The Perseverance rover has spotted a solar halo on Mars, the first time such a phenomenon has been recorded outside of Earth. According to scientists, it is created by micrometer-sized hexagonal crystals of water ice in the atmosphere of the Red Planet. The article was published in Geophysical Research Letters.
The phenomenon of the atmospheric halo is well known to the inhabitants of the Earth – it occurs around bright sources of light, such as the Sun or the Moon, due to the scattering and refraction of light on ice crystals in the Earth’s troposphere. The earth’s halo can take on a wide variety of forms – rings, arcs or spots. In the case of Mars, simulation data show that the particles of the Red Planet’s ice clouds can be micrometer-sized complex polyhedrons that resemble terrestrial cirrus clouds and can create conditions for the formation of a halo for an observer on the surface of Mars. However, this phenomenon has never been recorded before in the atmosphere of the Red Planet.
A team of planetary scientists led by Mark Lemmon of the Space Science Institute in Boulder reported the first recorded solar halo on Mars on the morning of December 15, 2021. Observations were carried out for 3.3 hours using the cameras of the Perseverance rover, which operates in the Lake crater near the planet’s equator.
The halo on Mars appears as a circle within 22 degrees of the Sun and has been observed in cold and cloudy weather. According to scientists, it could be created by ice ice crystals in the form of hexagonal prisms (or columns), measuring up to more than 11 micrometers in length and in diameter. The crystals were originally located at altitudes of 40–50 kilometers, and during the observation period they could descend to heights of 3–12 kilometers.
The researchers concluded that the clouds that form a halo on Mars are quite rare due to the high levels of water saturation required, but can be common in the northern subtropical regions of the planet in midsummer.