Mars clouds are very much like Earth’s cirrus clouds but thinner. While ground clouds may contain liquid water, the low temperatures and pressures on Mars allow only the formation of ice-water clouds (and CO2 ice). However, these water ice clouds are optically thin due to the small amounts of water present in the atmosphere of Mars; If all the water was on the surface, it would make a thinner coat than just one strand of hair.
Looking at clouds helps us understand the atmosphere and the current functioning of the water cycle on Mars, the manner in which water vapour is carried by atmospheric circulation and the manner in which temperatures and water abundance vary according to height. By observing the movement of clouds, we can also find out about the velocities and directions of the wind in the atmosphere, which we have no other way of measuring. In spite of his slenderness, these clouds still have an impact on current atmospheric warming and cooling, and in the past, clouds have perhaps played a much more important role in maintaining a warmer atmosphere that has allowed liquid water to drain onto the surface of Mars.
Source: NASA/JPL-Calgary Tech. For a few months around the northern summer solstice, orbiting spacecraft observe heavy cloud activity from ~10° south to 30° north latitude. Because perseverance is to explore the crater of Jezero, which is about 18 degrees to the north, we are in an ideal spot to observe these clouds from the surface! There is over a month left before the beginning of this cloudy season. Already starting to see more activity in the cloud.
The image presented was taken by the mission's ground-based navigation camera (navcam) 691, just before the sun rises looking east, and shows thin layers of clouds lit up by the sunrise. We regularly take pictures and films of navcam to study the synchronization, movement and morphology of the clouds above the jezero crater. When there are many clouds surrounding, we also take pictures of mastcam-z (that contain more spectral information) to find out more about the composition of these clouds, as the mean size of the particulate matter. We also monitor clouds by means of sensors for environmental dynamics (meda). The meda radiation and dust (rds) sensor measures incoming solar radiation at various wavelengths and can detect when clouds block or disperse a portion of the solar light reaching the collectors. The meda thermal infrared (fire) sensor measures thermal radiation from the sky and the surface and can also provide cloud information. For example, whether clouds are present when the sun goes down, surface temperature decreases slower than normal after sunset, because even these thin clouds emit sufficient descending thermal radiation to keep the surface warm.
At last, the meda skycam camera, oriented upwards, takes daily images in search of clouds. We expect more and more clouds as the cloud season approaches, so we will look for some interesting cloud activity in our observations. Towards the end of last year's cloudy season, we saw something that had never been discovered beyond the earth: a halo around the sun, which lasted many hours. Halos are caused by the refraction and reflection of light by large ice crystals, which can only occur when the concentration of water vapour is sufficiently high.
Written by Priya Patel (UCL, JPL), Leslie Tamppari (JPL), Claire Newman (Aeolis Research), and Mark Lemmon (SSI)