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Raindrops also fall on exoplanets
One day, man may step on another habitable planet. That planet looks very different from the earth, but one thing will feel familiar – the rain.
In a recent article published in JGR Planete, Harvard researchers have found that raindrops are remarkably similar in different planetary environments, even planets that differ so drastically from the earth and Jupiter. Understanding the behavior of raindrops on other planets is the key to not only revealing the ancient climate on planets March but the identification of potentially habitable planets outside our solar system.
“The life cycle of clouds is very important when we consider the habitability of the planet,” said Kaitlyn Loftus, a graduate student in the Department of Earth and Planetary Sciences and lead author of the article. ‘But clouds and precipitation are really complicated and too complex to completely model. We are looking for simpler ways to understand how clouds develop, and a first step is whether cloud droplets evaporate into the atmosphere or when rain comes to the surface. ”
“The humble raindrop is an important component of the precipitation cycle for all planets,” said Robin Wordsworth, associate professor of environmental science and engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and senior author of the article. . . “If we understand how individual raindrops behave, we can better represent rainfall in complex climate models.”
An essential aspect of raindrop behavior, at least for climate modelers, is whether the raindrop reaches the surface of the planet or not, because water in the atmosphere plays a major role in the planet’s climate. For the purpose, size is important. Too large and the fall will break apart due to insufficient surface tension, whether it is water, methane or overheated liquid iron as on exoplanet Called WASP-76b. Too small and the droplet evaporates before falling to the surface.
Loftus and Wordsworth identified a Goldilocks Raindrop Gold Zone using only three traits: droplet shape, fall speed and evaporation speed.
“The insights we gain by thinking about raindrops and clouds in different environments are key to understanding the habitability of exoplanets.”
– Robin Wordsworth, Associate Professor of Environmental Science and Engineering
Droplet shapes are the same in different rain material and depend mainly on how heavy the droplet is. Although many of us may imagine a traditional teardrop-shaped drop, raindrops are actually spherical as they get smaller, while getting larger until they turn into a shape like the top of a hamburger bun. False speed depends on this shape as well as the gravity and thickness of the surrounding air.
Evaporation rate is more complex, influenced by atmospheric composition, pressure, temperature, relative humidity, and more.
Taking all these properties into account, Loftus and Wordsworth found that the mathematics of the fall of raindrops over a wide range of planetary conditions could mean only a very small fraction of the possible droplet sizes in a cloud.
“We can use this behavior to guide us as we model cloud cycles on exoplanets,” Loftus said.
“The insights we gain from thinking about raindrops and clouds in diverse environments are key to understanding the habitability of exoplanets,” Wordsworth said. “In the long run, it can also help us gain a deeper understanding of the climate of the Earth itself.”
Reference: “The Physics of Falling Reindrops in Diverse Planetary Atmospheres” by Kaitlyn Loftus and Robin D. Wordsworth, March 15, 2021, JGR Planete.
DOI: 10.1029 / 2020JE006653
This research was supported by the National Science Foundation through grant AST-1847120.