Eighth Edition
by Harold L. Levin

Chapter 8 - page 4

The Earliest Earth: 2,100,000,000 Years of the Archean Eon

The Solar System Tour, From Center to Fringe

The Sun

The Sun is a star.

Composition: about 70% hydrogen and 27% helium, and 3% heavier elements.

Size: About 1.5 million km in diameter.

Contains about 98.8% of the matter in the Solar System.

Temperature: may exceed 20 million ^oC in the interior.

Sun's energy comes from fusion, a thermonuclear reaction in which hydrogen atoms are fused together to form helium, releasing energy.

The Sun's gravity holds the planets in their orbits.

Sun's energy is the force behind many geologic processes on Earth:

• Evaporation of water to produce clouds, which cause precipitation, which causes erosion.
• Uneven heating of the Earth's atmosphere causes winds and ocean currents.
• Variations in heat from sun may trigger continental glaciations or change forests to deserts.
• Sun and moon influence tides which affect the shoreline.

The Planets

Know the names of the nine planets, in order, from the sun outward.

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

It may help to have a mnemonic device to keep them straight. Use the letters M V E M J S U N P. Examples:

• Mark's very extravagant mother just sent us nine parakeets
• My very elegant mother just sent us nine pizzas

The planets can be grouped into two types. (Pluto falls into a category called "Other" as it is different from these two types).

• Terrestrial planets (Earth-like): Mercury, Venus, Earth, Mars
• Small
• Dense (4 - 5.5 g/cm³)
• Rocky + Metals
• Jovian planets: Jupiter, Saturn, Uranus, Neptune
• Large
• Low density (0.7 - 1.5 g/cm³)
• Gaseous

For a density comparison, the mineral quartz has a density of 2.6 g/cm³ and water has a density of 1 g/cm³.

The Terrestrial Planets

Mercury

• Smallest of the terrestrial planets
• Revolves rapidly around the sun; its year is 88 Earth days
• Densely cratered
• Thin atmosphere of sodium and lesser amounts of helium, oxygen, potassium, and hydrogen
• Weak magnetic field and high density suggest an iron core
• No moons

This mosaic of Mercury was taken by the Mariner 10 spacecraft during its approach on 29 March 1974. The mosaic consists of 18 images taken at 42 s intervals during a 13 minute period when the spacecraft was 200,000 km (about 6 hours prior to closest approach) from the planet. Image courtesy of NASA NSSDC Photo Gallery

Venus

• Similar to Earth in size, mass, volume, density, and gravity
• No oceans or liquid water
• Very high atmospheric pressure
• Atmosphere is 98% carbon dioxide
• Dense clouds of sulfuric acid droplets in atmosphere
• Greenhouse effect causes temperature on planet's surface to reach 470^oC, hot enough to melt lead
• Rotates once on its axis (one day on Venus) in 243 Earth days
• Rotates on axis in opposite direction to other planets, possibly due to collision with other object
• Has volcanoes
• Has craters
• Surface rocks resemble basalt
• No moons

Ultraviolet image of Venus' clouds as seen by the Pioneer Venus Orbiter (Feb. 5, 1979). Image courtesy of NASA NSSDC

Topographic Map of Venus from Pioneer Venus (Mercator Projection). Click on image to see larger version. Image courtesy of NASA NSSDC

Earth

• Diameter = nearly 13,000 km (8000 mi)
• Oceans cover 71% of surface
• Atmosphere = 78% nitrogen and 21% oxygen
• Surface temperature ranges between roughly -50 and +50 ^oC.
• Average density = 5.5 g/cm³; surface rocks are 2.5-3.0 g/cm³
• Core is denser (and about 7000 km in diameter)
• Mantle surrounds core. Extends from base of crust to depth of 2900 km.
• Geologically active. Plate tectonics.
• Only body in the Universe known to support life.

Several factors make Earth hospitable for life:

• Distance from the Sun helps maintain temperatures in the range where water is liquid. Temperature has remained relatively constant for billions of years.
• Rotation allows all sides of Earth to have light and heat.
• Atmosphere absorbs some heat from the Sun and reflects some solar radiation back to space.
• Magnetic field protects life from dangerous high energy particles and radiation in the solar wind.

