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The innermost and smallest of the seven major moons of the Saturn system, Mimas, is a battered iceball orbiting just outside the main rings, and has been dubbed the "Death Star moon" due to a massive impact crater deeply gouging one of its faces.  It is a stark, dreamlike object utterly dwarfed in the looming presence of its host planet, and yet all the more amazing as a visual cue to the scales involved.  

The progress of our adventure so far (current in bold):

1.  The Sun
2.  Mercury
3.  Venus
4.  Earth (Vol. 1)
5.  Earth (Vol. 2)
6.  Earth (Vol. 3)
7.  Earth (Vol. 4)
8.  Earth (Vol. 5)
9.  Earth (Vol. 6)
10.  Luna
11.  Mars (Vol. 1)
12.  Mars (Vol. 2)
13.  Mars (Vol. 3)
14.  Phobos & Deimos
15.  Asteroids (Vol. 1)
16.  Asteroids (Vol. 2)
17.  Asteroids (Vol. 3)
18.  Ceres
19.  Jupiter (Vol. 1)
20.  Jupiter (Vol. 2)
21.  Io
22.  Europa (Vol. 1)
23.  Europa (Vol. 2)
24.  Ganymede
25.  Callisto
26.  Saturn (Vol. 1)
27.  Saturn (Vol. 2)
28.  Saturn (Vol. 3)
29.  Rings of Saturn
30.  Mimas
31.  Enceladus
32.  Tethys
33.  Dione
34.  Rhea
35.  Titan
36.  Iapetus
37.  Minor Moons of Saturn
38.  Uranus
39.  Miranda
40.  Ariel
41.  Umbriel
42.  Titania
43.  Oberon
44.  Neptune
45.  Triton
46.  The Kuiper Belt & Scattered Disk
47.  Comets
48.  The Interstellar Neighborhood
An artificially-colorized image of Mimas in front of Saturn, approximating what we would see if we were there:


I.  Context

Mimas is the ninth moon of Saturn, but the innermost of the major moons.  Its orbit around the planet has a radius of about half the average distance between Earth and Luna.  Out of all major moons in the solar system, it is the second deepest within its planet's gravity well after Io.  Orbital and gravity well diagrams:



From a large perspective, Mimas is just a tiny speck among several:











As we get closer, Mimas resolves into an appreciable disk:











Against the ring shadows projected on to the blue North, with the ring plane below:


Then it becomes clear enough that we realize with a shock that this little object is an actual world:




















With the ring plane in the background:


Saturn is colossal in the Mimantean sky on the Near Side, covering 38.5° of arc - and that's just the planet, not including the rings.  This is 71 times larger than the full Moon as seen from Earth, and almost twice the apparent size of Jupiter as seen from Io.  Let's imagine what this would look like by superimposing Saturn into the black sky of an Apollo image from the lunar surface - the ring plane would look thinner than this from Mimas, but this gives a general idea of the scale involved:

Imagining Saturn in the Mimantean Sky

The Sun, however, is small from everywhere in the Saturn system, so if we were to use the same image to show the Sun, it would look something like this (ignoring shadows):

Imagining the Sun in the Lunar Sky As Seen from Mimas

The Sun illustration works just as well for any moon of Saturn except Titan, whose thick, hazy atmosphere probably diffuses sunlight too much to actually see the Sun.


II.  History

At some point in the formative history of Saturn, icy material just outside of the planet's Roche limit (see the discussion on this in the Rings of Saturn entry) was able to coalesce into a small iceball moon without being torn apart from tidal forces.  However, the tidal forces were and continue to be very strong, and as a result Mimas formed as visibly somewhat egg-shaped (noticeable from some perspectives) due to water ice being more deformable than rock.  

Then, at an unknown time in the past, a giant impact occurred on Mimas and formed its distinctive crater, Herschel.  An impact energetic enough to create such a large crater would have very nearly destroyed the moon, and only the relative flexibility and thermal absorptive properties of water allowed it to remain a single object rather than being shattered to pieces.  As it was, the impact was so traumatic that shockwaves passed completely around it and converged on the other side, causing surface deformations on the opposite side of Mimas from Herschel.  

Its surface has before and since then been pockmarked with numerous, probably ancient craters, but nothing else even approaching the scale of Herschel has happened - fortunately for Mimas.  Its life in recent eons has probably been uneventful, making it something of a museum piece, although the very topmost surface layer continues to absorb icy material from its surroundings and emit trace amounts of hydrogen and oxygen due to solar radiation acting on the ice: A process called sputtering that is common to icy bodies in vacuum.


