ASTR 1210 (O'Connell) Study Guide 20

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Jupiter with erupting Io in foreground
(New Horizons Mission image)

"And now for something completely different," as they used to say on Monty Python. The outer solar system is different indeed.

The outer solar system we can now explore (out to about 50 AU) is vast -- over 35,000 times the volume of the terrestrial realm out to Mars -- and sparsely populated.

The four large "Jovian" planets (Jupiter, Saturn, Uranus, and Neptune) are entirely unlike the terrestrial planets (see more discussion in Study Guide 11). They may have rocky cores, like larger versions of the Earth, at their centers, but these are enveloped in giant gaseous atmospheres. Only the outermost skins of these atmospheres can be studied directly. This is meteorology, instead of the geology/topography we discussed for the terrestrials. However, it can be just as extreme with respect to Earth-bound meteorology as are the canyons and mountains of Mars compared to those of Earth.

Another major distinction of the Jovians is the large number of satellites they possess. The satellites, observed at close range by spacecraft, display an astonishing diversity of surface types and features. Unlike the three terrestrial planet satellites, the larger Jovian satellites are rich in water ice and exhibit many different phenomena as a consequence. They may even harbor biospheres. The ring systems, which are present around all 4 Jovians, are probably the remnants of distintegrated satellites.

Many examples of a third kind of planet have recently been discovered outside the orbit of Neptune. These are perhaps most aptly called the "ice dwarf planets," of which Pluto is the archetype.

A. History of Exploration Beyond Mars

Prehistory: Jupiter, Saturn known
1610-1700: Galileo discovers four large satellites in orbit around Jupiter. Later in the 17th century Huygens and Cassini discover the rings and several large satellites of Saturn and the giamt "Red Spot" on Jupiter.
1704: Edmond Halley determines the orbit of the eponymous comet and discovers that it travels in a huge, highly elongated orbit.
Halley's work dramatically expands the horizon of the Solar System. The comet's aphelion (greatest distance from the Sun) is 35 AU's, over three times farther than Saturn, at 9.5 AU's. But remember that in 1704, neither Uranus nor Neptune had been discovered. So, the long orbit of Halley's Comet plunged it into a completely mysterious outer realm that offered the promise of many discoveries yet to come.

1781: Amateur astronomer William Herschel discovers Uranus (accidentally) using a small telescope---the first new planet in recorded history. Its orbital semi-major axis is 19 AU's. (Drawing on royal patronage because of his subsequent fame, Herschel became a remarkably productive professional astronomer.)
1790+: Deviations from the strict Newtonian predictions are observed in Uranus' orbit. This could be evidence of a flaw in Newtonian theory of gravity -- or it might be caused by the gravitational attraction of yet another (unknown) planet. The Newtonian theory of gravity had been refined to the point where, given enough data on Uranus' motion, astronomers could predict the location of the unknown planet. After 1843, astronomers in France and Britain compete to locate and observe the new planet.
1845: Neptune is discovered by French astronomers Le Verrier and Galle, as predicted, at 30 AU from the Sun --- a triumph of Newtonian mechanics and a resounding demonstration of the power of science: a new world discovered through mathematics.
1930: Working at Lowell Observatory, Clyde Tombaugh discovers Pluto. A much more difficult proposition than Neptune because it is 700 times fainter, both because it is much smaller and has an orbital semi-major axis of 39 AU's. Tombaugh had been an amateur astronomer before being hired by the observatory. He was only 24 at the time and did not yet have a college degree.
Although the search for Pluto followed the methods established for Neptune, based on apparent abnormalities in the orbits of Neptune and Uranus, it quickly became clear that its discovery was accidental: Pluto is too small to have affected the other planets' motion significantly, nor were the claimed distortions in their orbits accurate.
Pluto also broke the pattern established by the four other planets beyond Mars (Jupiter, Saturn, Uranus, and Neptune): it was not a gas giant planet. Instead, it was so small it could not be resolved by telescopes of the time and showed indications of having a rocky surface.

