The size of the raindrops helps determine potentially habitable planets outside our solar system


Raindrops continue to fall on the outer planets

One day, mankind may set foot on another habitable planet. This planet might look very different from Earth, but one thing will look familiar – rain.

In a recent research paper published in JGR Planets, Harvard researchers have found that raindrops are remarkably similar across different planetary environments, even planets are as drastically different as Earth and Jupiter. Understanding the behavior of raindrops on other planets is key not only to uncovering ancient climates on such planets Mars But identifying potentially habitable planets outside of our solar system.

“The life cycle of clouds is really important when we think about a planet’s habitability,” said Caitlin Loftus, a graduate student in the Department of Earth and Planetary Sciences and lead author of the paper. “But clouds and precipitation are really complex and too complex to be fully designed. We are looking for simpler ways to understand how clouds develop, and the first step is whether cloud droplets evaporate into the atmosphere or reach the surface as rain.”

“The humble raindrop is a vital component of the rainfall cycle for all planets,” said Robin Wordsworth, associate professor of environmental science and engineering at Harvard’s 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 raindrops behavior is, at least for climate designers, whether or not a raindrop is reaching the planet’s surface because water in the atmosphere plays a large role in a planet’s climate. To this end, size matters. Too large and the droplet will disintegrate due to insufficient surface tension, regardless of whether the water, methane, or liquid iron is extremely hot Exoplanet It is called WASP-76b. Too small and the droplet will evaporate before hitting the surface.

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Loftus Wordsworth defined the Goldilocks region for raindrop volume using only three characteristics: drop shape, drop velocity and evaporation velocity.

“The insights we gain from thinking about raindrops and clouds in diverse environments are key to understanding the habitability of exoplanets.”
Robin Wordsworth, Associate Professor of Environmental Science and Engineering

The falling shapes are the same across different rain materials and mainly depend on how heavy the drop is. While many of us might imagine a traditional teardrop-shaped droplet, raindrops are actually spherical when young, becoming squeezed as they grow larger until they transition into a shape like the top of a hamburger bun. The speed of falling depends on this shape as well as the gravity and thickness of the surrounding air.

The speed of evaporation is more complex, and is affected by the composition of the atmosphere, pressure, temperature, relative humidity, etc.

By taking all of these properties into account, Loftus and Wordsworth found that across a wide range of planetary conditions, calculating the fall of raindrops means that a very small fraction of the potential droplet sizes in the cloud could reach the surface.

“We can use this behavior to guide us while modeling cloud cycles on exoplanets,” said Loftus.

“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 term, they can also help us gain a deeper understanding of Earth’s climate itself.”

The reference: “Physics of Raindrops Fall in Various Planetary Atmosphere” by Caitlin Loftus and Robin D. Wordsworth, March 15, 2021 JGR Planets.
DOI: 10.1029 / 2020JE006653

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This research was supported by the National Science Foundation with an AST-1847120 grant.

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