This moon that looks like the "Death Star" in Star Wars would hide an ocean under its surface
Numerical simulations of the Herschel impact basin, the most visible feature of Mimas' surface, have shown that Saturn's satellite may have a thinning shell of ice and a geologically young ocean.
Mimas is a natural satellite of Saturn , the smallest – around 400 kilometers – and the closest to the planet – at around 185,500 kilometers – of the seven having a spheroidal shape . Its modest size and lack of geological activity suggest a frozen and inactive history because,mimesbeing currently separated from its planet by less than 3 radii fromSaturnand hisorbitbeing “highly” eccentric (0.02), this should stimulate intense activity oftides. Furthermore, Mimas is heavily cratered and does not have the typical characteristics of amoonoceanic, like thesurface activeof its neighbor Enceladus .
However, the Mimas measurements made by the Cassini mission are best explained by an ocean under a relatively thick shell of ice (see our 2014 article below). A more recent study has also shown that, thanks to the tides, the small moon can generate the right amount ofheatto harbor an internal oceanliquid(see our 2022 article below as well). These results then encouraged the study of the surface of Mimas to understand how its interior could have evolved.
A young ocean under a thinning shell of ice
Of thenumerical simulationsfrom the Herschel impact basin , the 139 kilometer diameter crater that gives Mimas its Death Star look , showed that the structure of the basin and the lack of tectonics on Mimas are consistent with an ice shell that s thins and a geologically young ocean.
Adeene Denton, then a student in the Department of Earth Sciences,atmosphereand planets from Purdue University in West Lafayette (Indiana, USA), worked with Alyssa Rhoden, a planetary scientist at the Southwest Research Institute in Boulder (Colorado, USA), to better understand how Mimas could possess an internal ocean . Their findings led to a new paper in Geophysical Research Letters .
Adeene Denton modeled the formation of the Herschel impact basin using thesoftwareiSALE-2D simulator. Models showed that Mimas' ice shell must have been at least 55 kilometers thick at the time of the impact that formed Herschel. In contrast, observations of Mimas and models of its internal warming limit the current thickness of the ice shell to less than 30 kilometers if it currently hosts an ocean. These results imply that a current ocean within Mimas must have warmed and expanded since the formation of the basin. It is also possible that Mimas was completely frozen at the time of impact and is still so today. However, A. Denton found that including an inner ocean in the impact models helped produce the shape of the basin.
models of the interior of mimas, after denton and rhoden (2022). option a: ice shell between 24 and 31 kilometers thick above an ocean less than 40 kilometers thick. option b: shell of ice 71 kilometers thick surmounting a core that would not be in hydrostatic equilibrium.
nasa/jpl/ssi/swriConstraints for other moons
Denton, who is now a postdoctoral researcher at the University of Arizona, said: " We found that Herschel could not have formed in an ice shell of the current thickness without annihilating the ice shell on the impact site. If Mimas has an ocean today, the ice shell has been thinning since the formation of Herschel, which could also explain the absence of fractures on Mimas. If Mimas is an ocean worldemerging, this imposes significant constraints on the formation , evolution, and habitability of all of Saturn's mid-sized moons ”.
Rhoden, co-lead of the Ocean Worlds Research Coordination Network of theNaa and who previously served on the National Academies Committee onastrobiologieand planetology, specifies that, " although [their] results support a current ocean within Mimas, it is difficult to reconcile the characteristicsorbitalsand geology of the moon with [their] current understanding of its thermal-orbital evolution. Assessing Mimas' status as an oceanic moon would allow comparisons of patterns of its formation and evolution. It would help [them] better understand Saturn's mid-sized rings and moons as well as theprevalencepotentially habitable ocean moons, especially around Uranus . Mimas is an unavoidable target to continue the investigation ”.
This Saturn moon would also hide an ocean under its surface
Around Saturn there was alreadyEnceladeAndTitan. Among the moons of the ringed planet that hide a liquid ocean under their surface, it will now probably also be necessary to count the little Mimas.
Several years ago, the hypothesis was raised. Under hiscrustof ice, Mimas, a giant name for a very small satellite of Saturn -- its diameter is only around 400 kilometers --, would hide an ocean. The hypothesis was supported by measurements returned by the Cassini probe (Nasa). But theastronomerswere puzzled. Nothing. There was absolutely nothing on the surface of the satellite to confirm this.
"The surface of Mimas is riddled with craters, one of which is particularly huge - some even see a resemblance to theStarof Death from Star Wars. We thought it was just a block of ice , ”says Alyssa Rhoden, specialist in the geophysics of this type of celestial object, in a press release from the Southwest Research Institute (SwRI, United States). "Nothing like the surfaces we know from other 'inner ocean worlds' that are fractured and show signs of geologic activity. And it is precisely by wanting to demonstrate this that astronomers have discovered new evidence that the satellite does indeed have a liquid internal ocean.
?Did you know
In our Solar System, several natural satellites are now known to hide oceans. Among them Enceladus and Titan, two other moons of Saturn. And maybe Dione too. But also Europe, Callisto and Ganymede, moons of Jupiter or even Triton, a moon of Neptune.
