Scientists have learned a lot about our solar system's inner worlds, including Earth and Mars. But far less is generally known about icy water-rich outer planets, Neptune and Uranus.
In a world first, a team of scientists recreated the pressure and temperature of both planet's interiors in a lab, unveiling new knowledge about the chemistry of the outer-planets' deep water layers, according to a recent study published in the journal Nature Astronomy.
The new findings may also hint at the compositions of oceans on distant water-rich exoplanets, far beyond our solar system.
Magnesium could be like the Earth's saltwater in Uranus and Neptunes' water layers
Generally, scientists consider Neptune and Uranus to have distinct separate layers, involving an atmosphere, ice or fluid, then a rocky mantle and metallic core in the center. The latest study saw the researchers investigate the possibility of interactions between water and rock deep inside the two gas giants' interiors. "Through this study, we were seeking to extend our knowledge of the deep interior of ice giants and determine what water-rock interactions at extreme conditions might exist," said Taehyun Kim of Yonsei University in South Korea, who is also lead author of the study, in a Phys.org report. "Ice giants and some exoplanets have very deep water layers, unlike terrestrial planets."
"We proposed the possibility of an atomic-scale mixing of two of the planet-building materials (water and rock) in the interiors of ice giants," added Kim. To simulate the instant-death conditions of the deepwater layers on Uranus and Neptune in the lab, the team initially immersed conventional rock-forming minerals like ferropericlase and olivine in water. Then they compress the sample in a diamond anvil to extremely high pressures. Once this happens, the team observed the interaction of water and minerals, and took X-ray measurements while a laser brought the sample to unconscionably high temperatures.
The resulting chemical reaction brings forth high concentrations of magnesium in the pressurized water, from which the team concluded that oceans on water-rich planets might be characterized by substantially different chemical properties than the Earth's ocean. In other words, the high pressure might make those oceans rich in magnesium. "We found that magnesium becomes much more soluble in water in high pressures," said Sang-Heon Dan Shim of Arizona State University's school of Earth and space exploration, who is also a study co-author. "In fact, magnesium may become as soluble in the water layers of Uranus and Neptune as salt is in Earth's ocean."
Unlocking the secrets of distant exoplanets could point to life
These unique chemical characteristics might help scientists better grasp why Uranus' atmosphere is so much colder than Neptune's despite both of them sharing a water-rich composition. If Uranus' water layer possesses much more magnesium beneath its atmosphere, it might block heat from escaping the interior, further heating the interior. "This magnesium-rich water may act like a thermal blanket for the interior of the planet," added Shim.
Outside of our solar system, high-pressure and high-temperature experiments could also help expand our understanding of exoplanets with masses smaller than Neptune (sub-Neptune exoplanets). This is significant because sub-Neptune planets are the most commonly known type of exoplanets, and scientists think many of them might feature a thick water-rich layer with a rocky interior. In essence, the new study suggests that deep oceans on distant exoplanets might be very different from Earth's ocean, rich in magnesium.
"If an early dynamic process enabled a rock-water reaction in these exoplanets, the topmost water layer may be rich in magnesium, possibly affecting the thermal history of the planet," said Shim. To push the research further the team plans to switch up the high-pressure/high-temperature experiments, and experiment with shifting conditions to simulate a wider variety of planetary interiors. "This experiment provided us with a plan for further exploration of the unknown phenomena in ice giants," said Kim.
If scientists can better grasp how minerals and water interact in the core of distant exoplanets, then we might discern the conditions of alien worlds with space telescopes like the forthcoming James Webb Space Telescope, and more. Knowing at a glance whether a planet could support life — whether like the Earth or drastically different — might finally give us real answers to the most basic scientific questions about our place in the universe: are we alone? The best is yet to come.