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Category Archives: 07. The Jovian planets: windswept giants

Adapted from an article by Kenneth Chang published in The New York Times, September 14, 2017:

NASA’s Cassini spacecraft, the intrepid robotic explorer of Saturn’s magnificent beauty, has finally ended its 20-year journey. By design, the probe vanished into Saturn’s atmosphere, disintegrating moments after its final signal slipped away into the background noise of the Solar System. Until the end, new measurements streamed one billion miles back to Earth, preceded by the spacecraft’s last picture show of dazzling sights from around the Sun’s sixth planet.


Credit: Credit NASA/JPL-Caltech/Space Science Institute


The mission for Cassini, in orbit since 2004, stretched far beyond the original four-year plan, sending back multitudes of striking photographs, solving some mysteries and upending prevailing notions about the Solar System with completely unexpected discoveries.

Even at the end, 20 years after launch, Cassini and its instruments remained in good working shape. The plutonium power source was still generating electricity. But there was not enough propellant fuel left to safely send Cassini anywhere except into Saturn.

In the very last phase of the mission, Cassini dove through the gap between Saturn and the planet’s innermost ring. This provided new, sharp views of the rings and allowed the craft to probe the planet’s interior, as another NASA’s Juno spacecraft is doing at Jupiter.

Links: read the full article; also NYT’s ‘100 Images from Cassini’ feature.


From JPL press releases, July 4, 2016:

While Americans celebrated the evening of Independence Day, 1.7 billion miles (2.7 billion kilometres) NASA’s Juno spacecraft, launched nearly five years ago, reached its final destination: the most massive planet in our Solar System, Jupiter.


Credit: NASA/JPL-Caltech

Juno now starts its tour of Jupiter in a 53.5-day orbit. The spacecraft saves fuel by executing a burn that places it in a capture orbit with a 53.5-day orbit instead of going directly for the 14-day orbit that will occur during the mission’s primary science collection period. The 14-day science orbit phase will begin after the final burn of the mission for Juno’s main engine on October 19.

Most of Juno’s instruments deal with Jupiter’s particles and magnetic field, which is 20,000 times more powerful than Earth’s. The main instruments are in a vault made of 400 pounds of titanium to protect them from the strong radiation. The Junocam, its imaging camera, is outside that protection, and may not last as long as other instruments; further, it will give images as it rotates that will have to be transformed to the equivalent of steady views.

Links: Full details via the JPL press release; NASA Juno mission page; NY Times: Jupiter and its moons graphic.

Credit: Lunar and Planetary Insitute

Credit: Lunar and Planetary Insitute

Planetary Science Nuggets are PowerPoint slides that have been provided to NASA Science Mission Directorate’s Planetary Science Division by members of the scientific community to highlight important science results or mission activities. A subset of these submissions are selected by the Planetary Science Division to be presented to SMD leadership and, potentially, NASA leadership, OSTP and the White House. This collection represents those selected Nuggets.

Link: Planetary Science Nuggets hosted by the Lunar and Planetary Institute.

In this NY Times slideshow, Kenneth Chang curates a selection of images from NASA’s Cassini spacecraft’s farewell tour. Cassini entered orbit around Saturn 11 years ago; last fall, the space agency granted a final extension, through 2017, when the spacecraft will have exhausted the fuel for its thrusters. On August 17, it made its last flyby of Dione, the fourth largest of Saturn’s more than 60 moons, at 700 miles wide. Dione has its own mysteries that planetary scientists hope to unravel.

See the slideshow here (external link).

