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Category Archives: 07. The Jovian planets: windswept giants

p. 563, Appendix 3C Our Solar System: Orbital Properties of Planets

The units of the ‘Semimajor Axis’ second column are out by a factor of 10; they should be 10^6 km, i.e. millions of kilometers. (The equivalent column is shown correctly in Appendix 3D.)


The opening sentence misstates the size of Triton relative to our Moon. The sentence should read:

“Neptune’s largest moon, Triton, is slightly smaller than our Moon and has a retrograde (backward) orbit.”

The pictorial chart on pp. 228-229 and the accompanying table on p. 230 give the correct size order.

Mission Juno website hosts a wealth of Jupiter resources, including news, discussions, images, movies, and explanatory animations about the mission, its science goals and what we know about the Solar System’s largest planet. A special feature is a series of 9 short movies from Bill Nye “the Science Guy”!

Credit: NASA/JPL-Caltech

See the Mission Juno website from the Southwest Research Institute (SwRI).

More than 400 years after its discovery by Italian astronomer Galileo Galilei (see Figure 3-2, p. 38), the largest moon in the Solar System – Jupiter’s moon Ganymede – has finally been fully mapped (see Section 7.1g(iii), pp. 175–176). Since its discovery in January 1610, Ganymede has been the focus of repeated observation, first by Earth-based telescopes, and later by the flyby missions and spacecraft orbiting Jupiter. These studies depict a complex, icy world whose surface is characterized by the striking contrast between its two major terrain types: the dark, very old, highly cratered regions, and the lighter, somewhat younger (but still very old) regions marked with an extensive array of grooves and ridges.

Credit: USGS Astrogeology Science Center/Wheaton/NASA/JPL-Caltech

Scientists have now produced the first global geologic map of Ganymede, Jupiter’s seventh moon. The map combines the best images obtained during flybys conducted by NASA’s Voyager 1 and 2 spacecraft (1979) and Galileo orbiter (1995–2003) and is now published by the U.S. Geological Survey as a global map. It technically illustrates the incredibly varied geologic character of Ganymede’s surface and helps planetary scientists to make sense of the apparent chaos of its complex surface, in order to decipher the icy world’s evolution. It will also enable researchers to compare the geologic characters of other icy satellite moons in the Solar System.

The European Space Agency’s Jupiter Icy Moons Explorer mission is slated to be orbiting Ganymede around 2032, with instrument contributions from NASA.

Earth-bound astronomers can observe Ganymede (with binoculars) in the evening sky this month, as Jupiter is in opposition and easily visible.

Links: JPL press release; a rotating Ganymede movie; the geologic map.

On a recent public radio broadcast, Philip Marcus, professor of fluid dynamics at the University of California Berkeley, explains the persistence of Jupiter’s Great Red Spot (see Section 7.1b, p. 170).


Credit: NASA

The Great Red Spot is huge – 36 times larger than the United States, and its winds, which clock in at 250 miles per hour, surpass those of the most violent hurricane. But, its most baffling property is its multi-century lifetime.

Scientists believe that the Red Spot should have lasted just a few years. It avoids being torn apart because it is sandwiched between layers of cold and hot air. But, those layers should have warmed and cooled their ways to oblivion in only 5 years or so and taken the Red Spot along with them.

So why is the Red Spot still here? One explanation is that the Great Red Spot merges with, and absorbs, smaller “spots”. It was thought that by cannibalizing smaller spots, the Red Spot could stay alive indefinitely, but now we know.  That diet is too meager.

Dr. Marcus’s group recently found a new explanation for the Red Spot’s longevity – it has weak, vertical winds. Like storms on Earth, the vertical winds of the Great Red Spot appear to be negligible; they are hundreds of times smaller than the horizontal winds. Therefore, previous studies ignored them.  To their surprise, when they accurately calculated the vertical winds using computer models, the Red Spot’s lifetime increased from 5 years to 800 years. The vertical winds escaped the Great Red Spot and then threaded through the atmosphere, where they harvested energy from the surrounding air. When the winds returned to the Red Spot, they brought their bounty, and that excess energy has sustained the Great Red Spot for centuries.

Links and source: WAMC’s Academic Minute.

This spring, NASA officials will conduct a review of the spacecraft that have outlived their original missions. For the 2015 fiscal year, which begins October 1, the agency faces particularly tough choices, in order to balance their books.

