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


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.”

A New Venusian Look-Alike

A New Venusian Look-Alike [1]

A new member of the exoplanet club is sparking excitement in the astronomical community. Angelo et al. published in The Astronomical Journal (DOI: 10.3847/1538-3881/aa615f) their discovery of an object 219 light years from Earth—Kepler-1649b — that is strikingly similar to our bright sister planet, Venus. With the help from the Kepler mission transit data and observations from the Mount Palomar Observatory in California, the team was able to analyze the flux of radiation onto the planet and the planet’s radius, concluding that the size and the amount of radiation it receives from its sun is consistent with the values for Venus. This Venus doppelgänger has a few notable differences, however. Kepler-1649b takes just nine days to orbit around a sun that’s one quarter the size of our own. The group noted that by default the planet must travel much closer to its pint-sized host star to receive the amount of radiation comparable to Venus. This might subject the planet to solar flares, coronal mass ejections, and large tidal effects, which can influence seasonality and geologic activity of the star. This Venusian look-alike is now on the research docket, to understand how it differs from Earth-like planets and what conditions might lead to habitability on a planet.

[1] See “Research News: A New Venusian Look-Alike”, APS News (26, 5,May , 2017, p. 1, 7)

Cassini’s Deep Dive

Cassini’s Deep Dive[1]

The recording starts with the patter of a summer squall. Later, a drifting tone like that of a not-quite-tuned-in radio station rises and for a while drowns out the patter.

These are the sounds encountered by NASA’s Cassini spacecraft as it dove through the gap between Saturn and its innermost ring on April 26, the first of 22 such encounters before it will plunge into Saturn’s atmosphere in September [2017].

What Cassini did not detect were many of the collisions of dust particles hitting the spacecraft as it passed through the plane of the rings.

“You can hear a couple of clicks,” said William S. Kurth, a research scientist at the University of Iowa who is the principal investigator for Cassini’s radio and plasma science instrument.

The few dust hits that were recorded sounded like the small pops caused by dust on a LP record, he said. What he had expected was something more like the din of “driving through Iowa in a hailstorm,” Dr. Kurth said.

Since Cassini had not passed through this region before, scientists and engineers did not know for certain what it would encounter. Cassini would be traveling at more than 70,000 miles per hour as it passed within 2,000 miles of the cloud tops, and a chance hit with a sand grain could be trouble.

The analysis indicated that the chances of such a collision were slim, but still risky enough that mission managers did not send Cassini here until the mission’s final months. As a better-safe-than-sorry precaution, the spacecraft was pointed with its big radio dish facing forward, like a shield.

Not only was there nothing catastrophic, there was hardly anything at all. The few clicking sounds were generated by dust the size of cigarette smoke particles about a micron, or one-25,000th of an inch, in diameter.

To be clear: Cassini did not actually hear any sounds. It is, after all, flying through space where there is no air and thus no vibrating air molecules to convey sound waves. But space is full of radio waves, recorded by Dr. Kurth’s instrument, and those waves, just like the ones bouncing through the Earth’s atmosphere to broadcast the songs of Bruno Mars, Beyoncé and Taylor Swift, can be converted into audible sounds.

Dr. Kurth said the background patter was likely oscillations of charged particles in the upper part of Saturn’s ionosphere where atoms are broken apart by solar and cosmic radiation. The louder tones were almost certainly “whistler mode emissions” when the charged particles oscillate in unison.

The dust particles create their own distinctive noises.

Upon hitting Cassini, the dust and a bit of the spacecraft vaporize in small clouds of ultrahot gas where electrons are ripped away from atoms and generate radio waves.

The actual physics is still somewhat contentious. “People aren’t really sure what they’re seeing,” said Sigrid Close, an associate professor of aeronautics and astronautics at Stanford.

Her idea, supported by laboratory experiments and computer simulations, is that the lighter electrons initially speed away faster, setting up an electric field that pulls the electrons back toward the ions, and then they oscillate back and forth.

Above is a picture taken of Earth through Saturn’s rings. The arrow points to Earth.

Similar radio frequency pulses are generated by lightning, she said.

For spacecraft closer to home, like commercial and military satellites in orbit around the Earth, this could be an important process to understand, because pulses generated by large dust strikes could cripple their electronic systems.

When Cassini passed through a faint Saturn ring in December, the dust impacts numbered in the hundreds per second.

With the knowledge that the gap between Saturn and its innermost ring is safe, Cassini does not need to use its antenna as a shield, allowing it to make additional scientific measurements.

