Raindrops can help identify potential habitable planets

The study led by researchers from Harvard University in Massachusetts, US, found that raindrops are remarkably similar across different planetary environments, even planets as drastically different as Earth and Jupiter.

Understanding the behaviour of raindrops on other planets is key to not only revealing the ancient climate on planets like Mars but also identifying potentially habitable planets outside our solar system.

"The lifecycle of clouds is really important when we think about planet habitability," said lead author Kaitlyn Loftus, a graduate student in the Department of Earth and Planetary Sciences.

"But clouds and precipitation are really complicated and too complex to model completely. We're looking for simpler ways to understand how clouds evolve, and a first step is whether cloud droplets evaporate in the atmosphere or make it to the surface as rain," Loftus added.

For the study, the team identified three properties of raindrops: drop shape, falling speed, and evaporation speed.

Drop shapes are the same across different rain materials and primarily depend on how heavy the drop is. Falling speed depends on shape as well as gravity and the thickness of the surrounding air.

Evaporation speed is more complicated, influenced by atmospheric composition, pressure, temperature, relative humidity and more.

The researchers found that across a wide range of planetary conditions, the math of raindrop falling means only a very small fraction of the possible drop sizes in a cloud can reach the surface.

"The humble raindrop is a vital component of the precipitation cycle for all planets," said Robin Wordsworth, Associate Professor at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS).

"If we understand how individual raindrops behave, we can better represent rainfall in complex climate models," Wordsworth said.