Space scientists reveal new evidence of icy sea worlds and moons can host life

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July 5, 2022

Space scientists reveal new evidence of icy sea worlds and moons can host life

The earth's sea is a huge, uniform electrolyte solution. They contain salt (sodium chloride) and other nutrients such as magnesium, sulfate and calcium. We cannot survive without electrolytes, and life on earth can look very different without the electrolyte content of the oceans. It may even be non-existent.

On Earth, electrolytes are released into the oceans from rocks through various processes such as volcanism and hydrothermal activity.

Are these life-giving nutrients available on water worlds?

What is a water world?

Water worlds are exoplanets with enough water to form a hydrosphere. Many of the exoplanets we have discovered are super-Earths and / or mini-Neptunes, and scientists expect some of them to be water worlds. On these planets, electrolytes probably play a similar role in habitability as they do on Earth's sea.

TRAPPIST-1e can be an icy exoplanet that can sustain life.NASA / JPL-Caltech / R. Hurt, T. Pyle (IPAC)

But the problem is that super-Earths and mini-Neptunes are more massive than Earth, and their interiors are under more pressure than Earth. These planets can form deep planetary ice mantles between the rocky cores and surface oceans. These ice barriers are not ordinary ice. Instead, the high-pressure ice is like ice VII, and the dense ice can be a barrier that prevents important mineral electrolytes from moving from the cores to the oceans, where they would be available for life.

These high-pressure ice barriers can limit the habitability of the marine worlds. But a new study suggests that electrolytes can flow through these icy mantles on water worlds. If this is true, there is another reason to be optimistic about life in these compelling worlds.

What is new – The study is “Stability of salt ice at high temperature indicates electrolyte permeability in water-rich exoplanetary mantles. ” It is published in the journal Nature communicationand the lead author is Jean-Alexis Hernandez, a researcher at the European Synchrotron Radiation Facility.

“Electrolytes play an important role in the internal structure and dynamics of water-rich satellites and potentially water-rich exoplanets,” the paper begins.

“But on planets, the presence of a large high-pressure ice mantle is believed to prevent the exchange and transport of electrolytes between different liquid and solid deep layers.”

Exotic is

The ice in these mantles is different from the ice on Earth. A series of ice types are formed under higher pressure on more massive planets. Ordinary, atmospheric earth ice is called Ice I. Scientists have created other types in laboratory experiments, from Ice II to Ice VII. In one experiment, scientists exposed a drop of water to a powerful shock wave and created Ice VII, even if it only lasted a moment.

This diagram shows what the interior of Ganymede, the largest moon in the solar system, can look like. Image credit: NASA / JPL – Caltech

In marine worlds that are super-Earths or mini-Neptunes, the deeper layers of the ocean are likely frozen to high-pressure ice such as Ice VII. Ice VII is structurally different from Ice I. In Ice VII, water molecules break apart, oxygen ions crystallize and the hydrogen atoms move freely in the oxygen crystal lattice. According to the study's simulations, nutrients can get into the ice.

Ice VII has an important property when it comes to commercial transport. While ordinary earth ice expels salt as it forms, ice VII can hold about 2.5 weight percent NaCl in its structure. NaCl in Ice VII lowers the melting point of the ice and softens it. So convection currents from the planet's interior can drive NaCl upward through the ice and into the ocean. It creates a temperature difference, and the ice cools and sinks again. The result is a recycling stream of salt from the rocky interior of the planet, up through the ice mantle into the ocean and down again.

This can happen in our solar system. Researchers have found evidence that hydrated salt minerals stain the surface of some of the icy moons. Moons such as Ganymede, Callisto, Europa and Enceladus all probably have oceans beneath the surface under their frozen shells, with HP ice mantles at different depths forming barriers between their rocky cores and their oceans. So the minerals that colored the surfaces were transported through at least one layer of ice, maybe more. Some of the moons are not large enough to form Ice VII, but Jupiter's Callisto, Ganymede and Saturn Titan are massive enough to form mantles of high-pressure ice.

If nutrients such as sodium chloride can be transported from a rocky interior of a planet through a layer of ice VII to the ocean, it could be a game changer. Suddenly, there is more evidence that these marine worlds could support life.

Water worlds of potential

When we find more exoplanets, we see more potential water worlds. The well-known TRAPPIST-1 system can host several of them – TRAPPIST-1e and TRAPPIST-1f are strong candidates – even if researchers are not sure. Kepler-62e and Kepler-62f are also possible water worlds.

Baptiste Journaux is a researcher at the University of Washington, where he studies planetary science, including the conditions in deep planetary seas. Journaux wrote a comment about this new study in Nature communication.

In his article, he said that discoveries of exoplanets show that marine worlds are likely to be widespread. Our solar system has lunar moons but no marine worlds. And while the earth's surface is two – thirds of the ocean, our planet is actually remarkably dry.

These new results have increased the habitability potential for all marine worlds out there, according to Journaux.

“The study by Hernandez et al. Offers the most convincing argument to date for solving the dilemma of the habitability of the large planetary hydrosphere.”

The study of marine exoplanets is about simulations; there is no way to observe them in detail. But the James Webb Space Telescope may begin to change that. It may be able to detect spectroscopic fingerprints from interactions between the atmosphere of an ocean planet and its oceans. And more help is on the way.

The pursuit of life

NASA and ESA are developing missions for some of the solar system's icy / ocean moons. ESA: s Jupiter Icy Moons Explorer (JUICE) will be launched in about a year and reach the Jovian system in 2031. In 2034, it will go into orbit around Ganymede, the solar system's largest moon. It will eventually approach within 500 kilometers (310 miles) of Ganymede's surface.

NASA's Europe Clipper is scheduled to launch in 2024 and reach Jupiter in 2030. Although it will orbit Jupiter, it will study Europe, another sea moon with an icy shell.

This projection of the official USGS Europe base map is centered on the estimated source region for potential plumes that may have been detected using the Hubble Space Telescope.NASA / JPL-Caltech / SETI Institute

Ganymede and Callisto are probably massive enough to form high-pressure water mantles. Titan is, too, but it is far away, and although there is talk of a mission to Saturn's largest moon, it is far from certain.

The missions to these moons will begin to test some of the study's conclusions. If electrolytes can be transported through high-pressure ice deposits on Ganymede, it will be an important find in favor of habitability in water worlds. But life requires more than just Na and Cl. We still do not know if other important molecules can pass through these icy barriers.

What's next – The upcoming missions will tell a lot about the icy sea moons of our solar system and the permeability of ice mantles under high pressure. Some of the finds will also extrapolate to marine worlds in other solar systems.

“These missions will not only allow us to better understand how the hydrospheres of icy moons work internally, but will be the key to understanding the largest oceans in our universe in water-rich exoplanets, their potential for habitability and their future characterization of modern and next generation telescope, ”Journaux said in his article.

The authors of the new study conclude by talking about some of the other factors involved in the habitability of the marine world.

They point out that electrolyte transport depends “… on the actual size, composition and surface temperature of the planet in question, which may result in different scenarios at the interface between the sea and the ice sheet and between the ice sheet and the rocky core.”

Many factors must be just right for a sea planet to transport nutrients to the surface ocean. But they at least showed with their simulations that it is possible.

We have to wait to find out if their simulations are correct and how widespread the phenomenon is.

This article was originally published on The universe today by Evan Gough. Read original article here.

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