The list of places scientists should look for signs of extraterrestrial life just got a lot longer, as new research suggests 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 thought 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 penetrating water closer to the surface reach.
In a new study published on Tuesday nature communicationHowever, French and Norwegian researchers used new modeling techniques to show that salts such as sodium chloride could be transported from a planet’s rocky core through a hard ice shell to enrich an exoplanet’s water. This is a crucial step for any life that might evolve in an alien ocean.
“The presence of electrolytes in the ocean, such as dissolved salts, is a necessary but obviously not a 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.
One reason Europa and Enceladus are piqued the curiosity of scientists is that they believe they not only harbor global water oceans beneath their icy skins, but that those oceans are in direct contact with these moons’ rocky mantles. The minerals from this rock, especially when combined with an energy source such as geothermal vents on the sea floor, could create an environment in which life could develop and survive.
However, scientists believe that similar exoplanets of the frigid ocean world between the size of Earth and Neptune – super-Earths and mini-Neptunes – have different structures. About half of the known exoplanets fall into these size classes, 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 the 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 rocks from the ocean above.
These high-pressure ices would exist in exotic phases, making them very different from the ice that occurs naturally on Earth.
“In contrast to the ice that we have on the surface of the earth (Ice I), all of this [high pressure] Ice is denser than liquid water,” which leads to the formation of this ice mantle beneath an exoplanet’s subsurface ocean, said Dr. hernandez “These ices are also much stiffer than Ice I.”
While ice I forms when water reaches freezing point, high pressure ice can form from ambient temperature water under high pressure. Compressing it at 2,000 times Earth’s atmospheric pressure yields Ice VII, which is composed of cubic crystals. Further pressure turns Ice VII into “supersonic ice,” in which the hydrogen atoms move freely in the water while the oxygen atoms remain locked in a crystalline structure.
“This transition makes the ice conductive, which could be the origin of the non-dipolar magnetic fields of Uranus and Neptune,” said Dr. hernandez
Importantly, while terrestrial ice and other forms like Ice VI eject salt ions from their structure when they then crystallize, he added, his well-known Ice VII and supersonic ice can retain larger concentrations of dissolved salts. If it could retain the salts, then a known mechanism could put them in contact with the ocean on an exoplanet.
Whether of Ice VII or rock, the mantles of planets flow by convection patterns, albeit very slowly over geological timescales. This is due to the temperature difference between the top and bottom of the mantle.
“Like in a pan, the material at the bottom is hotter to a point where it becomes less dense than the surrounding area and starts to rise,” said Dr. hernandez “As it gets to the top, it progressively cools and becomes denser than the surrounding area and begins to sink.”
What the modeling in the paper shows, he added, is that high-pressure ice could retain dissolved salt over the full range of pressure and temperature conditions it might experience during convection. Instead of sealing off the mineral-laden rocky core from the subsurface ocean, high-pressure ice mantles could transport those minerals to the water.
The results mean that scientists shouldn’t remove an exoplanet from their list of potentially habitable worlds just because it’s a mini-Neptune, but they’re not definitive either. dr Hernandez is quick to point out that the study is limited and other factors could still make such planets inhospitable to life.
“Our study is limited to [sodium chloride]and other electrolytes should be studied,” he said. “Life requires many other conditions besides the presence of electrolytes in the ocean, so we do not pretend to judge the habitability of these planets, we merely show that chemically they may not be as simple as expected.”