THE ATMOSPHERE of JUPITER

There are holes in Jupiter’s atmosphere – regions that don’t have a lot of clouds – that allow us to see deep into the planet. Because it gets hotter the farther you go into Jupiter, these regions, which are gray-blue in color, appear hotter than the cloudy areas that cover the rest of the planet. Called hotspots, these areas glow brightly in infrared light.

In 1995, the Galileo spacecraft dropped a probe into Jupiter and it happened to fall into one of these hotspots. The probe wanted to measure the amount of water in Jupiter’s atmosphere, but the Jovian air in the hotspot was much drier than expected. It was akin to an alien craft visiting Earth for the first time and landing in a desert. Although the probe couldn’t determine how much water there was, it was still able to learn about the winds, the temperature, and the composition of Jupiter down to a depth of about 100 kilometers (60 miles) below the cloud tops.

We have a general idea of what happens in a hotspot. When Jovian air enters such a region, it sinks into the hotter depths of Jupiter, where it warms up and dries out. As it flows out of the hotspot a few days later, it rises back to its original altitude.

400MPH WINDS, 15 MILE WIDE LIGHTNING, AND PLANET SIZED HURRICANES    

The bands of white, orange and brown clouds that wrap around Jupiter are the result of a variety of complex phenomena and chemical reactions.

A process called convection drives the formation of the clouds – the same basic way that clouds on Earth form.

Hot material deeper in Jupiter’s interior rises while cooler material sinks. As the hot gases rise, they cool and condense into liquid droplets or ice crystals to form clouds. Jupiter’s fast rotation – spinning once every 10 hours – creates strong jet streams, smearing its clouds into bands across the planet. Jet streams on Earth are generated in a similar fashion.

Jupiter seems to have at least three major cloud layers made out of different chemicals. Each layer sits at an altitude where the temperature is cold enough for the respective chemicals to condense.

The highest layer, where it’s the coldest, is composed of bright, white clouds of ammonia crystals. The next layer consists of ammonia hydrosulfide, which smells like rotten eggs. Farther down are water-ice clouds, which likely sit on top of a blanket of water-ammonia fog that covers the planet. Juno will determine how much water is in each of these layers.

No one really knows why Jupiter’s clouds are so colorful, since the ices that form the clouds should be white. One possible reason is that a combination of lightning and sunlight somehow alters the ice at high altitudes, producing orange and brown hues. Organic compounds – molecules containing carbon – and sulfur compounds are most likely the chemicals that give the clouds their colors.

Jupiter is filled with swirling storms that originate in the water-cloud layer – or perhaps even deeper. These storms range in width from 200 kilometers (125 miles) to over 1,000 kilometers (600 miles), generating lightning that flashes 100 times brighter than on Earth.

These enormous storms release huge amounts of heat from the planet’s interior.

Many aspects of Jupiter’s atmosphere are still a mystery. For example, no one knows how its jet streams remain so stable over time or what determines their widths and speeds. By studying the clouds and weather on Jupiter, Juno will help us better understand how atmospheres work in general – including our own. What we learn about Jupiter’s atmosphere will also help us interpret data from telescopes observing gas giants orbiting other stars.    

Jupiter is mostly made out of hydrogen and helium – the same light gases that make up a star. In fact, if Jupiter were about 80 times more massive, the pressure and temperature at the center would be high enough for nuclear fusion to ignite, and the planet would become a star. The rest of Jupiter includes water, ammonia and methane – compounds that arise when you mix oxygen, nitrogen and carbon in a hydrogen-rich atmosphere. Deeper inside the planet, these chemicals can change form with increasing pressure and temperature. At its center, Jupiter may even have a rocky core.

In 1995, the Galileo spacecraft dropped a probe into Jupiter, which measured the planet’s composition down to about 100 kilometers (60 miles) below the cloud tops, where the pressure is 20 times greater than at Earth’s sea level.

Unfortunately, the probe entered a dry region called a hotspot so it couldn’t determine the atmosphere’s water content. But it was able to measure other chemicals, finding that all gases – except for hydrogen, helium and argon – were two to four times more abundant than on the Sun.

But Jupiter was born early in the solar system’s history, from the same cloud of gas and dust that formed the Sun. Since this would imply that Jupiter’s composition should mirror the Sun’s, how Jupiter became enriched in these other elements remains a big mystery. We have many ideas as to how it acquired these elements, and it turns out that they all predict different amounts of water on the planet. So by measuring Jupiter’s water content, Juno will allow us to test our different theories.    

Jupiter is called a gas-giant planet because it’s almost entirely composed of hydrogen and helium. But as you go deeper inside, the increasing pressure causes the gas to behave in strange ways. When the pressure becomes about 1,000 times greater than that at Earth’s sea level, the gas acts more like a liquid. As a result, only a thin, outer layer is what we would recognize as an atmosphere.    

Jupiter’s storms can be gigcantic. Case in point: the Great Red Spot. Perhaps Jupiter’s best known feature, the Great Red Spot is a giant vortex wider than planet Earth.

View the Great Red Spot Section

We can’t say for sure, but the Jovian environment is so inhospitable that the possibility of life is slim. Any Jovian life would have to be quite different from any organism on Earth. Since Jupiter has no solid surface, Jovian aliens would have to float or fly, surviving in extreme conditions too harsh for any Earthling.

Still, at certain altitudes in Jupiter’s atmosphere, there do exist chemicals that are necessary for some sort of exotic life form. The temperatures are warm enough and flashes of lightning could provide energy that drives the chemical reactions needed for life.

But staying at such a favorable depth would be a problem for a microbe or any other simple organism. Winds could carry them deep into the planet’s interior, where the heat would cook them. Or the winds could lift them higher, where they would not only freeze but would be killed by radiation from space.    

A brown band of clouds, called the Southern Equatorial Belt, wraps around Jupiter. Twice as wide as Earth, the belt is one of Jupiter’s most prominent features. Between the end of 2009 and the middle of 2010, it suddenly disappeared. Even more puzzling, the eastward and westward jet streams that drive many of Jupiter’s atmospheric features stayed the same.

This vanishing act has actually happened before. People have seen the band fade every few years, only to return accompanied by spectacular outbursts of storms and whirling vortices.

But scientists don’t think that the belt was actually gone. Instead, it might have been obscured by wispy white clouds of icy ammonia crystals. The same sort of clouds – called cirrus clouds – also form on Earth and are made of water ice instead of ammonia.

Researchers suspect that changes in the upwelling circulation inside Jupiter somehow brought ammonia-rich material into the clear, cold zone above the brown belt. There, the ammonia could freeze into crystals.