A mechanism for water removal from Mars has been verified. The surface of Mars has features that show water once flowed freely on the planet. Now, however, the amount of of water on Mars is small. Observations by an instrument on the ESA Mars Express satellite that is designed to measure the penetration of the solar wind into the Martian atmosphere has found that the solar winds penetrates deeply enough to account for Mars's loss of water.
Mars Express had earlier observed features on Mars that are evidence of large-scale water erosion, with evidence of an ocean in the planet's northern hemisphere. The amount of water on Mars today, however, is not sufficient to cause these features. What water still exists on the planet appears in seasonal polar ice caps. The question is what happened to the water on Mars.
Because Mars is very small, with only 11% of the mass of Earth, the escape velocity is small, at 45% of Earth's escape velocity. Because of this, to escape from Mars, an atom only needs 20% of the energy required to escape from Earth. But is there a source of energy that can give an atom escape velocity. In the case of removing water, all that is required is an energy source that dissociates water into hydrogen and oxygen, and then accelerates the hydrogen to the escape velocity.
One possible source of this energy is the solar wind. In a process called solar wind scavenging, the high-velocity particles in the solar wind collide with water molecules in the upper atmosphere of Mars, dissociating the molecules and producing energetic hydrogen atoms. Mars Express has an instrument on board, ASPERA-3, that is designed to test this hypothesis. The instrument consists of plasma spectrometers and an imager that detects neutral atoms. By observing the solar wind, the penetration of the solar wind into the Martian atmosphere, and the flow of atmosphere away from Mars, scientists working with the instrument were able to determine that the solar wind passed through Mars's ionosphere and penetrated very deep into the atmosphere to an altitude of 270km. This observations supports the solar wind scavenging hypothesis.