Mars could've looked like Earth 2 billion years ago, wetter and warmer

Our red neighbor planet Mars most probably had an atmosphere, rivers and water 2 billion years ago, according to very recent discoveries thanks to the Mars Exploration Rovers like Opportunity and Curiosity, and orbiting satellites like the European Mars Express. Just last week, a study of the Martian meteorite NWA 7034 revealed that it surprisingly contained between 10 and 30 times more water than other Martian meteorites.
The Red Planet may have even supported life. But even if not, Mars could have looked quite similar to Earth.

A beautiful thought, which became artistic reality thanks to Kevin Gill, a software engineer who used real elevation data to plot out Mars' oceans, mountains, valleys, and other geological features - plus using his own judgment to paint in the deserts, forests and other vegetation.
Kevin Gill's beautiful attempt to picture a billion year old version of Mars are thus somewhere between an artist's concept and a scientific approximation. In his corresponding post on Google+ he explains:

 "There is no scientific reasoning behind how I painted it; I tried to envision how the land would appear given certain features or the effects of likely atmospheric climate. For example, I didn't see much green taking hold within the area of Olympus Mons and the surrounding volcanoes, both due to the volcanic activity and the proximity to the equator (thus a more tropical climate). For these desert-like areas I mostly used textures taken from the Sahara in Africa and some of Australia. Likewise, as the terrain gets higher or lower in latitude I added darker flora along with tundra and glacial ice. These northern and southern areas' textures are largely taken from around northern Russia. Tropical and subtropical greens were based on the rainforests of South America and Africa."

To be sure, Gill said that "this wasn't intended as an exhaustive scientific scenario" - but nonetheless hopes some of his assumptions will prove to be true.