During its second dive through the gap on Tuesday [May 2, 2017], another instrument, the cosmic dust analyzer, made the first direct analysis of the ring particles. The dust analyzer is able to record particles smaller in size than could have been detected via radio waves during the first pass. In addition, Cassini performed magnetic measurements that will help determine the length of a Saturn day. That is still a mystery, because Saturn’s clouds obscure how quickly the underlying planet is rotating.

Cassini got back in touch with Earth on Wednesday morning [May 3, 2017] and is sending the results of the second dive.

[1] See “The ‘[Sounds’ of Space as NASA’s Cassini Dive by Saturn,” New York Times (May 3, 2017)

Pluto and its Moons

Pluto and its Moons[1]

The editors of Astronomy deem the flyby of Pluto and its moons, the number one most important astronomy story of 2015.

When NASA’s New Horizons spacecraft flew by Pluto, Earth watched and celebrated. “The target didn’t disappoint,” says Principal Investigator S. Alan Stern. “It’s absolutely stunning.” And even though the science collection lasted just months, the New Horizons mission had been decades in the making. NASA chose the mission in 2001, the spacecraft launched in 2006, and it reached Pluto on July 14, 2015.

Seeing the pixelated blobs of Pluto and its largest moon, Charon (below), evolve into complex worlds through the eye of New Horizons was rewarding, satisfying, and awesome, says Stern. That’s because everything about Pluto surprised scientists. They expected a frozen, cratered, and long-dead world with an equally old-looking system of moons. Instead, Pluto’s surface is young, with smooth frozen plains, icy mountains as high as   the U.S. Rockies, topography that resemble dunes, a glacial lake, and ice that has recently flowed around other features in the same way that glaciers move on Earth’s surface. The scientists estimate that the uncratered swaths of terrain are 100 million years old, while other regions are billions of years in age.

Pluto’s varied surface with such youthful areas means that something internal must be warming it to make it pliable. And while all the objects in our planetary system would have been warm shortly after the solar system formed 4.5 billion years ago, scientists didn’t think such a small object could stay warm all these years.

Pluto’s largest moon Charon also came into sharper view, including glimpses of the dark region near its north pole informally known as Mordor Macula.

“We expect small planets to typically run out of energy a lot sooner than the big planets. It’s like a small cup of coffee cools off faster than a bucket of coffee,” says Stern. But what New Horizons has revealed about Pluto, he adds, changes the expectations of planetary geology.

Scientists have also created a map of methane ice distribution, and this material seems to prefer a region of young terrain that scientists have informally named “Sputnik Planum.” Outside of this area methane is still present and congregates on crater rims and brighter regions but avoids crater centers and darker regions for unknown reasons.

The up-close photos of Pluto have also let scientists precisely measure the width of the dwarf planet 1,473 miles (2,370Icm. This secures Pluto as the largest known object orbiting beyond Neptune.

After New Horizons flew by Pluto, it looked back and watched the dwarf planet eclipse the Sun. This alignment let scientists study Pluto’s atmosphere as sunlight filtered through it. Above the surface lie distinct haze layers that extend to about 80 miles out, several times farther than resarchers expected. And New Horizons detected wisps of a nitrogen-rich atmosphere 1,000 miles out.

While Pluto has been the main focus, Charon also has shown surprises. It too has a varied surface, with some regions void of impact craters. Cliffs stretch hundreds of miles across the surface, indicating the crust has fractured. A deep canyon, 4 to 6 miles deep, also scours Charon’s surface.

New Horizons snapped photos of Pluto’s four smaller moons as well: Nix, Hydra, Styx, and Kerberos. While Charon is 751 miles across, each of these four is just a few dozen miles wide.

Most of New Horizons’ data is still on board the spacecraft and will be downloaded piece by piece over the next several months. Researchers will pore over the additional data in the next few years, learning more every day about Pluto and its moons. Even though humans saved this dwarf system for last in our exploration of the solar system, just the first views exceeded and upended expectations and have given researchers a treasure-trove of new science.

[1]See “Pluto and Its Moons Revealed,” Astronomy (44, 1, 2016, p. 31)

Red Planet Under Water

The Red Planet Under Water[1]

The editors of Astronomy deem the discoveries on Mars by the Mars rovers and orbiters the 10th most important astronomy story of 2015.

