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Mass of the Moon is about 1/80 that of the Earth, and its diameter is about 1/4 that of the Earth. The orbit is very nearly circular (eccentricity ~ 0.05) with a mean separation from the Earth of about 384,000 km, which is about 60 Earth radii. The plane of the orbit is tilted about 5 degrees with respect to the ecliptic plane. The Moon
Moon is the primary cause of tides Tides due to the Sun are two times weaker The interior of the Moon and Earth is heated by tides. (Compare to Io!)
As the Earth rotates beneath the tidal bulges, it attempts to drag the bulges along with it. A large amount of friction is produced which slows down the Earth's spin. The day has been getting longer and longer by about 0.0016 seconds each century. The Earth’s day was 18 hours long 900 million yr ago Eventually the Earth will keep one face towards Moon. Moon already keeps one face towards Earth – rotational period equals orbital period of 29.5 days.
Fig. 17-4b, p.353 Since the synodic rotational period of the Moon is 29.5 days, Lunar day and Lunar night are each about 15 Earth days long. During the Lunar night the temperature drops to around -113 degrees Celsius, while during the Lunar day the temperature reaches 100 degrees Celsius. The temperature changes are very rapid since there is no atmosphere or surface water to store heat.
Moon has too weak gravity to keep the atmosphere. That is why it also does not have liquid water. It may have ice though (important for future stations!)
History of Impact Cratering Most craters seen on the moon’s (and Mercury’s) surface were formed within the first ~ 1/2 billion years. Rate of impacts due to interplanetary bombardment decreased rapidly after the formation of the solar system.
Moon Rocks All moon rocks brought back to Earth are igneous (= solidified lava) No sedimentary rocks => No sign of water ever present on the moon. Different types of moon rocks: Vesicular (= containing holes from gas bubbles in the lava) basalts, typical of dark rocks found in maria Breccias (= fragments of different types of rock cemented together), also containing anorthosites (= bright, low-density rocks typical of highlands) Older rocks become pitted with small micrometeorite craters
Lunar maria (“seas”) – huge flows of dark basalt lava
Formation of Maria Impacts of heavy meteorites broke the crust and produced large basins that were flooded with lava
The Moon's density (3.3 g/cm 3 ) is fairly uniform throughout and is only about 3.3 times the density of water. If it has an iron core, it is less than 800 kilometers in diameter. This is a sharp contrast from planets like Mercury and the Earth that have large iron-nickel cores and overall densities more than 5 times the density of water. The Moon's mantle is made of silicate materials, like the Earth's mantle, and makes up about 90% of the Moon's volume. The temperatures do increase closer to the center and may be high enough to partially liquify the material close to the center. Its lack of a liquid iron-nickel core and slow rotation is why the Moon has no magnetic field. Lunar samples brought back by the Apollo astronauts show that compared to the Earth, the Moon is deficient in iron and nickel and volatiles (elements and compounds that turn into gas at relatively low temperatures) such as water and lead. The Moon is richer in elements and compounds that vaporize at very high temperatures. The Moon's material is like the Earth's mantle material but was heated to very high temperatures so that the volatiles escaped to space. Strange peculiarities in the Moon’s composition
Our Moon could have been formed in a giant collision 4.5 billion years ago
proposes that a large Mars-sized object hit the Earth and blew mantle material outward which later recoalesced to form the Moon. The Earth had already differentiated by the time of the giant impact so its mantle was already iron-poor. The impact and exposure to space got rid of the volatiles in the ejecta mantle material. Such an impact was rare so is was not likely to have also occurred on the other terrestrial planets. Giant impact theory:
Modern Theory of Formation of the Moon The Large-Impact Hypothesis
Impact heated material enough to melt it
consistent with “sea of magma”
Collision not head-on
Large angular momentum of Earth-moon system
Collision after differentiation of Earth’s interior
Different chemical compositions of Earth and moon
Mercury Very similar to Earth’s moon in several ways:
Small; no atmosphere
lowlands flooded by ancient lava flows
heavily cratered surfaces
Most of our knowledge based on measurements by Mariner 10 spacecraft (1974 - 1975) View from Earth
The surface conditions are among the harshest in the Solar System. During the long Mercurian day the temperature rises to about 425 degrees Celsius, hot enough to melt lead and hotter than any planet except Venus. Because there is no substantial atmosphere to retain heat, during the equally long nights, the temperature drops quickly to around -180 degrees Celsius, which is among the coldest found in the Solar System. This range of -180 Celsius at night to 425 Celsius in the day is the largest surface temperature variation in the Solar System.
Almost all planets rotate counterclockwise, i.e. in the same sense as orbital motion.
Exceptions: Venus, Uranus and Pluto
Venus rotates clockwise, with period slightly longer than orbital period.