1950+: Deeper searches yield no large members of our Solar System beyond Neptune. In 2014, the WISE infrared survey mission announced that it detected no planets larger than Saturn to a distance of 10,000 AU's from the Sun and none larger than Jupiter to 26,000 AU's from the Sun.
1973-4: The Pioneer 10 & 11 spacecraft fly by Jupiter and Saturn, providing the first close-up observations of the Jovian planets. Only limited data are obtained, but fascinating hints of the complexity of these worlds emerge.


Flyby orbits of Voyager Missions to outer planets

1979-89: The Voyager 1 & 2 spacecraft fly by all 4 Jovians. These were highly successful missions but required elaborate planning and exacting in-flight operation (over 12,000 person-years of effort). They made numerous discoveries, including the volcanos of Io. The Voyager spacecraft have continued on out of the Solar System, with Voyager 1 officially reaching interstellar space in September of 2013. Voyager 2 followed in 2018.
1992: The first "Kuiper Belt Object" (KBO) is discovered. Over a thousand of these small, rocky-icy objects have been identified to date, a number of them large enough to qualify as "ice dwarf planets." These lie in a thick region centered on the ecliptic plane, mostly beyond the orbit of Neptune. Pluto is now recognized to be a large KBO.
1994: Comet Shoemaker-Levy 9 collides with Jupiter, acting as a natural, explosive atmospheric probe.
1995: The Galileo orbiter/probe mission arrives at Jupiter and sends a probe ~50 miles into the atmosphere; the orbiter continues to study Jupiter and its satellites until 2003.
2004: The Cassini-Huygens mission enters orbit around Saturn and returns outstanding data for over a decade.
2005: The Huygens probe lands safely on Titan, Saturn's largest satellite. The Cassini orbiter continues its studies of the Saturn system and discovers a water plume extending from Enceladus, indicating a reservoir of liquid water below the surface of this small satellite.
2006: The International Astronomical Union debates the status of Pluto and other KBO's. Pluto is redefined to be a "dwarf planet."
2015: The New Horizons mission flies by Pluto in July 2015 and returns astonishing data on its surface and surroundings.
2016: The Juno Mission goes into close orbit around Jupiter on July 5. Over 7 years, Juno will make extensive observations of Jupiter's atmosphere, gravity field, and magnetosphere.
2017: After 13 years of continuous observations of Saturn and its satellites, the Cassini spacecraft makes a series of final, risky passages through the rings and then is commanded to plunge into Saturn's atmosphere on 15 September.
2019: New Horizons flies by a second KBO, Arrokoth and discovers that it is a binary planet, about 36 km across, with two lobes connected by a narrow bridge.

Spacecraft images of the four Jovian planets (from left: Jupiter, Saturn, Uranus, Neptune),
scaled to their correct relative sizes (but not distances from the Sun).

B. Jovian Planets: Properties

The four Jovian planets share gross properties. Pluto is entirely different (see below).

Distant from Sun: 5-30 AU. (Pluto is at 39 AU.)

Large: The Jovians are 4-11 times larger in diameter than Earth. Jupiter's volume is 1300 times the Earth's. Masses are 15(U)-318(J) times Earth's. Jupiter contains twice as much mass as all other planets combined. An animated timelapse image of Jupiter's rotation and surface features is shown at the right. Click on the image for a more recent, high resolution video.

Jupiter is midway between terrestrial planets and stars on a power of ten mass scale. Objects only 13 times more massive are considered to be small stars, not planets.

Structures

• Their low mean densities (~ 1 gram/cc) imply that Jupiter and Saturn are mainly composed of H and He, with only small rocky cores, perhaps Earth-size. That is a product of their formation out of the cool regions of the solar nebula, which were dominated by icy (H-rich) solids. Uranus and Neptune have larger fractional complements of heavy elements than Jupiter and Saturn.

