2000 February 1, SPS 1020 (Introduction to Space Sciences) - Read TNSS Ch. 11 (Mars) by Thursday, February 3. - Read TNSS Chs. 7 (Mercury) and 8 (Venus), and PBD Chs. 11 and 12 by Tuesday, February 8. - There will be a reading quiz on TNSS Chapters 7 (Mercury), 8 (Venus), 10 (The Earth's Moon), 11 (Mars), and PBD Chapters 11 and 12 on Tuesday, February 8. ------------------------------------------------------------------------- The Earth's Moon (TNSS Chapter 10): ---------------------------------- Lunar phases: strictly an illumination effect (handout) Tides: caused by _difference_ of grav. forces, on opposite sides of a body Creates ocean bulges on both sides of Earth. High tide therefore comes about every 12 hours. Because of the Earth's rapid (24-hr) rotation, the tidal bulges lag behind the Moon's position, so it's really > 12 hours. Because the tidal bulge rotates faster than the Moon orbits Earth, the tidal bulge tries to accelerate the Moon, _increasing its orbital radius_. The Moon "pulls" on the tidal bulge => it slows down Earth's rotation. K3 => Moon is receding from Earth, at about 3 cm/year! Earth's spin is also slowing down: 500 Myr ago, a "day" was ~ 22 hours. Eventually, Earth's spin will be at the rate the Moon orbits Earth (at its increased distance). This "day" will equal 47 present (24-hr) days. D(Moon) = 55,000 km, vs. 384,400 km today. The Earth's Moon: ----------------- diameter = 3476 km ~ 1/4 Earth mass = 1/81.3 Earth distance = 384,400 km = 238,000 mi P(synodic) = 29.5 days (time between Full Moons). Almost no atmosphere, because M is too low, and T is too high. => T (day) = +130 C (250 F) T(night) = -200 C (-300 F) Moon's rotation is tidally locked (synchronized), so period of rotation (day) = period of revolution (orbital period) => P(synodic) = length of 1 day and 1 night. Average density = 3.3 gm/cm^3 ~ Earth's mantle (Earth 5.52 g/cm^3) Only very small global magnetic field => Fe core only 2% mass of Earth's (dynamo that generates magnetic field is so much weaker). Two basic types of lunar terrain: -------------------------------- 1) Highlands: rugged, ancient cratered areas (albedo = 9-12 %) 2) Maria (Latin for "seas"; singular Mare): relatively smooth, dark, lava plains (albedo = 5-8 %) Most maria are on the Near Side (facing Earth): no one knows why. Perhaps tides involved? Crust _is_ thinner on Near Side. Everywhere is coated with regolith (often wrongly called "soil"), a "very, very fine grained powder." [N. Armstrong] ~10m deep on maria, ~100 m deep in highlands Of the 4 principal geological processes, 1) Impact cratering dominates, by far! 2) Little tectonism; some faults and some scarps (cliffs, from shrinkage of crust when lava cooled), but no plate tectonics, because there is no internal heat! 3) No active volcanism. Most maria formed > 3 billion years ago, like lava plateaus on Earth (oozed). Some activity as recent as ~1 b.y.a. - Rilles: lava channels. - Lunar domes may be cinder cones? e.g. Marius Hills - May be residual outgassing (Transient Lunar Phenomena or TLPs), esp. near Aristarchus. However, recent Sky & Telescope article casts doubt on _all_ such claims. 4) Only source of geological gradation (or erosion) = micrometeorites and Solar wind => Astronauts' footprints will last 100,000 - 1 million years! A Too-Brief History of Lunar Exploration: ----------------------------------------- 1959: Soviet Lunas 1, 2, and 3 (see inside front cover of PBD). Luna 1 first artificial object in solar orbit, flew past Moon. Luna 2 first artificial object to strike another world. Luna 3 first imaged lunar farside. 1964: U.S. Ranger 7 finally made it! Impact. 1966: Soviet Luna 9 soft lander; Luna 10 orbiter. 1966-7: U.S. Lunar Orbiter, Surveyor (lander) series. 