What are raindrops like on other planets?

The earth is not the only planet that will rain. Physicists believe that there must be a phenomenon similar to the earth's rainfall in other parts of the universe. The kind of matter falling from the sky is almost as different as the planet itself. The research team led by Caitlin loftus of Harvard University found that all the liquid substances that make up rain in the whole solar system have one similarity-all raindrops, no matter what they are made of, are about the same size.

Rainfall is one of the main features of the earth's weather, and the rainfall varies greatly in different regions. The rainfall process not only brings different phenomena, but also shapes the landscape of the ground. Rain eroded the valley and flooded rivers and lakes. Scientists already know that there are similar processes on other planets. For example, Titan also has rivers, lakes, canyons and rainfall, but the liquid in it is liquid methane, not water.

Mars also seems to have traces of rain, although it can be traced back to billions of years ago. Gas giants such as Jupiter and Saturn don't have surfaces like rocky planets, but raindrops play a vital role in the huge storms in their atmosphere because they can transfer heat in the atmosphere.

This leads to an interesting question: What are the raindrops on these planets? How similar are they to raindrops on the earth? The research of Caitlin loftus and Robin Wordsworth provides us with the answer. They pointed out that in any given atmosphere, no matter how raindrops are formed, there are only three factors that determine their size. Because of this, raindrops on other planets are likely to have obvious similarities with raindrops on earth.

Dynamic mechanism of cloud

First, let's learn some background knowledge. When water vapor condenses around smaller particles (such as aerosols), raindrops form and grow in the clouds. This is a complicated nonlinear process. The condensation process will redistribute the heat and humidity of the cloud, which will affect the formation of water droplets in the future. Scientists know little about how this phenomenon occurs in the vast cloud hosting.

However, the droplets formed in this process are very similar. The size of most raindrops is around 1 mm, and it will hardly be larger than 4 mm, because any larger raindrops will be broken. The size of raindrops is the same, because no matter how they are formed, they all follow the same physical laws when passing through the atmosphere.

Loftus and Wordsworth said that only three attributes-shape, final speed and evaporation speed when falling-determine the size of raindrops. They said: "From these properties, we prove that only relatively small raindrops can reach the surface from the clouds under various planetary conditions."

Relatively speaking, scientists already know the shape of raindrops very well. "According to the size of raindrops, they can choose various shapes, but they are never the shape of tears as people think," the researchers said. "As the quality of raindrops increases, it will evolve into an oblate sphere, similar to the upper layer of Hamburg."

The researchers also determined the final velocity of raindrops in different planetary atmospheres, which depends on the downward force (gravity) and the aerodynamic resistance in the opposite direction. The resistance depends on the cross-section of raindrops, the speed of raindrops passing through the atmosphere, the density and viscosity of the atmosphere and other factors. When these forces are balanced, raindrops will fall at a steady speed.

On the earth, raindrops fall very fast. Loftus and Wordsworth pointed out that even the largest raindrop will accelerate to 99% of its final speed within 1% of the total falling distance. They also studied how changes in the size of raindrops, such as those caused by evaporation, affect the falling speed. Facts show that the difference is not great. Therefore, raindrops in the atmosphere of other planets are likely to be very similar to raindrops on the earth.

Finally, they studied the evaporation rate of raindrops. This is a bit complicated, because the evaporation rate of the droplet surface depends on the surrounding humidity, atmospheric density, water vapor diffusion mode, and the temperature of the droplet and the surrounding air. When water droplets move in the air, they will change their temperature through conduction; At higher temperatures, heat can also be transferred by radiation. Therefore, the physics involved is complicated. However, the researchers pointed out that the evaporation rate can be characterized by a single parameter after considering all factors.

Their model results show that, as expected, raindrops with a radius of less than110 mm will evaporate before falling to the surface, while larger raindrops with a radius of more than several millimeters will split into raindrops with a closer average size.

surface tension

The two researchers continued to calculate the volume growth of raindrops under various conditions. An important factor they consider is the surface tension of liquid, which can keep the shape of raindrops unchanged. However, when the external force exceeds the surface tension, the droplet will break. It is this process that makes the raindrops on the earth only a few millimeters in diameter.

The two researchers pointed out that there are similar restrictions in the atmospheres of other planets, and the scope is very wide. Generally speaking, raindrops in the extraterrestrial world are limited by some factors, which are similar in size to raindrops on the earth, and there will be no order of magnitude difference. "For terrestrial planets, there is only an order of magnitude difference in the size of raindrops that can transport condensed objects to the surface," they said. "We have proved through a series of condensate and planetary parameters that the maximum raindrop volume will not change significantly."

They also pointed out that the same physical principle applies to raindrops made of non-aqueous liquids. Astronomers have found that an exoplanet named WASP-76b has a very high temperature atmosphere, the temperature can exceed 2750 degrees Celsius, which is enough to evaporate iron and float to the colder side of the planet to condense and drop the "iron rain". The raindrops of this "iron rain" are likely to follow the same rules as the raindrops of the earth. In fact, the same high temperature and similar "iron rain" phenomenon will appear in the process of meteor hitting the earth.

Even for larger planets, there is no obvious difference in the size of raindrops. The raindrops on Jupiter or Saturn are similar in size and shape to those on Earth or Mars. Similarly, the composition of raindrops has little effect on their size. For example, on Titan, raindrops are mainly composed of methane. It has been found that although Titan's gravity and weather patterns are completely different from those of the Earth, the largest of these methane droplets is only more than twice the size of the average raindrop on the Earth.

At present, the exact reason for this uniformity is not clear, but the research team believes that it is related to the density and surface tension of raindrop components. Understanding how raindrops form on other planets will help planetary scientists understand the atmosphere of exoplanets. With the launch of more powerful exoplanet observation satellites in the near future, this will become a more predictable topic.

"When considering raindrops and clouds in different environments, the information we get will be the key to understanding the habitability of exoplanets." Wordsworth explained, "In the long run, they can also help us to know more about the climate of the earth itself." (Ren Tian)