The depleting atmosphere
Mars was once a planet with water flowing across the surface and had a suitable atmosphere that kept it relatively warm, which could have supported life. Today, Mars is a cold, dry planet with an atmosphere 150-times lower in pressure than on Earth, so Mars Express set out to find out what happened during its evolution to transform the planet.
Mars Express’ Analyser of Space Plasma and Energetic Atoms (ASPERA) instrument took valuable data about the interaction between the solar wind and the upper atmosphere and ionosphere. Data showed that in just a few minutes the upper atmosphere can alter and open a path for the solar wind to penetrate deeper into the atmosphere, interacting with water ions and causing them to escape into space.
However, this ion escape rate depends on varying solar winds and cannot fully account for the drastic depletion over the last 3.5 billion years. This means another mechanism must have been at work, possibly removing neutral atoms in the upper atmosphere. A recent theory even suggests that dust storms play a part in kicking up atoms to higher altitudes where they then escape Mars.
The intense solar winds strip away the atmosphere of Mars