Raindrops continue to fall on the outer planets
One day, humanity may step on another habitable planet. This planet may not look like Earth at all, but one thing will seem familiar – rain.
A recent study published in 2005 Planets JGRHarvard researchers have found that raindrops are very similar in different planetary environments, even though the planets are quite different, such as Earth and Jupiter. Understanding the behavior of raindrops on other planets is a key factor not only in identifying the ancient climate on such planets. Mars But the discovery of potentially habitable planets outside our solar system.
“The life cycle of clouds is really important when it comes to the habitable nature of the planet,” said Caitlin Loftus, a graduate student in the Department of Earth and Planetary Sciences and lead author of this article. “But clouds and precipitation are really complex and too complex to be fully formed. We’re looking for easier ways to understand how clouds evolve, and the first step is whether the clouds evaporate in the atmosphere or come to the surface like rain.”
“The humble raindrop is an important part of the planetary rainfall cycle,” said Robin Wordsworth, an associate professor of science and technology at the John A. Paulson School of Engineering and Applied Sciences (SEAS) and lead author of the paper. . “If we understand how individual raindrops behave, we can better represent precipitation in complex climate models.”
A key aspect of raindrop behavior, at least for climate designers, is whether raindrops hit the planet’s surface, as atmospheric water plays an important role in the planet’s climate. For this, size matters. Too large and the drop will disintegrate due to insufficient surface tension, whether extremely hot water, methane or liquid iron Exoplanar It is called WASP-76b. Too small and the droplets evaporate to the surface.
Wordsworth Loftus identified a zone of moderate raindrop size using only three characteristics: droplet shape, fall rate, and evaporation rate.
“The understanding we gain from thinking about raindrops and clouds in different environments is key to understanding exoplanar habitat.”
– Robin Wartworth, Associate Professor of Environmental Sciences and Engineering
The forms of fall are the same for different precipitation materials and mainly depend on the severity of the fall. Although many of us can imagine a traditional drop-shaped drop, raindrops are actually spherical, increasing until they turn into a shape similar to the top of a hamburger bun. The rate of fall depends on this shape, as well as the force of gravity and the thickness of the surrounding air.
The evaporation rate is more complex and depends on the atmospheric composition, pressure, temperature, relative humidity and so on.
Taking all these properties into account, Loftus and Wordsworth found that calculating the fall of a raindrop over a wide range of planetary conditions means that a very small fraction of the potential size of the cloud droplets can reach the surface.
“We can use this to control us by modeling cloud cycles on exoplanets,” Loftus said.
“The understanding we get from thinking about raindrops and clouds in different environments is key to understanding exoplanar habitat,” Wordsworth said. “In the long run, they can also help us better understand the Earth’s own climate.”
Reference: Catelin Loftus and Robin D. “Physics of raindrops in different planetary atmospheres.” Worthworth, March 15, 2021 Planets JGR.
DOI: 10.1029 / 2020JE006653
This study was supported by the National Science Foundation grant AST-1847120.