Earth - The Blue Marble. Credit: NASA Goddard Space Flight Center Image by Reto Stöckli. Visible Earth.

Earth's Moon

Basic facts about the Moon

• Diameter = about one quarter that of Earth
• Density = about 3.3 g/cm³ (similar to Earth's mantle)
• Rotates on its axis at same rate as it revolves around Earth (29.5 days), which results in same side of Moon always facing Earth
• Far side of moon is more densely cratered
• No atmosphere
• Ice is present at the poles

Geology of the Moon

Dominant rock type is anorthosite (related to gabbro, rich in calcium plagioclase feldspar). Basalt is also present.

1. Lunar highlands
1. Light-colored
2. Rough topography
3. Highly cratered
4. Rocks more than 4.2 billion years old
2. Maria (singular = mare)
1. Large, dark areas
2. Immense basins covered with basaltic lava flows
3. Age of basalt is 3.8 to 3.2 billion years
4. Few craters indicating decrease in meteorite bombardment after about 3.8 billion years ago

The Moon. Photograph taken by Apollo 11 in 1969. Image courtesy of NASA National Space Science Data Center.

Origin of the Moon

Moon may have formed as a result of an impact of a large body with Earth about 4.4 billion years ago.

Debris from the impact was thrown into orbit around Earth and collected to form the Moon.

Heat from impacts led to melting and differentiation (or segregation of materials of different density; low density materials rose and high density materials sank).

• Low density alumino-silicates rose to surface to form a crust.
• Higher density iron-rich minerals sank to form the lunar mantle, and possibly a small core.
• Basaltic lava flows of the maria erupted following large impacts, somewhat later.

Mars

• Has white polar caps made of frozen carbon dioxide ice.
• Has seasonal changes in which polar ice caps expand and contract.
• Rusty orange color due to iron oxides on surface.
• Heavily cratered due to early bombardment by meteorites and asteroids.
• Diameter is about half that of Earth.
• Mass is only about 10% of Earth's mass, so gravity is much less.
• Thin atmosphere (less than 1% as dense as Earth's). Dominant gas is carbon dioxide; small amounts of nitrogen, oxygen and carbon monoxide. No greenhouse effect.
• Previously has a denser atmosphere.
• Evidence of abundant liquid water in the past.
• An ocean once existed, at least 0.5 km deep and larger than all 5 U.S. Great Lakes.
• Temperatures range from -85^oC to 21^oC (about room temperature at the high).
• Lower density than other terrestrial planets.
• Little to no magnetic field, suggesting only a small iron-rich core.
• Lack of magnetic field exposed planet to solar winds which swept away atmosphere and liquid water.
• Has volcanoes, canyons, channels, sand dunes, and layered rock.
• Two small moons, Phobos and Deimos.

Global image of Mars taken by the Wide-Field Planetary Camera 2 on the Hubble Space Telescope. The image shows the Valles Marineris region (centered on roughly 60 degrees longitude). Note the polar ice caps (white). Click on image to see larger version. Image courtesy of NASA NSSDC.

The largest known volcano in the Solar System, Olympus Mons, on Mars. The cental edifice of Olympus Mons has a summit caldera 24 km above the surrounding plains. Surrounding the volcano is an outward-facing scarp 550 km in diameter and several kilometers high. Beyond the scarp is a moat filled with lava, most likely derived from Olympus Mons. Farther out is an aureole of characteristically grooved terrain, just visible at the top of the frame. Click on image to see larger version. Image courtesy of NASA NSSDC.

Link to information on evidence for liquid water found by Mars Rover Opportunity in 2004.
Link to images indicating recent liquid water on Mars.

The Asteroid Belt

There is an asteroid belt with thousands of asteroids, primarily between Mars and Jupiter.

Asteroids are composed of rocks and metal, such as iron and nickel.

Size of asteroids ranges from a few km in diameter to about one tenth the size of Earth.

