Yes its true. It hasent been proven yet but theoretically Mars should have rainbows.
The planet that has practicly no water in the air has ranbows on its surface...and morover...they are propably everywhere and everytime. How's that possible?
The answer is: Dust.
There are few places on earth where you can see rainbow on a desert.
Instead of water droplets in the air, there are dust particles from sandstone that had been eroded for many milenia.
Such long erosion gives them almost ideal spherical shape, and thus - they work just like water droplets.
There is sandstone on mars and erosion, so there should be rainbows also.
But becouse Mars is one big desert, and the erosion has been taking place for over several houndrets of milions of years,
the rainbows should be even more common than on Earth.
It is possible that the first thing that the astronauts to mars will see will be a rainbow made out of settling dust that had been raised by ERV's rocket engines...
Preety neet huh?