The list of worlds with subsurface oceans in our solar system is getting longer by the year. Of course, many people are familiar with the most obvious cases: The icy moons Enceladus and Europa are literally bursting at the seams with water.
Now, scientists argue in a paper in Nature that we may have reason to add yet another long-shot to the list: Saturn’s “Death Star” moon, Mimas. A lack of clear evidence on its surface made scientists skeptical it could be hiding an interior ocean.
The paper, which contains fresh analysis of observations made by the Cassini probe, says changes in the moon’s orbit over time are best explained by the presence of a global ocean deep below its icy crust.
The team believes the data also suggests the ocean is very young, explaining why it has yet to make its presence known on the surface.
How exactly do frozen moons on the outskirts of the solar system come to contain whole oceans of liquid water? In short: Combine heat and a good amount of ice and you get oceans.
We know there is an abundance of ice in the outer solar system, from moons to comets. Interior ocean worlds depend on another source of heat-gravity.
The friction from this grinding, called tidal flexing, produces heat which melts ice to form salty oceans.
The more we look, the more we find evidence of hidden oceans throughout the outer solar system.
Speculation that Mimas might be an ocean world isn’t new.
A decade ago, small shifts in the moon’s orbit measured by Cassini suggested it either had a strangely pancake-shaped core or an interior ocean. So they looked to the interior ocean hypothesis and modelled a range of possibilities.
The models not only fit Mimas’s orbit well, they also suggest the ocean likely begins 20 to 30 kilometers below the surface.
The team believes the ocean would likely be relatively young, somewhere between a few million years old and 25 million years old.
What accounts for this youth? The team suggests relatively recent gravitational encounters-perhaps with other moons or during the formation of Saturn’s ring system, which some scientists believe to be relatively young also-may have changed the degree of tidal flexing inside Mimas.
The associated heat only recently became great enough to melt ice into oceans.
If these measurements match predictions made in the paper, scientists might confirm the ocean’s existence as well as its depth below the surface.
Studying a young, still-evolving interior ocean could give us clues about how older, more stable oceans formed in eons past.