The Terrestrial Planets Introduction v The four terrestrial

The Terrestrial Planets

Introduction v The four terrestrial planets – Mercury, Venus, Earth (The largest), and Mars – have similar sizes and structure v These rocky worlds orbit in the inner part of the Solar System v They have very few satelites – the Earth has the relatively large Moon and Mars has two small captured asteroids as moons The Terrestrial Planets 2

Portraits of the Terrestrial Planets The Terrestrial Planets 3

Portraits of the Terrestrial Planets The Terrestrial Planets 4

Portraits of the Terrestrial Planets v Although the four terrestrial planets are comparable in size, they have very different atmospheres and surfaces: l l Mercury – smallest terrestrial planet, looks like Moon (gray, bare, cratered), essentially no atmosphere Venus – covered with deep sulfuric acid clouds in a dense CO 2 atmosphere, hottest planet, immense volcanic peaks tower over desolate plains Mars – polar caps of ice and CO 2, vast red deserts with craters and dunes, canyons and dry river beds, ancient volcanoes, thin CO 2 atmosphere Earth – blue seas, white clouds and ice caps, red deserts, green jungles, mountains v Planetary size along with distance from Sun is the cause for these differences The Terrestrial Planets 5

Mercury v Introduction l Mercury’s radius is 1/3 and its mass 1/20 that of Earth l Circular craters cover the surface l Congealed lava flows are found in many of Mercury’s old craters and pave much of its surface l Enormous cliffs, formed as Mercury cooled, and shrank, wrinkling like a dried apple The Terrestrial Planets 6

Mercury v Mercury’s Temperature and Atmosphere l Mercury’s noon temperatures at the equator (about 800° F) and nighttime temperatures (-280° F) l These extremes result from Mercury’s proximity to the Sun and its lack of atmosphere l Its low mass and proximity to the Sun do not allow Mercury to retain an atmosphere of any significance l Its lack of volcanoes suggests that Mercury never had a significant atmosphere The Terrestrial Planets 7

Mercury v Mercury’s Rotation l Mercury spins very slowly with a rotation period of 59 Earth days l Its orbit around the Sun takes 88 Earth days l Consequently, Mercury spins 3 times for every 2 trips around the Sun l Mercury’s solar day (sun to reach it's highest point in the sky twice) twice is 176 Earth days, longer than its year! l Because of Mercury’s slow rotation, near perihelion (when Mercury is closest to the sun) the Sun will briefly reverse direction in Mercury’s sky. The Terrestrial Planets 8

Venus v Introduction l Venus has a mass and diameter very close to that of Earth l However, the two planets have very different surfaces and atmospheres v The Atmosphere l 96% CO 2, 3. 5% N 2, and small amounts of H 2 O and other gases l The clouds of Venus are sulfuric acid droplets with traces of water l The atmosphere is extremely dense, reaching pressures about 100 times that of Earth’s l The lower atmosphere is very hot with temperatures of 900° F at the surface, enough to melt lead l Spacecraft have landed on Venus, but do not survive long The Terrestrial Planets 9

Venus v The Greenhouse Effect l Large amounts of CO 2 in the atmosphere creates an extremely strong greenhouse effect l The effect is so strong Venus’ surface is hotter than Mercury’s although Venus is farther from the Sun The Terrestrial Planets 10

Venus v The Surface of Venus l Ground features can be mapped with radar from Earth and spacecraft orbiting Venus since radar can penetrate the clouds l Venus’s surface is less mountainous and rugged than Earth, with most of its surface low, gently rolling plains The Terrestrial Planets 11

Venus v The Interior of Venus l Interior of Venus probably very similar to Earth – iron core and rock mantle v Rotation of Venus l Venus is the slowest rotating planet, taking 243 Earth days to rotate once, and its spin is retrograde (“backward”) l Solar day on Venus is 117 Earth days The Terrestrial Planets 12

Mars v The Surface of Mars l Although its diameter is ½ and its mass 1/10 that of Earth, Mars is the planet that most resembles the Earth On a warm day, the temperature hits about 50° F l Winds sweep dust and patchy ice crystal clouds through a sky that generally is clear enough for its surface to be seen from Earth l Sparkling white polar caps contrast with the reddish color of most of the planet l The Terrestrial Planets 13

Mars v The Surface of Mars l Valles Marineris A rift running along the equator stretching 1000 km long, 100 km wide, and 10 km deep l This canyon, named after Mariner, dwarfs the Grand Canyon and would span the U. S. l l Martian Polar Ice Caps Change in size with seasons (Mars’ tilt similar to Earth’s) l Thin atmosphere creates more severe extremes in the seasons leading to large ice cap size variations l Water contained in Mars caps is far less than that in Earth’s caps l The Terrestrial Planets 14

Valles Marineris The Terrestrial Planets 15

Mars v The Surface of Mars l Huge channels and dry river beds From winding nature of features that often contain “islands”, it is inferred that water once flowed on Mars l No surface liquid is now present l Huge lakes and small oceans thought to have once existed – evidence comes from smooth traces that look like old beaches around edges of craters and basins l The Terrestrial Planets 16

The Terrestrial Planets Photo taken from Mars Rover 17

Mars v The Martian Atmosphere l Clouds and wind blown dust are visible evidence that Mars has an atmosphere l CO 2 (95%) with traces of N 2 (3%), oxygen and water l The atmosphere’s density is about 1% that of the Earth’s l The lack of atmospheric density and Mars distance from the Sun make the planet very cold l l l Noon temperatures at the equator reach a bit above the freezing point of water Night temperatures drop to a frigid -67° F Thus, most water is frozen, locked up either below the surface as permafrost or in the polar caps as solid ice The Terrestrial Planets 18

Mars v. The Martian interior Differentiated like the Earth’s interior into a crust, mantle, and iron core l Having a mass between that of dead Mercury and lively Earth/Venus implies Mars should be intermediate in tectonic activity l The Terrestrial Planets 19

Mars v The Martian moons l Phobos and Deimos are about 20 km across and are probably captured asteroids l Their small size prevents gravity from pulling them into spherical shapes l Both are cratered, implying bombardment by smaller objects, and Phobos has cracks suggesting an impact by an object large enough to nearly split it apart The Terrestrial Planets 20

Phobos Deimos The Terrestrial Planets 21

Mars v Life on Mars? l Interest in life on Mars grew enormously with the misinterpretation of observations made by astronomer Giovonni Schiaparelli in 1877, who called certain straight-line features on Mars “canalli” meaning “channels l l English-speaking countries interpreted this as “canals” and the search for intelligent life on Mars began Spacecraft photos later revealed features on Mars to be natural land structures Viking spacecraft landed on Mars to search for life up close – no evidence found In 1996, a meteorite was found on Earth with a Mars origin l l Certain meteorite structures suggested Martian bacteria Most scientists today are unconvinced The Terrestrial Planets 22

Update: Exploring Mars v Mars Pathfinder/Sojourner landed in 1997 l Pathfinder studied the atmosphere and noticed wispy cirrus clouds like those on Earth and large variations in surface temperature even over distances as small as 1 meter l Sojourner, a small motorized cart, examined rocks and found some indications that the landing site was once covered by water v Mars Global Surveyor In Orbit in 1997 l Detailed images show evidence of soil and rock layers as well as many wind-caused erosion features v Mars Rovers l Launched June 2003 and arrived on Mars January 2004 l The rovers were designed to last for 90 days on the Martian surface. l The rovers investigate the landscape and collect samples, sending data back to scientists on Earth. The Terrestrial Planets 23

The Terrestrial Planets 24