[amalia (OP)]

We received this question through our mailbox, and we need your help:
I understand that daytime color of Mars is red and sunrise and sunsets are blue. Also Mars atmosphere is filled with iron oxide dust which is an efficient absorber of blue light scattering red light making day time red. How can sunset be blue when all the blue is absorbed by the iron oxide dust?
Do you know the answer?

[Janus]

Well, quite obviously, the dust doesn't absorb all the blue light. Mars atmosphere is very thin compared to our own, and there just isn't enough dust in it to absorb all the blue in the sunlight. 

During the day on Mars, the dust scatters the red light and lets the blue through. The scattered red makes the sky look pink.  On the Earth the thicker air scatters the blue, making the sky look blue.
On Mars during sunset, the light has to pass through more atmosphere, which scatters out more red light, filtering it out from the light coming at you from the direction of the Sun.  Thus looking in the direction of the sunset, you see the blue light that isn't scattered.

[Petrochemicals]

I have never heard of this. Probably due to the lack of water and predominantly carbon dioxide atmosphere, water and co2 absorb infra red but are not so good at ultra violet, thus through a longer stretch of atmosphere blue is predominant as a guess

Edit:

For clarity on the above answer, mars has a predominantly co2 atnosphere(little water), so a longer stretch through the atmosphere at evening mean that the blue hues pass through and the red stopped.

[Bored chemist]

the dust scatters the red light and lets the blue through.

Why/ how?

[evan_au]

On the Earth the thicker air scatters the blue, making the sky look blue.

To add to this, on Earth, the dominant form of scattering on a clear day is Rayleigh scattering.
- This is due to microscopic variations in the density of Earth's atmosphere, on a scale smaller than the wavelength of light.
- This effect varies as (wavelength)⁴, so affects blue light about 16 times more strongly than red light.

Most of the scattering in the atmosphere of Mars is from fine dust particles, which are larger than the wavelength of light, so the behavior is very different from Rayleigh scattering.
See: https://en.wikipedia.org/wiki/Scattering#Electromagnetic_scattering

[alancalverd]

Rayleigh scatter actually varies with λ^-4, which is a bit more logical!

This means that during the day our sky is blue "off axis". The red component of sunlight is less scattered and mostly arrives "on axis". At sunset the longer oblique track of the sun's rays through a dusty atmosphere means that the on-axis beam is attenuated by dust* and water with the blue component more strongly scattered away from the axis, so we can look more or less directly at the solar disc with the immediately surrounding sky being devoid of blue.

During the Martian day the negligible particle content of a very thin and waterless atmosphere produces very little attenuation of the primary beam: white sun in a black sky. As the oblique track lengthens at sunset, we see more Rayleigh scatter - the optics begins to look more like a terrestrial atmosphere with an overhead sun. 

*which is why harvest sunsets can be spectacular.

[Bored chemist]

Most of the scattering in the atmosphere of Mars is from fine dust particles, which are larger than the wavelength of light, so the behavior is very different from Rayleigh scattering.

Indeed, and the wiki page says
"Both Mie and Rayleigh scattering are considered elastic scattering processes, in which the energy (and thus wavelength and frequency) of the light is not substantially changed."

So, why the blue colour?
The dust is reddish, it should preferentially absorb blue light.
The scattering is not  colour selective.

I wasn't kidding when I asked this

the dust scatters the red light and lets the blue through.

Why/ how?

[alancalverd]

The dust is pretty irrelevant. The tenuous atmosphere  (1% of earth pressure) cannot support much dust against 0.38 g of gravitational acceleration.

Rayleigh scattering is predominantly from molecules, not mesoscopic particles.

[Bored chemist]

https://www.nasa.gov/feature/goddard/the-fact-and-fiction-of-martian-dust-storms
Given that the dust can blot out enough of Mars that you can see the dust from Earth (with a telescope) I think it's fair to say it would be enough to alter the look of the Sun.
The images taken on Mars make it clear that, no matter what Alan thinks, the dust is significant.
https://earthsky.org/space/understanding-mars-dust-storms

[alancalverd]

Martian dust storms occur about once in 5 days. They are visible and remarkable, as are small whirlwinds, because the atmosphere is usually clear.

NASA first saw a global dust storm up close in 1971 when our Mariner 9 spacecraft — the first to orbit another planet — arrived at a dust-engulfed red planet. Since then, we’ve seen global storms in 1977 (twice), 1982, 1994, 2001, 2007 and 2018.

ibid

[Bored chemist]

So, which are you going with?
Is it irrelevant or not?

[alancalverd]

To the extent that large dust storms are rare (7 in 40 years) and local storms and twisters are visible (i.e. not obscured by persistent thick dust), and the atmosphere doesn't have enough water or solar input to sustain significant daytime thermal vortices, and the atmosphere is thin and gravity is significant, I  would tend to the view that the martian sunset is not always dominated by dust. This leads to the prediction of a blue sunset, which we are told is the norm, without recourse to unknown physics or magic.     

Searching for pictures of "sun seen from mars" produced some low-sun images as I predicted, with predominant blue sky close to the solar disc.

Given that the surface is red,  the light reflected upward from the surface will have very little blue content, so the Rayleigh backscatter  with the sun overhead will be reddish, with or without atmospheric dust.

[chris]

This question - what are sunsets like on Mars - was covered recently on the Naked Scientists Podcast; that answer will prove informative.

[Bored chemist]

This leads to the prediction of a blue sunset,

How?

I keep asking that, and nobody has actually answered.

Incidentally, people on earth have to clean up dust in their houses even in places like England where dust storms are practically unheard of.

You don't need a storm to have dust.
The fact that they sometimes have storms proves that your contention

The tenuous atmosphere  (1% of earth pressure) cannot support much dust against 0.38 g of gravitational acceleration.

is wrong.
It can support so much dust that you can see it from here.

It doesn't have to do it every day to show that you are wrong.
It only has to do it once.