Previous: Using Sunlight to Propel Spaceships
Next: Great Minds: Tycho Brahe, the Astronomer With a Pet Elk



View count:174,408
Last sync:2019-06-14 01:00
Carl Sagan predicted some amazing things including the aftermath of nuclear war.

Hosted by: Hank Green
Dooblydoo thanks go to the following Patreon supporters -- we couldn't make SciShow without them! Shout out to Justin Ove, Justin Lentz, David Campos, Philippe von Bergen, Chris Peters, Lilly Grainger, Happy Birthday!!, and Fatima Iqbal.
Like SciShow? Want to help support us, and also get things to put on your walls, cover your torso and hold your liquids? Check out our awesome products over at DFTBA Records:

Or help support us by becoming our patron on Patreon:
Looking for SciShow elsewhere on the internet?

[SciShow Space theme music playing]

[Hank] November 9, 2015 would've been Carl Sagan's 81st birthday.  We've talked a lot about Sagan here on SciShow Space and on SciShow cause it's hard to make a show about space science without his contributions coming up.

He's probably best known for his work explaining the nuances of astronomy, cosmology, and astrophysics as well as helping found SETI. But Sagan was also involved in plenty of academic research.

We've spent some time exploring his studies of Venus' weather and Mars' wind, for example.  But he was also part of a team that figured out Saturn's moon Titan could contain amino acids, the building blocks of proteins.  

And he helped develop the concept of nuclear winter, a possible effect of widespread nuclear explosions that made everyone during the Cold War years realize just how high the stakes were when it came to the potential for nuclear war.

But before Sagan studied apocalyptic chemistry, he was busy naming a whole new class of chemicals.  In the 1970s, Sagan's research involved taking the compounds you'd find out in space, like between the stars or on other worlds, and blasting them with ultraviolet radiation.

He and other researchers noticed that, a lot of the time, they'd end up with weird, brownish, tar-like substances that didn't seem to form naturally on Earth.

They had trouble figuring out exactly what this gunk was made of, and there was no official name for it.  So in 1979, Sagan and another astronomer Bishun Khare proposed that these compounds be called Tholins after the Greek word for muddy.

Tholins are now known as collections of complex organic molecules sometimes including amino acids that form when simpler molecules like methane or ethane are exposed to UV radiation.

Sagan and Khare pointed out that if Tholins were found in space, which is full of UV radiation, it might explain why astronomers kept finding evidence of complex gases in the interstellar medium, the material that's often found between stars.

Before their discovery, some scientists were trying to figure out how those complex gases formed from simpler ones.  But Sagan and Khare were the first to show that they might actually have formed from Tholins that were breaking down.

Tholins, it turns out, are also the compounds responsible for making reddish worlds look red.  In 1986, Sagan and Khare led a group of astronomers who figured out that it was Tholins who give Titan's atmosphere its reddish haze, and there could be amino acids in those Tholins, which could mean that the moon also contained amino acids.

So to test their hypothesis, they decided to re-create Titan's atmosphere here on Earth.  Not the whole thing - that would've killed everyone.

Their team combined nitrogen and methane to simulate Titan's atmosphere and then added energy in the form of electricity.  In the end, they were able to isolate 16 different amino acids from their experiment.  And to this day, partly because of this research, many astronomers believe that Titan probably has amino acids in its atmosphere.

In the paper announcing the results published in the journal Nature, researchers explored an implication that was Sagan's trademark.  If the building blocks of protein are on Titan, does that mean there could be life there too?

And they didn't think so, at least not yet.  Tholins on ancient Earth might have produced amino acids that led to evolution of life, but they thought Titan was too cold for that.  Nonetheless, they said, as the sun ages and Titan warms up, conditions could become a lot more favorable to life.  

Which was encouraging in its own way because back in 1983, Sagan was among the authors of a paper that for the first time described how nuclear winter could have cooled the planet way down.

In effect, Sagan and his colleagues warned the world that the Cold War could have literally made the planet cold.  For those of you who weren't around at the time, the threat of nuclear war was scary back in the 1980s.  

But the conventional wisdom with it, at least some people like those in remote areas would probably survive the initial fiery radiation-filled aftermath of a nuclear exchange.

Scientists, including Sagan, suggested that the survival of the human race might not be that simple.  After all, they pointed out, other mass extinctions may have had a lot to do with particles in the atmosphere that absorbed sunlight and cooled the planet down so much that many species couldn't survive anymore.

Couldn't the smoke and dust from a series of nuclear blasts and the subsequent firestorms in cities do the same thing?

According to Sagan and other authors of the paper, the answer was yes.  And this time around, humans probably wouldn't make it through.

They combined climate models with several different scenarios ranging from just a few bombs on major cities to large scale nuclear war where everybody emptied their silos.  And it turned out that detonating 5,000 megatons of nuclear bombs equal to 100 explosions of the most powerful bomb ever made would start a nuclear winter.

Around a month later, they calculated, average land temperatures around the world would drop to negative 23 degrees Celsius, and it would take about a year to get close to normal again.

Even 100 megatons would be enough if the targets were major cities, they concluded, because the fires would spread, creating more particulate matter in the atmosphere.

That's not a pretty picture.  And the paper reignited discussions of the consequences of nuclear war.

Go science!  And go Carl Sagan.  Thanks a lot, man.

And thank you for watching this episode of SciShow Space news and especially to all of our patrons on Patreon who make this show possible.  If you want to help us keep making shows like this, you can go to, and don't forget to go to and subscribe.

[SciShow Space theme music playing]