Directions for teachers:

Before this discussion, students should read the online Science News article “How the laws of physics constrain the size of alien raindrops.” 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.  

With a partner, students will answer the first set of questions comparing and contrasting rain on the planets and moon mentioned in the Science News article with rain on Earth. Pairs should work together to build or draw the structures of the different molecules that compose rain droplets, using outside resources as needed. Based on the structures and their knowledge about bond polarity, pairs will determine the intermolecular attraction forces between molecules in a raindrop. They should use that information to discuss conditions that might allow rain to form on other planets and create a brief weather forecast for a planet of their choice that they can present to the class.

Note: Students will need background knowledge of Lewis Dot Structures and the Valence Shell Electron Pair Repulsion model.

Want to make it a virtual lesson? Post the online Science News article to your virtual classroom. Discuss the article and questions with your class on your virtual platform.

Rain: alien or not

1. How would you define rain? What is meant by the phrase “alien rain?”

Rain on Earth is primarily water (H₂O) that condenses to form clouds in the atmosphere and then falls to the planet’s surface. Alien rain refers to rain on other planets. That rain may form through a similar condensation process, but is not necessarily made of water.

2. Based on the first generalized physical model of alien rain covered in the Science News article, how do droplet shape and size vary across planets? How does the rain’s chemical makeup affect these attributes?

According to the model, falling droplets of a liquid tend to take on the same shape regardless of what the liquid is made of or what planet the droplets fall on. The model predicts that raindrops fall within a fairly narrow size range that varies by less than a millimeter. Worlds with higher gravity produce slightly smaller drops.

3. In the table below, compare and contrast the makeup, shape and maximum size of raindrops on Earth with raindrops on another planet mentioned in the Science News article. Consult outside resources as needed, and be sure to cite your sources. Use the last column to explain why you think each similarity or difference occurs.

The sample answer below compares Earth rain with Jovian rain.

Rain attributes	Earth	Planet: Jupiter	Explain
Makeup	Primarily water	Ammonia and water	Since the atmospheric composition and condition of some other planets are drastically different than Earth’s, atoms or molecules other than H2O may be present in high atmospheric concentrations, and under the relatively extreme conditions of another planet may be able to condense into clouds and eventually fall toward the planet’s center as a liquid.
Shape	Oblate spheroid (like the top of a hamburger bun)	Oblate spheroid (like the top of a hamburger bun)	The physical laws of fluid mechanics and thermodynamics allow rain on any planet to take on a similar shape.
Maximum size	About 11 millimeters	About 7 millimeters	Jupiter’s gravity is significantly higher than Earth’s gravity. The model predicts that worlds with higher gravity produce slightly smaller drops. This may be due to the fact that the external forces acting on falling liquid drop may be more compressive or that higher gravity environments tend to produce droplets composed of fewer atoms or molecules.

Sources:
K. Loftus and R. D. Wordsworth. The physics of falling raindrops in diverse planetary atmospheres. JGR Planets, Volume 126, Issue 4, April 2021. doi:10.1029/2020JE006653.

American Geophysical Union. A comparison of alien raindrops. Eurekalert. Published 2021.

NASA. ‘Shallow lightning’ and ‘Mushballs’ reveal ammonia to NASA’s Juno scientists. Published August 5, 2020. 

4. What affects the evaporation rate of a raindrop? Which do you think would evaporate faster: Earth raindrops or raindrops falling on the planet you chose?

Student answers will vary, but students should note that the evaporation rate depends on the droplet’s surface area. Students may mention that droplets with larger surface areas could perhaps initially evaporate faster than droplets with smaller surface areas.

Chemical breakdown

Answer the following questions with your partner to explore the molecular composition of rain on different planets compared to rain on Earth. This information will help you predict conditions that might exist on the planet you chose. 

1. Make a list of the planets and moons mentioned in the Science News article that have rain. State what the rain is made of for each celestial object, whether the substance is an atom or a molecule and give its chemical formula. If the substance is a molecule, state how many atoms of each element are within one molecule of it and what types of bonds exist between the atoms.

Earth, ancient Mars and exoplanet K2 18b rain water. Water is a molecule composed of 2 hydrogen atoms and one oxygen atom. Its chemical formula is H₂O.   

