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The size of raindrops helps identify habitable planets outside our solar system.



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Raindrops

Raindrops continue to fall on exoplanets.

Humanity may one day tread on other habitable planets. That planet may look very different from Earth. But one thing that feels familiar is the rain.

In the latest articles published in JGR planetHarvard researchers found that raindrops are highly similar in different planetary environments, even those that are extremely different as Earth and Earth. JupiterUnderstanding the behavior of raindrops on other planets is key not only But revealed the ancient climate on the planet such as Mars Rather, it can identify planets that live outside of our solar system.

“The life cycle of clouds is very important when we think about the habitability of planets,”

; said Kaitlyn Loftus, a graduate student in the Department of Earth and Planetary Sciences and lead author of the paper. Too complicated to simulate at all We’re looking for an easier way to understand how clouds evolve, and the first step is whether cloud droplets either evaporate in the atmosphere or cause the surface to rain. ”

“Humble rainwater is a key component of all planetary precipitation cycles,” 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. “If we understand the behavior of individual raindrops, we can better express rainfall in complex climate models.”

A key feature of raindrops behavior, at least for climate modelers, is whether rainwater falls to the Earth’s surface, as atmospheric water plays an important role in the planet’s climate. For this reason, size matters. Too large and the droplets will rupture due to insufficient surface tension, either water, methane or overheated liquid iron, as well as Exoplanets Called the WASP-76b, it’s too small and drops will evaporate before it hits the surface.

Loftus and Wordsworth identified the Goldilocks zone for rainwater size, using just three properties: drop shape, falling speed and evaporation speed.

“The insights we gain from thinking about raindrops and clouds in a variety of environments are key to understanding exoplanet habitats.”
Robin Wordsworth, Associate Professor of Environmental Science and Engineering

The droplet shape is the same in different rainproof materials and largely depends on the weight of the fall. As many of us may have imagined traditional teardrops. But raindrops actually look spherical when they are small, they become grainy as they grow larger to the shape of a hamburger bun. The falling speed depends on this shape, as well as the gravity and the thickness of the surrounding air.

Evaporation velocity is more complex, influenced by atmospheric composition, pressure, temperature, relative humidity, etc.

By looking at all of these properties, Loftus and Wordsworth found that in a wide variety of planetary conditions, the mathematics of falling rain means that only a small fraction of the size of the droplets possible in a cloud can reach the surface.

“We can use this behavior to guide us as we simulate the cloud cycle on exoplanets,” Loftus said.

“The insights we get from thinking about raindrops and clouds in a variety of environments are key to understanding exoplanet habitats,” Wordsworth said. Have a deeper understanding of the global climate. “

Reference: “Physics of Raindrops Falling in the Atmosphere of Various Planets” by Kaitlyn Loftus and Robin D.Wordsworth, 15 March 2021, JGR planet.
DOI: 10.1029 / 2020JE006653

This research was supported by the National Science Foundation providing AST-1847120.




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