The Why Files The Why Files --

A bolt from the black

22 DEC 2004

Scared of lightning? Then don't move to the ringed planet. The spacecraft Cassini found that the radio signals of lightning on Saturn are about 1 million times as intense as they are on Earth. edge of sphere with distinct rings visibleThat finding, along with other news from the distant planet, were released Dec. 16 by several Cassini science teams.

Cassini rounded Saturn on Oct. 27, 2004 and photographed the dark portion of the rings. Some of Saturn's atmosphere is visible, and its shadow on the rings. The infrared image was shot from about 618,000 kilometers away (click on photo to enlarge). Photo: NASA

Last July, seven years after its launch, Cassini became the first satellite to orbit Saturn. The $3.3 billion mission was supported by NASA, the European Space Agency and the Italian Space Agency. It's a piggy-back mission.

Cassini itself is chockablock with 12 instruments. On Dec. 24, Cassini is set to release its hitchhiker, Huygens, which will carry another six instruments on a short descent to the frozen hydrocarbon hell of Titan, a large moon discovered by Dutch astronomer Christiaan Huygens in 1655. Cassini itself got its name from Italian-French astronomer Jean Dominique Cassini, AKA Giovanni Cassini, who first saw four other Saturnian moons.

Even without Huygens, Cassini is bespeckled with scientific instruments. Photo: JPL
many labelled futuristic metallic ship with dish antennae-looking contraption on top

Doing the wave
Among Cassini's dozen instruments is the radio and plasma wave science instrument, which was designed by a team directed by Don Gurnett, a professor of astronomy at the University of Iowa who has listened to radio signals in space for several decades.

Even before Cassini reached Saturn, the gadget started bringing home the scientific bacon regarding lightning. As people who fish know, it's easy to "hear" lightning on the AM radio band.

Stronger lightning makes stronger radio signals, and Cassini heard Saturn's lightning from 161 million kilometers, compared to just 90,000 kilometers for Earth's lightning. Simple math says Saturn's radio noise, and presumably its lightning as well, was more than 1 million times as intense as Earth's.

dish antennae looking instrument: The Cassini Spacecraft. rpws section highlightedThe three arms of this antenna feed signals to Cassini's radio and plasma wave science instrument. Image: NASA-JPL

That, Gurnett says, is a "staggering intensity."

Staggering, perhaps, but why hasn't anybody seen it? Perhaps because the bolts occur deep in Saturn's dense atmosphere, where light can't escape. Deep lightning could also be more intense, Gurnett says, because a denser atmosphere would allow a greater charge to build up.

While optical astronomers have not seen Saturn's lightning, they have seen storm clouds on Saturn that coincided with the signals of lightning.

All hail the ringed planet
Saturn is not famous for lightning, but for rings, and Cassini has also helped flesh out our understanding of those enormous and beautiful circles of dust. For its own safety, Cassini's orbit passes through gaps in the rings, but even here, the radio receiver recorded heavy dust impacts.

When microscopic flecks of dust hit Cassini in the range of 20 kilometers per second, they create a cloud of charged gas (plasma) and a tiny electrical signal, which is picked up by the nearby antenna, Gurnett explains. Even in a gap between two rings, the impact sounded like hail in an Iowa thunderstorm: "We detected, right at the ring plane crossing... between the F and G rings, up to 1,200 impacts per second," Gurnett says. "It sounds like hail hitting a metal roof."link to first hail sound file

Click to hear the radio sound of dust striking Cassini as it slipped between two of Saturn's rings (544 KB). Courtesy Don Gurnett , University of Iowa

The crescendo of impacts showed that the dusty region between rings was about 300 kilometers thick. While the legendary spacecraft Voyager I and II detected this dust back in 1980-81, the modern instruments give a fuller picture of particle size, Gurnett says.

Polar opposites
Beyond dust and lightning, Cassini has also come up with intriguing data on Saturn's rotation and magnetic field, says Gurnett, who also worked on Voyager and Galileo. On the rocky inner planets, it's easy to measure one day -- one complete rotation.

pitted portion of grey planet (click to enlarge)On 14 December, 2004, Cassini shot this photo of battle-scarred Dione, a moon of Saturn. Photo: CICLOPS

But on the gas-bag outer planets: Jupiter, Uranus, Neptune, and Saturn, "It's a big issue," says Gurnett. Astronomy books give "a very accurate number, within seconds," for the rotation rate, he says, "But you might wonder how they get that, these are big balls of gas, you can't see a surface, there are clouds moving, the jet stream is different at different altitudes and latitudes."

If you can't see a surface -- if you can't even be sure the planet has a surface -- the "standard way" to find rotation is to study periodic changes of radio signals, Gurnett says. "The idea is that radio emissions are produced by charged particles, and their motion is controlled by the magnetic field of the planet, which is linked to the deep interior," and thus can be expected to rotate at the same rate as the planet's big belly.

So far, so good. Jupiter's rotation -- unlike Saturn's -- is detectable from Earth, and 50 years of measurements have found a rotation that is identical "within a matter of seconds."

Not Saturn. When Voyager cruised past in 1980 and 1981, the "day length" was 10 hours, 39 minutes and change. But when Cassini came calling, a quarter-century later, the "day" somehow seemed six minutes longer, Gurnett says. It's inconceivable that Saturn slowed six minutes in 23 years, but the change is "difficult to understand," Gurnett says. "Saturn is an anomaly; it's a big puzzle for us."

Still to come: intensive exploration of exotic Titan, a big moon that is emerging as one of the oddest objects in the solar system. Cassini is to drop Huygens Dec. 24. The little disc is scheduled to parachute onto Titan on Jan. 14.

-- David Tenenbaum little alien head

Hear, here (the sounds of space):
If you think space is just sitting out there taking up empty, well, space, think again. It's full of radio waves and plasma waves. Don Gurnett from the University of Iowa has been tuning into these waves for years. Here are a few of his greatest hits:

Gurnett uses radio emissions to figure out how fast Saturn is rotating. The rotation rate is obtained by analyzing the periodic rotational change in radio emissions. Radio emissions are generated by charged particles, and their motion is controlled by the planet's magnetic field.

link to bowshock sound file When the Cassini spacecraft crossed the bow shock of Saturn on June 27, 2004, this sound recording was made. A bow shock is a break in the solar wind, where the solar wind, an outflow of charged particles, meets a planet's magnetic field.

link to earth whistlers sound file Whistlers are produced by lightning and travel along Earth's magnetic field from one hemisphere to the other. In the ionized gas in this region of space, high frequencies travel faster than low ones, so the wave from the lightning stroke becomes a whistling tone. Notice how the frequency drops as time passes.

--Megan Anderson

Radio and Plasma Wave Observations at Saturn from Cassini's Approach and First Orbit Don Gurnett et al, Science Express, published online December 16 2004. 10.1126/science.1105356

ESA does Cassini

Cassini-Huygens mission

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