1. Planets in The Solar System
In this chapter we take a quick look at all the planets in the solar system.

2. Surface Features on Mercury
Mercury cannot be imaged well from Earth; best pictures are from Mariner 10
Figure 8-9. Mercury, Up Close (a) Mercury is imaged here as a mosaic of photographs—a composite image constructed from many individual images—taken by the Mariner 10 spacecraft in the mid-1970s during its approach to the planet. At the time, the spacecraft was 200,000 km away from Mercury. (b) Mariner 10’s view of Mercury as it sped away from the planet after each encounter. Again, the spacecraft was about 200,000 km away when the photographs making up this mosaic were
taken. (NASA)

3. Rotation Rates
Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false.
Rather, Mercury’s day and year are in a 3:2 resonance; Mercury rotates three times while going around the Sun twice.
Figure Mercury’s Rotation Mercury’s orbital and rotational motions combine to produce a day that is 2 Mercury years long. The red arrow represents an observer standing on the surface of the planet. At day 0 (center right in Year 1 drawing), it is noon for our observer and the Sun is directly overhead. By the time Mercury has completed one full orbit around the Sun and moved from day 0 to day 88, it has rotated on its axis exactly 1.5 times, so that it is now midnight at the observer’s location. After another complete orbit, it is noon once again on day 176 (center right in Year 3 drawing). The eccentricity of Mercury’s orbit is not shown in this simplified diagram.

4. The Surface of Mercury Mercury is less heavily cratered than the Moon
Some distinctive features:
Scarp (cliff), several hundred kilometers long and up to 3 km high
Figure Mercury’s Surface Discovery Scarp on Mercury’s surface, as photographed by Mariner 10. This cliff, or compressional feature, seems to have formed when the planet’s crust cooled and shrank early in its history, causing a crease in the surface. Running diagonally across the center of the frame, the scarp is several hundred kilometers long and up to 3 km high in places. (NASA)

5. Evolutionary History of the Moon and Mercury
Mercury much less well understood:
Formed about 4.6 billion years ago
Melted due to bombardment, cooled slowly
Shrank, crumpling crust

6. Mercury Interior
Mercury is much denser than the Moon and has a weak magnetic field—not well understood!
Figure Terrestrial Interiors The internal structures of Earth, the Moon, and Mercury, drawn to the same scale. Note how large a fraction of Mercury’s interior is the planet’s core.

7. Physical Properties Moon Mercury Earth Radius 1700 km 1440 km 6380 km
Mass
7.3 × 1022 kg
3.3 × 1023 kg
6.0 × 1024 kg
Density
3300 kg/m3
5400 kg/m3
5500 kg/m3
Escape Speed
2.4 km/s
4.3 km/s
11.2 km/s

8. Venus
Chapter 9 opener. Often called Earth’s sister planet, Venus is nothing like Earth. When it comes to surface temperature, it’s hot enough there (730 K) to melt lead. We now know that Venus’s climate, like Earth’s, has varied over time—largely the result of geological activity and atmospheric change. What we do not know well is why Venus became so very much hotter than Earth—or if Earth could someday heat up similarly. Here, this global view of the surface of Venus was created when the Magellan spacecraft’s radar data were mapped onto a computer-simulated globe. ( JPL)

9. Venus is much brighter than Mercury, and can be farther from the Sun
Called morning or evening star, as it is still “tied” to Sun
Brightest object in the sky, after Sun and Moon
Figure 9-1. Venus at Sunset The Moon and Venus in the western sky just after sunset. Venus clearly outshines even the brightest stars in the sky. ( J. Schad/Photo Researchers, Inc.)

10. Rotation period: 243 days, retrograde
Radius: 6000 km
Mass: 4.9 x 1024 kg
Density: 5200 kg/m3
Rotation period: 243 days, retrograde
Figure 9-3. Terrestrial Planets’ Spins The inner planets of the solar system—Mercury, Venus, Earth, and Mars—display widely differing rotational properties. Although all orbit the Sun in the same direction and in nearly the same plane, Mercury’s rotation is slow and prograde (in the same sense as its orbital motion), that of Venus is slow and retrograde, and those of Earth and Mars are fast and prograde. Venus rotates clockwise as seen from above the plane of the ecliptic, but Mercury, Earth, and Mars all spin counterclockwise. This is a perspective view, roughly halfway between a flat edge-on view and a direct overhead view.

