Why oceans may exist on dwarf planets – 04/11/2024 – Science

For a long time, it was thought that Earth was the only planet in our Solar System with an ocean, but it is beginning to appear that there are subsurface oceans on even the most surprising icy bodies.

In fact, the icy moons and dwarf planets of the outer Solar System appear to have liquid oceans beneath layers of thick ice. Recent research suggests that there may even be oceans inside bodies beyond Pluto. This is surprising, since these bodies have surface temperatures well below -200°C.

70 years ago, it seemed plausible that Venus’s hot atmosphere was hiding a global ocean. But that idea was discarded in 1962, when the Mariner 2 spacecraft passed by Venus and discovered that its surface was too hot to have liquid water.

It wasn’t long before we realized that whatever ocean there may have once been on Venus, and also on Mars, disappeared billions of years ago due to major changes in their climates.

Tidal heating

The revolution in thinking that paved the way for our new view of the Solar System’s oceans dates back to a 1979 paper by astrophysicist Stan Peale. He predicted that Jupiter’s innermost moon, Io, would be so hot inside that it could be volcanically active.

The heat source that makes this possible is a gravitational effect — the repeated tidal force interaction between Io and Jupiter’s nearby moon, Europa.

Europa completes exactly one orbit for every two of Io. Io therefore passes Europa every two orbits, being regularly and repeatedly “pulled” by Europa’s tidal force, which prevents Io’s orbit from becoming circular.

This means that the distance from Io to Jupiter is continually changing — and therefore so is the strength of Jupiter’s (much stronger) tidal force, which actually distorts Io’s shape.

The repeated distortion of the tide inside heats Io by internal friction, much as if you bend a stiff wire back and forth several times — and then touch the newly bent part to your lip (try it with a coat hanger or a paper clip), you will be able to feel the heat.

Peale’s tidal warming prediction was confirmed just a week after publication, when Voyager-1, which performed the first sophisticated flyby of Jupiter, sent back images of erupting volcanoes on Io.

Io is a rocky world without any form of water, so it may seem like it has nothing to do with the oceans. However, the Jupiter-Io-Europa tidal interaction works both ways. Europa is also heated by tides, not just by Io, as well as the next moon, Ganymede.

There is now very good evidence that between Europa’s ice cap and its rocky interior lies an ocean 100 km deep. Ganymede may have up to three or four liquid layers, squeezed between layers of ice.

In these cases, the heat that prevents liquid water from freezing is probably caused mainly by the tides.

There is also evidence of a zone of salty liquid water within Callisto, Jupiter’s most distant moon. This is not likely to be due to tidal heating — but rather to the heat emitted by the decay of radioactive elements.

Saturn has a relatively small (504 km radius) icy moon called Enceladus, which has an internal ocean thanks to tidal heating resulting from interaction with the larger moon, called Dione. We are absolutely certain that this ocean exists because Enceladus’ icy shell oscillates in a way that is only possible because this layer is not fixed to the solid interior.

In addition, the Cassini probe collected samples of water and residual components from this internal ocean. Their measurements suggested that Enceladus’ ocean water must have reacted with hot rocks beneath the ocean floor, and that the chemistry down there appears well suited to supporting microbial life.

Other oceans

Surprisingly, even for moons that should not have tidal heating, and for celestial bodies that are not moons, the evidence for internal oceans continues to mount. The list of worlds that may have, or once had, internal oceans includes several of Uranus’ moons, such as Ariel, Triton, Neptune’s largest moon, and Pluto.

The closest internal ocean to the Sun may be within the dwarf planet Ceres, although it may already be largely frozen, or it may consist only of saline ooze.

Particularly surprising to me are indications of oceanic worlds far beyond Pluto. They come from recently published results from the James Webb telescope, observing ratios of various isotopes (atoms with different numbers of particles called neutrons in their cores) in the frozen methane that coats Eris and Makemake, two slightly smaller and considerably smaller dwarf planets. more remote than Pluto.

The authors say their observations are evidence of chemical reactions between inner ocean water and the rocky ocean floor, and also of very recent, possibly even current, water plumes. The authors suggest that the heat from the decay of radioactive elements in the rock is enough to explain how these internal oceans were kept warm enough to avoid freezing.

You might be wondering if all this could increase our chances of finding alien life.

Sorry to spoil the party, but several papers were presented at this year’s Lunar and Planetary Science Conference in Houston (March 11-15) arguing that the rock beneath Europa’s ocean floor must be too strong for damage to break. it, in order to create the types of hot springs (hydrothermal fissures) on the ocean floor that fueled microbial life on Earth’s earliest days.

It is possible that other subsurface oceans are equally inhospitable. But so far, there is still hope.

_{This article was originally published on the academic news site The Conversation and republished here under a Creative Commons license. Read the original version here (in English)}