III.  Properties

1.  Orbital and Rotational Features

The Mimantean month is 22 hours, 36 minutes, which is also its day because it's tidally locked to Saturn - i.e., always presents the same face to the planet, and thus only rotates once for every orbit it completes.  Herschel crater is always on the leading face of Mimas that points in the direction of motion, which probably played a significant role in the energy of the impact that created it since Mimas' orbital speed is nearly 52,000 km/h.  Mimas casts a small but noticeable shadow on Saturn:


Because Saturn covers such a huge angle of sky from Mimas, at times of the planet's year closer to equinox every orbit will involve an extended solar eclipse when Mimas enters Saturn's shadow.  As a result, a significant fraction of the Mimantean day during these seasons is spent with the entire surface in darkness.  However, the rest of the year the orbital plane is facing the Sun and Saturn's shadow doesn't extend beyond the ring plane.

Since all the major moons except Iapetus have only small orbital inclinations, visually compelling conjunctions often occur.  Even Titan can be seen in association with the other moons despite being substantially farther out than five of the seven because it's comparatively huge.  From left to right, Mimas, Dione, and Rhea:


Rhea and Mimas:


Mimas and Rhea:


Mimas and Dione:


Titan way in the background with Mimas on the right:



Mimas passes in front of Dione:


Mimas passes in front of Rhea:


Mimas (top) and Dione (bottom):


Mimas and minor moon Prometheus:


Smorgasbord of moons - Mimas is second from the bottom, peeking out from behind Rhea:


A few videos showing Mimas in motion - Mimas passes in front of Rhea:

Mimas and minor moon Pandora zip by outside the ring plane:

Enceladus and Mimas (the smaller two) moving against Rhea:

Another important feature of Mimas' motion is that its gravitational influence causes an orbital resonance with a significant region of the ring plain between the A and B ring, and as a result tends to clear it of material.  This relatively sparse region is called the Cassini Division, and was the first gap in the rings spotted by early astronomers.  However, don't get the mistaken impression that Mimas is in the Cassini Division - it's well outside the visible ring system.  It just exerts a gravitational "tug" on the region in such a way that material that ends up there will experience forces with just the right direction and timing to "slingshot" them out of the rings.  The darkest, empties regions of the Division are those where the resonance is strongest.  

Below is an image of the Cassini Division.  Coincidentally, the thin black linear feature extending across the rings is the shadow of Mimas, which is outside the frame:


2.  Size and Mass Characteristics

Mimas is the smallest of the major moons of Saturn, but the seventh largest of Saturn's moons in general.  Its average diameter (396 km) is a little bigger than Ohio, and its total surface area (about 490,000 km2) is comparable to California.  However, because of the strength of tides from Saturn, its shape is very oblate (egg-shaped), with its radial diameter (the one pointing toward Saturn) being 9% longer than the diameter tangent to its orbit.  An image showing the oblateness of Mimas:


The mass of Mimas is 3.75 x 1019 kg, or about 0.00063% of Earth, which is on the same order as the mass of Saturn's rings.  In other words, if the ring material were coalesced into a single object, it would be roughly like Mimas.  Due both to its low mass and the low density of water - especially in ice form - the surface gravity on Mimas is very low, at roughly 0.0065 g.  If you weigh 150 lbs on Earth, you would weigh less than 1 lb on Mimas, and it would take nearly 6 seconds to fall 1 meter.  So if you were in a base on Mimas, you would basically be ping-ponging yourself off the floors, walls, and ceilings to get places rather than walking, but would still have the benefit of a little gravity to stabilize yourself and have a sense of Up and Down.

Rough size comparisons with comparable objects - mouse over to see the name of the image if you don't recognize something:






















3.  Temperatures

For an airless moon with no geological activity, one would assume the daylit hemisphere would be considerably warmer than the side in shadow, but this isn't necessarily what happens on Mimas: The biggest temperature differences occur between the leading and trailing hemispheres - i.e., the side facing forward along its orbit vs. the side facing backward.  Even when the leading hemisphere is in full daylight and the trailing hemisphere in night, the latter is much warmer - by tens of kelvins / Celsius degrees.  Temperature map:

Temperature Map

Researchers think the discrepancy is a result of the surface layer of ice on the leading hemisphere being compacted by continuous particle bombardment, making it less thermally conductive and thus both slower to heat in daylight and slower to cool in shadow than the trailing hemisphere.  Testing this hypothesis would be pretty straightforward: All they have to do is take the same kind of temperature map when the leading hemisphere is in night and see if it cools down more slowly than the trailing hemisphere did in the previous cycle.  I don't know when the next Cassini encounter with Mimas will be, so it might be a while before the issue is definitively settled.

