Saturn’s moon Mimas may have ocean under its crust

Subject: Science and tech

Section: Space tech

What is Mima?

Mimas is the smallest of the major moons of Saturn-known as the “big seven” or the “inner moons”.
Nicknamed “Death Star” for the resemblance of its cratered surface to a space station in the Star Warsfranchise.

Presence of ocean under crust:

Mimas was an unlikely candidate to host an ocean because the presence of these water bodies is generally marked by modifications of the surface by internal dynamics.
Scientists analysed Mimas’s orbital motion based on data from NASA’s Cassini spacecraft and determined that the moon’s liberations — or oscillations — indicate it either has an elongated silicate core or a global ocean.

Possibility 1:

If Mimas were solid, its librations would depend on gravity coefficient and changes in orbit on this and another factor called J2.
By measuring its librations and orbital changes, scientists could determine the values for these numbers if Mimas is entirely solid.
But calculations reached a dead end. Possibility of being elongated silicate layer is thus omitted.

Possibility 2: Mimas hosting a subsurface ocean.

They used a theoretical approach to understand how an underground ocean would affect the moon’s movement, depending on the shape of three deformed layers, the gravity coefficients, and the number of liberations.
The outer Icy layer is considered viscoelastic (i.e. both viscous and elastic) in the liberational model.
And the interior interfaces are assumed to be hydrostatic (i.e. at rest).
The resulting calculations suggested the ice sheet would have to be 20-30 km thick to match observations.
These figures agreed closely with the findings of a 2022 study as well.
Further, the amount of heat released at the surface of Mimas was estimated to be around 25milliwattper sq. m, which should reduce the moon’s eccentricity — a measure of the extent to which its orbit’s shape deviates from a perfect circle — by a factor of two in 4-5 million years.
Based on these values, the scientists ran simulations and found that its ocean may have formed 2-25 million years ago.
The predicted surface heat also matched observations of Enceladus — a Saturn moon that has a global ocean beneath its crust — indicating Mimas may have a hydrothermally active core even if surface activity doesn’t indicate that.

Conclusion:

Mimas may have a liquid ocean around 20-30 km under its heavily cratered ice shell.