The NASA/ESA Hubble Space Telescope has helped solve the mystery of Mars’ escaping water.
Scientists have discovered that the escape rates of hydrogen and “heavy hydrogen,” called deuterium, change rapidly when Mars is close to the Sun. This upended the classical picture that scientists previously had, where these atoms were thought to slowly diffuse upward through the atmosphere to a height where they could escape. Extrapolating the escape rate backwards through time helped the team to understand the history of water on the Red Planet.
These are far-ultraviolet Hubble images of Mars near its farthest point from the Sun, called aphelion, on December 31, 2017 (top), and near its closest approach to the Sun, called perihelion, on December 19, 2016 (bottom). The atmosphere is clearly brighter and more extended when Mars is close to the Sun.
Reflected sunlight from Mars at these wavelengths shows scattering by atmospheric molecules and haze, while the polar ice caps and some surface features are also visible. Hubble and NASA’s MAVEN showed that Martian atmospheric conditions change very quickly. When Mars is close to the Sun, water molecules rise very rapidly through the atmosphere, breaking apart and releasing atoms at high altitudes.
[Image description: Split image of two panels stacked vertically. In the left corner of the top image is the label Mars Corona, Hubble Space Telescope. This label pertains to both panels. In the top panel, on a black background, an orange and white orb is surrounded by a small, diffuse, grainy, orange halo. The halo appears to have more material on its left side than its right. Under the orb is the label Aphelion: December 31, 2017. In the bottom panel, on a black background, a larger orange and white orb is also surrounded by a diffuse, grainy, orange halo. This halo is wider than the one in the top panel. The halo appears to have more material on its right side than its left. Under the orb is the label Perihelion: December 19, 2016. In both panels, white, polar ice caps and some surface features are visible.]