But one thing is obvious from the temperature scale above: Mimas is cold.  Which is to say it's typical of the system, and one of the many frigid wonders that makes it such a saturnine place.  The radically low temperatures may some day prove useful for human purposes, as I explain later.

4.  Internal Structure

Tidal forces acting on Mimas should theoretically heat its interior more greatly than Enceladus, and yet the Mimantean surface is ancient while that of Enceladus is relatively young and geologically active.  As a result, researchers aren't yet confident they know what's going on inside Mimas, if anything, and what explains the discrepancy.  Although the very low density of the moon guarantees a composition overwhelmingly dominated by water - and thus, very likely, a relatively small rocky core - I haven't even been able to find speculation about the internal structure, let alone hypothetical models.  This is probably due both to a paucity of information and much lower interest compared to Enceladus.  Mimas is apparently The Little Engine That Couldn't, and attracts attention mainly for the fact that it's anomalously dead.

5.  Surface Features

Mimas has the rare distinction of approaching the maximum possible density of craters without turning into a fragmentary body like an asteroid or a comet: So dense that new craters simply obliterate old ones, meaning that it's not structurally possible to achieve a greater level of cratering with the same surface materials.  Most other major solid objects in the solar system resist building up this kind of surface, either through dynamic resurfacing processes (like those of Io, Europa, and Enceladus), or by having enough gravity that craters fall apart by themselves or else cause temporary liquid upwellings that smooth over the terrain.  A high-resolution image of the leading hemisphere illustrates "maximum crater density":


The most obvious feature is, of course, Herschel crater: 139 km across, with the crater walls descending 5 km to the edge of a basin that sinks another 5-7 km before the 6-8 km rise of the central peak.  An important thing to notice about craters like this is that they don't appear to be blasted out of a brittle surface, but rather sloshed - like someone dropped a stone into water and then froze it mid-splash.  That is actually, literally what happens with icy body impacts below a certain energy, and the Herschel impactor was right on the border of that limit.  But let's get a broader sense of the terrain before zooming in.  Some global views in phase:









One thing to notice about Mimas from these high-angle lighting conditions is how deep the craters are relative to the size of the moon.  It might be that the exact same impact on a larger moon would leave a crater with the exact same dimensions, but if it occurs on a moon that's twice as large, it will look like a slight dimple in the surface rather than a traumatic gouge.  This makes the Mimantean surface distinctive, and gives the amateur one way to know whether any given image of a heavily-cratered iceball moon is Mimas: Are the craters deep or shallow?

This helps in another way, because its fellow icy moon Tethys not only has a very similar surface, but also has a huge, dominating, mid-latitude impact crater that vaguely resembles the Death Star.  But that crater, Odysseus, is much shallower than Herschel, and thus easy to distinguish.  Another way to tell Mimas apart is the ovoid shape visible from some perspectives, although this can be deceiving due to the play of shadows.  

Below are mosaic images of the polar regions to complete our general survey, North followed by South.  Note that these are not global views from a polar perspective, but only show the highest 45° of latitude:

North Polar

South Polar  

As we zoom in to the lower latitudes, the alien starkness of the Mimantean surface comes into sharp relief:

High-res 10

high-res 11

High-res 9

high-res 13

high-res 14

high-res 12

Zooming in even further...

High-res 5

High-res 1

High-res 7

High-res 3

High-res 4

High-res 6  

Something to notice about Herschel crater is that the interior doesn't have that many craters of its own, meaning that it's younger than the rest of the surface.  However, given how old the rest of Mimas is, that doesn't necessarily mean a lot.  It could be actually young (in geologic time), or it could be old and simply younger than the rest of the moon - the latter being more likely.

There are thus far 42 officially-named features on Mimas, according to its IAU nomenclature page.  With the exception of Herschel, Mimantean feature names are based on people and places from Arthurian legend.  Labeled maps:

Mid-Latitude Map 1

Mid-Latitude Map 2

South Polar Map

The North Pole map doesn't have any labeled features and is much less clear than the mosaic image of the North shown above, so I've left it out.


IV.  Modern Relevance to Humanity

Due to its maintenance of the Cassini Division, Mimas played an indirect role in giving early modern astronomers in the 17th century the first clues that the rings of Saturn are not a solid structure.  However, aside from its role in the handful of scientific mysteries discussed above, Mimas today is not an especially active area of research compared to other bodies in the Saturn system.  However, it is proving useful as a test for theories regarding the internal activity of Enceladus: Any theory that seeks to explain the geyser activity of Enceladus must also explain why Mimas doesn't have any.