Off-center collision with massive protoplanet
Tidal forces of the sun on molten core
Venus is the second planet from the Sun, with a nearly circular orbit having an average radius of 0.7 A.U. This gives it an orbital period of 225 days. Venus is peculiar in that its rotation is retrograde (in the opposite sense of the Earth and all other planets except Uranus) and because it is very slow: a day on Venus corresponds to 243 Earth days. At present, we have no solid explanation for why this is so. The most plausible theories invoke the collision of two large masses to form Venus in just such a way to cancel most of the rotation for the two masses. Like Mercury, but unlike the other planets, Venus has no moons. UV Radio image
Venus is about 95% the size of the Earth and has 82% of the Earth's mass. Like the Earth, Venus has a rocky crust and iron-nickel core. But the similarities stop there. Venus has a thick atmosphere made of 96% carbon dioxide ( CO2 ), 3.5% nitrogen ( N2 ), and 0.5% other gases. At Venus' surface, the air pressure is 91 times the Earth's surface atmospheric pressure. Venus' surface atmospheric pressure is the same as what you would feel if you were 1 kilometer below the ocean surface on the Earth. A human cannot survive at depths greater than just 70 meters below the ocean surface without special diving suits or a submarine. If you want to send someone to Venus, that person would need to be in something like a diving bell. The Venus explorer would also need a very powerful cooling system: the surface temperature is 737 K (= 477° C)! This is hot enough to melt lead and is over twice as hot as it would be if Venus did not have an atmosphere. Why does Venus have such a thick atmosphere and why is it so hot on its surface?
The Atmosphere of Venus UV image Extremely inhospitable: 96 % carbon dioxide (CO 2 ) 3.5 % nitrogen (N 2 ) Rest: water (H 2 O), hydrochloric acid (HCl), hydrofluoric acid (HF) 4 thick cloud layers ( surface invisible to us from Earth). Very stable circulation patterns with high-speed winds (up to 240 km/h) Extremely high surface temperature up to 745 K (= 880 o F) Very efficient “greenhouse”! UV image
The Surface of Venus Early radar images already revealed mountains, plains, craters. Venera 13 photograph of surface of Venus: Colors modified by clouds in Venus’s atmosphere More details from orbiting and landing spacecraft: After correction for atmospheric color effect:
Axis tilted against orbital plane by 25 o , similar to Earth’s inclination (23.5 o )
Seasons similar to Earth Growth and shrinking of polar ice cap
Crust not broken into tectonic plates
Volcanic activity (including highest volcano in the solar system)
"It will be possible to see cities on Mars, to detect navies in [its] harbors, and the smoke of great manufacturing cities and towns... Is Mars inhabited? There can be little doubt of it ... conditions are all favorable for life, and life, too, of a high order. Is it possible to know this of a certainty? Certainly." Samuel Leland 1895
Tales of Canals and Life on Mars Early observers (Schiaparelli, Lowell) believed to see canals on Mars This, together with growth/shrinking of polar cap, sparked imagination and sci-fi tales of life on Mars. We know today: “canals” were optical illusion; do not exist! No evidence of life on Mars.
Mars is about half the diameter of the Earth and has 1/10th the Earth's mass. Mars' thin atmosphere (just 1/100th the Earth's) does not trap much heat at all even though it is 95% carbon dioxide ( CO2 ). The other 3% is nitrogen ( N2 ). Because the atmosphere is so thin, the greenhouse effect is insignificant and Mars has rapid cooling between night and day. When night comes the temperature can drop by over 100 K (180° F)! The large temperature differences create strong winds. The strong winds whip up dust and within a few weeks time, they can make dust storms that cover the entire planet for a few months.
The Geology of Mars Giant volcanoes Valleys Impact craters Vallis Marineris Reddish deserts of broken rock, probably smashed by meteorite impacts.
Volcanism on Mars Volcanoes on Mars are shield volcanoes. Olympus Mons: Highest and largest volcano in the solar system.
The Geology of Mars (2) Northern Lowlands: Free of craters; probably re-surfaced a few billion years ago. Southern Highlands: Heavily cratered; probably 2 – 3 billion years old. Possibly once filled with water.
Hidden Water on Mars No liquid water on the surface: Would evaporate due to low pressure. But evidence for liquid water in the past: Outflow channels from sudden, massive floods Collapsed structures after withdrawal of sub-surface water Splash craters and valleys resembling meandering river beds Gullies, possibly from debris flows Central channel in a valley suggests long-term flowing water
Hidden Water on Mars (2) Gusev Crater and Ma’adim Vallis: Giant lakes might have drained repeatedly through the Ma’adim Vallis into the crater.