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Juror Number 8
2004-Jan-06, 09:33 PM
Evening all,

First post from a beginner, so please be gentle 8-[ .

I've just read Philip Plait's book and found it fascinating. Even managed to (just about) understand why the sky is blue!

Anyway, when the colour pictures of Mars came in I noticed the red sky. Can anyone gently explain why the same principles don't apply on Mars as they do on Earth with respect to the colour of the sky. I've had a good old beginner's think about it and can only think:

- Mars may have an almost constant cloud cover which may glow red if reflected off the surface. Although surely sufficient light wouldn't penetrate the cloud cover to turn the whole sky red?

- The Martian atmosphere is thicker than the Earth's, giving rise to a much greater dispersement of 'red' light photons. Similar perhaps to the 'red sky at night' effect on Earth as the Sun's light has to fight its way through a thicker atmosphere?

Or am I miles out? A bit like Beagle 2?

Glom
2004-Jan-06, 09:37 PM
Welcome.

Mars's atmosphere is thinner than Earths, but it's filled with dust. That dust is rust like the surface and so turns the sky red.

tofu
2004-Jan-06, 09:52 PM
yep, it's fine dust in the atmosphere. Take a look at some of the pictures on this page. They were taken during the 2003 wild fires in califorina. A lot of them show an orange sky similar to what you see on mars.

http://fire.textamerica.com/

Juror Number 8
2004-Jan-06, 10:02 PM
#-o Thanks.

Sorry to be a pain, but its horrible being in the position when you're interested in something but are absolutely clueless (but if you ever want to know anything about the films of Akira Kurosawa or about Hull City Football Club, please feel free to drop me a line!)

Firstly, would it be possible for winds on Mars to blow this dust away revelaing a patch of bright blue sky, as it would make a corking photo! Secondly, if the planet's atmosphere is covered in red dust, how does this affect observsations of the Martian surface?

Again, thanks!

AGN Fuel
2004-Jan-07, 12:05 AM
#-o Thanks.

Firstly, would it be possible for winds on Mars to blow this dust away revelaing a patch of bright blue sky, as it would make a corking photo! Secondly, if the planet's atmosphere is covered in red dust, how does this affect observsations of the Martian surface?

Again, thanks!

No, 'fraid not. The scattering of blue light that gives our atmosphere it's distinctive colour is primarily due to nitrogen & to a lesser extent oxygen molecules. The atmosphere of Mars (which is at some 1/100th the pressure) is primarily Carbon Dioxide, which is not as efficient at scattering blue light. IIRC, the true colour of the Mars sky is sort of salmon-y.

George
2004-Jan-07, 01:17 AM
#-o Thanks....
Firstly, would it be possible for winds on Mars to blow this dust away revelaing a patch of bright blue sky, as it would make a corking photo! Secondly, if the planet's atmosphere is covered in red dust, how does this affect observsations of the Martian surface?

Again, thanks!

As I understand, light scattering is very wavelength dependent (1/ ^4). So, blue light, at close to the wavelength of red, will scatter 16 times better than red light.

Also, the molecule sizes play a key role, apparently, as well.


Rayleigh scattering theory is applicable to scattering of UV and visible radiation by air molecules, infra-red radiation by small aerosols, and microwave radiation by cloud and rain drops.
From >>> here (http://www.ess.uci.edu/~cmclinden/link/xx/node20.html) <<<.

As AGN Fuel has pointed out, the molecules are larger. Rayleigh scattering, to a much lesser extent, may be playing a roll in the atmosphere of Mars but in the red portion.

The dust is likely the bigger factor. Dust storms last for months there as upper winds can be over 200 mph and surface winds of 80 mph I believe are not too uncommon. Although a 100 mph wind there is about equal in force to a 20 or 25mph wind here due to the air density difference.

I am sure the weaker gravity plays a role in the length of time the dust stays air-born, too.


... IIRC, the true colour of the Mars sky is sort of salmon-y.

I'm only slightly glad you did not say peachy pink! :roll: [Sun's supposed true color based on computer modeling - [-X ] :)

Senor Molinero
2004-Jan-07, 01:18 AM
Here's an off-the-wall prediction from around 1965.
I recall an episode of "My Favorite Martian" in which Uncle Martin is telling Tim what it is like on Mars. He says, in his description "...the pink sky..."
Not bad for a throw-away sit-com.

George
2004-Jan-07, 01:24 AM
It would be nice to see some true color images from Mars if anyone has them.

I found only this one from Surveyor. >>> here (http://themis.asu.edu/zoom-20031114a.html)<<<.

Not quite as pink or red as you would guess.

Another point might be made regarding the imaging equipment. I understand that most will favor red over other colors. Is this correct?


[Edit. Hmmm-why did I think it was Surveyor?]

Glom
2004-Jan-22, 06:42 PM
Please move me.