Homogeneous ice nucleation from water vapour suggested by elongated clouds on Mars

It is widely accepted that water ice cloud particles below the triple point of water form by heterogeneous nucleation (with the aid of a pre-existing particle). Clouds formed from water vapour by homogeneous ice nucleation (without a pre-existing particle) have been shown to be theoretically possible for mesospheric clouds on Earth and Venus, but they have never been observed in nature, and this process is not typically considered viable under real atmospheric conditions on any planet, as it requires high levels of supersaturation. Here, we use a model of homogeneous ice nucleation from water vapour in a meteorological model of Mars and show that this successfully reproduces the unique characteristics of the Arsia Mons Elongated Cloud, a water ice cloud that occurs recurrently on Mars and that had remained difficult to reproduce in a model under conventional cloud microphysics. This finding strongly suggests that homogeneous ice nucleation from water vapour occurs on Mars, challenging some current assumptions about cloud formation processes for certain clouds on Earth and potentially on other planets. The Arsia Mons Elongated Cloud on Mars probably forms by homogeneous ice nucleation, with implications for other planets including Earth and Venus, according to a model of homogeneous ice nucleation from water vapour in a meteorological model of Mars.