Today, we think of Mars as having a cold, dry, and desolate environment (because it does).
But that was not always the case. Four billion years ago, while our Sun was still in its infancy, Mars was covered with water.
Back then, it had a much thicker atmosphere, which kept the planet warm enough for water to exist in its liquid form. Some estimates say that at one point, up to 1640 ft (about half a kilometer) of water covered the whole planet.
NASA will launch its Mars Atmosphere and Volatile EvolutioN (MAVEN) on November 18. MAVEN’s job is to determine exactly what happened to Mars’ atmosphere during those four billion years.
In the meantime, they had their Goddard Conceptual Image Lab create a video showing what Mars might’ve looked like four billion years ago and how it changed as the atmosphere thinned out over time:
There are a number of theories as to why Mars’ atmosphere disappeared, including a major asteroid impact and the loss of its magnetic field as a result of solar winds.
NASA hopes that the data collected by MAVEN will help them solve the issue once and for all.
(h/t IFL Science)