APOD: The First Color Panorama from Mars... (2012 Aug 11)

[Markus Schwarz]

Hello,

coming back to the "you landed on Mars and opened your eyes" part, the following question arose during lunch with my colleagues: why does it look so bright on Mars? From the picture I get the feeling that it is as bright on Mars as on a normal day on Earth. From Wikipedia I got that Mars is about 50% farther away from the sun than Earth (1.5 AU compared to 1AU). So I would expect it to receive only 44% of the sunlight intensity. Would this compare to dawn or dusk or a cloudy day on Earth? Maybe our eyes can easily adopt to such a reduced intensity? Or did they use a longer exposure time to make the picture as bright?

Thanks for your help!

[Chris Peterson]

coming back to the "you landed on Mars and opened your eyes" part, the following question arose during lunch with my colleagues: why does it look so bright on Mars? From the picture I get the feeling that it is as bright on Mars as on a normal day on Earth. From Wikipedia I got that Mars is about 50% farther away from the sun than Earth (1.5 AU compared to 1AU). So I would expect it to receive only 44% of the sunlight intensity. Would this compare to dawn or dusk or a cloudy day on Earth? Maybe our eyes can easily adopt to such a reduced intensity? Or did they use a longer exposure time to make the picture as bright?

44% of the intensity is still very bright. The amount of light hitting the ground on Earth varies that much just with minor weather and even seasonal changes. If you were actually on Mars, I doubt you would notice the Sun was less bright than on Earth, unless you had some sort of reference to compare it to.

Aside from that, however, imagers use a sort of zone system. At the simplest, that means ignoring the actual intensity value of a pixel, and setting the light and dark points to correspond to white and black. That effectively normalizes the intensity of the image. Even the pictures we get back from Pluto in a few years will range from black to white, and appear to be as bright as a terrestrial image.

Instrumentally, you would take a longer exposure on Mars than on Earth to get the same signal on the sensor. But the difference is only about one f-stop.