What betrayed Mimas' secret was thelibrationrecorded a few years ago now, still by the Cassini probe. A libration is the name that astronomers give to slowmovementsamount of oscillation a satellite can have when viewed from the body around which it orbits. But thephysicistsknow that the phenomenon of the tides -- part of which is caused by the libration in question -- tends to dissipateenergyas heat in a satellite. And in this case, enough for a liquid ocean to persist beneath the icy crust of Mimas.
Go see on Mimas what's going on there
According to the models developed by the researchers, the ice crust on the surface of Mimas would thus be between 20 and 30 kilometers thick. And it would completely cover a liquid ocean. However, the conditional remains in place. Because as astronomers recognize, “it is difficult to reconcile the orbital and geological characteristics of Mimas with our current understanding of its evolution” . The hypothesis of a nucleus in the form ofballrugby also formulated to explain recorded libration remains possible. Only a mission sent on the spot seems able to decide. It could at the same time study, if it exists, the composition of this ocean, undoubtedly filled with water trapped there for billions of years.
Understanding Mimas' model better could enlighten astronomers on how many "inner ocean worlds" they can expect to find. Not only in our solar system . Maybe on the side ofUranusand ofMiranda,ArielOrumbria. And on smaller objects than what researchers had previously imagined. But also around more distant stars.
What also open a little more the field of possibilities for life. Indeed, the range defined by the researchers as "habitable" around a star could well be seen rather largely enlarged. And no longer confined to an area located around an equivalent of the Earth-Sun distance. Thanks to underground water which would prove to be much more common than surface water.
Around Saturn, Mimas would hide an ocean... or a flattened core
Based on observations made by the Cassini space probe, an international team of researchers measured the rotation of Mimas, one of Saturn's many moons, and detected oscillations there. Not consistent with predictive models, they suggest that this small satellite could harbor either a strongly flattened core or, under its layer of ice, an ocean.
mimas is one of many natural satellites orbiting saturn. in this image taken in 2010 by the cassini space probe, about 16,000 km away, we can see on its surface the large herschel crater (130 km), characteristic of this moon 396 km in diameter. its orbit is synchronized with the giant planet.
nasa, jpl, space science instituteMimas' rotation was measured from Cassini images (Esa, NASA), using a little-known technique called stereophotogrammetry. It makes it possible to reconstruct in three dimensions a portion of the surface of an object when it is observed at least twice from different angles. Just like the Moon around the Earth, Mimas is in rotationsynchronousaround Saturn. This means that this satellite rotates on itself at the samespeedthat it performs a revolution around thegiant planet, thus always showing the same face to his planet.
However, this uniform average movement is superimposed by oscillations. These are called librations, because they result from the gravitational force couple exerted by Saturn on the satellite of about 400 km in diameter. The work carried out by an international team involving French researchers from the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE) of the Paris Observatory (CNRS, Pierre etMarie Curie, Université Lille 1) and the AIM laboratory (CEA, CNRS, Université Paris Diderot) and scientists from the Royal Observatory of Belgium, the University of Namur (Belgium) and the University of Cornell (United States) have enabled to highlight two types of librations: one at low frequency, the other at high frequency. However, of these two types, the high frequency one has an amplitude twice as large, incompatible with the rotation model of a satellitesolid, at hydrostatic equilibrium. This amplitude is indicative of the distribution ofmassinside the body and the presence or absence of liquid layers.
mimas is one of the closest spherical natural satellites to saturn. although photographed by cassini more than 3 million km away, the herschel crater which dominates the surface of this small moon is easily noticed.
nasa, jpl, space science instituteMimas could hide an ocean
These observations are therefore surprising and reveal an intriguing internal structure. After having explored several hypotheses, it appears that this strong amplitude can be explained either by the presence, under thecoatof Mimas ice , a very elongated rock core, or by the existence of an internal ocean hidden between its icy surface and its core. Indeed, planetary scientists assume that the core of the small satellite must be in hydrostatic equilibrium (where the forces of gravity, centrifugal andpressurebalance each other in the body), hence from a very early formative age. However, the high amplitude of the high-frequency libration could indicate a core with an elongation of 20 to 60 km greater than in the hydrostatic case.
If the core of Mimas is indeed elongated, then it would have frozen since its formation and would have largely retained its initial shape. On the other hand, if this moon has an ocean, it would then join the club of " internal ocean satellites " of the Solar System, which includes several moons ofJupiter (Europe, Ganymede...) and, around Saturn, Titan and Enceladus . Such a global ocean would be a real surprise, as the surface of Mimas shows no signs of recent geological activity. Additional observations from Cassini will help refine Mimas' interior models.
Whether it is one or the other of these two solutions, we now know that Mimas, despite its apparently ancient surface riddled with craters and its small size, is not the cold and inert star that we imagined. Uncovering the secret of its interior will surely shed light on its formation, and thereby on the formation of the Saturn system as a whole.
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