Saturn is the only planet we know of whose magnetic field is almost exactly aligned with its axis of rotation. In a new analysis of data collected during the final orbits of NASA’s Cassini mission, astronomers propose the field’s unique nature might come from a thick layer of helium “rain” that’s falling slowly onto the planet’s metallic hydrogen core. The results appear in the June AGU Advances.
Scientists think the dynamo effect in the rotation and convection of a conducting fluid generates planetary magnetic fields. Earth’s magnetic field, for example, originates within its vigorously convecting core of molten iron, whereas Jupiter’s and Saturn’s cores contain hydrogen heated and compressed to a metallic (fluid) state. Uranus and Neptune are too small to provide the right conditions for metallic hydrogen; their internal dynamos are instead thought to be powered by an unusual form of ionized water.
The magnetic axis of almost all of these planets — except Saturn — tilts at least several degrees from the planet’s rotation axis. Uranus and Neptune are the most extreme, 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, including their orientation, decades ago. But data from the more recent Cassini mission showed that the maximum possible tilt of Saturn is exceptionally small, under 0.007°, says Hao Cao (Harvard University), who was not involved in the current study.
For decades, planetary scientists thought Saturn’s convecting interior was 99% hydrogen and helium. However, the distribution of those elements and their physical states inside the planet remained unknown. Now, the data from the closing months of the Cassini mission is helping scientists understand the interior conditions and distribution of the materials that leads to Saturn’s unusually aligned magnetic field.
During its final 22 orbits, the Cassini spacecraft came closer and closer to Saturn’s polar region. “It gathered beautiful data on the magnetic field, more detailed than we had ever had before,” says Sabine Stanley (Johns Hopkins University).
Chi Yan (also at Johns Hopkins) and Stanley incorporated this data in the computer models they developed to calculate the magnetic fields that various forces inside the planet would produce. They tweaked their models until they provided the best match to observations. The final passes, when Cassini came closest to Saturn, helped narrow the range of conditions possible within the planet.
The Strange Layers of Saturn
Yan and Stanley find their best model of Saturn’s interior has four layers. The inner core is solid, or possibly a stratified fluid without convection. It contains the rock and ice around which Saturn originally condensed and extends about a quarter of the way to the surface.
Next is a convective outer core of metallic hydrogen and dissolved helium, the layer that hosts the planet’s dynamo. The temperature and pressure are so high that this layer is in an unusual state called a supercritical fluid, which is neither liquid nor gas. It reaches up to about 42% of the way toward the surface.
The third layer is also a supercritical fluid but the helium in it doesn’t dissolve into the hydrogen, instead remaining separate like oil in water. Here, helium “rains” down through the fluid hydrogen, but only very slowly — a phenomenon previously predicted but never definitely observed.
Helium rain is strange and fascinating stuff, which Stanley calls “one of my favorite things in the universe.” It can only happen at pressures larger than a megabar, roughly a million times Earth’s atmospheric pressure at sea level, but at lower pressures than the extremely dense core. In Saturn, this transition occurs at about 70% of the way up to the surface. While hydrogen and helium mix together at lower pressures closer to the surface and mix again within the core, at a million atmospheres they become immiscible, like oil and water. Helium, which accounts for about a quarter of the fluid, forms blobs within the metallic hydrogen that fall deeper into the planet.
“The process is really slow,” Stanley says. The helium doesn’t rain so much as settle slowly to the bottom of the helium-rain layer, where pressures become high enough that the helium becomes metallic and can 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 result does not rule out other ways to explain Saturn’s weird interior. “We would love it if other people could find other solutions.”
She has already talked with another group that’s studying not magnetic fields but 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 currently under scientific review), Christopher Mankovich and Jim Fuller (both at Caltech) suggest the transition between Saturn’s ice/rock core and metallic-hydrogen envelope is diffuse, a mixed-together region that extends to 60% of Saturn’s radius. They estimate this region contains some 17 Earth masses of ice and rock.
NASA has extended the Juno mission at Jupiter to 2025, which Cao says will provide “a lot more detailed information about Jupiter and Saturn to do comparative planetology.” That could lead to a better understanding of the two gas giants’ inner layers and shed more light on the planet’s formation 4.5 billion years ago.