Definately needs improvement...
1: The land... Its not generated. Solely using the elevation tool wont get you the results you want. Theres also not enough of it.
2: The lighting... Its too dark, and mars isnt quite that red...its more of a rust.
3: Clouds... Too fuzzy, and look as though theyve been 'painted on.'
4: JPEG... Quality is quite low. MSpaint cannot improve the quality of JPEG's beyond its preset value. Use a different prog (Or if you arent using paint...Change the qulaity.)
Try some of the links at Terragens home page: http://planetside.co.uk especially Luc Bianco's page to see what can be accomplished when all goes well. Also take a look at Xerraire's page at http://www.Xerraire.com
I also have a few here at WC. Search the wallpaper library for: EventHorizon.