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Cassini's last gasp is a unique chance to understand Saturn's atmosphere

NASA/JPL-Caltech/Space Science Institute

For the past 13 years the Cassini spacecraft has been exploring Saturn and its many moons, but this Friday engineers at the Jet Propulsion Laboratory in Pasadena are crashing the probe into the planet's complex atmosphere, on purpose.

While that sounds bad, it's arguably one of the probe's most important moves of the entire mission because it'll help scientists understand the gas giant as they never have before.

The reasons for the crash are twofold. One, it protects Saturn's moons from contamination by the spacecraft, as there could be life on Titan and Enceladus. Two, it'll let scientists sample Saturn's atmosphere, something that's never been done before.

"When the scientists heard that, it was like kids in a candy store," said Cassini's deputy project scientist Scott Edgington. "Their eyes just opened up and said that's it. We could learn so much more about Saturn by being up close and personal."

When Cassini hits Saturn's atmosphere, it'll be traveling at about 77,000 miles per hour, live-streaming its readings back to Earth. It'll be measuring the makeup of Saturn's atmosphere as well as surveying the planet's magnetic and gravitational fields. But the type of data that we'll get back, and how much, is questionable. The friction caused by the atmosphere will eventually disorient Cassini, cutting off the live-stream. The question for scientists is how long until that happens? Edgington said that they're hoping the transmission lasts for two minutes.

If it does, the data could be crucial for our understanding of the gas giant, of which we have a lot left to learn.

Scientists are pretty sure that the planet’s made up mostly of hydrogen and helium with a smattering of methane and complex molecules, but they're not 100 percent sure. They've never sampled the atmosphere first hand. Their theories are informed by the behavior of the atmosphere and measurements of how sunlight refracts as it passes through the planet's atmosphere. 

We think that a Saturn day is only 10 hours long, but we're unsure because of how the planet's magnetic field and spin axis are nearly perfectly aligned.

We've have observed epic, six-month storms that make their way around the planet every 20 to 30 years, but they're unsure of why they form or last so long.

It's known that there's a giant hexagonal storm on the planet's north pole, but we don't know why it's there or how deep it goes.

The interior of the planet is even more of a mystery, since we can't see it or send a probe down to measure it.

We do have some theories though.  

Imagine you had a magical spacecraft. With it, you could head past Saturn's upper atmosphere and directly to the planet's core. What we do know is that, even though it's a gas giant, the interior of Saturn isn't like a smoke bomb -- ephemeral and easy to pass through. In fact, it would be like nothing you'd experienced before.

Travel beyond the first part of the atmosphere and you'd eventually reach puffy clouds, tinted by complex molecules reacting to sunlight, according to Edgington. The clouds are a component of the massive storms that we've observed engulfing the planet for months at a time.

The deeper you go, and the more planet that's above you, the hotter things start to get. The pressure increases, and the atmosphere starts to thicken.

"I imagine pea soup," said Edgington. 

The temperature could reach thousands of degrees, the pressure rises and the molecules around you begin to collide so strongly that electrons are coming off, the first indications of the planet's magnetic field.

"And then at some point, and this is still a controversial theory, it gets to a temperature and pressure where helium starts to condense from a gas phase into a liquid phase," said Edgington. "It's possible that there's helium rain."

You'd be in complete darkness. No sunlight could penetrate the dense layers of gas above you. But even if your magical spacecraft had a spotlight, it's not clear what you'd see when you finally reached the core.

"You're probably talking something you're familiar with, rock ice, but probably very very hard and probably very very molten," said Edgington, but its unclear how the material would behave at such a high temperature and pressure.

"Is a solid really a solid?" he said. "We’re learning that it’s more complex inside than our previous models have predicted."

The theories about the interior are obviously just that, as we don't have a spacecraft to send deep in Saturn.

But, what we do have is a curious probe that's running out of fuel, sacrificing itself for a crucial two minutes of science. Because when it samples the atmosphere and takes measurements of the gravitational and magnetic fields, we'll get a much clearer picture of what's going on deep inside the gaseous planet.

Due to the distance between Saturn and Earth, it'll take us 90 minutes after Cassini's last transmission to know what data it was able to gather. And it'll likely be the last time, at least in our lives, that we get such intimate details of the sixth largest planet in our solar system.