• Internal structures are entirely different from terrestrial planets because of their predominant hydrogen composition. Click on the Jupiter cross-sectional drawing at right for an enlarged version. The high internal pressures in Jupiter and Saturn convert hydrogen gas to liquid or "metallic" form in their interiors. The lower-pressure interiors of Uranus and Neptune contain mantles composed of ammonia, methane, and water. Unlike Terrestrial planets, the Jovians have no solid surfaces. They are "gas giants".

Visible Surfaces

• The visible surfaces of the Jovians are cloud layers, about 150 miles deep. The clouds consist of 3 main types of ice crystals: ammonia, ammonium hydrosulfide, and water. Colors are from trace compounds. Thin, white clouds on Neptune are methane crystals, which freeze out at the lower atmospheric temperature there.

• "Spots": The most famous is Jupiter's Red Spot (the large oval in the image at right: 22,000 mi long ~ 3x Earth's diameter). It is a long-lived cyclonic storm. The transient "Great Dark Spot" on Neptune was a similar feature, and many smaller spots or ovals, mostly transient and ranging from dark red to white in color, are visible in the Jovian atmospheres (most conspicuous on Jupiter; 8 are present in the image at the right).

• Atmospheric banding is caused by powerful lateral windstreams fed by rising/falling convection currents. Winds reach 300-600 mph on Jupiter and Saturn and up to 1300 mph on Neptune. See the enhanced pseudocolor image of Saturn's atmospheric banding below right.
  
• The Juno spacecraft, skimming in an elliptical orbit to within 2600 miles of Jupiter's cloud layers, is now returning amazingly detailed images of the atmosphere.
  Here is a beautiful recent Juno image of Jupiter's Red Spot with Io in the foreground, casting a shadow.

• Videos of Jupiter atmosphere:
  • Voyager 1 "Blue Movie" (time lapse of approach to planet)

  • Voyager 1 Red Spot Movie

  • Cassini Full Surface Movie

  • Videos from Juno mission

Special Probes of Jupiter

• Comet Shoemaker-Levy-9: was captured by Jupiter and hit the planet in 1994. The impact acted as a "natural" atmospheric probe; infalling fragments produced multiple enormous, concentrated energy releases in the atmosphere. The explosions were bright and the scars long-lived.

• The Galileo spacecraft probe was sent into Jupiter's atmosphere in 1995 and returned pressure, temperature, and composition profiles to a depth of almost 100 miles before it was crushed by the pressure.

Magnetospheres

Strong magnetic fields are generated by motions in the liquid metallic hydrogen interiors of Jupiter & Saturn. Because the magnetic fields trap energetic protons and electrons, these produce powerful radiation belts, up to 100x stronger than those of Earth.

Pseudo-color infrared image of Saturn

C. Ring Systems

Saturn has the brightest rings, but rings are present around all 4 Jovians

• The rings are not solid: the inner rings revolve faster than the outer ones, as expected for objects in Keplerian gravitational orbits

• The rings are huge in width but by comparison are remarkably thin, mostly less than 1 km (3300 feet) thick. They have an aspect ratio of about 300,000:1. This is over 110 times thinner than standard printer paper. Here is an article on the ring geometry by Phil Plait.

• The rings are composed of billions of ice-coated particles (typically about 10 cm in size). Different particle sizes and coatings produce some of the structure visible in the rings.

• Origin: the rings are primarily debris from tidally or collisionally fragmented satellites
  The rings lie inside the planet's "Roche Limit". Closer than this distance from the planet's core, gravity tides would pull apart a large body, such as a satellite.

• The rings have a complex structure (at right), consisting of numerous gaps and ringlets. The biggest gaps are "resonance" effects produced by the cyclical gravitational tug of the satellites outside the ring. The ringlets may be produced by the self-gravity of the material in the rings.

• A video of Saturn's rings from the Cassini mission

Spacecraft images of the four Galilean satellites of Jupiter, shown to scale
(Io, Europa, Ganymede, and Callisto).

