A crater that dominates a moon

Herschel · close crater mosaic — NASA/JPL/Space Science Institute
Processed image. Eight Cassini photographs from 13 February 2010 form this orthographic mosaic: seven narrow-angle images plus a wide-angle fill at lower right. The camera views were acquired around 30,000 kilometres from Mimas, despite the encounter’s closer minimum distance. Native sampling is about 180 metres per pixel. Bright crater walls contrast with darker floors. Downslope sorting and impact melt are interpretations of the photographed textures, not processes filmed here.
NASA factual/editorial media guidelines · credited mission imagery, resized to WebP; no endorsement implied
· Original image source · Image licence

A dramatic circular scar belongs to a much broader battered surface.

World: Mimas
Case date or period: Camera observations · 2009–2016

Look closer

Mimas’s large Herschel crater is instantly recognisable, but the surrounding smaller craters offer their own intricate landscape. Cassini’s views and global map connect the landmark to the rest of the moon.

Follow the evidence

The cylindrical map is a projection assembled from observations, so polar regions stretch. Use it to locate terrain while returning to the camera views for an intuitive sense of shape.

Using a crater to test an interior

C. A. Denton and Alyssa Rhoden modelled Herschel’s formation in 2022 using iSALE-2D, varying the thickness of ice above a hypothetical ocean. Their assumed vertical impact at 15 kilometres per second breached thin shells; a shell at least about 55 kilometres thick retained a basin resembling Herschel. Compared with contemporary ocean-model thickness estimates, that suggests subsequent thinning if Mimas has an ocean. The threshold depends on the adopted impact and thermal parameters; it is a simulation constraint, not a measured ancient ice thickness.

The peak is not an ocean detector

Denton and Rhoden cautioned that a central peak cannot generally identify an ocean. Their inference depends on Mimas’s gravity, tidal heating and ice-shell structure, and on how impact models treat warm and cold ice. They published model output separately for scrutiny. The camera records the basin’s shape; the proposed interior history comes from testing models against it.

Change the viewpoint before changing the explanation

Start with the close Herschel mosaic, then compare the later limb and ultraviolet views. Their different lighting, wavelengths and camera geometry change the appearance of the same basin; they are not before-and-after photographs of an impact. Follow the bright rim, central peak and crater-floor textures, then use the cylindrical map only as a locator. The image can constrain a simulated basin shape, but a proposed ancient ice thickness or buried ocean remains a model inference.