An Asteroid. Image courtesy of NASA Jet Propulsion Laboratory

The Five Outer Planets

Jupiter

• Largest planet in the Solar System. (Diameter 11 times greater than Earth)
• Low density. (Density is about one quarter that of Earth)
• Rotates on axis rapidly. One day on Jupiter is 10 hours on Earth.
• Rotation causes bands in atmosphere.
• Note Great Red Spot, a cyclonic storm.
• Atmosphere composed of hydrogen, helium, with lesser amounts of methane and ammonia.
• Most of planet's interior is probably liquid metallic hydrogen.
• Has a faint ring of debris which encircles the planet.
• Has 63 or more moons.
• Four largest moons are (from innermost, outward, with first letters following the mnemonic "I eat green carrots"):
  1. Io- covered by sulfur volcanoes.
  2. Europa - has sea of liquid water beneath an icy surface.
  3. Ganymede - cratered with sinuous ridges; has sea of liquid water beneath an icy surface.
  4. Callisto - highly cratered; has sea of liquid water beneath an icy surface.

Image of Jupiter taken by Voyager in 1979. The Great Red Spot shows prominently below center, surrounded by what scientists call a remarkably complex region of the giant planet's atmosphere. An elongated yellow cloud within the Great Red Spot is swirling around the spot's interior boundary in a counterclockwise direction with a period of a little less than six days, confirming the whirlpool-like circulation that astronomers have suspected from ground-based photographs. Ganymede, Jupiter's largest satellite, can be seen to the lower left of the planet. Ganymede is a planet-sized body larger than Mercury. Click on image for larger version. Image courtesy of NASA NSSDC.

Saturn

• Second largest planet
• Has prominent rings of debris encircling planet in equatorial plane
• Density is less than that of water; it could float. (Density = 0.7g/cm³)
• Mostly hydrogen and helium; also contains methane, ammonia, and water; may have iron core
• Has magnetic field, radiation belts, and internal heat source
• Has 47 or more moons

Full-color view of Saturn and its rings. NASA's Voyager 2 took this photograph of Saturn on July 21, 1981, when the spacecraft was 33.9 million kilometers (21 million miles) from the planet. Two bright, presumably convective cloud patterns are visible in the mid-northern hemisphere and several dark spoke-like features can be seen in the broad B-ring (left of planet). The moons Rhea and Dione appear as blue dots to the south and southeast of Saturn, respectively. Voyager 2 made its closest approach to Saturn on Aug. 25, 1981. Image courtesy of NASA NSSDC.

Uranus

• About 4 times larger than Earth
• Low density (density = 1.3 g/cm³)
• Axis of rotation is tipped on its side, possibly due to collision with another Solar System object
• Has 27 or more moons
• Atmosphere of hydrogen, helium, and methane
• Has planetary ring system

Uranus. Image courtesy of NASA Jet Propulsion Laboratory

Neptune

• Similar in size and color to Uranus
• Low density (density = 1.6 g/cm³)
• Atmosphere of hydrogen, helium, and methane
• Has 13 or more moons
• Has Great Dark Spot, a cyclonic storm
• Has planetary ring system

Color image of Neptune showing its "Great Dark Spot." Click on image to see larger version. Image courtesy of NASA NSSDC.

Pluto

• Smallest planet.
• Orbit is very elliptical and crosses the orbit of Neptune.
• Orbit is inclined or tilted at 17^o to the rest of the Solar System.
• Axis of rotation lies sideways.
• Has a moon, Charon.
• Atmosphere is methane and nitrogen.
• No rings.
• May be an escaped moon of Neptune, rather than an original planet.

Pluto and its moon Charon as revealed by NASA's Hubble Space Telescope (HST). The Hubble observations show that Charon is bluer than Pluto. This means that both worlds have different surface composition and structure. A bright highlight on Pluto suggests it has a smoothly reflecting surface layer. Though Pluto was discovered in 1930, Charon wasn't detected until 1978. That is because the moon is so close to Pluto that the two worlds are typically blurred together when viewed through ground-based telescopes. Pluto typically is called the double planet because Charon is half the diameter of Pluto (our Moon is one-quarter the diameter of Earth). Credit: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA Image courtesy of NASA NSSDC.

Planets have also been detected orbiting stars outside our Solar System, but they are too far away to observe directly. They are detected by observing their gravitational effect on the stars around which they revolve.