[Intro plays]

Caitlin: Between the discovery of gravity waves on Pluto and radioactive interstellar rain on Earth, there’s a whole lot of atmospheric science to talk about this week. So let’s start with everyone’s favorite dwarf planet.

After analyzing some photos from the New Horizons spacecraft, NASA scientists say that there’s evidence of gravity waves in Pluto’s atmosphere, which had only been detected on Earth and Mars before. But, wait. Let me be clear. These gravity waves aren’t the same as gravitational waves -- y’know, the ones we just managed to see for the first time a few months ago. Whoever named this stuff really didn’t make things easy.

Gravitational waves are created when large objects like stars move around in space, causing energy ripples in spacetime itself -- which you can imagine like ripples on the surface of a lake after something like a boat goes through. Gravity waves, on the other hand, are the actual ripples that a boat makes in an actual lake here on Earth.

To make gravity waves, all you need is a place where two fluids with different densities are touching, like air and water -- since air can flow, it’s considered a fluid in physics. So, when the pond water is still, the force of gravity pulling the air down is balanced by the water’s force of buoyancy pushing up. But when a boat speeds through, it pushes the water in front of it up a bit. The force of gravity pulls the water back down, trying to restore balance, but the water ends up below the original surface level.

Then, buoyancy pushes that water back up, but it gives the water so much momentum that it overshoots, and ends up back above the original surface level. And then the whole thing repeats, so you get a section of the water moving up and down -- and that’s a gravity wave. And waves in water are just one way to visualize gravity waves. When a layer of air is pushed over mountains -- which causes that initial bump -- and interacts with a colder, denser layer of air, it can create these rippling clouds -- another form of gravity wave.

These gravity waves have been seen in the atmospheres of Earth and Mars, but not on any other planets. At least, not until the New Horizons team noticed hazy layered structures moving up and down in Pluto’s skies -- which they think are because of gravity waves. So even though New Horizons passed by Pluto last July, we’re still learning a lot from it.

Now, let’s move on to that radioactive rain. A few weeks ago, an international team of scientists published a study about detecting the radioactive remnants of faraway stars at the bottom of Earth’s oceans. And just last week, a new paper suggests that these radioactive elements may still be raining down on Earth. Just -- very slowly.

See, when a star much bigger than our Sun reaches the end of its life, it dies in a fantastic explosion called a supernova and sprays its guts out into the surrounding cosmos. Most of this stuff is hydrogen or helium, since those make up most of the star. But there’s also some heavier elements that the star synthesized in its core right before exploding -- like a radioactive form of iron called iron-60, which was recently found deep in the Earth’s crust. So how did it get there?

The international team of scientists reported that the iron-60 might have come from a series of relatively recent, nearby supernovae -- which would have exploded sometime between 1.7 and 3.2 million years ago. This means that Earth was bombarded by dust and gamma rays from supernovae right around the last ice age. These researchers also found evidence of iron-60 from at least one other supernova -- from around 6.5 to 8.7 million years ago. Again, right when the Earth was going through some big climate changes.  So they think there’s a small chance that nearby stars, after their fiery deaths, might have affected the climate here on Earth -- but they don’t have much more evidence for it yet.

Then, last week, scientists analyzing data from NASA’s Advanced Composition Explorer probe found evidence that more iron-60 has been raining down on us for the past 17 years -- from supernovae a couple hundred light years away. But only at a rate of about 1 nucleus per year, which is not enough to do very much at all, and definitely not enough to hurt us.

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