### Sizing up alien rain

This exercise is a part of Educator Guide: Physics Helps Alien Rain Stay In Shape / View Guide

Directions for teachers:

Ask students to read the online Science News article “How the laws of physics constrain the size of alien raindrops,” which explores a new model for rain on planets across the Milky Way, and answer the following questions. A version of the story, “Physics helps alien rain stay in shape,” appears in the May 8, 2021 & May 22, 2021 issue of Science News.

1. What is the first generalized model of alien rain?

The model is a set of equations for what happens to a falling raindrop after it has left a cloud. The equations can be applied to any planet.

2. What does the model suggest about the shape of raindrops across the Milky Way?

All liquid raindrops have a similar shape and behave similarly, regardless of what the liquid is made of or what planet the rain falls on.

3. Why is this model important, according to astronomer Tristan Guillot?

It will help scientists understand what happens in the atmospheres of other worlds.

4. What role does rain play in planets’ atmospheres?

Raindrops help transport chemical elements and energy.

5. What types of rain did scientists consider when making the model? What celestial objects is the rain found on?

Researchers considered water rain on Earth, ancient Mars and exoplanet K2 18b, methane rain on Saturn’s moon Titan, ammonia “mushballs” on Jupiter and iron rain on exoplanet WASP 76b.

6. What does the model indicate about raindrops’ size? How does a planet’s gravity affect raindrop size?

Raindrops’ radii fall within a narrow range, from a tenth of a millimeter to a few millimeters. Planets with higher gravity tend to produce smaller raindrops and planets with weaker gravity tend to produce larger raindrops.

7. What happens to small and large raindrops that fall beyond the size range determined by the model?

Larger raindrops break apart into smaller droplets, and smaller raindrops quickly evaporate.

8. Why do the different raindrops behave similarly, according to planetary scientist Kaitlyn Loftus?

All of the raindrops behave similarly because they are governed by the same physical equations.

9. What do the researchers want to use the model to study next?

The scientists would like to study solid precipitation such as hail.

10. What goal does this model bring scientists a step closer to achieving?

To develop an understanding of how planets and atmospheres work that isn’t solely influenced by our knowledge of how Earth works.

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