The list of places where scientists should look for signs of extraterrestrial life has gotten much longer, as new research suggests that certain classes of exoplanets may be more hospitable than previously 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 formation deep inside the planet would prevent important minerals in its rocky core from reaching water closer to the surface. .
In a new study published Tuesday in Nature CommunicationsHowever, French and Norwegian researchers have used new modeling techniques to show that salts like sodium chloride can be transported from a planet’s rocky core, through a hard shell of ice, in order to enrich an exoplanet’s water. This is a crucial step for any life that could develop in an alien ocean.
“The presence of electrolytes in the ocean, as dissolved salts, is a necessary but obviously not sufficient condition for habitability,” said Jean-Alexis Hernandez, mineral physicist at the European Synchrotron Radiation Facility in Grenoble, France and lead author of the study. studying.
One reason Europa and Enceladus attract scientists’ curiosity is that they believe they not only host oceans of global water beneath their icy shells, but that these oceans are in direct contact with the rocky mantles of these moons. The minerals in this rock, especially if combined with an energy source such as geothermal sources on the ocean floor, could provide an environment where life could evolve and survive.
But scientists believe that similar exoplanets from the icy ocean world between the size of Earth and Neptune — super-Earths and mini-Neptunes — exhibit different structures. About half of the 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 on the ocean floor on super-Earths and mini-Neptunes, the high-pressure ice would form a thick mantle around the exoplanet’s core, sealing off the mineral-rich rock of the ocean above.
These high-pressure ices would exist in exotic phases, making them very different from the ice found naturally on Earth.
“Unlike the ice we have on the Earth’s surface (ice I), all these [high pressure] ices are denser than liquid water,” leading to the formation of this icy mantle beneath an exoplanet’s subsurface ocean, Hernandez said. “These ices are also much stiffer than ice I.”
While ice I forms when water reaches freezing point, high pressure ices can form from the ambient temperature of water under high pressure. Compression to 2,000 times Earth’s atmospheric pressure produces ice VII, which consists of cubic crystals. More pressure turns the VII ice into “supersonic ice”, where the hydrogen atoms in the water move freely while the oxygen atoms remain trapped in a crystalline structure.
“This transition makes the ice conductive that could be at the origin of Uranus and Neptune’s non-dipolar magnetic fields,” said Hernandez.
Importantly, while land ice and other forms such as ice VI expel salt ions from their structure when they crystallize, he added, their familiar ice VII and supersonic ice can retain higher concentrations of dissolved salts. If it could retain the salts, then the known mechanism could 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, albeit very slowly on geological timescales. This is due to the temperature difference between the upper and lower part of the mantle.
“Like in a pan, the material at the bottom is hotter to the point where it becomes less dense than the environment and starts to rise,” Hernandez said. “Upon reaching the top, it progressively cools down and becomes denser than its surroundings and begins to sink.”
What the modeling in the paper shows, he added, is that high-pressure ice can retain dissolved salt over the full range of pressure and temperature conditions it might encounter during convection. Rather than sealing off the mineral-laden rocky core of the subterranean ocean, high-pressure ice sheets can act to transport these minerals into 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 aren’t definitive either. Dr. Hernandez is quick to point out that the study is limited, and other factors could still make these planets inhospitable to life.
“Our study is limited to [sodium chloride], and other electrolytes should be investigated,” he said. “Life requires many other conditions besides the presence of electrolytes in the ocean, so we do not intend to assess the habitability of these planets, but simply show that they may not be as chemically simple as expected.”