especially Europa, which is thought to have beneath its icy surface a liquid-water ocean that might conceivably support life. Juno’s immolation will avoid any possibility of contamination in the future.

All of this drama is to serve the study of a planet that remains mysterious. Last time, with Galileo, “we learned enough to realise that we don’t understand a lot of things”, says Scott Bolton, an experimental physicist who is the Juno mission’s chief. One particularly mysterious thing is Jupiter’s origin.

Jupiter belongs to a class of planets called gas giants. (Saturn is another such, and many more have been identified in planetary systems surrounding stars other than the sun.) Researchers know that it was formed from the same primordial cloud of hydrogen and helium (with a scattering of other, heavier elements) as gave birth to the sun. But how exactly this happened is unclear.

A theory called “core accretion” holds that a rocky core formed first, assembling itself under the influence of gravity from dust grains, then pebbles, then boulders and so on. Once this core acquired sufficient mass, it began attracting hydrogen and helium from the primordial cloud, and would have enough gravity to hold onto them. If this view is correct, Jupiter might be thought of as a rocky planet similar in a way to Earth, but with an absolutely humongous atmosphere. The core-accretion theory, though, has a timing problem. Light exerts pressure, and the pressure of light from the infant sun should, calculations suggest, have driven off most of the hydrogen and helium of the primordial cloud before Jupiter had a chance to grab it.

A rival hypothesis argues that Jupiter formed without the need for a large rocky core, from a knot in the gas cloud itself. That would make it quite a different beast from an overblown terrestrial planet. One of Juno’s jobs, then, is to try, by measuring subtle variations in Jupiter’s gravitational field, to determine whether the planet has a core, and if so how big it is. This will not, of itself, be enough to resolve the question of how it formed. But it should narrow the range of possibilities.

Jupiter’s atmosphere is another part of the puzzle. Back in 1995 Galileo dropped a probe into that atmosphere, and this probe reported back comparatively larger helpings of certain heavy elements, including nitrogen and argon, than are found in the sun. This suggests either that Jupiter formed in the cool outer reaches of the early solar system, where such elements would have been more abundant, before migrating to its current position, or that the heavy elements in question were supplied by comets and asteroids from those outer reaches. But there was much less of one heavy element—oxygen—than there should have been. The probe detected little water, the compound into which gas-cloud oxygen is overwhelmingly bundled. So, either astronomers’ theories of why Jupiter is blessed with so many heavy elements are wrong, or else, by sheer bad luck, Galileo’s probe dropped into a particularly dry part of the planet’s atmosphere.

There is evidence that something like that may, indeed, have happened. Observations by terrestrial telescopes suggested that the probe, which survived for less than an hour before contact was lost, ended up in the downdraft of a giant atmospheric convection cell. This might well have been drier than the surrounding atmosphere because much of the water in it would have condensed and fallen as rain or snow when it was on the upward side of the convention cell.

Either way, says Dr Bolton, “all we can do is go back and do it again”. And Juno will attempt just that, sampling a different part of the atmosphere with each of its diving loops. Combining measurements from all over the planet should help sort the theoretical sheep from the goats.

Nor is it theories of the formation of Jupiter alone that are at stake. The chance to poke and prod a gas giant up close could help to shed light on how planetary systems other than the sun’s have formed. One of the big surprises of exoplanetology, as the study of such systems is called, has been the discovery of a type of planet known as “hot Jupiters”. These are gas giants which orbit close to their parental stars—in some cases having orbital periods measured in mere handfuls of days. (By contrast, the orbital period of Mercury, the planet closest to the sun, is 88 days.) Everything researchers think they know about planet formation suggests such worlds could not have formed in their present locations. The radiation from their parent stars would have disassembled them as fast as they formed.

The assumption, then, is that they must have come into being elsewhere and then migrated closer to their stars. But how that happens, or how common it is, is still unclear. Reconstructing the history of the solar system’s own biggest gas giant could help astronomers understand how billions of other planets in the galaxy came into being, too.

Dig deeper: A probe to Jupiter has arrived successfully (July 5th 2016)

This article appeared in the Science & technology section of the print edition under the headline "By Jove!"

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