Raindrops on alien worlds will obey Earth-like rules

Their size will be similar no matter what they’re made of or on which planet they fall, a new analysis finds.

Raindrops on alien worlds will obey Earth-like rules

Raindrops behave much the same way across the Milky Way, a new analysis finds. This should hold whether we’re talking about a methane torrent on Saturn’s moon Titan or a drizzle of iron on the exoplanet WASP 78b. Regardless of what they’re made of, droplet sizes will always be close to the same size.

“You can get raindrops out of lots of things,” says Kaitlyn Loftus. She’s a planetary scientist at Harvard University in Cambridge, Mass. She and Harvard colleague Robin Wordsworth just published new equations to show what happens to a falling raindrop after it leaves a cloud. Their findings appear in the April JGR Planets.

Previous studies had looked at rain in specific cases. Perhaps it was the water cycle on Earth. Or it might have been methane rains on Saturn’s moon Titan. This analysis is the first to consider rain made of any liquid.

These authors “are proposing something that can be applied to any planet,” says astronomer Tristan Guillot. “That’s really cool,” he adds. He works at the Observatory of the Côte d’Azur. It’s in Nice, France.

Clouds can heat or cool a planet’s surface. Rains help move chemical elements and energy around and through the atmosphere. Scientists want to understand the atmospheres of other worlds, including their clouds and climate, And for that, Guillot notes, understanding rain size “is something that’s needed, really.”

Raindrops follow the law

Clouds are complex. Scientists don’t really understand how they grow and evolve, even on Earth. Raindrops, though, are governed by a few simple laws of physics. Falling drops of any liquid tend to take the same spherical shape. And the rate at which a droplet evaporates depends on its surface area.

“This is basically fluid mechanics and thermodynamics,” Loftus says. And those, she says, “we understand very well.”

She and Wordsworth considered a variety of different rains. This included water droplets on early Earth, on modern Mars and on a gaseous exoplanet called K2 18b. That last planet may host clouds of water vapor. The pair also considered Titan’s methane rain, ammonia “mushballs” on Jupiter and iron rain on an ultrahot gas giant exoplanet called WASP 76b. “All these different [rains] behave similarly,” she finds. That’s because they all must follow the same laws of physics.

Worlds where gravity is stronger tend to produce smaller raindrops, the pair found. Still, all the raindrops they analyzed fell within a narrow range. Their radius spanned only from about a tenth of a millimeter (a few thousandths of an inch) to a few millimeters. Droplets that get much bigger will break apart as they fall, Loftus and Wordsworth found. Much smaller drops, in contrast, may evaporate before hitting the ground (for planets that have a solid surface, anyway). That would keep this moisture in the atmosphere.

Eventually the researchers would like to extend the study to solid precipitation, such as snowflakes and hail. But it won’t be easy. The math to do that is much harder. “That adage that every snowflake is unique is true,” Loftus says.

This new study is a first step toward understanding precipitation in general, says Björn Benneke in Canada. He’s the University of Montreal astronomer who discovered water vapor in the atmosphere of K2 18b. Understanding alien atmospheres, he says, is “what we are all striving for.” Astronomers want to develop a fairly universal picture of how atmospheres and planets work. It’s important, he says, to “not just be completely Earth-centric.”

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Staying grounded in space requires artificial gravity

On TV, people in space walk around like they’re on Earth. How can science give real astronauts artificial gravity? Spin right round, baby.

Staying grounded in space requires artificial gravity

In lots of books, movies and TV shows, people on spaceships walk around like they would on Earth. In real life, though, astronauts in space float. The difference isn’t just because the books, movies and TV are fiction. It’s that in those fictional worlds, artificial gravity exists. In our world it doesn’t — yet. But it may be coming.

Gravity is a fundamental force. It attracts objects with mass toward each other. Objects with a lot of mass — such as Earth — attract other objects toward their centers. This is why we stand firmly on the ground no matter where on Earth we are. Gravity decreases with distance, though. So as people travel to the Moon or Mars, their pull toward Earth quickly weakens, which leaves them floating.

Scientists Say: Gravity

This might seem like fun. But life without gravity isn’t great. In the long term, our bones and muscles don’t work as hard in a gravity-free environment. This weakens them. Without gravity, blood and other bodily fluids don’t flow normally and can collect in the upper body. This can cut off hearing.

Also, floating around in zero gravity makes you puke.

In fact, notes Mika McKinnon, “We know a lot of ways to have the same effect as gravity using other forces.” She is a physicist with the Search for Extraterrestrial Intelligence (SETI) Institute. It’s in Mountain View, Calif. And at least a few of the simpler tactics might not be that far off.

Mass attraction

One approach would be to “use electricity and magnetism as a way of substituting for gravity,” McKinnon explains. “You can create that magnetic field by running electricity around in circles,” she says. The flow of electric current produces magnetism. All an astronaut would have to do is wear metal boots. The attraction between the metal and the magnet would help someone walk along the floor.

The work required to walk against a magnet might also limit bone and muscle loss in space. But being stuck to the floor isn’t the same as gravity. Fluids would still be able to collect in the upper body. And your stomach would still be awfully confused.