11. Slow, retrograde rotation of Venus results in large difference between solar day (117 Earth days) and sidereal day (243 Earth days); both are large compared to the Venus year (225 Earth days)
Figure 9-4. Venus’s Solar Day Venus’s orbit and retrograde rotation combine to produce a solar day on Venus equal to 117 Earth days, or slightly more than half a Venus year. The red arrows represent a fixed location, or an observer standing, on the planet’s surface. The numbers in the figure mark time in Earth days.

12. Long-Distance Observations of Venus
Dense atmosphere and thick clouds make surface impossible to see
Surface temperature is about 730 K—hotter than Mercury!
Figure 9-5. Venus This photograph, taken from Earth, shows Venus with its creamy yellow mask of clouds. No surface detail can be seen, because the clouds completely obscure our view of whatever lies beneath them. (AURA)

13. The Surface of Venus Surface mosaics of Venus:
Figure 9-7. Venus Mosaics (a) This image of the surface of Venus was made by a radar transmitter and receiver on board the Pioneer spacecraft, which is still in orbit about the planet, but is now inoperative. The two continent-sized landmasses are named Ishtar Terra (upper left) and Aphrodite (lower right). Colors represent altitude: blue is lowest, red highest. The spatial resolution is about 25 km. (b) A planetwide mosaic of Magellan images, colored in roughly the same way as part (a). The largest “continent” on Venus, Aphrodite Terra, is the yellow dragon-shaped area across the center of this image. See also the full-page, chapter-opening photo. (NASA)

14. The Surface of Venus
Photographs of the surface, from the Venera landers:
Figure Venus In Situ (a) The first direct view of the surface of Venus, radioed back to Earth from the Soviet Venera 9 spacecraft, which made a soft landing on the planet in The amount of sunlight penetrating Venus’s cloud cover is about the same as that reaching Earth’s surface on a heavily overcast day. (b) Another view of Venus, in true color, from Venera 14. Flat rocks like those visible in part (a) are seen among many smaller rocks and even fine soil on the surface. This landing site is not far from the Venera 9 site shown in (a). The peculiar filtering effects of whatever light does penetrate the clouds make Venus’s air and ground appear peach colored—in reality, they are most likely gray, like rocks on Earth. (Russian Space Agency)

15. The Atmosphere of Venus
Venus is the victim of a runaway greenhouse effect—just kept getting hotter and hotter as infrared radiation is reabsorbed
Figure Greenhouse Effect on Earth and Venus Because Venus’s atmosphere is much deeper and denser than Earth’s, a much smaller fraction of the infrared radiation leaving the planet’s surface escapes into space. The result is a much stronger greenhouse effect than on Earth and a correspondingly hotter planet. The outgoing infrared radiation is not absorbed at a single point in the atmosphere; instead, absorption occurs at all atmospheric levels. (The arrows indicate only that absorption occurs, not that it occurs at one specific level; the arrow thickness is proportional to the amount of radiation moving in and out.)

16. Earth
Chapter 7 opener. Photographs like this one, showing Earth hovering in space like a “blue marble,” help us appreciate our place in the universe. The air, water, land, and life comprise a complex, interactive system that constantly changes. Scientists are now trying to understand not only the details of regional events, such as volcanoes, earthquakes, and weather, but also to learn more about the global changes affecting the whole planet. This image is a mosaic of many photographs taken by the GOES-7 environmental satellite. Note the hurricane in the Gulf of Mexico. (NOAA/NASA)

17. Overall Structure of Planet Earth
Mantle
Two-part core
Thin crust
Hydrosphere (oceans)
Atmosphere
Magnetosphere
Figure 7-1. The Main Regions of Planet Earth At the center lies our planet’s solid inner core, about 2600 km in diameter, and surrounding this is a liquid outer core, some 7000 km across. Most of the rest of Earth’s 13,000-km interior is taken up by the mantle, which is topped by a thin crust only a few tens of kilometers thick. The liquid portions of Earth’s surface make up the hydrosphere. Above the hydrosphere and solid crust lies the atmosphere, most of it within 50 km of the surface. Earth’s outermost region is the magnetosphere, extending thousands of kilometers into space.

