8 The giant planets

8 The giant planets

8.1Giant planets are large, cold, and massive

8.1.1 Overview

Brown, Mike. How we discovered planet X,” Astronomy, 44, 6, June 2016, pp. 20-25

Sheppard, Scott S. “The hunt for planet X,” Sky and Telescope, 134, 4, October 2017, pp. 16-21

8.1.2 Characteristics of the giant planets

Dunham, David. “Preliminary results from Triton cover-up,” Sky and Telescope, 135, 2, February 2018, p. 13

8.1.3 Composition of the giant planets

8.1.4 Rotation of the giant planets


Ocean on Ganymede

Hubble Telescope Spots Ocean on Jupiter Moon Ganymede[1]

The biggest moon in the Solar System harbors a salty ocean beneath its frozen surface, according to a study that examined the moon’s flickering auroras to probe its interior.

A number of Solar System objects are thought to have oceans. But this is the first clear-cut data of its kind to suggest that a sea lies hidden under the icy shell of Jupiter’s moon Ganymede, which is 50% bigger than our own Moon. Scientific models predicted an ocean on Ganymede, and when NASA’s Galileo spacecraft visited Ganymede in the 1990s, it collected data that hinted at an ocean. But new images from the Hubble Space Telescope offer strong confirmation of a liquid body of water inside Ganymede, scientists say.

Galileo’s observations “provide inconclusive evidence for the ocean,” says study co-author Joachim Saur of the University of Cologne. “The Hubble data require an ocean.”

Finding an ocean on a celestial body hundreds of millions of miles from Earth is no easy feat. Saur and his team turned to the space-going Hubble, which trained its keen eyes on Ganymede in 2010 and again in 2011. The Hubble focused on Ganymede’s two auroras, shimmering patterns in the sky similar to the earthly phenomenon known as the Northern Lights. A person standing on Ganymede’s surface and looking up would see a red glow, Saur says.

Ganymede has two auroras, one around its north pole and one around its south pole, both created in part by the moon’s own magnetic field. These auroras don’t stay fixed in place. Instead, they wander slightly across Ganymede’s face. With the help of supercomputers, the scientists calculated how much Ganymede’s auroras would shift if the moon had a salty sea. A layer of salty water could carry electrical current, generating another magnetic field that would affect the auroras.

The researchers found that the aurora shift witnessed by Hubble nicely matched the prediction of what should happen if Ganymede has an ocean. Just as importantly, the Hubble data did not match the prediction for an ocean-less Ganymede, the scientists reported online February, 2015 in the Journal of Geophysical Research.

Ganymede’s ocean is sandwiched between two layers of ice. That’s not particularly hospitable to life, says planetary scientist William McKinnon of Washington University in St. Louis, who didn’t work on the new study. But Saur says it’s still possible that Ganymede’s waters are habitable.

Other scientists praise the study for revealing important new evidence about Ganymede’s hidden interior.

Previous evidence of an ocean on the gigantic moon was “somewhat ambiguous,” University of California-Santa Cruz planetary scientist Francis Nimmo, who was not part of the study, says via e-mail. “So this study is … nice in that it provides independent confirmation of a subsurface ocean on Ganymede.”

The new findings are “very significant,” McKinnon agrees, though he thinks they’re not “air-tight. … What we need to do is go back to Ganymede and take measurements on site.”

Fortunately for Ganymede partisans, the European Space Agency is planning to launch a spacecraft in 2022 to explore this supermoon and its neighbors. Saur says he welcomes the mission as a chance to confirm his study’s findings, which he calls “solid science” but based on an “indirect method.”

A Solar-System sea is “really only 100% certain,” he says, when “you have a finger in the water.”

