Photo (left): Gills' depiction of the western hemisphere of Mars.
Photo (right): Gills' depiction of the eastern hemisphere of Mars.
According to very recent discoveries from the Mars Exploration Rovers like Opportunity and Curiosity and orbiting satellites like European Mars Express, Mars likely had an atmosphere, rivers and water 2 billion years ago.
Mars could have even supported life, but even if it did not, Mars may have looked quite similar to Earth.
Kevin Gill, a software engineer who used real elevation data to plot out Mars' oceans, mountains, valleys and other geological features, created a depiction of what he thinks a billion year old version of Mars may have looked like, using his own judgement to paint in the deserts, forests, and other vegetation.
In a post on Google+, Gill explained:
"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."
Gill also made sure to point out that "this wasn't intended as an exhaustive scientific scenario", but adds that he hopes that some of his assumptions will prove to be true.