Saturn is the one planet we all know of whose magnetic discipline is nearly precisely aligned with its axis of rotation. In a brand new evaluation of knowledge collected through the closing orbits of NASA’s Cassini mission, astronomers suggest the sector’s distinctive nature would possibly come from a thick layer of helium “rain” that’s falling slowly onto the planet’s metallic hydrogen core. The outcomes seem within the June AGU Advances.
Scientists assume the dynamo impact within the rotation and convection of a conducting fluid generates planetary magnetic fields. Earth’s magnetic discipline, for instance, originates inside its vigorously convecting core of molten iron, whereas Jupiter’s and Saturn’s cores comprise hydrogen heated and compressed to a metallic (fluid) state. Uranus and Neptune are too small to supply the precise situations for metallic hydrogen; their inner dynamos are as a substitute considered powered by an uncommon type of ionized water.
The magnetic axis of just about all of those planets — besides Saturn — tilts not less than a number of levels from the planet’s rotation axis. Uranus and Neptune are probably the most excessive, their magnetic axes tilted at 45° to 60° from their spin axes.
The Pioneer and Voyager spacecraft measured the magnetic fields of the outer planets, together with their orientation, a long time in the past. But information from the more moderen Cassini mission confirmed that the utmost doable tilt of Saturn is exceptionally small, below 0.007°, says Hao Cao (Harvard University), who was not concerned within the present examine.
For a long time, planetary scientists thought Saturn’s convecting inside was 99% hydrogen and helium. However, the distribution of these components and their bodily states contained in the planet remained unknown. Now, the information from the closing months of the Cassini mission helps scientists perceive the inside situations and distribution of the supplies that results in Saturn’s unusually aligned magnetic discipline.
During its final 22 orbits, the Cassini spacecraft got here nearer and nearer to Saturn’s polar area. “It gathered beautiful data on the magnetic field, more detailed than we had ever had before,” says Sabine Stanley (Johns Hopkins University).
Chi Yan (additionally at Johns Hopkins) and Stanley integrated this information within the pc fashions they developed to calculate the magnetic fields that numerous forces contained in the planet would produce. They tweaked their fashions till they offered the perfect match to observations. The closing passes, when Cassini got here closest to Saturn, helped slim the vary of situations doable inside the planet.
The Strange Layers of Saturn
Yan and Stanley discover their finest mannequin of Saturn’s inside has 4 layers. The internal core is strong, or presumably a stratified fluid with out convection. It incorporates the rock and ice round which Saturn initially condensed and extends a couple of quarter of the way to the floor.
Next is a convective outer core of metallic hydrogen and dissolved helium, the layer that hosts the planet’s dynamo. The temperature and strain are so excessive that this layer is in an uncommon state known as a supercritical fluid, which is neither liquid nor gas. It reaches as much as about 42% of the way towards the floor.
The third layer can be a supercritical fluid however the helium in it doesn’t dissolve into the hydrogen, as a substitute remaining separate like oil in water. Here, helium “rains” down via the fluid hydrogen, however solely very slowly — a phenomenon beforehand predicted however by no means undoubtedly noticed.
Helium rain is unusual and engaging stuff, which Stanley calls “one of my favorite things in the universe.” It can solely occur at pressures bigger than a megabar, roughly 1,000,000 occasions Earth’s atmospheric strain at sea degree, however at decrease pressures than the extraordinarily dense core. In Saturn, this transition happens at about 70% of the way as much as the floor. While hydrogen and helium combine collectively at decrease pressures nearer to the floor and blend once more inside the core, at 1,000,000 atmospheres they develop into immiscible, like oil and water. Helium, which accounts for a couple of quarter of the fluid, kinds blobs inside the metallic hydrogen that fall deeper into the planet.
“The process is really slow,” Stanley says. The helium doesn’t rain a lot as settle slowly to the underside of the helium-rain layer, the place pressures develop into excessive sufficient that the helium turns into metallic and may dissolve into the metallic hydrogen.
Solving the Puzzle of Saturn’s Core
“We have found one solution,” says Stanley of her and Yan’s work. But their consequence doesn’t rule out different methods to clarify Saturn’s bizarre inside. “We would love it if other people could find other solutions.”
She has already talked with one other group that’s finding out not magnetic fields however gravity-induced waves in Saturn’s rings, which act as seismometers to probe the planet’s insides. In a paper posted on the arXiv preprint server (and presently below scientific overview), Christopher Mankovich and Jim Fuller (each at Caltech) recommend the transition between Saturn’s ice/rock core and metallic-hydrogen envelope is diffuse, a mixed-together area that extends to 60% of Saturn’s radius. They estimate this area incorporates some 17 Earth lots of ice and rock.
NASA has prolonged the Juno mission at Jupiter to 2025, which Cao says will present “a lot more detailed information about Jupiter and Saturn to do comparative paleontology.” That might result in a greater understanding of the 2 gas giants’ internal layers and shed extra mild on the planet’s formation 4.5 billion years in the past.