Mars is the only other planet in the solar system apart from the Earth that combines a clear – albeit thin – atmosphere and a solid surface. Therefore, an astronaut standing on the surface may observe and record phenomena on the martian sky, both astronomical and atmospheric. One of these phenomena happens to be..clouds!
Martian clouds are much less ubiquitous on Mars than they are at the Earth but they do exist. There are two types of martian clouds, these can generally be distinguished by colour. Yellowish dust clouds are similar to sandstorm clouds on the Earth’s deserts, while bluish-white clouds are made of water ice crystals, similar to the familiar wispy cirrus clouds of Earth. Either can grow large enough to be visible to Earth-based telescopes.
Water ice clouds have now been documented by robotic rovers roaming the surface. They are most frequently seen at certain times of the (martian) day, for example in the early morning hours. They also tend to come and go with the seasons; one example is the so-called Arsia Mons Elongated Cloud or AMEC for short, a 1000-mi long cloud formation that has been recorded by spacecraft since the 1970s. As its name suggests, the AMEC forms on the flanks of Arsia Mons, a 12-mi high extinct martian volcano, and always appears at the time of solstice in the southern hemisphere, the equivalent of the winter solstice on Earth. This phenomenon may be an example of an orographic cloud, formed as moisture-laden warm air is forced to flow upwards by the rising terrain, causing cooling and forming the ice crystals. Scientists still do not know for how long the AMEC has been disappearing and reappearing or, indeed, why it only forms in the early morning hours.
Last seen in 2018, the AMEC was picked up again just a few weeks ago by a camera onboard the European Space Agency (ESA) Mars Express probe. Mars will be coming close to the Earth this Autumn (more about the upcoming Mars apparition in a future Astronote) and, if the AMEC persists until then, it will likely become visible to amateur and professional observers of the planet.