Here is a consolidated list of errors from the text’s first printing. Many of these have already been posted here as separate chapter updates. (Our publisher will make the necessary corrections to the printed book at the earliest opportunity.):

p. 25, Figure It Out 2.3: The last paragraph (about Fraunhofer) shouldn’t be there. Instead, it should be at the end of the caption of Figure 2-4 on p. 26.

p. 64, Q34: 1 Angstrom should be listed as 1010 m, not 108 m.

p. 64, Q41: Ditto

p. 78: There is an error in the equation relating the apparent magnitude and brightness of stars in Figure It Out 4.1.  In this equation, 2.512 should be raised to a power equal to (mB−mA).

p. 92, Q1: We could more clearly say “On the top picture” instead of just “On the picture” – since there are now two pictures on the opening page of the chapter (and the stars are somewhat too dense for individual clarity in the bottom picture).

p. 190: First sentence of Section 7.4d: “a little larger” should be “a little smaller” for the relative sizes of Triton and the Moon.

p. 309, Q53, there is a printing error when going from the bottom of column 1 to the top of column 2. At the top of column 2, the “(e)” should be boldface, there should be a period after “1/16”, and the remainder of the text should be deleted.

p. 363, column 1, second line from the bottom: When referring to the event horizon: “1/3” should be “2/3”, i.e. the sentence should read “Its radius is exactly 2/3 times that of the photon sphere…”

p. 391, column 2, second line from the bottom: for Spitzer, “Section 3.8c, Figure 3-32a).” should say simply “Section 3.8c).”

p. 392, column 1, line 5: At the end, add “(See Section 3.8c, Figure 3-32a.)”

Appendix 3C, column 3 header: “105 km” should be “106 km”

From a JPL press release dated November 21, 2014:

Scientists have produced a new version of what is perhaps NASA’s best view of Jupiter’s ice-covered moon, Europa. The mosaic of color images was obtained in the late 1990s by NASA’s Galileo spacecraft. This is the first time that NASA is publishing a version of the scene produced using modern image processing techniques.

Credit: NASA/JPL-Caltech/SETI Institute

Credit: NASA/JPL-Caltech/SETI Institute

The new image more closely approximates what the human eye would see than the earlier version (released in 2001). The image features many long, curving and linear fractures in the moon’s bright ice shell. Scientists are eager to learn if the reddish-brown fractures, and other markings spattered across the surface, contain clues about the geological history of Europa and the chemistry of the global ocean that is thought to exist beneath the ice.

In addition to the newly processed image, a new video details why this likely ocean world is a high priority for future exploration.

Links: more details about the image above; Europa exploration movie; NASA’s Europa homepage.


A new interactive posting from National Geographic tells the story of the twin Voyager spacecraft, sent to explore the outer planets and the edge of our Solar System.

Links: The Voyager feature on the National Geographic website; a short movie.

From a JPL press release, August 21, 2014:

NASA’s Voyager 2 spacecraft gave humanity its first close-up look at Neptune and its moon Triton in the summer of 1989. Like an old film, Voyager’s historic footage of Triton has been “restored” and used to construct the best-ever global color map of this strange moon. The map, produced by Paul Schenk, a scientist at the Lunar and Planetary Institute in Houston, has also been used to make a movie recreating that historic Voyager encounter, which took place 25 years ago, on August 25, 1989. (See pp. 190-192)

Map of Triton

Credit: NASA/JPL-Caltech/Lunar & Planetary Institute

The new Triton map has a resolution of 1,970 feet (600 meters) per pixel. The colors have been enhanced to bring out contrast but are a close approximation to Triton’s natural colors. Voyager’s “eyes” saw in colors slightly different from human eyes, and this map was produced using orange, green and blue filter images.

In 1989, most of the northern hemisphere was in darkness and unseen by Voyager. Because of the speed of Voyager’s visit and the slow rotation of Triton, only one hemisphere was seen clearly at close distance. The rest of the surface was either in darkness or seen as blurry markings.

The production of the new Triton map was inspired by anticipation of NASA’s New Horizons encounter with Pluto, coming up a little under a year from now. Among the improvements on the map are updates to the accuracy of feature locations, sharpening of feature details by removing some of the blurring effects of the camera, and improved color processing.

Although Triton is a moon of a planet and Pluto is a dwarf planet, Triton serves as a preview of sorts for the upcoming Pluto encounter. Although both bodies originated in the outer solar system, Triton was captured by Neptune and has undergone a radically different thermal history than Pluto. Tidal heating has likely melted the interior of Triton, producing the volcanoes, fractures and other geological features that Voyager saw on that bitterly cold, icy surface.