D. The Jovian Planet Satellites

In many ways, the satellites of the Jovian planets are more interesting than the planets themselves. The four largest satellites of Jupiter (the "Galilean satellites") were discovered by Galileo and were the first additions to the planetary inventory of the Solar System in recorded history. Galileo saw them only as points of light, and good information on their surfaces was not obtained until the spacecraft flyby missions of the 1970's and 80's.

The composite picture above shows spacecraft images of the four Galilean satellites (scaled to the correct relative sizes). It illustrates the outstanding feature of these four satellites: they are amazingly different from one another. Each is a unique world in its own right. They are typical of all Jovian satellites in that they are diverse, and they often have had violent histories.

Here are some of the other characteristics of the Jovian satellites:

The satellite complement for a given planet is large: 14-79
Click here for a Java animation of orbits of the satellites of each planet

Larger moons are mixtures of rocky/icy materials
• Two are larger than Mercury. See this comparison of satellites and planets.

• The large moons formed at the same time as their parent planet

• Because of their large ice content, their surfaces are more plastic than those of the terrestrial planets; some show extensive evidence of melting and resurfacing.

Smaller moons, e.g. Hyperion (Saturn), are irregular in shape
• Most of these are rocky or icy planetesimals/asteroids, which were gravitationally captured by a planet over time.

Important examples of satellites: (click on the names for additional illustrations)

• Io (J): Io is the innermost of the four Galilean satellites of Jupiter. It suffers continual volcanic eruptions caused by heating from tidal flexing in Jupiter's gravitational field. Io is much more active today than even the Earth. There are presently over 400 active volcanoes on Io. Its huge volcanic plumes at the time of their discovery by Voyager 1 in 1979 are shown at the right.

• Europa (J): The second Galilean satellite is completely ice-coated and extraordinarily smooth (i.e. has a small range of elevation). Few craters, indicating a young surface. Most scientists believe the ice shell covers an underlying ocean, kept warm by tidal flexing (less severe than for Io). Long, dark lines on the surface may be places where the shell has cracked, allowing filling by younger ice. In 2013, NASA announced the detection of water vapor plumes jetting off of Europa's surface (like the similar features found earlier on Enceladus). Because of the presence of water, there is much speculation about a possible biosphere on Europa (see Study Guide 23).
  
• Titan (S): Saturn's largest moon has a thick atmosphere(!), visible as a blue haze in the infrared image at the right. It is mostly nitrogen with a small amount of methane. Titan is the only object in the Solar System other than Earth to have a predominantly nitrogen atmosphere. The atmosphere can be retained, despite Titan's small mass, because of its low temperature at Saturn's distance from the Sun.
  Titan was the main target of the European Cassini-Huygens Mission. While the primary spacecraft stayed in orbit around Saturn, the Huygens probe was detached and successfully landed on Titan's surface in January 2005, relaying data during its descent and for a short period on the ground.
  Solar UV light interacting with methane has produced a rich mixture of hydrocarbon clouds and obscuring haze. There is probably hydrocarbon rain on Titan. Click here for an atmospheric profile.
  Recent radar data from the orbiting Cassini spacecraft shows that there are large lakes on Titan, probably of methane or ethane.
  In company with Europa and Enceladus (see below), Titan is now regarded as a possible site of a biosphere --- but with lifeforms based on utilizing methane rather than carbon dioxide.

Left: Enceladus; Center: water vapor plume from Enceladus; Right: possible internal structure of Enceladus

• Enceladus(S): Although only a small satellite, Enceladus became a focus of astronomer interest when it was unexpectedly discovered by the Cassini orbiter to possess huge water/ice geysers jetting into space from its surface. The plumes contain water vapor, complex hydrocarbons, and sodium salts. The plumes are thought to originate from warm liquid water reservoirs beneath the surface which are heated by tidal flexing; jets escape through deep vents. See the picture above. The outflow from Enceladus feeds material into Saturn's "E ring."
  In April 2014, scientists announced that gravity measurements deduced from tiny accelerations of the Cassini spacecraft in the vicinity of Enceladus confirm the presence of a liquid ocean with a volume comparable to Lake Superior lying under its south pole.

