Earlier this year, researchers at the Paris observatory revealed that Mimas, the small moon of Saturn reminiscent of the ‘Death Star’ from ‘Star Wars’, is not a solid rock as previously believed, but rather hosts a relatively young underground ocean (formed between 2 and 25 million years ago) and currently still evolving. And now, the same team of scientists has discovered how this ocean could form in such a small body (barely 400 km in diameter) and apparently so unsuitable.
In a paper just published in Planetary Science Letters, researchers suggest that Mimas’s inner ocean formed as its outer ice shell melted, becoming increasingly thinner as the small moon’s orbit around Saturn tightened. It was becoming less ‘flattened’ (that is, less eccentric and more circular) because of the gravitational attraction of the giant planet.
No one expected that such a small moon would be capable of hosting an ocean under its surface, so its mere presence has already redefined the idea that scientists had about what the so-called ‘ocean worlds’ should be like. Something that will affect our way of searching for water and life both inside and outside the Solar System.
An ocean 45 km deep
“In our previous work,” explains Matthew E. Walker, of the Planetary Science Institute in Tucson, Arizona, and co-author of the study, “we discovered that for Mimas to be an ocean world today, it must have had a much thicker ice sheet in the past.” past. But because Mimas’s eccentricity would have been even greater in the past than it is today, the path for moving from thick ice to thinner ice was less clear. In this work, we show that there is a pathway for the ice sheet to be currently thinning, even as the eccentricity is decreasing due to tidal warming. However, from a geological point of view, the ocean must be very young.
Mimas, also known as the ‘Death Star’ because of the Herschel crater, which gives it an appearance similar to that of the enormous space station in the ‘Star Wars’ saga, acquired its enormous scar (140 km in diameter and with a peak 6 km high in the center) due to the impact with an object about 5 km away around 4.1 billion years ago.
Calculations indicate that the underground ocean is between 20 and 30 km below its icy surface, and that it has a depth of between 40 and 45 km, which means that it occupies almost half of the total volume of Mimas.
The ‘warming tides’
According to researchers, the unexpected Mimas ocean was created as a result of a process known as ‘tidal heating’, the same process that maintains enormous oceans of liquid water under the surface of other larger moons of Saturn and Jupiter, and that occurs when a body (such as a moon) is distorted and ‘stretched’ due to variations in the gravitational forces it experiences along an ‘eccentric’, i.e. elliptical or oval, orbit.
«Eccentricity – says Walker – drives the warming of the tides. And right now, on Mimas that heating is very high compared to other active ocean moons, such as neighboring Enceladus. “We believe that tidal warming is the heat source responsible for the current thinning of the ice cap.”
The scientist adds that the problem is that tidal heating is not ‘free’ energy, which means that when the icy layer of Mimas melts, tidal heating extracts the necessary energy from the orbit of the moon. around Saturn. And that, according to Walker, will reduce the orbital eccentricity even further until, in the end, Mimas’s orbit is completely circular and the whole process stops forever.
Orbital eccentricity
Orbital eccentricity is measured in values ranging from 0 to 1, where 0 represents a perfect circle and 1 represents a parabola. Any value in between is an ellipse. The team estimates that the onset of ice melt must have begun when Mimas’s orbital eccentricity was two to three times greater than its current value. Which represents the last 10 million years of the small moon’s history and bears witness to an evolution that is consistent with the geology we observe today.
«Generally – continues Walker – when we think about oceanic worlds we do not see many craters, because the environment renews them and ends up erasing them, as happens in Europa or at the south pole of Enceladus. “Therefore, the shape, central peak, and intact interior of Herschel crater require that the ice shell have been much thicker in the past, when Herschel formed.”
According to the study, in fact, for the crater to be as we see it today, the ice layer that covers the moon of Saturn must have been at least 55 km thick when it was hit by the body that created the crater. Herschel about 4.1 billion years ago.
“Craters – explains Walker – can provide clues to the presence of an ocean (…).” In summary, the data indicate that the Mimas subsurface ocean is still very young, and that it could not have begun to form more than 25 million years ago.
«In other words – concludes the scientist – we believe that Mimas was completely frozen until between 10 and 25 million years ago, at which time its ice cap began to melt. What changed to start that era of melting is still being investigated. “We may be seeing Mimas at a particularly interesting time in its history.”
