Ocean worlds in disguise: Could there be a secret ocean on the moon Mimas?

Title: The case for an ocean-bearing Mimas from tidal heating analysis

Authors: Alyssa R. Rhoden, Matthew E. Walker

First Author’s Institution: Southwest Research Institute, Boulder, CO, USA

Status: Published in Icarus [closed access]

Water plays such an important role in life on Earth that astronomers have long considered oceans on other planets to be promising places to search for life. You might think it should be really obvious if a planet or moon has an ocean, but it turns out that many of the oceans in our solar system are hidden beneath solid layers of ice, up to kilometers thick. So far, finding ocean worlds has involved checking the surfaces of icy moons for signs of cracked ice or spewing water, and these methods have uncovered oceans on several moons in the solar system

In today’s paper, the authors Alyssa Rhoden and Matthew Walker investigate the possibility of a new kind of ocean hiding beneath the icy surface of Mimas (figure 1). Mimas is the smallest of Saturn’s regular moons (its diameter is only a little bigger than Switzerland!). If Mimas does have an ocean, it could signal the existence of a whole new group of “stealth” ocean worlds, which give no signs on the surface of underground water.

Figure 1: 3D model of Mimas. Image Credit: NASA Visualization Technology Applications and Development (VTAD)

How to heat a rock by squishing it

Tidal heating occurs when a moon has an eccentric (non-circular) orbit around its host planet. As it orbits, the moon will be pulled into different shapes (figure 2), and all of this stretching can release heat. As far as oceans are concerned, tidal heating is really important because the released heat can help keep liquid water from freezing.

Tidal Forces!

Figure 2: Cartoon of the effects of tidal forces for a moon (blue) on an eccentric orbit around a planet (yellow). The changes in the planet’s shape release heat, allowing oceans to remain liquid. Image Credit: Toby Smith, University of Washington

Most other moons with sub-surface oceans show signs of cracking on the surface due to those same tidal forces. As the moon is stretched and squished, the surface ice can fracture and allow water to erupt onto the surface.

Mimas doesn’t show any of these surface cracks or water jets, so at first glance it doesn’t seem like it would have an ocean. However, some measurements of Mimas indicate there may be more going on under the surface. 

Good librations

The key data used in this paper is the libration of Mimas. Like our moon, most solar system moons are tidally locked, meaning they show the same face to their host planet at all times. However, depending on the shape of the moon’s orbit and the moon’s internal structure, the moon can oscillate, appearing to rock back and forth in the sky (see figure 3). Those oscillations are called libration. 

The libration of Mimas was previously measured with the Cassini spacecraft, and the result suggested that Mimas’ outer ice shell is about 20-30 km thick. So the authors’ goal was to model possible tidal heating scenarios that could lead to a liquid ocean beneath a 20-30 km ice shell. If the required heating is within existing estimates, it is quite likely that Mimas has a sub-surface ocean!

Moon libration

Figure 3: Images showing the libration of the moon. Image Credit: Carsten Arnholm

A few leagues under the ice

The authors model the ice shell thickness that can be maintained by various levels and types of heating. They look at both heating due to tides (“surface” heating in figure 4) and heating that could come from below the ice shell (“basal” heating). 

Figure 4 shows the results of their heating models. Generally, high levels of heating can only maintain thin shells, while lower levels of heating can sustain much thicker shells. The shaded region in figure 4 shows the observed ice shell thickness estimates. A shell of 25-30 km requires tidal heating of about 20-30 mW/m2 and very little basal heating from beneath the ice. The low basal heating is in line with previous estimates for Mimas. While the real amount of tidal heating on the moon has not been measured, the authors’ calculated range of 20-30 mW/m2 should be observable by current instruments, so it is possible to check this estimate with future observations of Mimas. 

Heating model
Figure 4: The amounts of heat required to sustain various thicknesses of ice shells on Mimas. Tidal forces generate the surface heating, while any heat sources below the ice layer provide the basal heating. The ice thickness measured by libration is shown as the shaded region. Image Credit: Figure 3 in today’s paper

Overall, the heating levels suggest it’s plausible that Mimas could host a sub-surface ocean beneath a thick ice shell, but if that’s the case, some more puzzles remain. Why does Mimas’ surface lack the usual signs of sub-surface oceans? How could an ocean with a thick ice shell  have formed in the first place given Mimas’ eccentric orbit and proximity to Saturn? Would Mimas’ ocean have any effects on the craters on the surface? If this secret ocean exists on Mimas, it means that astronomers should be on the watch for other potential “stealth oceans,” and reconsider what types of bodies can form and sustain them.

Astrobite edited by: H Perry Hatchfield

Featured Image Credit: Getty Images

About Isabella Trierweiler

I'm a fourth year grad student at UCLA. I'm interested in planet formation and I study the compositions of exoplanets using polluted white dwarfs. In my free time I like knitting, playing train games, and growing various fruit trees.

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