• Miranda (U): has a remarkable patchwork surface, probably from a shattering collision & reassembly or possibly surface scars from internal convection
  Animation of Miranda's terrain

• Triton (N): the largest satellite of Neptune is a captured KBO. (It has a "retrograde" orbit -- it orbits in the opposite direction to Neptune's spin -- implying it did not form with Neptune but was gravitationally captured later.) Triton has a young, nitrogen-rich surface with diverse structural features caused by ice melting, shifting, and re-freezing. During Voyager 2's brief passage, it featured several geysers or "cryovolcanos" of nitrogen, methane, ice, and dust jetting out of its surface. Its surface has much in common with Pluto's (see below).

Artist's Concept of Huygens Probe Landing On Titan

E. Pluto and the Kuiper Belt

Pluto is entirely unlike the four large outer planets. With a diameter of only 1475 miles, it is smaller by a factor of 2 than any of the terrestrial or Jovian planets. It is a rocky/icy object rather than a gas giant. Its orbit is the most highly inclined to the ecliptic plane of any of the classical "9 planets."

When first discovered, Pluto was thought to be isolated at the edge of the Solar System. However, in the last 25 years, after wide-field digital detectors were installed on large telescopes, astronomers have identified many more similar bodies, some with sizes comparable to Pluto. These, including Pluto, are all members of the "Kuiper Belt".

• The Kuiper Belt is a huge volume beyond the orbit of Neptune, centered on the ecliptic plane, but extending many AU's above and below the plane. It is much larger than the asteroid belt, which lies between Mars and Jupiter. Over 1000 "Kuiper Belt Objects" (KBO's) have been discovered in this volume to date. A number of these are larger than any of the asteroids; Pluto, for instance, is 2.5 times the diameter of Ceres, the largest asteroid.
  Here is a comparison of the eight largest KBO's and their satellites to one another. The plot at the right shows the known KBOs with the Belt both face-on and edge-on (click for an enlargement).

• The most massive known KBO---yes, it's more massive than Pluto---is Eris, also the second most distant known KBO (97 AU). It was discovered in 2005. Its size (1400 miles diameter) is slightly smaller than Pluto's. Click here for a page describing Eris by its discoverer, Mike Brown.

• Sedna, a 600-mile diameter object discovered in 2003, has an aphelion (greatest orbital distance from the Sun) of 937 AU, although at present it is at only 90 AU. Its orbital period is about 11,400 years. It is distant enough that it may be a member of the "Oort Cloud" rather than the Kuiper Belt. The object 2012 VP113 is similar to Sedna, with a perihelion (minimum orbital distance from the Sun) of 80 AU, currently the most distant perihelion of any Solar System body. Some of the known KBO orbits are shown individually here.

These discoveries, particularly that of Eris, precipitated the messy discussion at the International Astronomical Union in the summer of 2006. Astronomers held a debate over the meaning of the term "planet"---specifically whether or not Pluto and the other large KBO's should be placed in a separate category. In the end, the IAU voted to create a new category of "dwarf planet" for these objects but was then forced to add the asteroid Ceres for consistency. All this was handled very clumsily, and it generated needless controversy. It turns out many non-astronomers were fond of Planet Pluto and protested the demotion.

Long before the discovery of Pluto, we had already identified many "minor planets" or "asteroids," small, rocky objects with orbits lying mostly between the orbits of Mars and Jupiter. Now, we know about many similar, but icy, objects beyond Neptune. Sensible designations for these types, above some threshold in size, are as "rock dwarf planets" and "ice dwarf planets," respectively.

