Plain language summary
The rate of water loss from Mars depends on hydrogen, the main product
of the H2O photodissociation escaping from the upper
atmosphere. The ability of water to reach high altitudes and to be a
direct source of atomic hydrogen is limited by cloud formation, holding
water vapor in the lower atmosphere. This process is regulated by
temperature and pressure. The condensation starts when the temperature
is cold enough and condensation nuclei (for example, small dust
particles) are available. Or it does not, lacking condensation nuclei,
their size being too small, or the temperature drops too fast. Then
water vapor becomes supersaturated. Recent studies have shown that
Martian water vapor is often supersaturated. Here we present the first
seasonal cycle of the saturation state from simultaneous measurements of
water and temperature during two Martian years by ACS on the Trace Gas
Orbiter. Our results show that supersaturation is typical on Mars. It
occurs above clouds both in aphelion and perihelion seasons, as well as
in the lower polar atmosphere. We demonstrate supersaturation to be an
important factor, facilitating the escape of water. Unlike on the Earth,
water easily penetrates through the cold trap where clouds form to reach
photodissociation altitudes.