What would happen if you tried to land on a gas giant?
Our solar system contains three types of planets. Between the four terrestrial planets–Mercury, Venus, Earth, and Mars–and the distant ice giants of Neptune and Uranus, sit two gas giants: Saturn and Jupiter.
These planets are mostly composed of hydrogen and helium gas. Researchers now appreciate that gas planets are more complex than first thought. New findings have implications for our understanding of how these planets formed and will help design future missions to potentially visit them.
Gas giants originate from one of two processes. The first method is called core accretion, explains Ravit Helled, a professor of theoretical astrophysics at the University of Zürich. This starts with the birth of a new star, when molecular clouds collapse under gravitational pressure. Whorls of gas–called protoplanetary disks–start to spin around these new stars. Within these gas disks will be heavier particles–dust, rock, or any elements heavier than helium. These particles can clump together and then suck in gas from the surrounding disk, forming a giant planet mainly composed of gas.
A second method that may form gas giants called disk instability–this is a newer theory that still causes some controversy among planetary theorists. According to this idea, when massive protoplanetary disks cool down, they become unstable and can produce clumps of rock and gas that evolve into gas giants. Importantly, this proposed formation process happens much more quickly than core accretion. Helled says that Saturn and Jupiter likely formed via core accretion, but that disk instability may 'explain very massive planets at large orbits or giant planets around small mass stars.'
Regardless of how they form, the structure of gas giants is nothing like that of terrestrial planets like Earth. Jupiter and Saturn don't have a surface in the same way Earth does. Instead, their atmosphere simply gets thinner until there isn't enough density left to call the surrounding air part of the planet anymore. 'There is no location where you can say, okay, this is where the planet stops,' says Helled.
A spaceship attempting to 'land' on Jupiter's 'surface' would have to overcome some significant obstacles. Once you enter the cloud of gas that roughly marks the beginning of a giant like Jupiter, temperature and pressure steadily increase as your head toward the planet's core, and gaseous hydrogen and helium morph into liquid form. While our solar system's gas giants are far from the sun, the core of a gas giant is likely to be incredibly hot–Jupiter's is estimated at around 43,000 degrees Fahrenheit. You'd also have to pass through the thick clouds of ammonia found in Jupiter's upper atmosphere.
If you make your ship from tough stuff–tougher than any known substance on Earth–that could survive these conditions, it might make it to a gas giant's core. What it would find there in the alien murk is still unclear.
'For decades, it was assumed that there was a defined core,' says Helled. Recent probe missions, like Juno and Cassini, have orbited Jupiter and Saturn, respectively. The information these probes sent back has changed that view.
'We now think that they have what we call fuzzy or diluted cores,' says Helled. This means that there isn't a clear transition point between the upper layers of liquid gas and liquid hydrogen and helium and the planet's core.
In truth, Juno and Cassini's data has revolutionized our understanding of these planet's structures. Helled explains that they likely have complex heat and composition gradients. Jupiter is famously wracked with massive storms, like the Great Red Spot, which produces winds up to 425 mph (640 km/h). Some of these shifts can produce dramatic phenomena. Jupiter and Saturn likely have regions in which helium gas separates from hydrogen. Here, the helium becomes a rain of droplets that pour towards the planet's core.
These insights can reveal more about our solar system's giants, as well as similar planets outside our solar system.
'Now we realize that some of the simple assumptions that we've made to model these planets are wrong, and we need to modify the models,' says Helled.
This story is part of Popular Science's Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you've always wanted to know? Ask us.
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