18. Earth’s Interior Mantle is much less dense than core
Mantle is rocky; core is metallic—iron and nickel
Outer core is liquid; inner core is solid, due to pressure
Volcanic lava comes from mantle, allows analysis of composition

19. Surface Activity
Earth’s upper mantle, near a plate boundary; this is a subduction zone, where one plate slides below another
Figure Earth’s Upper Mantle The outer layers of Earth’s interior. The rocky lithosphere comprises both the crust and part of Earth’s upper mantle. It is typically between 50 and 100 km thick. Below it lies the asthenosphere, a relatively soft part of the mantle over which the lithosphere slips.

20. Surface Activity Plate motion is driven by convection
Figure Plate Drift The motion of Earth’s tectonic plates is probably caused by convection—in this case, giant circulation patterns in the upper mantle that drag the plates across the surface.

21. Surface Activity
If we follow the continental drift backwards, the continents merge into one, called Pangaea
Figure Pangaea Given the current estimated drift rates and directions of the plates, we can trace their movements back into the past. About 200 million years ago, they would have been at the approximate positions shown in (a). The continents’ current positions are shown in (d).

22. The Tides
The Sun has less effect because it is farther away, but it does modify the lunar tides
Figure Solar and Lunar Tides The combined effects of the Sun and the Moon produce variations in the high and low tides. (a) When the Moon is either full or new, Earth, Moon, and Sun are approximately aligned, and the tidal bulges raised in Earth’s oceans by the Moon and the Sun reinforce one another. (b) At first- or third-quarter Moon, the tidal effects of the Moon and the Sun partially cancel each other, and the tides are smallest. Because the Moon’s tidal effect is greater than that of the Sun (since the Moon is much closer to us), the net bulge points toward the Moon.

23. Mars
Chapter 10 opener. The search for life on Mars continues unabated. “Follow the water” is a good guide when prospecting for life—where there is water, there may well be life. But Mars today is as dry as any desert on Earth. However, there is growing evidence for a wetter Mars billions of years ago when the Martian climate was perhaps warmer. One such piece of evidence is seen in this true-color image taken in 2004 by the Spirit robot on one of the Mars Exploration Rovers now exploring Mars’s surface. Amid the many scattered rocks, which hold “memories” of the ancient events that formed them, we see a smooth area (at center) that might be a dried up lakebed that held shallow “puddles” of water. (JPL)

24. Physical Properties of Mars
Radius: 3400 km
Moons: Deimos, Phobos
Mass: 6.4 x 1023 kg
Density: 3900 kg/m3
Length of day: 24.6 hours

25. Long-Distance Observations of Mars
From Earth, can see polar ice caps that grow and shrink with the seasons
Much better pictures from Mars missions, close-up
Figure Mars (a) A deep-red (800-nm) image of Mars, taken in 1991 at Pic-du-Midi, an exceptionally clear site in the French Alps. One of the planet’s polar caps appears at the top and a few other surface markings are evident in this ground-based telescopic view. (b) A visible-light Hubble Space Telescope image of Mars, taken while the planet was near opposition in (c) A view of Mars taken from a Viking spacecraft during its approach in The planet’s surface features can be seen clearly at a level of detail completely invisible from Earth. (CNRS; NASA)

26. Water on Mars
Figure Martian Outflow (a) An outflow channel near the Martian equator bears witness to a catastrophic flood that occurred about 3 billion years ago. (b) The onrushing water that carved out the outflow channels was responsible for forming these oddly shaped “islands” as the flow encountered obstacles—impact craters—in its path. Each “island” is about 40 km long. (NASA)
Current thinking: Open water (rivers, lakes) once existed on Mars