The Pluto Flyby

New Horizons, the first mission to Pluto and the Kuiper Belt, was launched by NASA in 2006 and, having received a gravity assist from Jupiter, finally reached Pluto on 14 July 2015. (Yes, nine years later. The fact that New Horizons was launched with the brisk velocity of over 36,000 miles per hour gives you some appreciation for the scale of the solar system.) The spacecraft could not carry enough fuel to decelerate into orbit around Pluto (that would have made it much more massive and expensive), so it was always planned as a "flyby," with a closest approach of about 7800 miles. It took over 15 months for all the data collected during the brief flyby to be telemetered back to Earth.

New Horizons returned a wealth of data on the surface and atmosphere of Pluto and the nature of its 5 satellites. Although the strangeness of Pluto's surface was anticipated by the 1989 Voyager images of Neptune's moon Triton, the close-range, high resolution imaging of New Horizons revealed amazing features.

Much of the surface is covered by rugged terrain (reddish-brown in the image above), with mountains ranging up to 16000 feet high. The surface is not made of rock, however --- rather of a mixture of ices of nitrogen, water vapor, carbon monoxide, and methane, which have frozen out at the very low temperature of Pluto (-230C). The cratering density in the mountainous regions indicates ages up to 4 billion years old.
The smooth, light-colored regions in the image above (part of the heart-shaped "Tombaugh Regio") almost entirely lack cratering and are therefore much younger, less than 10 million years old. They consist mainly of frozen nitrogen and carbon monoxide. The surface is broken into polygonal cells (see image below right), which indicate relatively recent convective motions. The force of the rising ice was apparently enough to float the mountains of water ice lying at the margins of the region.
Overall, Pluto is revealed as much more geologically active than expected, with exchanges continuing to occur between the surface and interior (including through apparent "cryovolcanos") and between the surface and the atmosphere. The thin atmosphere, consisting mainly of nitrogen, methane, and carbon monoxide, partially freezes out on the surface and then sublimes as Pluto's distance from the Sun changes.
Charon, the largest (750 miles diameter) of the five satellites of Pluto, also features surprisingly complex terrain, with rugged canyons --- in this case almost exclusively formed of water ice and without much change in the last 4 billion years.

The Arrokoth Flyby

Following the Pluto encounter, New Horizons was retargeted to fly by another KBO, 2014 MU69M, which was located in a special 2014 search for objects near to the spacecraft's outgoing trajectory.

New Horizons performed a highly successful flyby of this second KBO, now officially named "Arrokoth," on 1 January 2019. Data taken will be streaming back to Earth through mid-2020. But it was immediately clear that Arrokoth is a fascinating object. It is a binary, with two lobes joined at a narrow neck; the largest lobe is about 22km across. Both lobes are significantly flattened. The reddish color (also conspicuous on Pluto and Charon) is caused by tholins, organic compounds produced by solar UV irradiation of carbon-based materials like methane and carbon dioxide.

Arrokoth as imaged by New Horizons

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Reading (this material covers 2 lectures):

Study Guide 20
Bennett textbook, Chapter 11

Reading for next lecture:

Study Guide 21
Bennett textbook, Chapter 12

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Web Links:

The Race for Neptune (MacTutor)
Happy first birthday, Neptune! (2011 marked the end of the first Neptunian year, which is
165 Earth years, since it was discovered; see this congratulatory note from Phil Plait)
JPL Photojournal Image Site
Information & Images at Views of the Solar System
Selected Images of Europa (O'Connell)
The Voyager Missions

Voyager Gee-Whiz Stuff
Voyager Grand Tour Videos
Current locations of Voyager and Pioneer spacecraft

The Galileo Mission to Jupiter
The Cassini-Huygens Mission to Saturn and Titan

Landing on Titan: Huygens Descent Imager Movies

Interactive Global Maps of Jupiter and Galilean Satellites (New York Times)
The New Horizons Mission to Pluto and beyond
International Astronomical Union explanation of planet types
Songs---yes, songs---about the demise of Pluto
Interview with the "Man Who Killed Pluto" (NPR, "On Point")

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Last modified January 2024 by rwo

Cross section drawings copyright © Pearson Education. Text copyright © 1998-2024 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.