High pressure ice on super Earth and mini-Neptune exoplanets could make them more habitable, rather than killing chances for life to arise
The list of places where scientists should search for signs of extraterrestrial life just got a lot longer, as new research suggests certain classes of exoplanets might be more hospitable than once thought.
Water-rich exoplanets and icy Moons like Jupiter’s Europa and Saturn’s Enceladus are potential targets for astrobiologists looking for signs of life elsewhere in the cosmos. But until recently it was assumed that for many water-rich exoplanets larger than Earth but smaller than Neptune, ice forming deep in the planet would keep important minerals in their rocky core from reach water closer to the surface.
In a new study published Tuesday in Nature Communications, however, French and Norwegian researchers used new modeling techniques to show that salts such as as sodium chloride could be transported from a planet’s rocky core, through a hard shell of ice, in order to enrich the water of an exoplanet. That’s a crucial step for any life that might develop in an alien ocean.
“The presence of electrolytes in the ocean, like dissolved salts, is a necessary but of course not sufficient conditions for habitability,” said Jean-Alexis Hernandez, a mineral physicist at the European Synchrotron Radiation Facility in Grenoble, France and lead author of the study.
One reason Europa and Enceladus compel the curiosity of scientists is that its believed they not only host global water oceans beneath their tick, icy shells, but that those oceans are in direct contact with those moons’ rocky mantles’. The minerals from that rock, especially if paired with an energy source such as geothermal vents on the ocean floor, could provide an environment where life could evolve and survive.
But scientists believe similar icy ocean world exoplanets between the size of Earth and Neptune — super-Earths and mini-Neptunes — exhibit different structures. About half of known exoplanets fall into these size categories, although it is not known how many of them are water rich or contain global oceans.
Due to the immense pressure at the bottom of oceans on super Earths and mini-Neptunes, high pressure ice would form a thick mantle around the exoplanet’s core, sealing off the mineral rich rock from the ocean above.
These high pressure ices would exist in exotic phases, making them vastly different than the ice encountered naturally on Earth.
“Contrary to the ice we have at the surface of the Earth (ice I), all these [high pressure] ices are denser than liquid water,” leading to the formation of that icy mantle beneath an exoplanet’s sub-surface ocean, Dr Hernandez said. “These ices are also much stiffer than ice I.”
While ice I forms when water reaches freezing, high pressure ices can form from ambient temperature water under high pressure. Compressing at 2,000 times Earth’s atmospheric pressure pressure yields ice VII, which consists of cubic crystals. Further pressure turns ice VII into “supersonic ice,” where the hydrogen atoms in water move freely while the oxygen atoms stay locked in a crystalline structure.
“This transition makes the ice conductive which might be at the origin of the non-dipolar magnetic fields of Uranus and Neptune,” Dr. Hernandez said.
Importantly, while terrestrial ice and other forms such as ice VI expel salt ions from their structure when then crystallize, he added, its known ice VII and supersonic ice can retain larger concentrations of dissolved salts. If it could retain the salts, then known mechanism just might bring them into contact with the ocean on an exoplanet.
Whether made of ice VII or rock, the mantles of planets flow through convection patterns, albiet very slowly over geologic time scales. This is due to the difference in temperature between the top and bottom of the mantle.
“Like in a pan, the material at the bottom is hotter up to a point it becomes less dense than the surrounding and starts to rise,” Dr. Hernandez said. “When reaching the top, it cools down progressively and becomes denser than surroundings and starts to sink.”
What the modeling in the paper shows, he added, is that high pressure ice could retain dissolved salt over the entire range of pressure and temperature conditions it might encounter while undergoing convection. Rather than sealing off the mineral-laden rocky core from the subsurface ocean, high pressure ice mantles could act to transport those minerals to the water.
The findings mean scientists shouldn’t cross an exoplanet off their list of potentially habitable worlds just because it’s a mini-Neptune, but they’re also not definitive. Dr Hernandez is quick to point out that the study is limited, and other factors might still render such planets inhospitable to life.
“Our study is limited to [sodium chloride], and other electrolytes should be investigated,” he said. “Life requires many other conditions in addition to the presence of electrolytes in the ocean, so we do not pretend to assess the habitability of these planets, but we simply show that they may not be as chemically simple as expected.”