Help Shape the Future of Space Exploration

Join The Planetary Society Now  arrow.png

Join our eNewsletter for updates & action alerts

    Please leave this field empty

Headshot of Emily Lakdawalla

Mimas and the F ring

Posted by Emily Lakdawalla

02-01-2008 12:37 CST


This Cassini photo, released today, of the F ring crossing Mimas is from the "just plain cool" department. The F ring is a narrow, dim, dusty, braided thing that orbits Saturn just outside the main A and B rings; the main rings, and Saturn, would be off the screen to the left of this photo.

Mimas and the F ring


Mimas and the F ring
In this unusual perspective from Cassini, the F ring crosses in front of Saturn's moon Mimas. The two are actually not close to each other in space; Mimas orbits Saturn more than 40,000 kilometers (25,000 miles) beyond the center of the F ring. The view is from the sunlit side of the rings, so the dusty F ring appears dark where it crosses the trailing hemisphere of Mimas. Cassini was 772,000 kilometers (480,000 miles) from Mimas when this image was taken on November 18, 2007.
I think what I like about this image is how it plays with your perceptions of what "bright" and "dark" mean in an image. If you look at the F ring against the blackness of space, its core looks "bright." But if you follow the core across the face of Mimas, which, like most of Saturn's moons, has a surface made primarily of water ice, the F ring core looks "dark." This is an optical illusion; our eyes (or more accurately, our brains) measure contrast in a relative sense, looking out at the world, comparing one thing to another, and determining which things are brighter than other things. Seen against space, the F ring is the brightest thing around, so it looks bright, but where it crosses Mimas, the moon is much much brighter than the F ring, so the F ring looks dark.

I checked the actual brightness of the pixels in Photoshop, to see if it was true that the core of the F ring has the same brightness all the way around, and I got a surprise. Where the F ring crosses space, the pixels have values of around 100 out of 255, which is fairly dark. But where the F ring crosses Mimas, it's actually brighter -- not darker, as my brain is trying to convince me -- with a value of about 120 out of 255; there must be some light from Mimas leaking through the dusty ring, or maybe the image compression bled some light from adjacent areas into the narrow stripe of the F ring. Shows you what my brain knows.

See other posts from January 2008


Or read more blog entries about:


Leave a Comment:

You must be logged in to submit a comment. Log in now.
Facebook Twitter Email RSS AddThis

Blog Search

Planetary Defense

An asteroid or comet headed for Earth is the only large-scale natural disaster we can prevent. Working together to fund our Shoemaker NEO Grants for astronomers, we can help save the world.


Featured Images

Ice Flows and Dunes in Mars' Northern Polar Region
The TRAPPIST-1 system: Where might liquid water exist?
The TRAPPIST-1 system
Saturn in infrared
More Images

Featured Video

Intro Astronomy 2017. Class 5: Venus & Mars

Watch Now

Space in Images

Pretty pictures and
awe-inspiring science.

See More

Join The Planetary Society

Let’s explore the cosmos together!

Become a Member

Connect With Us

Facebook, Twitter, YouTube and more…
Continue the conversation with our online community!