Celestia Forums

Hopefully, this link will take you to some Mimas images that were acquired by Cassini in the past couple of days... there are some Enceladus images at the top, the Mimas ones are at the bottom. The Mimas ones start at:
http://saturn1.jpl.nasa.gov/multimedia/ ... geID=29856

and end at:
http://saturn1.jpl.nasa.gov/multimedia/ ... geID=29871

Three things stand out - first, Herschel crater. It's smack bang on the day/night terminator, 10 km deep with a 6km high central peak that is still partially illuminated by the sun.

Second, the distinctly non-spherical shape of Mimas - it's quite obviously stretched out (by tides, presumably?)

Third, if you stretch the contrast on many of those images, you can actually see a dimly-lit crescent on the dark side that is illuminated by saturn-shine - sunlight reflecting off Saturn.

All those craters on Mimas make it kinda reminiscent of a big golf ball...

Mimas does look curiously egg-shaped in this view

http://saturn1.jpl.nasa.gov/multimedia/ ... geID=29962

Is it exaggerated somehow? Like an xy axis jpeg failure? I does have an odd shape...

lostfisherman wrote:Mimas does look curiously egg-shaped in this view

http://saturn1.jpl.nasa.gov/multimedia/ ... geID=29962

Is it exaggerated somehow? Like an xy axis jpeg failure? I does have an odd shape...

Nope, it's egg-shaped alright. Though this was known beforehand - the NSSDC data sheets claim that Mimas' radius is 209 x 196 x 191 km, so the shortest axis is about 91% of the longest axis. I'd just never noticed its non-sphericity in the Voyager images.

It's such a low mass moon anyway, and is composed of much more ice than rock if I remember right. I'm not sure of its exact mass, but it is right around the point of being forced into a spherical shape, and yet can have these large bumpy features, correct?


...John...

Yes, but if you look at Mimas, it isn't exactly that irregular in shape, it sort of looks like the missing link between irregularly and spherically shaped objects.

There are bigger not quite spherical objects though, such as Proteus and Ceres.

Michael Kilderry

Evil Dr Ganymede wrote:the NSSDC data sheets claim that Mimas' radius is 209 x 196 x 191 km, so the shortest axis is about 91% of the longest axis.

Does it mean we can add the line :

Oblateness 0.09

for mimas in the solarsys.ssc file ?

@+
Vincent

That egg-shape is pretty much far enough away from a typical spherical moon to qualify as irregular in my book. At any rate, all of the bodies you listed are indeed irregular to some degree. In Earth units, the mass cut-off for an object being spherical is 0.01 -- give or take a bit.

Maybe there is enough tidal flexing going on in this system to work some effects on even the outwardly appearing inert bodies?

...John...

Michael Kilderry wrote:Yes, but if you look at Mimas, it isn't exactly that irregular in shape, it sort of looks like the missing link between irregularly and spherically shaped objects.

There are bigger not quite spherical objects though, such as Proteus and Ceres.

Michael Kilderry

Vincent wrote:Does it mean we can add the line :

Oblateness 0.09

for mimas in the solarsys.ssc file ?

That wouldn't be quite right, since Mimas isn't symmetric about its rotation axis, and is closer to a prolate spheroid than an oblate one. Giving it a negative oblateness should elongate it north-south, but unfortunately, as far as I know, there's currently no way in the .ssc file to make a prolate ellipsoid whose long axis is perpendicular to the rotation axis, which is what we'd need here.

I suppose that if precession is modeled, you could do it by giving the axis a tilt of 90 degrees, turning off rotation entirely, and setting the precession period equal to Mimas's orbital period, but that seems like a bad kludge (it would also need a weirdly transformed texture map).

Since Mimas's topography is so dramatic, I suppose one might as well go the whole hog and use a 3D mesh, if one is available.

Matt McIrvin wrote:That wouldn't be quite right, since Mimas isn't symmetric about its rotation axis, and is closer to a prolate spheroid than an oblate one. Giving it a negative oblateness should elongate it north-south, but unfortunately, as far as I know, there's currently no way in the .ssc file to make a prolate ellipsoid whose long axis is perpendicular to the rotation axis, which is what we'd need here.

I suppose that if precession is modeled, you could do it by giving the axis a tilt of 90 degrees, turning off rotation entirely, and setting the precession period equal to Mimas's orbital period, but that seems like a bad kludge (it would also need a weirdly transformed texture map).

That's exactly what I did for my only ever add-on: Fudge! See topic Add-on Announcement: Fudge!.

I had found a way to get the .ssc's to produce tide-locked prolate bodies*, and applied it to both contact binaries stars and double planets. Yet, the responses were so unappreciative that I gave up (since it also degenerated into a discussion about ASCII and Unicode instead).

[Calvin-mode]No one understands the brilliance of my accomplishments![/Calvin-mode] (Hobbes rolls eyes...)

* Point of astronomy. The shapes of these bodies are actually defined by Roche lobes, which are more egg-shaped with the nearer end being more narrow and pointy. For bodies that are small compared to their distance from the other body, the Roche lobe tends more to a prolate ellipsoid. My Fudge example's use of prolate ellipsoids is quite an inaccurate substitute for Roche lobes for almost contacting bodies, but for the Mimas' case it's probably very accurate.

Matt McIrvin wrote:Giving it a negative oblateness should elongate it north-south, but unfortunately, as far as I know, there's currently no way in the .ssc file to make a prolate ellipsoid whose long axis is perpendicular to the rotation axis, which is what we'd need here.

Yes, I've tried with "Oblateness -0.09" but as you say, there's no way to elongate it in 'the 3rd dimension' ...

Matt McIrvin wrote:Since Mimas's topography is so dramatic, I suppose one might as well go the whole hog and use a 3D mesh, if one is available.

I'll make one with anim8or..

@+
Vincent

The Raw Images page is back on the Cassini Website now at:
http://saturn.jpl.nasa.gov/multimedia/i ... /index.cfm

This piccie is one of a sequence that looks really awesome though. It's of Mimas crossing Saturn's disk. You can really appreciate how big Saturn is when you consider that it's looming over Mimas in these views but it's still 185,000 km behind it...