On Earth, it rains water. But on Venus, it tumbles sulfuric acid. The thick atmosphere of Jupiter is traversed by showers of helium or hailstones of ammonia and water mixed together. And on Titan, the largest satellite of Saturn, we can wipe a grain of liquid methane … With their atmospheres varied in composition, pressure, temperature, the planets of the solar system and their satellites offer sometimes very exotic rains. But what do the drops that carry them look like? According to a study conducted by researchers at Harvard University, and published in the Journal of Geophysical Research, they would ultimately be quite similar to ours …
20mm drops on Titan
To reach this conclusion, the American planetologists Kaitlyn Loftus and Robin Wordsworth modeled the formation of these drops according to the characteristics of the atmosphere and the gravitational pull of the planet. First observation: the more massive the star, the smaller the drops. This is understandable: for a drop to detach from the cloud where it forms, it must be sufficiently attracted by the gravitational field of the planet. However, the more massive a star, the more intense this field. Thus, a drop of methane on Titan (0.22 times the Earth’s mass) is twice as large (20 mm maximum) than on Earth, according to the researchers’ simulations. Severity is not the only criterion that matters. This work emphasizes that the size of the drops is also constrained by their fall. Too small, they evaporate before hitting the ground. Too big, they break into several smaller droplets. Applied to the Earth, this means that a drop measures between a tenth of a millimeter and 11 millimeters. This is why the Breton drizzle is so different from the stormy downpour …
The researchers also studied how drops composed only of water would fall on different planets in the solar system.
By comparing with the Earth, Mars (when the climatic conditions were compatible with the presence of water), and the gas giants (Jupiter and Saturn), they found that the raindrops behaved in their fall (speed, shape of gout) in a similar way in these atmospheres which have nothing to do with it. As for their size, it would vary from 7 mm on Jupiter to 30 mm on Titan. This remains a modest difference given the difference between these stars. This relative homogeneity could be explained by the fact that the characteristics of the drops are more dependent on molecular interactions within the fluid than on external conditions.
Sizes of water drops on different planets. Credits: AGU. Click on it to see in big
Rains of iron or diamonds
“This work is useful because it will make it possible to include rain and hail in the modeling of the atmospheric circulation of the planets., comment for Science and the Future Tristan Guillot, planetologist at the Observatory of the Côte d’Azur, at the origin of the theory of hailstones mixing water and ammonia on Jupiter. However, we have seen with the Juno space probe that it is essential to understand the deep atmosphere of Jupiter, and by extension those of all the gas giant planets that have no surface. They are also valuable a few months before the launch of the James Webb Space Telescope (JWST), scheduled for the end of 2021, and which should observe many exoplanets, many of which are gas giants.“By finely analyzing the light coming from these planets orbiting other stars, astrophysicists should be able to deduce the composition, but also the dynamics of their sometimes very exotic atmospheres.”For this, we will need tools like these models developed by Harvard researchers “, concludes Tristan Guillot. When we think that on some exoplanets, it could rain iron, or diamonds, we can not wait for the JWST to turn to them.