Why is this photo of the Sun taken from the surface of Mars so exciting to scientists?


In December 2021, scientists were astonished to discover an optical phenomenon well known on Earth but never seen on another planet: in a photograph taken by the Perseverance rover on Mars, the Sun appears surrounded by a luminous halo. A phenomenon that allows us to make important hypotheses about the composition of the Martian atmosphere. But on closer inspection, one detail caught the scientists’ attention. In the panorama, the Sun is clearly visible. So far, nothing exceptional. Except that the luminous disk of our star appears surrounded by a halo. And it is precisely this very characteristic optical phenomenon that has aroused the excitement of the scientific community A phenomenon well known on Earth but observed for the first time on Mars This phenomenon is well known on Earth, where it is often observed. It happens that the ice crystals suspended in our atmosphere distort the light emitted by the Sun, creating a luminous circle around the solar disk. Our atmosphere is actually made up of tiny ice crystals, grouped within cirrus clouds, present at a very high level. altitude These crystals have a hexagonal structure which, through the phenomenon of refraction, refraction or reflection of light, will dictate the size and type of halo observed. Usually, a circle displaced by 22° relative to the light source is observed, but it is also possible to observe arcs or a column of light. However, the crystals must be of a very specific size for a halo to be observable. And it is in this parameter, which depends on the atmospheric conditions, where everything is played. For example, if the crystals are too small or the clouds are too thick or too hot, nothing will be visible around the Sun. However, water is not the only element capable of producing this phenomenon. Indeed, it is conceivable that crystals of other chemical elements could produce a similar effect. However, each crystal structure will have its own signature, creating a halo of a different size. The study of the halo thus allows finding the chemical element in question, the size and shape of the crystals, but also the temperature of the cloud. Evidence of the presence of large water ice crystals in the Martian atmosphere Earth, Mars has an atmosphere, although it is much thinner and of a very different composition. In theory, this optical phenomenon is therefore very possible on Mars. However, it had never been observed, despite the countless photographs of the sky taken by the various Mars missions carried out so far, until December 15, 2021. At first, scientists thought that the observed halo was produced by the presence of dry ice in the atmosphere of Mars, a molecule molecule much more common in the red planet’s atmosphere than water molecules. But the optical signature doesn’t match. The scientists therefore considered the different possible options: artifact due to the camera, presence of dust in the atmosphere… but none of these hypotheses allows to reproduce the halo photographed by PerseverancePerseverance. Except for the presence of large water ice crystals Other plans needed to better understand the origin of this halo The observation of this halo therefore has strong implications, even if it is difficult to give definitive conclusions about a single example For the researchers, who present their results in a paper published in Geophysical Research Letters, this halo captured by the Perseverance camera implies an oversaturation of water in the upper atmosphere of Mars. A rare situation for the Red Planet, but one that could be more frequent than expected in late summer in the subtropical regions of the Northern Hemisphere-Northern Hemisphere. It is precisely in this situation that Perseverance captured this snapshot of the Sun. Now scientists are eagerly waiting for Mars to enter this special season again so they can take new pictures of the Sun. This type of data complements the atmospheric measurements acquired from the Martian orbit by the probes and allows to study in more detail the composition but also the functioning of the Martian atmosphere.
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