[1] Traci Watson, in USA TODAY (March 12, 2015)

NASA’s Jupiter Mission

NASA’s Jupiter Mission[1]

Multiple images combined show Jupiter’s south pole, as seen by NASA’s Juno spacecraft from an altitude of 32,000 miles. The oval features are cyclones. (Credit NASA/JPL-Caltech/SwRI/MSSS/Betsy Asher Hall/Gervasio Robles)

The top and bottom of Jupiter are pockmarked with a chaotic mélange of swirls that are immense storms hundreds of miles across. The planet’s interior core appears bigger than expected, and swirling electric currents are generating surprisingly strong magnetic fields. Auroral lights shining in Jupiter’s polar regions seem to operate in a reverse way to those on Earth. And a belt of ammonia may be rising around the planet’s equator.

Those are some early findings of scientists working on NASA’s Juno mission, an orbiter that arrived at Jupiter last July.

Juno takes 53 days to loop around Jupiter in a highly elliptical orbit, but most of the data gathering occurs in two-hour bursts when it accelerates to 129,000 miles an hour and dives to within about 2,600 miles of the cloud tops. The spacecraft’s instruments peer far beneath, giving glimpses of the inside of the planet, the solar system’s largest.

“We’re seeing a lot of our ideas were incorrect and maybe naïve,” Scott J. Bolton, the principal investigator of the Juno mission, said during a NASA news conference on Thursday, May 25, 2017).

Two papers, one describing the polar storms, the other examining the magnetic fields and auroras, appear in this week’s issue of the journal Science. A cornucopia of 44 additional papers are being published in the journal Geophysical Research Letters. The papers describe findings based largely on the first two close passes of Jupiter in which Juno was able to make measurements. Juno has now made five, with the next on July 11, 2017, when it is to pass directly over the Great Red Spot.

Scientists are puzzled to see that the familiar striped cloud patterns of Jupiter may be only skin deep. An instrument collecting microwave emissions probes the top layers of the atmosphere, but that data does not reflect what is seen in the clouds. “These zones and belts either don’t exist or this instrument isn’t sensitive to it for some reason,” Dr. Bolton said.

The microwave instrument did detect a band of ammonia rising in the equatorial region from at least a couple of hundred miles down—“the most startling feature that was brand-new and unexpected,” Dr. Bolton said.

In measuring the gravitational field, scientists hoped to learn what lies at the center of Jupiter. Some predicted a rocky core, perhaps the size of Earth or several Earths. Others expected no rocky core, but hydrogen, the planet’s main constituent, all the way down. “Most scientists were in one camp or the other,” Dr. Bolton said, “and what we found is neither is true.” Instead, the data suggests a “fuzzy core,” one that is larger than expected, but without a sharp boundary, perhaps partly dissolved.

The magnetic field is also not simple. “What scientists expected was that Jupiter was relatively boring and uniform inside,” Dr. Bolton said. “What we’re finding is anything but that is the truth.”

John E.P. Connerney, a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and the deputy principal investigator on the mission, reported spatial variations in the magnetic field that were much stronger than expected in some areas and much weaker in others.

The magnetic field is generated by the churning of electrically charged fluids at the core. On Earth, that comes from the convection of molten iron in the outer core. On Jupiter, the currents come from hydrogen, which turns into a metallic fluid under crushing pressures.

The spatial variations suggest that the dynamo of churning currents is larger than had been thought and may extend beyond the metallic hydrogen region, Dr. Connerney said.

For the magnetic field and gravity measurements, a glitch that has greatly slowed the pace of data gathering could turn out to be beneficial. A final engine burn last October was to put Juno in a 14-day orbit, but a pair of sluggish valves in the fuel system led mission managers to forgo that, and Juno remains in the 53-day orbit instead. The spacecraft is to make the same number of orbits and collect the same amount of data, and the longer mission means that Juno may be able to detect slow changes in the magnetic field.

More surprises were found at the top and bottom of Jupiter.

With Juno’s orbits passing almost directly over the north and south poles, scientists can better study the powerful auroras, which are generated by charged particles traveling along Jupiter’s magnetic field and colliding with molecules in the atmosphere. In Earth’s case, charged particles from the Sun speeding outward through the Solar System are diverted by the planet’s magnetic field toward the poles, generating light when they collide with air molecules. The expectation was that the same would occur at Jupiter, and it does to some extent.

