A Day's A Day The World Around — But Shorter On Saturn | KERA News

# A Day's A Day The World Around — But Shorter On Saturn

Mar 28, 2015
Originally published on April 14, 2015 3:38 pm

Researchers have answered a question that has been nagging them for years: Exactly how long is a day on the planet Saturn? The result (10 hours and 32 minutes or so) was published this week in the journal Nature, and could teach scientists more about the giant, ringed planet.

A day is simply how long it takes a planet to spin all the way around. On Earth, one rotation takes 23 hours, 56 minutes and 4 seconds — though combined with the planet's motion around the sun, we earthlings experience 24-hour days.

Measuring a day on rocky planets like Earth is fairly simple, according to Ravit Helled, a planetary scientist at Tel Aviv University in Israel: "You can, you know, identify a mountain or a stone or whatever it is, and just check how long it takes it to come back."

Those tricks don't work on Saturn because it's a gas giant, which means its surface is deep below layers of clouds.

When the Voyager spacecraft first flew by in the 1980s, it estimated the day using the planet's magnetic field. That showed the day was 10 hours and 39 minutes long. But in 2004, another mission called Cassini arrived at Saturn and measured a different value: 10 hours and 45 minutes.

Six minutes makes a big difference. The speed at which Saturn's rocky core rotates determines how fast the winds blow above. It also changes estimates of the planet's internal structure.

"If you want to understand, you know, giant planet formation and the origin of the solar system, these 10 minutes or six minutes are actually quite crucial," Helled says.

So she and her colleagues set out to make a better estimate of Saturn's day. They looked at the planet's gravitational pull and combined that with estimates of the core's density. After some tricky statistical analysis, they found that a day on Saturn is 10 hours, 32 minutes and 45 seconds — more or less.

"We have an uncertainty of 46 seconds, but it's much better than the 15 minutes' uncertainties that we had until now," she says.

Helled eventually wants to use the technique to learn the length of days on planets outside the solar system — though, she adds, scientists will need quite a few Earth days to figure out exactly how to accomplish that.

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SCOTT SIMON, HOST:

I think we've heard enough Earth-based stories for today. Let's take Saturn for a spin. This week, researchers announced they had precisely determined the length of a day on Saturn. NPR's Geoff Brumfiel reports they hope this'll teach them more about the past of the ringed planet.

GEOFF BRUMFIEL, BYLINE: Planets spin, and a day is simply how long it takes to spin all the way around. Ravit Helled is a planetary scientist at Tel Aviv University in Israel. She says it's easy to measure a day from space.

RAVIT HELLED: You can, you know, identify a mountain or a stone or whatever it is and just check how long it takes it to come back and that will be, you know, one day.

BRUMFIEL: On Earth, one rotation takes 24 hours - actually, 23 hours, 56 minutes and four seconds if you want to be precise. And Helled does want to be precise because that's her job, which brings us to Saturn. It has those pretty rings, but the planet itself is a beige ball of gas. It's surface is covered in clouds. You can't see any landmarks. So when the Voyager spacecraft first flew by in the 1980s, it estimated the day using the planet's magnetic field.

HELLED: The number was 10 hours, 39 minutes.

BRUMFIEL: Then in 2004, another mission arrived on Saturn and measured a different value.

HELLED: They found a rotation period of 10 hours and 45 minutes.

BRUMFIEL: Six minutes makes a big difference. The speed at which Saturn's rocky core rotates determines how fast the winds blow above. It also changes estimates of the giant planet's internal structure.

HELLED: If you want to understand, you know, giant planet formation and the origin of the solar system, these 10 minutes or six minutes are actually quite crucial.

BRUMFIEL: So Helled and her colleagues set out to make a better estimate of Saturn's day. They measured the planet's gravitational pull, combined that with estimates of the core's density and did some very tricky statistical analysis.

HELLED: It's tough, you know. (Laughter) The concept is not - yeah, it's a bit complicated. I'm sorry.

BRUMFIEL: But the bottom line published in the journal Nature - a day on Saturn is 10 hours, 32 minutes and 45 seconds. Helled eventually wants to use the technique to learn the length of days on planets outside our solar system, though, she adds, she'll need quite a few Earth days to figure out exactly how to make it work. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.