Mars like Earth in the Past?

Simulation of Mars with an atmosphere by Kevin Gill, NASA

Fox News mentions NASA Engineer Kevin Gill was inspired to render a simulation of what Mars might have looked like with an atmosphere. While the image is pure fancy, the inspiration for it is a growing body of evidence that Mars had liquid water in the past. The article states:

As we send more missions to Mars, it's becoming clear that the planet was once a wet world with features that were very Earth-like. For example, NASA's recently landed Mars Science Laboratory rover Curiosity touched down on an ancient riverbed inside Gale Crater where water, perhaps two feet deep, used to flow. Evidence of clays near Mars Exploration Rover Opportunity are also evidence that minerals have interacted with surface water some time in the past. (Read more)

Herein lies a mystery which I explore in my novel, Beyond Earth: Liquid water requires an atmosphere at an Earth-like pressure or the water immediately boils away. Yet with only 1/3 of Earth's gravity, Mars can't hang on to a thick atmosphere for very long. Over time, the excess gas diffuses into space. How long such an atmosphere would last is merely guesswork at this point, but estimates range from hundreds of years to millions. At the high range, this makes Mars' ancient atmosphere too young to fit the conjectured 4-billion-year age of Mars. At the low range, it suggests sometime in the past Mars gained and lost an atmosphere over a relatively brief time--an intriguing idea I'm not aware of anyone exploring in science fiction, apart from Beyond Earth.