Titan: Rains methane. Methane is a molecule composed of 4 hydrogen atoms covalently bonded to one carbon atom. The chemical formula is CH₄.

Exoplanet WASP 78b: Rains iron. Iron is an element, and has the elemental symbol of Fe.

Jupiter: Rains “mushballs” of ammonia and water. Ammonia is a molecule composed of three hydrogens covalently bonded to a nitrogen atom. Its chemical formula is NH₃ Water is a molecule composed of 2 hydrogen atoms and one oxygen atom. Its chemical formula is H₂O.

2. Using your knowledge of Lewis Dot Structures and the Valence Shell Electron Pair Repulsion model (VSEPR), explore the shape of each molecule in the table below. Make sure to include bond angles and the common name for the molecule’s shape in your VSEPR model. Finally, state whether you would expect the molecule to be polar or nonpolar and why. Note that iron is not a molecule, rather it is a metal atom.

Molecule	Chemical formula	Lewis Dot structure	VSEPR model	Polarity
Methane	CH4	Four solid lines connecting each H to the central C.	Tetrahedral, with bond angels of 109.5°.	Based on the nonpolar bonds and the symmetrical tetrahedral shape, we would assume this molecule is nonpolar.
Ammonia	NH3	Three solid lines connecting each H to the central N, and one lone pair of electrons (two dots) on the central N.	Trigonal pyramidal, with bond angles of 107°.	Based on the highly polar bonds and non-symmetrical shape, you would expect the molecule to be polar.
Water	H2O	Two solid lines connecting each H to the central O, and two lone pairs of electrons (four dots) on the central O.	Tetrahedral bent, with bond angles of 104.5°.	Based on the polar bonds and bent shape, you would expect the molecule to be polar.

3. Draw the general shape of a rain drop. Then draw a zoomed in view of the atoms or molecules within each type of raindrop. Indicate the forces of attraction between the atoms or molecules within the liquid raindrops.

The raindrops should be shaped like the top of a hamburger bun. Zoomed in to the molecular level for each type of rain, students should show discrete molecules or atoms of the substance randomly moving around each other. The attractive forces between the molecules for each type of rain are:

Methane: London dispersion forces, the weakest attraction force.

Ammonia: Dipole-dipole interaction forces with strong hydrogen bonding effects from the bonds between nitrogen and hydrogen. Students’ diagrams should show attraction between the partially negative N in one molecule to the partially positive H in another molecule.

Water: Dipole-dipole interaction forces with strong hydrogen bonding effects from the bonds between hydrogen and oxygen. Students’ diagrams should show attraction between the partially negative O in one molecule to the partially positive H in another molecule.

Iron: We would expect it to bond metallically to other atoms, which means that delocalized electrons will be shared across its lattice structure. Students’ diagram could show individual iron atoms among a sea of delocalized electrons. 

Make it rain

Discuss the following questions with your partner, then create a short weather forecast about a planet other than Earth that you will present to your class. Be sure to include high and low temperatures, and whether you expect there to be a chance of rain.

1. Could the various types of alien rain mentioned in the Science News article exist as rain on Earth? Explain why or why not. (Hint: Consider each type of rain’s intermolecular forces of attraction and the typical conditions on Earth.)

Student answers will vary, but they should focus their answers on the relative difference in intermolecular forces of the substances. The weak forces between methane molecules would not allow it to condense to a liquid in Earth’s atmosphere. The strong metallic bonding between atoms in iron would not allow it to liquefy on Earth, etc. 

2. Based on the intermolecular attraction forces in an alien raindrop, predict a few conditions that might exist on your chosen planet that would allow rain to form. Examples of conditions include high and low temperatures, air pressure, weather patterns, wind speed and direction, and humidity. Use those conditions to create a short weather forecast. Note that you may need to look up additional information, like the boiling point, or full phase diagram of a substance.

Student answers will vary but should include at least a high and low temperature that is in the range of the boiling point of the alien rain substance. Students should also note an air pressure that would allow the substance to exist as a gas in the atmosphere, then condense to a liquid. Encourage students to have fun creating their forecast.