Let’s learn about gravity

Scientists could try to harness real gravity, McKinnon says. Everything with mass has gravity, she points out. So one simple idea would be to have a lot of mass. “Build yourself a planet and then you’ve got enough gravity,” she notes. Then again, she adds, That’s not very convenient to have to build a planet or carry a planet around with you.” Instead, she explains, the key might be to get a lot of mass into a very small area.

Neutron stars, for example, are extremely dense. A teaspoon of neutron-star material might be enough to give us gravity, she says. Or a “tiny pencil prick” of a black hole. Both of these exert vast amounts of gravity for their size.

But how could you contain a black hole — even a tiny one — in the middle of a spaceship? “That’s an engineering problem,” McKinnon says. “And we have no idea what the engineering would be.”

Ring around the spaceship

If you’ve ever been on a carnival ride like the spinning teacups, you’ve felt artificial gravity. When you are inside a large, spinning object, you will feel a pull toward the outside wall. This is because of inertia. Your body is resisting the change in motion of the object spinning around you.

We feel inertia as something that doesn’t exist — centrifugal force. This force seems to pull us to the outside edge of the rotating teacup.

Centrifugal force is really inertia. But if all you need is artificial gravity, then such an imaginary force works fine. All you need is either a small ship, rotating very fast, or a very large ship rotating slowly. Either way, the spin would pull someone feet-first toward the outside wall.

This is an improvement over magnets, because the whole body would feel the effect. Blood and fluids would move through the body just as they do on Earth. Bones and muscles would feel the pull when someone walked or ran.

A large version of such a system is called an O’Neill cylinder. It’s named for physicist Gerard O’Neill, who came up with the idea. A pair of these vast rotating cylinders would sit aimed toward the sun and spin in opposite directions. Those opposite spins would help hold them in place.

“The only reason we won’t have them is they are huge,” explains Joalda Morancy, who uses they/them pronouns. A junior at the University of Chicago in Illinois, they are studying physics and astronomy. Morancy also is an intern at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

And Morancy isn’t kidding when they say O’Neill cylinders are huge. O’Neill’s original idea was to create space habitats eight kilometers (five miles) across and 32 km long. “About a million people could live there,” Morancy says. “I really wish I could get to see one.”

Jeff Bezos, the founder of Amazon and the space company Blue Origin, is interested in building O’Neill cylinders. But that’s a long way off.

There’s also the problem of where to build them. Such a structure could probably be built on Earth. But how do you send something 32-km long into space? “It would cost so much and take a lot of rockets,” Morancy says.

An easier, lower cost option would be to assemble those giant habitats in space. But “we’re closer to the tech to get us to Mars than we are to building things in space,” Morancy notes.

On the International Space Station, astronauts have to do everything while floating free. A rotating room might give them artificial gravity — if they can take the spin.NASA

Spin your astronaut round and round

Smaller rotating objects can provide the same effect as O’Neill cylinders. The smaller the object is, though, the faster is must spin to give you the feeling of gravity. And that spin has its own challenges. Spend enough time in a small spinning teacup and your stomach may soon object.

What’s more, people in or on rotating objects suffer from the Coriolis effect. This is a deflection that occurs when objects not attached to the ground travel at high speeds or long distances relative to a rotating planet. As an object flew through the air, the ground below would be spinning. So the object would appear to deflect a little, landing to the side of where it was headed. Most of the time, this difference is so tiny you’d never notice. If you threw a baseball from New York City to the equator, though, you definitely would.

The Coriolis effect is another fake force. Like centrifugal force, it’s actually inertia (yes, again). And like centrifugal force, the Coriolis effect is noticeable. In a fast-spinning spaceship, your arms would be forced to one side as you lifted them. 

But the effect on your arms is nothing compared to the effect on your brain. People in a quickly rotating cylinder suffer from what’s called the cross-coupled illusion, notes Katherine Bretl. She’s an aerospace engineer at the University of Colorado in Boulder. When someone is inside a spinning ride — or a spinning space station — they often feel fine as they look forward. The cross-coupled illusion is that “tumbling feeling you get when you turn your head.”

Luckily, Bretl has found a way to overcome the problem. She and her colleagues have been putting people in a spinning chair and making them turn their heads for science. In one study, each recruit sat in the spinning chair for 25 minutes each day for 10 days. The chair started out spinning slowly — only once per minute. Over time, Bretl slowly increased the speed. After some 10 days, volunteers could tolerate being spun around more than 11 times per minute. After 50 days of training, they can spin more than 25 times per minute, on average. And to date, Bretl adds, “We haven’t had anyone puke.”

In 2019, her team described the 10-day procedure in the Journal of Vestibular Research. The 50-day results appeared last year in the journal npj Microgravity.

A room on the space station could rotate fast enough that astronauts would feel a gravitational force of about 1 g — the same as they would feel on Earth. The room wouldn’t have to be big, only about 2.6 meters (8.5 feet) across. That’s small enough to attach to the ISS. “Maybe a pair of modules opposite of one another rotate,” Bretly says. “Astronauts would be standing on a treadmill as this system is rotating.” The astronauts could work out in the rotating gyms, to make sure their muscles, bones and circulation stayed healthy. The rest of the time they would float throughout other parts of the space station.  

“I think a lot of people look at [artificial gravity] and think it’s super far off,” Bretl says. “But I don’t think it has to be.” Those huge O’Neill cylinders are probably a long way away. But “artificial gravity doesn’t require that large, super-expensive, massive system in order to provide benefits for the astronauts.”

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