Pluto is unlikely to be a copy of Triton, but some of the same types of features may be present. Triton is slightly larger than Pluto, has a very similar internal density and bulk composition, and has the same low-temperature volatiles frozen on its surface. The surface composition of both bodies includes carbon monoxide, carbon dioxide, methane and nitrogen ices.

Voyager also discovered atmospheric plumes on Triton, making it one of the known active bodies in the outer Solar System, along with objects such as Jupiter’s moon Io and Saturn’s moon Enceladus. Scientists will be looking at Pluto next year to see if it will join this list. They will also be looking to see how Pluto and Triton compare and contrast, and how their different histories have shaped the surfaces we see.

Links: the full JPL press release; Triton movie.

An article in New Scientist summarizes work by Michael Wong at Caltech, published in the journal Icarus, about how Titan keeps its surface methane liquid.

Saturn’s largest moon may once have been a giant snowball. Titan is already a frigid moon made mostly of ice. But methane gas in its atmosphere keeps the surface just warm enough for a scattering of lakes filled with liquid hydrocarbons. Scientists have puzzled over Titan’s atmospheric methane because the molecule is easily broken down by sunlight. Calculations suggest that all the methane Titan seems to possess should have been used up within tens of millions of years – a blip in the moon’s roughly 4-billion-year lifetime.

Adding to the mystery, the methane breakdown creates other compounds that rain over the surface, helping to fill the lakes. If used-up methane was replaced, this process would happen constantly, so Titan should be covered not by lakes, but by a global ocean hundreds of metres deep.

Michael Wong at Caltech says snowballs may be the missing piece. Scientists suspect Earth went through a snowball phase about 2 billion years ago, when the planet became covered in ice. A similar event could have taken place on Titan, says Wong. Methane levels may rise and fall if the gas is periodically released from inside the moon. If at some point the methane dropped by a factor of 100, temperatures would fall, and surface liquids would freeze over. A different mix of compounds would also be produced in the atmosphere. So this cold snap would mean the moon’s surface should host lots of compounds called nitriles, which would be solid rather than creating an ocean.

The New Horizons mission to Pluto could offer early clues. Like Titan, Pluto has an atmosphere that is mostly nitrogen with some methane. Pluto’s atmosphere is much thinner and colder, but the physics are similar enough that examining its composition could boost the snowball model.

Links: the New Scientist report; Icarus article.

An article in ScienceNews describes experiments on Earth about making a probe that can penetrate far enough into Europa’s surface ice to figure out what might be in Europa’s invisible ocean.

Credit: JPL-Caltech/NASA

Jupiter’s moon, Europa, looks just as desolate and uninviting as any other place in the outer Solar System. Its frozen façade is colder than the most frigid spot on Earth  by more than 100 degrees Celsius. Blasts of radiation sweep the surface. But beneath Europa’s inhospitable exterior, scientists think a vast ocean of liquid water flows. The moon’s seafloor might also bustle with activity from volcanoes and hydrothermal vents. If chemicals from the surface trickle down through the ice, as some scientists suspect, Europa could hold all the necessary ingredients for life.

Kevin Hand of NASA’s Jet Propulsion Laboratory in Pasadena, CA, says  “You’ve got incredible ecosystems of tube worms and crabs and fish and microbes [on Earth]. It’s anybody’s guess whether or not you’d find tube worms on Europa.”

The idea of exploring this vast ocean has launched a number of scientists on a quest for a space-ready ice drill. Somehow, such a device has to breach the moon’s icy shell — perhaps with blazing hot metal or the jagged teeth of a drill bit — and carry enough power for the job. The device has to be simpler and more reliable than anything used to bore through ice on Earth, and it will have to take care of itself — there’s no way to send a team of engineers to the far edges of the Solar System. And the entire ice-tunneling, power-toting, problem-free package needs to be light enough to launch beyond Earth’s gravitational grip. Research teams are now exploring the different approaches to penetrate, drill, burrow or melt through Europa’s icy shell.

Read more about these varied projects at ScienceNews.