But Juno also detected charged particles—mostly electrons—traveling in the opposite direction at Jupiter: out of the planet into space. “It’s a 180-degree turnabout from the way we were thinking about those emissions,” Dr. Connerney said.

He said a voltage differential in the atmosphere was drawing the electrons upward.

Earlier photographs of the polar regions were taken from a sharp angle, with details hard to make out. Juno revealed that the clouds there are very different from the usual Jupiter stripes. “What you see is incredibly complex features, the cyclones and anticyclones all over the poles,” Dr. Bolton said.

Planetary scientists had wondered whether Jupiter would have a giant hexagonal pattern like that spotted on Saturn by NASA’s Cassini spacecraft.

On Wednesday [May 24,2017], NASA released new images of Saturn’s north polar region, which has changed color in the last four years, possibly because summer has reached the northern hemisphere.

In the final stages of Cassini’s mission, which ends in September, it has shifted to a looping elliptical orbit, which will enable similar probing of Saturn’s interior.

“Eventually we will compare,” Dr. Bolton said. “We will really be able to advance our understanding of how these giant planets work.”

[1] Kenneth Chang, “NASA’s Jupiter Mission Reveals the ‘Brand-New and Unexpected’”, New York Times (May 25, 2017). A version of this article appears in print on May 26, 2017, on Page A20 of the New York edition with the headline: “Seeing the ‘Brand-New and Unexpected’ of Jupiter.”

Life on Europa?

Life on Europa?[1]

A few places in our Solar System could host microbes, and Jupiter’s moon Europa is a prime candidate.

Planetary scientists believe Europa has an extensive liquid water ocean beneath its icy crust. The sixth-largest moon in the Solar System, Europa is rich in silicates, probably has an iron core, and possesses a tenuous atmosphere and an icy surface striated by cracks and faults.

The extensive amounts of water ice on Europa and tidal flexing help to create the sub-surface ocean, the existence of which was bolstered in 2014 with the detection of plate tectonics on the moon’s thick ice crust the first detection of this activity on a planetary body other than Earth.

Further, in 2013, NASA scientists detected phyllosilicate clay minerals on Europa‘s surface—and these minerals on Earth are often associated with organic molecules. The moon also displays evidence of water vapor plumes similar to those of Saturn’s moon Enceladus.

[1] David J. Eicher at www.ASTRONOMY.COM. The picture shows a cracked icy surface in this Galileo spacecraft image. Picture from NASA/JPL-Caltech/SETI Institute (Europa); NASA/JPL-Caltech/R. Hurt (Cosmic seeds, 10 Tatooines; LLNL (Life from Comets)

Jupiter’s Red Spot

The Not-So-Great Red Spot[1]

Almost everyone has heard of Jupiter’s giant red spot. It is easily viewed with amateur telescopes. The red spot joins Saturn’s rings and the Martian polar caps as “must-see” features for Solar System enthusiasts. But the GRS is an inconstant icon. In fact, in recent years observers have become increasingly alarmed about how small (relatively speaking) the spot has become. Visually it’s far less impressive than it was two or three decades ago.

Is it really shrinking and, if so, might it someday disappear completely? Yes, it really is shrinking. Who knows what its future will be.

We know it is a gigantic storm. Arguably no planetary feature in the Solar System is better known that Jupiter’s Giant Red Spot, shown above in a contrast-enhanced composite of Voyager images from 1979. Note the vast regions of disturbed cloud flow to its north and south, along with the Earth-size white oval gliding nearby.

Studies of the internal dynamics of the spot will help us to model its fate, so we continue to make yearly Hubble observations. Amateur observations are critical for monitoring the GRS. Regular reports help astronomers to fill in the gaps between studies with HST and professional telescopes.

All things considered, we expect that the Great Red Spot will continue to shrink for some time to come. We don’t expect it to disappear completely—but for now, what happens in the years ahead is anyone’s guess.

[1] See Amy A. Simon, “Jupiter’s Shrinking Storm: The Not-So-Great Red Spot,” Sky and Telescope (131, 3 March 2016), pp. 8-21. Also “Jupiter’s Great Red Spot Viewed by Voyager I,” at the NASA web site.