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If you think about the planets in our solar system,
Earth definitely stands out, for a lot of reasons.
But one thing that isn’t unique about our planet is rain.
Like, scientists say there might be “mushballs” of water
and ammonia deep in Jupiter’s atmosphere.
And on the exoplanet WASP-76b, it appears to rain liquid iron.
And that’s not where the surprises end, either.
One research duo did some calculations, and they found that planets that seem
completely different may have rain that is eerily similar.
Now, precipitation is a super important part of
determining a planet’s overall climate.
By moving molecules like water through the atmosphere,
or from the atmosphere to the ground, rain changes things like
how much humidity is in the air, or how many clouds are in a part of the sky.
And that changes things like how much sun or starlight reflects off a planet,
and also how much heat from that light gets trapped by the atmosphere,
heat that’s crucial to life in the right amounts.
So, if scientists are interested in learning about a planet’s habitability,
they need to know about its precipitation.
Also, models of rain can act as a stand-in for more complex climate systems,
like cloud coverage, because clouds are just a manifestation of water…
or whatever it is that’s raining.
Scientists have tried to model precipitation on other worlds before,
but their studies were based on data from Earth, as well as cloud models,
which currently don’t capture all the complex physics involved.
So, one team took another approach:
They focused on individual raindrops, and just raindrops.
That’s because a drop’s behavior is governed by some decently well-understood,
general laws of thermodynamics and fluid dynamics.
And that makes the math more versatile if you want to apply it to multiple worlds.
Their study was published in the Journal of Geophysical Research:
Planets in March 2021, and it focused on three properties of a raindrop:
drop shape, falling speed, and evaporation speed.
Now, raindrops are not actually teardrop-shaped.
Instead, they start as spheres, and then
their bottoms generally flatten out as they fall.
If you want to figure out the exact shape, though,
you need to know things like the surface tension of the molecules in the raindrop,
or how much they like to stick to each other.
You also need to know how much gravity they’re experiencing,
and how dense the air around them is.
How fast they fall depends on some of that, too,
as well as what the air is made of and the local temperature.
And finally, how quickly the raindrop evaporates depends on even more factors,
like how easily the rain changes temperature,
how readily the air conducts heat, and what the humidity is.
And of course, there are even more effects to keep in mind
when you throw wind into the mix!
So, sure, this is based on some fundamental laws of physics,
but it is incredibly complex!
In their paper, the team started by testing their model with rainwater on Earth.
They plugged in numbers for a bunch of scenarios,
and got a range of possible raindrop sizes,
from the smallest size that could make it to the ground before evaporating,
to the largest that could fall through the sky without getting broken apart.
Then, they validated those data against modern observations,
experimental results, and other calculations for Earth.
But they didn’t stop there.
To help prove how universal their math was, they also tested their model against
data the Cassini-Huygens probe collected from Saturn’s moon Titan.
Titan has an atmosphere thicker than Earth’s, and it’s mostly nitrogen.
It’s also so cold that it doesn’t rain water: It rains methane.
But this team’s model still worked!
In fact, they eventually found a surprising trend:
They looked at terrestrial worlds and gas giants, as well as at rain made of water
and rain made of a few other common compounds.
And the researchers found that the range of possible raindrop size was eerily similar.
They didn’t give super specific numbers,
but they were all within a factor of ten of each other.
That said, the team acknowledges this is just the first step in their work
to understand the complexities of planetary precipitation.
They didn’t take into consideration how raindrops form,
and they only looked at liquid precipitation because
the shapes of solids would add a lot of complexity to the equations.
Also, scientists in general still need a better understanding of how
the properties of a planet create and distribute entire showers of raindrops.
But if this research holds water, it will open up
new ways of understanding alien worlds.
Like, we could use it to understand how ancient water on Mars
once produced surface features we can see today, like networks of valleys.
Or we could understand why Titan’s methane lakes and rivers look the way they do.
This model would also allow for a better understanding of
how rain works on planets that don’t have surfaces.
For instance, we could learn what consequences those
ammonia mushballs on Jupiter have on the atmosphere as a whole.
And of course, this could be used to study planets beyond our solar system, too.
If scientists can connect rain back to clouds,
then our observations of exoplanet clouds could tell us if it rains on that planet.
Not that you’ll ever get to visit and sing your rain-featuring lyrics of preference.
Even the closest exoplanet is light-years away.
Still, this model has the potential to teach us a lot about the worlds around us. And who knows?
Maybe someday, we will find an exoplanet where
the atmosphere is filled with potassium permanganate.
Because, you know. Purple rain.
Thanks for watching this episode of SciShow Space.
Before you go, you may be aware that July is almost over,
and with that, July’s Pin of the Month is almost gone.
If you want to get this incredibly cute design of the Crawler-Transporter that
crawled us all the way to the moon, and is still in service today,
you only have until the end of the month to preorder at the link in the description.
After that, it’s gone forever, but do not worry,
because we have another great pin planned for next month.
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