1968 Christmas Eve: Apollo 8 (orbiter), first human flight to another world (1998 book Genesis, by Robert Zimmerman) 1969-1972 Apollo landings: 11, 12, 14, 15, 16, 17 12 astronauts walked on the Moon (9 now still living). The last (Jack Schmitt) was the first geologist. All others were engineer/test pilots. 1969 July 20: Apollo 11 - first landing on another world by humans 1976: End of Golden Age of lunar exploration, with Soviet Lunokhod rovers and Luna sample return capsules Now: the Lunar Renaissance? 1994: Clementine: ice found at lunar poles (radar backscatter, from polar orbit; also laser altimetry). 1998-1999: Lunar Prospector. Confirmed ice at lunar poles, in permanently shaded craters. Moon's axis is nearly perpendicular to ecliptic: many craters have floors that never see sunlight, and have rims that are always sunlit. Best site for a base: "The Mountain of Eternal Light", at S.Pole. 2000?: Lunar A (Japan) seismograph penetrators Moon rocks: 383 kg (842 lbs) returned by 6 Apollo missions; ----------- < 1 kg from Luna 16 and 24; meteorites => All igneous rocks (2.7 g/cm^3 on average). Most > 3 billion years old; some "genesis rocks" (4.55 b.y.o.) *Extremely dry!* Earth rocks 1-2% water; Moon rocks << 1%. Not even hydrated minerals, requiring presence of water to form. (No sedimentary rocks because no water for sediementation; no metamorphic rocks since no tectonism, so no active resurfacing.) Rock types: 1) Anorthosite: crustal rock rich in Ca, Al, O; found mainly in highlands (older) 2) Basalts: dark, solidified lava from dense mantle rock (not unlike Earth's ocean floors) KREEP basalts: potassium (K), rare Earth elements, Phosphorus 3) Breccias: rocks welded together by impacts. Several previously unknown minerals (chemical substances in the rocks), all from impacts. 4) Regolith: powdered rock, lots of glass from impacts Interior: - Crust, 60 km thick (from Apollo seismographs & spacecraft tracking) - Mantle: has mass concentrations (mascons) => Moon's gravity field is surprisingly lumpy. This caused navigation problems: Apollo 11 landed 6 km long of their target; spacecraft in 60-km Apollo survey orbits need to "stationkeep" (fire thrusters periodically), or crash. - Fe core only 2% mass of Earth's, since magnetic field so much weaker; also no tectonism or active volcanism (no heat flow from interior). - Seismographs found few "moonquakes," except from impacts and tidal flexing. Origin: Was still a mystery, years after Apollo. ------ Supreme scientific embarrassment of the program! 3 models before Apollo: 1) Fission (early Earth rotated so fast, it split in two) 2) Capture (formed elsewhere in Solar System, captured by Earth) 3) Binary accretion (formed alongside of Earth). None could explain all the following: - Moon has same O16/O18 ratio as Earth => must have formed here => 2 wrong. - Moon's density is about that of Earth's mantle; apparently has no Fe core => 3 unlikely. - Observed angular momentum of Earth and Moon => 1 has problems. - Moon is rich in refractory material, and poor in volatiles (e.g. water) => was once hot. => Giant impact (Don Davis and Bill Hartmann 1975; accepted at Kona meeting, 1984) --------------- At end of planetesimal accretion, left with mostly big pieces => big collisions. "The Big Splat": object size of Mars collided with an early, smaller, Earth. => Threw up a disk of mainly Earth's mantle; Moon formed from this. Magma ocean (whole surface melted). Explains: O16/O18 ratio, density, refractories and volatiles; angular momentum no problem. Giant impacts may also explain tilt of Uranus (on side), rotation of Venus (backwards & v. slow) There are no other natural satellites of Earth (C. Tombaugh, 1960s), *except* Asteroid 3753 Cruithne, caught in an Earth-Sun horseshoe orbit. More on this when we discuss the Trojan asteroids of Jupiter.