27. Jupiter
Chapter 11 opener. Jupiter is certainly one of the most fascinating objects in the solar system. This is a true color mosaic, constructed from two dozen images taken by a camera onboard the Cassini spacecraft during its closest approach in It is the most detailed portrait of Jupiter ever produced, resolving features to as small as 60 kilometers. Everything seen here is a cloud, from the equatorial regions that show alternating light and dark belts, to high-latitude areas that appear more mottled. The varying structures are caused by differing cloud heights, thickness, and chemical compositions. (JPL)

28. Three views of Jupiter: From a small telescope on Earth; from the Hubble Space Telescope; and from the Cassini spacecraft
Figure Jupiter (a) Photograph of Jupiter made through a ground-based telescope, showing the planet and several of its Galilean moons. (b) A Hubble Space Telescope image of Jupiter, in true color. Features as small as a few hundred kilometers across are resolved. (c) A Cassini spacecraft image of Jupiter, taken while the vehicle was on its way to Saturn, shows intricate clouds of different heights, thicknesses, and chemical composition. (See also the full-page opening photo for this chapter.) (NASA; AURA)

29. Orbital and Physical Properties
Mass: 1.9 × 1027 kg (twice as much as all other planets put together)
Radius: 71,500 km (112 times Earth’s)
Density: 1300 kg/m3—cannot be rocky or metallic as inner planets are
Rotation rate: Problematic, as Jupiter has no solid surface; different parts of atmosphere rotate at different rates
From magnetic field, rotation period is 9 hr, 55 min

30. The Atmosphere of Jupiter
Major visible features:
Bands of clouds; Great Red Spot
Figure Jupiter’s Red Spot Voyager 1 took this photograph of Jupiter’s Great Red Spot (upper right) from a distance of about 100,000 km. Resolution is about 100 km. Note the complex turbulence to the left of both the Red Spot and the smaller white oval below it. (For scale, planet Earth has been superposed.) (NASA)

31. Internal Structure
Jupiter radiates more energy than it receives from the Sun:
Core is still cooling off from heating during gravitational compression
Could Jupiter have been a star?
No; it is far too cool and too small for that. It would need to be about 80 times more massive to be even a very faint star.

32. Internal Structure
No direct information is available about Jupiter’s interior, but its main components, hydrogen and helium, are quite well understood. The central portion is a rocky core.
Figure Jupiter’s Interior Jupiter’s internal structure, as deduced from Voyager measurements and theoretical modeling. The outer radius represents the top of the cloud layers, some 70,000 km from the planet’s center. The density and temperature increase with depth, and the atmosphere gradually liquefies at a depth of a few thousand kilometers. Below a depth of 20,000 km, the hydrogen behaves like a liquid metal. At the center of the planet lies a large rocky core, somewhat terrestrial in composition, but much larger than any of the inner planets. Although the values are uncertain, the temperature and pressure at the center are probably about 25,000 K and 60 million (Earth) atmospheres, respectively.

33. The Moons of Jupiter
Jupiter with Io and Europa. Note the relative sizes!
Figure Jupiter, Up Close Voyager 1 took this photo of Jupiter with ruddy Io on the left and pearllike Europa toward the right. Note the scale of objects here: Both Io and Europa are comparable in size to our Moon, and the Red Spot is roughly twice as big as Earth. (NASA)

34. The Moons of Jupiter Interiors of the Galilean moons:
Figure Galilean Moon Interiors Cutaway diagrams showing the interior structure of the four Galilean satellites. Moving outward from Io to Callisto, we see that the moons’ densities steadily decrease as the composition shifts from rocky mantles and metallic cores in Io and Europa, to a thick icy crust and smaller core in Ganymede, to an almost uniform rock and ice mix in Callisto. Both Ganymede and Europa are thought to have layers of liquid water beneath their icy surfaces.

