A super bright Io photo..why?

[Doodles23 (Dave Bleser)]

As Io approached the edge of the planet on its transit last week I did not get the orange color of the moon against the planet as Mike Salway did. Instead I got a super bright Io as you can see in the photo at

http://gallery.mac.com/davidbleser#1...&bgcolor=black

A Red channel photo is attached for comparison.

[iceman (Mike)]

I had that same effect when Io was right near the limb. Mine wasn't quite as close to the limb as yours but I noticed the same thing.

I'm sure it's something to do with the contrast - when it's on the planet the contrast is low, but when it's near the limb the contrast is higher.

I don't think it's entirely a processing artifact though, because I saw it appear brighter during the raw capture run.

Good question!

[Zac Pujic]

The moons are more easily visible near the limb (while still against the planet) because of the limb darkening that Jupiter shows. The limb darkening is caused by light absorption by higher levels of the Jovian atmosphere (troposphere etc). At the limb, the light reflected from Jupiter has to go through a longer distance and is therefore more highly absorbed, leading to a dark limb. Io will look brighter in the red channel since it is mostly red anyway. If you look in the blue channel, it will be much dimmer. Have I confused anyone

zac

[Paul Haese]

Yep fairly normal. I have numerous examples of this occurring and of course have numerous examples where colour is exhibited. Zacs explanation is spot on. Conditions and settings have to be right for colour showing on the moons.