I'm doing something on Mars too!
I think you should work more on the volume of your vehicles/units. They're too flat now. I made a small edit, to show you:
The turret hasen't really got a defined shape at the moment, so to it's right you will see the basic shape, without the detailing.
Is it a half dome shape, or something with sharp corners? That's up to you, but you should make that more clear with the right shading.
And I also put your 'truck' against a martian colored environment. That's what I learned here
See how the orange color of the vehicle doesn't make it stand out against the background? Something to keep an eye on. (Hell, for all I know your Mars could be covered with pink and purple plants
)
Hope this helps a bit and good luck!
-fellow martian freak-