Earth was long thought to be the only planet in our solar system with an ocean, but it is beginning to appear that there are subsurface oceans even in the most surprising icy bodies.
In fact, icy moons and dwarf planets in the outer solar system appear to have liquid oceans beneath layers of thick ice. Recent research suggests that there may even be oceans in bodies beyond Pluto. That’s surprising, since these bodies have a surface temperature well below -200°C.
Seventy years ago, it seemed likely that Venus’s steamy atmosphere obscured a global ocean from our view. This idea was thwarted in 1962 when the Mariner 2 spacecraft flew by Venus and discovered that the surface was too hot for liquid water.
It didn’t take long for us to realize that all the oceans that were once on Venus, as well as on Mars, had disappeared billions of years ago due to major changes in their climate.
Tidal heating
The revolution in thinking that paved the way for our new view of the solar system’s oceans can be traced to a 1979 paper by astrophysicist Stan Peale. This predicted that Jupiter’s inner large moon, Io, would be so hot inside that it could be volcanically active.
The heat source that makes this possible is a gravitational effect: a repeated tidal drag between Io and the next moon from Jupiter, Europa. Europa completes exactly one orbit to Io’s two. Io therefore overtakes Europa after every two orbits and receives a regularly repeated tidal drag from Europa that prevents Io’s orbit from becoming circular.
This indicates that Io’s distance from Jupiter is constantly changing, and therefore so is the strength of Jupiter’s much stronger tidal force, which actually distorts Io’s shape.
Repeated tidal deformation of the inside heats Io through internal friction, in the same way that 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 notice the heat can feel.
Peale’s prediction of tidal warming was confirmed just a week after publication when Voyager-1, the first advanced flyby of Jupiter, returned images of erupting volcanoes on Io.
Io is a rocky world, without water in any form, so it may seem to have nothing to do with oceans. However, the tidal tug Jupiter-Io-Europa works both ways. Europa is also warmed by the tides, not only by Io, but also by the next moon, Ganymede.
There is now very good evidence that between the icy shell of Europe and its rocky interior there is an ocean 100 km deep. Ganymede may have as many as three or four layers of liquid sandwiched between layers of ice. In these cases, the heat that prevents the liquid water from freezing is likely largely of tidal origin.
There is also evidence of a salty, liquid water zone within Callisto, Jupiter’s outermost large moon. This is probably not due to tidal warming, but possibly due to the heat released by the decay of radioactive elements.
Saturn has a relatively small (504 km radius) icy moon called Enceladus, which has an internal ocean due to tidal heating from interacting with the larger moon called Dione. We are absolutely certain that this ocean exists because Enceladus’ icy shell wobbles in a way that is only possible because it is not attached to its solid interior.
In addition, water and trace elements from this internal ocean were sampled by the Cassini spacecraft. The measurements showed that Enceladus’ ocean water must have reacted with warm rocks beneath the ocean floor, and that the chemistry down there appears to be suitable to support microbial life.
Other oceans
It is puzzling that even for moons that should not have tidal heating, and for bodies that are not moons at all, the evidence for internal oceans continues to pile up. The list of worlds that may have or once had internal oceans includes several moons of Uranus, such as Ariel, Triton, Neptune’s largest moon, and Pluto.
The closest internal ocean to the Sun may be in the dwarf planet Ceres, although it may now be mostly frozen or may be just salty silt.
I find the evidence for ocean worlds far beyond Pluto particularly astonishing. These come from recently published results from the James Webb Space Telescope, which looked at the proportions of different isotopes (atoms with more or fewer particles called neutrons in their nuclei) in the frozen methane surrounding Eris and Makemake, two dwarf planets that be slightly smaller and smaller. considerably further away than Pluto.
The authors claim that their observations are evidence of chemical reactions between internal ocean water and ocean floor rocks, as well as quite young, possibly even contemporary, water plumes. The authors suggest that the heat from the decay of radioactive elements in the rock is sufficient to explain how these internal oceans have been kept warm enough to prevent freezing.
You might be wondering if all this could increase our chances of finding extraterrestrial life. It saddens me to spoil the party, but there were several papers at the Lunar and Planetary Science Conference in Houston (March 11-15) reporting that the rock beneath the floor of the European ocean must be too strong to be broken apart by faults to fall. creating the kind of hot springs (hydrothermal vents) on the ocean floor that fueled microbial life on early Earth.
It is possible that other subsurface oceans are similarly inhospitable. But so far there is still hope.
Read more: An element essential to life discovered on one of Saturn’s moons, raising hopes of finding alien microbes
This article is republished from The Conversation under a Creative Commons license. Read the original article.
David Rothery is co-lead of the European Space Agency’s Mercury Surface and Composition Working Group, and co-investigator of MIXS (Mercury Imaging Space Agency. . He has received funding from the UK Space Agency and the Science & Technology Facilities Council for work on Mercury and BepiColombo, and from the European Commission under its Horizon 2020 program for work on geological mapping of planets. He is the author of Planet Mercury – from Pale Pink Dot to Dynamic World, Moons: A Very Short Introduction and Planets: A Very Short Introduction.