A decade after swinging into orbit around Saturn, the venerable Cassini spacecraft is still working, well beyond the four years of science the space agency had hoped to get. But the spacecraft is running low on maneuvering fuel, and its managers want to end with a scientific bang – an ambitious agenda that includes 22 orbits through a gap between the planet and its innermost ring before sending the craft on a death plunge into Saturn in 2017. For several months, however, scientists have worried that NASA, financially squeezed like the rest of the federal government, could terminate the mission sooner.

Credit: NASA/JPL/Space Science Institute

The Mars rover Curiosity, which will cost $68 million this year to operate, will complete its two-year primary mission in June 2014, so money for continued roving will come out of funds dedicated to “extended missions.” For this year, that amount is $140 million, which includes $58 million for Cassini. Other extended missions include the Messenger spacecraft at Mercury, the Mars rover Opportunity, and the Mars Reconnaissance Orbiter.

No one expects NASA to turn off Curiosity, which will not even arrive at its primary science destination until later this year, raising concerns that Cassini may be on the chopping block. More recently, NASA planetary science director James Green told scientists that the perception of Cassini versus Curiosity was inaccurate and that officials could instead scale back the cost and scope of the extended missions. The agency could also juggle other money to pay for both Cassini and Curiosity, but that could have consequences like delaying future missions, which themselves are under pressure to deliver the maximum scientific benefits for a smaller cost.

Links and source: NY Times op-ed by Kenneth Chang.

In this short article for The Conversation, Helen Maynard-Casely summarizes current efforts in exploring the Solar System, with missions underway to nearly every planet (and dwarf planet, Pluto).

On July 19, 2013, NASA’s Cassini orbiter passed into Saturn’s shadow and turned toward the Sun, capturing an image of the planet’s night side and the backlit semi-transparent rings. Cassini also captured seven of the moons and three planets. This was the third time our home planet was imaged from the outer Solar System; the second time it was imaged by Cassini from Saturn’s orbit; and it was the first time ever that inhabitants of Earth were made aware in advance that their photo would be taken from such a great distance.



In this video posted on her blog, Emily Lakdawalla of the Planetary Society talks us through some of the hidden features of this spectacular mosaic.

Credit: Emily Lakdawalla (via YouTube)

A newly released image of Titan, Saturn’s largest moon reveals details of seas or lakes near its northern pole.

Credit: NASA/JPL-Caltech/U. Arizona/U. Idaho

The false-color mosaic, made from infrared data collected by NASA’s Cassini spacecraft, reveals the differences in the composition of surface materials around these hydrocarbon lakes. Titan is the only other place in the Solar System that we know has stable liquid on its surface – but its lakes are made of liquid ethane and methane rather than liquid water. While there is one large lake and a few smaller ones near Titan’s south pole, almost all of Titan’s lakes appear near the moon’s north pole.

The image data suggest parts of Titan’s lakes and seas may have evaporated and left behind the Titan equivalent of Earth’s salt flats. They appear orange in this image against the greenish backdrop of Titan’s typical bedrock of water ice.

Launched in 1997, Cassini has been exploring the Saturn system since 2004. A full Saturn year is 30 years, and Cassini has been able to observe nearly a third of a Saturn year. In that time, Saturn and its moons have seen the seasons change from northern winter to northern summer.
Links: press release from Cassini imaging team; annotated image from NASA’s Cassini mission homepage.

NASA’s Voyager 1 spacecraft is officially the first human-made object to venture into interstellar space. The 36-year-old probe is about 12 billion miles (19 billion kilometers) from the Sun.


Credit: NASA/JPL-Caltech

New and unexpected data indicate Voyager 1 has been traveling for about one year through plasma, or ionized gas, present in the space between stars. Voyager is in a transitional region immediately outside the solar bubble, where some effects from our sun are still evident. A report on the analysis of this new data, led by Don Gurnett and the plasma wave science team at the University of Iowa, Iowa City, is published in the journal Science.

Voyager 1 does not have a working plasma sensor, so scientists needed a different way to measure the spacecraft’s plasma environment to make a definitive determination of its location. A coronal mass ejection, a massive burst of solar wind and magnetic fields, that erupted from the Sun in March 2012 provided scientists with the data they needed. When this blast from the Sun eventually overtook Voyager 1 some 13 months later, in April 2013, the plasma around the spacecraft began to vibrate like a violin string. On April 9, Voyager 1’s plasma wave instrument detected the movement. The pitch of the oscillations helped scientists determine the density of the plasma. The particular oscillations meant the spacecraft was bathed in plasma more than 40 times denser than what they had encountered in the outer layer of the heliosphere. This density is that which is expected in interstellar space.

Much more information is available on NASA’s Voyager page, and via this JPL press release, which includes more images and a short video. The sound of interstellar space may be heard here.