The rovers and orbiters at Mars have uncovered plenty of evidence that the planet once had liquid water on its surface, from etched river gullies and dried-up shorelines to minerals that need water to form. But a new study, some five years in the making, confirmed that the Red Planet hosts liquid water on its surface today. Since 2010 Lujendra Ojha, from Georgia State University, and colleagues have used Mars Reconnaissance Orbiter (MRO) data to study streaks running down martian crater walls. They suspected that the streaks, called “recurring slope lineae,” which appear to lengthen from one image to the next, mark flowing salt water. But they didn’t have proof. In the new study, published in the September 28 issue of Nature Geoscience, Ojha’s team provides the spectral signature (from MRO) of salty water at four locations of recurring slope lineae on the Red planet’s surface—confirming that flowing water is present today on Mars.

While little water remains today, scientists know that it must have been bountiful in the past. A study published in the April 10 (2015) issue of Science analyzed how much water the planet once had. Researchers used several Earth-based telescopes to look at the martian atmosphere in infrared light. Geronimo Villanueva of NASA’s Goddard Space Flight Center and colleagues were looking for specific colors: one that corresponds to normal water (H2O) and one that comes from a heavier form of water that has an extra neutron (hydrogen-deuterium-oxygen, or HDO).

The scientists mapped the ratio of these two types of water three times over six years (or three martian years) to compare the water in the atmosphere at different seasons.H2O is lighter than HDO and thus evaporates more easily. So by measuring the ratio of the two, the researchers could calculate how much water Mars has lost over time, and thus how much water it would have started with.

Villanueva’s team says that 4.5 billion years ago, some 6 million cubic miles of water pooled in a northern ocean covering nearly 20 percent of the surface. This martian ocean would have been a bit larger than Earth’s Atlantic Ocean.

This is more water than many researchers had expected. “[Mars] was very likely wet for a longer period of time than previously thought,” said co-author Michael Mumma of in a press statement, “suggesting the planet might have been habitable for longer.”

[1] See Astronomy (44, 1, January 2016, p. 24). “The Red Planet Under Water”

Changing View of Mars

Our Changing View of Mars[1]

Mars formed in the solar nebula and differentiated into a crust, mantle, and core. During the subsequent Noachian period, which began about 4 billion years ago, impact cratering, volcanism, and erosion were extensive. River valleys cut into surfaces already eroded and altered by water, some pervasively. The Hesperian period, starting about 3.7 billion years ago, saw continued extensive volcanism. Erosion and the formation of river valleys slowed dramatically, but catastrophic floods occurred; some of those floods emanated from subsurface reservoirs and produced temporary lakes and seas. Tectonic activity opened large canyons. Aqueous alteration of surface materials was more localized and reflected a growing scarcity of water.

The Amazonian period, starting about 3 billion years ago and continuing today, is icy and dusty. Water remains in the climate system, but predominantly as ice that is redistributed across Mars’s surface in climate cycles induced by the planet’s orbit and orientation to the Sun. Erosion rates are extremely slow, with wind in the thin atmosphere being the dominant agent. The decreasing abundance and stability of liquid water over time are tied to the gradual loss to space of much of the early Noachian atmosphere and the planet’s water inventory via thermal escape and ionization processes. The ionization processes were enhanced as the planet’s interior cooled and the magnetic field that deflects charged particles from the solar wind diminished. Declining global temperatures and the thinning atmosphere makes any liquid water on the planet’s surface vulnerable to freezing or evaporation.

The image above shows an ancient delta within the 45-m-diameter Jezero Crater, as photographed by the Mars Reconnaissance Orbiter in January 2007. The outside rim of the crater appears white. Lays of sedimentary rock on the crater floor contain spectral evidence of hydrous clay minerals and carbonates (green) among igneous minerals (yellow and blue). The sediments are thought to have come from nearby highlands, transported by water billions of years ago when the crater was a lake basin. The presence of clay minerals in such a deltaic setting is favorable for the accumulation preservation of organic material.

If life ever took hold on Mars, what environmental niches did it occupy? And what limited it? Did it ever become pervasive enough to modify its environment on the scale of Earth’s oxygenation, for example? Was it resilient to the loss of Mars’s atmosphere and hydrosphere? If life never took hold, what is that telling us, given Mars’s likely similarity to early Earth and the availability of habitable environments? The poet T. S. Eliot tells us that the end of exploration is to arrive where we started and to know the place for the first time. The end of Mars exploration is not in sight, but it is safe to say that we are learning more each day about where we started.

[1] See Ashwin Vasavada, “Our Changing View of Mars,” Physics Today (70, 3, pp. 34-41).