35. Saturn’s Atmosphere
This true-color image shows the delicate coloration of the cloud patterns on Saturn
Figure Saturn This spectacular image, acquired in 2005 by the Cassini spacecraft while approaching Saturn, is actually a mosaic of many images taken in true color. Note the subtle coloration of the planet and the detail in its rings. Resolution is 40 km. (NASA)

36. Orbital and Physical Properties
Mass: 5.7 × 1026 kg
Radius: 60,000 km
Density: 700 kg/m3—less than water!
Rotation: Rapid and differential, enough to flatten Saturn considerably
Rings: Very prominent; wide but extremely thin

37. Orbital and Physical Properties
View of rings from Earth changes as Saturn orbits the Sun
Figure Ring Orientation (a) Over time, Saturn’s rings change their appearance to terrestrial observers as the tilted ring plane orbits the Sun. At some times during Saturn’s 29.5-year orbital period the rings seem to disappear altogether as Earth passes through their plane and we view them edge-on. Numbers along Saturn’s orbit indicate the year. The roughly true-color images (inset) span a period of several years from the mid-1990s (bottom) to nearly the present (top), showing how the rings change from our perspective, from almost edge-on to more nearly face-on. See also Figure 12.2 for a closeup image of its tilted ring system relative to Earth. (NASA)

38. Saturn’s Interior Interior structure similar to Jupiter’s
Figure Saturn’s Interior Saturn’s internal structure, as deduced from Voyager observations and computer modeling.

39. The Moons of Saturn
The Huygens spacecraft has landed on Titan and is returning images directly from the surface
Figure The View from Huygens (a) Artist’s conception of the Huygens lander parachuting through Titan’s thick atmosphere. (b) This photograph of the surface was taken from an altitude of 8 km as the probe descended. It shows a network of channels reminiscent of streams or rivers draining from the light-shaded uplifted terrain (at center) into a darker, low-lying region (at bottom). Resolution is about 20 m. (c) Huygens’s view of its landing site, in approximately true color. The foreground rocklike objects are only a few centimeters across. (D. Ducros; NASA/ESA)

40. Uranus Image by Voyager 2 at a distance of 1 million km
Figure Uranus, Close Up This image of Uranus, taken from a distance of about 1 million km, was sent back to Earth by the Voyager 2 spacecraft as it whizzed past the giant planet at 10 times the speed of a rifle bullet. The image approximates the planet’s true color, but shows virtually no detail in the nearly featureless upper atmosphere, except for a few wispy clouds in the northern hemisphere. (NASA)

41. The Discovery of Neptune
Neptune was discovered in 1846, after analysis of Uranus’s orbit indicated its presence
Details of Neptune cannot be made out from Earth either; arrows again point to moons:
Figure Neptune from Earth Neptune and two of its moons, Triton (left arrow) and Nereid (right), imaged with a large Earth-based telescope. (UC/Lick Observatory)

42. Neptune
Chapter 13 opener. The outer planets have only briefly been explored as robot spacecraft glided by them. This photo of Neptune, in approximately true color, was taken by Voyager 2 when still at a distance of more than 6 million kilometers from the planet. Shown is the Great Dark Spot and its companion bright smudge, among other small clouds that persisted for as long as Voyager’s cameras could resolve them. Given other priorities of the U.S. space program—namely, sending humans back to the Moon and then on to Mars—it will likely be many years before robots return to these faraway worlds. (JPL)

43. Orbital and Physical Properties
Uranus and Neptune are very similar
Figure Jovian Planets Jupiter, Saturn, Uranus, and Neptune, showing their relative sizes compared to Earth. Uranus and Neptune are quite similar in their bulk properties, each one probably having a core about 10 times more massive than Earth. Jupiter and Saturn are both much larger, but their rocky cores are probably comparable in mass to those of Uranus and Neptune. Note the very different atmospheric features of these five worlds. (NASA)

44. Orbital and Physical Properties
Uranus
Neptune
Mass
14.5 x Earth
17.1 x Earth
Radius
4.0 x Earth
3.9 x Earth
Density
1300 kg/m3
1600 kg/m3