Winter is often synonymous with snow for people who are not used to seeing it all year round. While some regions of the Earth are permanently covered in snow, others hardly ever see a trace of it. Snow is defined as a precipitation phenomenon involving branched and crystallized ice particles, agglomerated into flakes. If it is relatively common on Earth, can we find this phenomenon elsewhere?
During the coldest seasons, many places on Earth observe more or less heavy snowfall. This phenomenon involves specific atmospheric mechanisms, and requires specific temperature and pressure conditions. It is therefore natural that astrophysicists have tried to find out if it could snow in other places in the Solar System, and even further.
Planetary scientists have already observed snowfall on Mars several times. With an average temperature of around -60°C, the red planet meets the temperature conditions to see snow appear. In 2008, NASA's Phoenix lander detected icy snow falling near the planet's north pole. While the Martian South Pole dons an ice cap (carbon dioxide ice) all year round. In 2012, researchers first spotted dry ice falling from Mars' atmosphere around the south pole.
Despite a constant rolling of clouds, snow rarely accumulates on the Red Planet's surface. Because Mars' atmosphere is thin — about 100 times thinner than Earth's — liquid water falls very slowly and tends to vaporize almost immediately. Astrophysicists have observed clouds dropping snow in the Martian atmosphere, only to see the precipitation disappear before approaching the surface (this also happens on Earth, in a phenomenon called virga).
Surface snow is possible on Mars under the right conditions, however, according to a 2017 study in the journal Nature Geoscience . Since Martian temperatures can drop nearly 111°C between day and night, cloud turbulence is common. “This can lead to strong winds, vertical plumes rising and falling inside and below clouds at around 10 meters per second “, explains Aymeric Spiga, planetary scientist at Pierre and Marie Curie University. In stormy conditions like these, snow could fall to the surface of Mars fairly quickly and stay on the ground overnight — but it would still vaporize by morning.
What about elsewhere in our solar system? The swirling clouds seen above Jupiter's surface in May 2017 would almost certainly be frozen over, planetary scientists said, and could drop an icy mixture of water and ammonia that could be thought of as something between snow and hail.
Saturn's sixth-largest moon, Enceladus, may be covered in snow, according to data taken from NASA's large Cassini probe in 2011. The spacecraft found that ice particles ejected from geysers on the icy moon fall on the surface of Enceladus in a predictable pattern, creating slopes of superfine crystals. The crystalline "snow," however, falls at an extremely slow rate by Earth's standards:less than a thousandth of a millimeter per year, according to data from the probe.
Accumulating about 100 meters would require a few tens of millions of years. On Kepler-13Ab, a huge exoplanet six times larger than Jupiter and located 1730 light years from Earth, titanium dioxide, one of the active ingredients in sunscreen, is snowing. While on Uranus and Neptune it could "rain" diamonds.
