Solving the puzzle of liquid water on a frozen planet

The James Webb Space Telescope continues to show the Outer Solar System in ways that originally required space probes.

A few days ago it detected a huge geyser—a high-pressure jet of liquid water—ejected from Enceladus, one of Saturn's moons. Jets of water ejected from that strange world have been known for some years. However, the big puzzle is why they are happening.

To better understand Enceladus, let's look at our moon.

Our moon is an airless rock ball. It is quite dark and absorbs about 85% of the solar energy falling on it. Its average temperature is about -48 C. The reason our world is warmer is the Earth has an atmosphere and the Moon does not.

The greenhouse effect makes our world liveable. Enceladus lies around 9.6 times farther from the sun than our moon and, consequently, it receives only around 1.1% of the solar energy. That is not the end of the story, because Enceladus is also covered with ice. That is far more reflective than the basalt rock covering our moon, so it captures less energy, making it colder—a frigid -200 C.

At that distance from the sun, water should be a permanently frozen rock mineral, with little prospect of it ever being a liquid. If there is liquid water on Enceladus, the heat must be from another source—inside.

Two possibilities are the decay of radioactive materials and tidal heating.

Our planet is hot in the middle for two main reasons. When it formed, it collected from its birth dust and gas cloud a collection of heavy, radioactive elements. Those sank down into the core while the earth was still molten, and they are producing heat as they decay.

There is also heat remaining from when the Earth was born. On average, a cubic centimetre of our planet weighs in at about 5.5 grams, that is, its density is 5.5 grams per cubic centimetre. Enceladus is far less dense, with an average density of 1.61 grams per cubic centimetre. Because rocks such as basalt have densities of around 2.9 grams per cubic centimetre, Enceladus must be made mostly of something less dense, such as ice. Such a low density also suggests it does not have a concentration of heavy, radioactive elements in its core, so heating by radioactive decay is probably not important.

The most likely candidate is tidal heating. Saturn, the planet it orbits, weighs in at the equivalent of about 95 Earths, and Enceladus orbits relatively close by, so tidal distortions are likely to be far more severe than the Moon inflicts on us, or us on the Moon.

Since Enceladus is probably mostly ice, tidal heating would have a lot to work on. So a possible picture for that interesting world would be a core consisting of rock and ice, surrounded by an ocean of liquid water, and sheathed in a shell of ice. Water does not tidally heat very much. It just happily slops around as it does in a bath. Most of the heat is probably generated in the flexing and cracking of the core of the body. There is probably a balance. If the core melts too much, tidal heating decreases and it freezes again.

Europa, one of Jupiter's moons, is another world sheathed in ice with an ocean underneath, due to tidal heating. Getting a sample of water from that ocean to see what it's like and to search for life involves a lander and a long drill.

Enceladus' geysers offer an easier option. The Cassini spacecraft flew though some of these plumes and took water samples. The information is still being analyzed, but we now know there is a deep, salty ocean under the ice, and that there are simple organic (carbon-based) chemicals in that water. These could be signs of life, or ingredients for life, but so far the jury is still out. Even if we search for just extraterrestrial carbon-based life, like life on Earth, it seems there are many exotic possibilities.

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• Venus and Mars lie close together in the west after sunset.

• Saturn rises in the early hours, and Jupiter appears low in the sky before dawn glow.

• The Moon will be new on June 17.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.