On alien worlds, an exotic form of ice can carry nutrients


Any potential extraterrestrial life in the waters of vast ocean worlds could receive vital nutrients from the molten cores of their planets via thick layers of high-pressure alien ice that can carry salts, new research suggests.

Water is one of the most common molecules in the universe, and worlds covered by global oceans hundreds of miles deep may greatly outnumber “drier” planets like Earth. Examples of such water worlds include several of the planets of the TRAPPIST-1 system, as well as GJ 1214b, Kepler-62nd and Kepler-62f.

The conditions at the bottom of these oceans are so extreme that the water is compacted enough to form high-pressure ice known as ice VII. Ice VII molecules are arranged in a cubic crystal structure, and it exists at pressures greater than 3 gigapascals (about 29,000 atmospheres) and temperatures up to 662 degrees Fahrenheit (350 degrees Celsius).

Related: Ultrahot ‘superionic’ ice is a new state of matter

However, scientists didn’t know if salts and other nutrients could move from a planet’s rocky core, through Ice Mantle VII and into the liquid ocean — and back down. Normal ice expels salt when it freezes, but new modeling shows Ice VII can hold salt crystals. In particular, research has shown that Ice VII can contain up to 2.5% by weight (weight percent) sodium chloride, more commonly known as table salt. The presence of salt lowers the melting point of ice, softening it and thus helping thermal convection currents to raise salt ice.

“Salt transport would not just be bottom-up, but also top-down,” said Jean-Alexis Hernandez, a scientist at the European Synchrotron Radiation Facility in France who led the new work. “The hot ice at the bottom of the mantle becomes gravitationally unstable because, being hotter, it is less dense than the surrounding ice, causing it to rise. A global flow develops and is maintained by the temperature difference between the top and the bottom of the mantle. coat,” Hernandez told Space.com.

This flow recycles nutrients and salts needed for biochemistry. Although their presence does not guarantee that life exists in the oceans of these aquatic worlds, it does increase the chances that these worlds are habitable.

ice crystals VII (Image credit: Baptiste Journals)

“The findings increase the number of planetary candidates for habitability by including super-Earths with high-pressure ice mantles,” said Baptiste Journaux, an instructor in the Department of Earth and Space Sciences at the Institute. University of Washington in Seattle, at Space.com. Journals did not participate in the research, but wrote a comment on new research for the journal Nature Communications.

The results can also bring new ideas much closer to home. The closest analogues of the ocean worlds of our solar system are some of the frozen moons of Jupiter and Saturn. Although many moons are too small to harbor high-pressure ice, the largest of them – that of Jupiter Callisto and Ganymede, and Saturn Titan — are massive enough to create an ice mantle VI, which forms at 1 GPa (about 10,000 atmospheres).

Although Ice VI is insoluble, researchers have already detected hydrated mineral salts that have stained the surfaces of some of these moons, presumably due to the oceans rising beneath their icy surfaces. Scientists believe that this finding indicates that some salt transport must be taking place. In the Jupiter system, some of these moons also have an induced magnetic field from interactions between their electrically conductive salty subterranean oceans and the planet’s immense magnetic field.

A cross-sectional diagram of Ganymede (Image credit: NASA/JPL–Caltech)

“Salts always play an important role in these objects because they change the phase boundaries [i.e., the transition regions between rock, ice and liquid] and water properties, which strongly influence the dynamics of the ice shell and oceans on these icy moons,” Hernandez said.

Callisto, Ganymede and Titan are therefore currently the best places to test our geophysical models describing high-pressure ice permeability, Journals told Space.com. In particular, scientists will soon have a wealth of new data from the European Space Agency. Explorer of the Icy Moons of Jupiter (JUS) mission, which will be launched in 2023. JUICE will be supported by NASA European Clipper mission, which will be launch in 2024 and visit Ganymede and Callisto as well as Europa, and by NASA’s Dragonfly helicopter mission to Titan, which will launch in 2027.

“The data we will get from icy moons will greatly help us understand the role of high-pressure ice mantles in controlling the composition and habitability of extraterrestrial oceans,” the Journals said.

However, the same studies cannot currently be done for distant exoplanets. So for now, determining whether ocean worlds might be habitable will remain primarily a matter of modeling rather than observation.

The research was published June 21 in the Nature Communication magazine (opens in a new tab).

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