This episode is sponsored by The Ridge.

Go to ridge.com/scishow to see their holiday guide and use promo code “scishow”  to get 10% off your next order. [♪ INTRO]. When you think about what  makes a planet habitable, you might come up with  liquid water on the surface, an oxygen-rich atmosphere, and maybe  what type of star it’s orbiting.

One thing that probably doesn’t make  your list is the planet’s radioactivity. But according to a new study, the abundance  of radioactive elements inside a planet could indicate whether it can support  life, and not in the way you might think. Because while we usually think  of radioactivity as a bad thing, planets like ours might not  be habitable without it.

This paper was published last week  in The Astrophysical Journal Letters, and it focused on two long-lasting,  radioactive elements: uranium and thorium. Elements like these are made in rare events like neutron star collisions  and possibly certain supernovas. So, depending on where a solar system forms, a star and its planets can be  made of different amounts of them.

And according to this team, the amount of  uranium and thorium a planet starts with can make all the difference  when it comes to habitability. They based their model on Earth, and  Earth’s interior has two main heat sources. One is the energy generated  by all that metal and rock squishing itself into a giant  sphere when the planet formed.

And the other is radioactive decay. This is when unstable atoms break down and  give off radiation and ultimately, heat. But it’s not actually how hot a  planet is that’s important here:.

It’s the change in temperature as you  move from the core up through the crust. For an Earth-like planet, you want a core  that’s hot enough relative to the mantle to drive the motion of a liquid metal dynamo; basically, a planet-sized lava lamp. Broadly speaking, this is when  metal in the outer core gets warmer, becomes less dense, and floats up.

Then, it gets cooler and denser,  sinks down, and the cycle repeats. On Earth, this dynamo is extremely important, because it creates our strong,  protective magnetic field. This field directly protects life  from harmful space particles, and it stops the atmosphere from being  stripped away by radiation from space.

So, according to this paper, how much heat radioactive elements  generate could be a big deal. Using computer simulations, they demonstrated that if Earth  formed with more uranium and thorium, a lot of it would have been trapped in  the mantle and acted as an insulator. That would have made the temperature of the mantle too similar to the core for the  churning dynamo to keep going.

So, we would have lost our magnetic field. On the flip side, the simulations also confirmed  that if Earth had too little radioactive material, it would cool too fast and  become geologically dead, with none of the processes that lead to volcanism. And volcanic activity was important  for creating Earth’s early atmosphere.

So, for Earth to work,  everything had to be just right! That said, these are only initial  predictions using a simplified model. So before we can say this is definitely  a thing, we’ll need more simulations.

But if we can get to that point, astronomers could use this information  to keep hunting for habitable planets, looking for signs of uranium  and thorium in other worlds. Meanwhile, in the journal Science, researchers published another paper last  week about a world closer to home: Mars. And this time, they’re challenging some  ideas about how Mars is losing its water.

Thanks to decades of exploration, we know that the Martian surface  used to be covered in liquid water. We also know that what water is  left is vanishing before our eyes. The classic idea is that this  happens in a slow, steady trickle:.

Exposed water ice on Mars turns into gas, which accumulates in the  lower part of the atmosphere. Then, those water molecules occasionally get  struck by energetic particles from space, which breaks them into lighter  compounds which then float up and away. Except, that’s not what  this team found support for.

In this paper, they looked at the abundance  of gasses in Mars’s upper atmosphere, using data collected by the MAVEN spacecraft. And among other things, they found that there was a lot more water in the  upper atmosphere than predicted. They also confirmed some older results, about how Mars’s water loss  changes with the seasons, something that doesn’t match the classic  idea about that slow, constant trickle.

According to them, all this helps  explain how Mars really loses its water. Instead of gathering in the lower  atmosphere, their evidence suggests that the water vapor rises high above  the surface, into the ionosphere. There, it lasts as briefly as a  few hours before it’s destroyed by reacting with charged carbon dioxide molecules.

That’s over ten times faster  than what we previously thought! The team also proposed that this  process would change with the seasons, with more water vapor moving to the upper  atmosphere as Mars approached the Sun. And finally, they confirmed just how important  the planet’s dust storms are to water loss.

They’re not clear on how this happens,  but they found that a global dust storm can transport as much water  into the upper atmosphere as what might normally happen in a whole year. Overall, this kind of information is great for  understanding what’s happening on Mars today, but the researchers used it  to look to the past, too. From the MAVEN data, they  extrapolated back one billion years.

And they estimated that thanks  to both of these processes,. Mars would have lost hundreds of  trillions of liters of water since then; the equivalent of a global  ocean 61 centimeters deep. Or a global puddle.

A really deep puddle. So, there’s still a lot of research to be  done in piecing together Mars’s history, and in understanding what it takes to  have a planet that can support life. But thanks to studies like these, we’re  getting closer and closer every week.

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