V.  Future Relevance to Humanity

Mimas has poor prospects for human colonization due to the very low surface gravity.  Unlike mid-sized asteroids in 1-10 km range, which can be hollowed out and spun up to create artificial gravity, Mimas is far too large and massive an object to do that, so it occupies a gravitational No Man's Land where it's too low for long-term habitation to be healthy and yet occurs in a body too large for engineered solutions.  However, its material abundance and location does make it an attractive resource base in some scenarios, in which case it would become an important industrial location with temporary inhabitants.

Due to its depth in Saturn's gravity, and the equal or greater abundance of materials elsewhere in the system, Mimas would be totally useless for supplying colonies that are located on other moons of Saturn.  However, it would be an ideal source of water, fuel derived from water, and oxygen for cloud cities in the atmosphere of Saturn itself (see this section of Saturn (Vol. 3) for a discussion of the concept).  Diffuse water from the atmosphere itself would probably be sufficient for many generations, but at a certain industrial scale it might be easier to import bulk quantities from orbit.

If that were the case, then you could see large-scale extraction operations on Mimas that feed the needs of civilizations in Saturn's clouds.  If your incredulity is itching at this idea, I should note this is on a timescale probably between 500 to 1,000 years from now, so I'm not imagining any sort of magic unless fusion power counts - and it really doesn't, given the level of credibility and global economic investment behind it.  Anyway, if such operations occurred, there would be a couple of options available that would depend on the economics involved: Either the Saturn-based societies could import blocks of water ice as-is, or - since Saturn's atmosphere is already full of hydrogen - they might find it more efficient to just mine the ice for oxygen and only bring back the O2.

I mentioned earlier that the frigid temperatures of the Saturn system might have benefits for human activity, and this is what I meant: You would only have to reduce the temperature of oxygen by a few tens of kelvins / Celsius degrees from the prevailing temperatures of Mimas to make it liquid (oxygen under pressure is liquid at 50.5 K, just 25-40 K lower than you get in the natural environment on Mimas), and thus both producing and storing it safely would be a lot easier than in terrestrial conditions.  You have to input energy to split H2O, and hence need whatever kind of nuclear technology is found to be most convenient out there - fission, fusion, RTG, what have you - but you can use the resulting gases as a fuel and recoup some of the energy input by recombining them in a rocket engine or other application.

Or, since hydrogen is ubiquitous in Saturn's atmosphere, just bring the oxygen and - very carefully - recombine it with ambient air for water and power.  This isn't a specific prediction, mind you: Just a possibility.  Any number of economic, technological, or even political reasons could make the scenario moot, so in making an actual prediction I'll say this: Either Mimas would be an industrial base for supplying water/oxygen to Saturn, or it would just be a tourist spot to bob around in the low gravity gawking at the awesomeness of Saturn dominating the Near Side sky.  Those are the only inherent economic uses of Mimas, as far as I know.  

Surely no one could possibly have the godlike sense of humor to turn Mimas into a military platform - a "fully armed and operational battle station"?  Most likely not, since it would be just way easier to turn much smaller, asteroidal objects into navigable installations that could be moved to arbitrary orbits.  On the other hand, a couple hundred kilometers of ice make for a pretty strong bunker and radiation/particle shield.

Of course, nothing says these possibilities are mutually exclusive: In fact, if Mimas became a crucial industrial base supplying the water, fuel, and oxygen for billions of people, that would pretty much guarantee its militarization.  However, there would be economic tension between the military/industrial applications and tourism, so I'll hazard a guess that the Near Side (the one with Saturn in the sky) would largely be for tourism and the Far Side would become industrial.  I realize I'm piling supposition on supposition here, but it makes good enough sense: If we posit that people end up colonizing Saturn, then it's a natural conclusion that Mimas would come to play a major economic role.  

VI.  Future of Mimas

Assuming humans don't strip-mine and/or belligerently blow it up - one of which we probably will - Mimas has at least a billion years of history ahead of it, after which the expansion of the Sun would start to strip away its ice, leaving behind just some pitiful little asteroid that had once been its rocky core.  At some point, if not well before then, its orbit would decay due to the greater amount of material it encounters along its orbit and it would spiral in to Saturn.


VII.  Catalog of Exploration

1.  Past & current probes:

Pioneer 11 (USA - 1979 flyby)
Voyager 1 (USA - 1980 flyby)
Voyager 2 (USA - 1981 flyby)
Cassini-Huygens (USA and Europe - entered Saturn orbit 2004, currently operating)

2.  Future probes:

(none planned)

Originally posted to Troubadour on Sat Apr 27, 2013 at 06:28 AM PDT.

Also republished by SciTech and Astro Kos.

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