The Moon. Tidal Coupling Surface Features Impact Cratering Moon Rocks History and Origin of the Moon

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1 The Moon Tidal Coupling Surface Features Impact Cratering Moon Rocks History and Origin of the Moon

2 Earth Moon Semi-major Axis 1 A.U. 384 x 10 3 km Inclination 0 Orbital period tropical year days Orbital eccentricity Rotational period 23 h 56 min 4.1 s days Tilt Radius 6378 km 1738 km Mass 5.97 x10 24 kg 7.35 x kg Bulk density 5.52 g/cm g/cm3 Atmosphere N 2, O 2 trace Ne, He Albedo Surface temperature K K Escape speed 11.2 km/s 2.4 km/s Magnetic moment (equator) 8 x G.km 3

3 Highlights of Lunar Exploration Soviets had first contact with the Moon: First spacecraft to fly past the Moon: January 1959 First spacecraft to (crash) land on the Moon: September 1959 First pictures of far side of the Moon: October 1959 The United States is (so far) the only country to send people to the Moon: First person on the Moon: July 1969 Last person on the Moon: December 1972

4 Lunar Interior Moon s density is relatively low, and it has no magnetic field cannot have sizable iron/nickel core Crust is much thicker than Earth s

5 Tidal Coupling From the Earth, we always see the same side of the Moon. The Moon rotates around its axis in the same time that it takes to orbit around the Earth. Tidal coupling: Tidal forces have slowed rotation down to same period as orbital period This is an example of 1:1 spin-orbit resonance.

6 When the Moon formed, the Earth s gravity, caused the Moon to form with its center of gravity closer to the Earth than its center of figure. This gravitational imbalance enabled the tidal force to slow the Moon into a 1:1 spin-orbit resonance. This diagram greatly exaggerates the difference. The center of mass and center of figure are separated by only ~7 km (0.2 %). Tidal Coupling

7 Acceleration of the Moon s Orbital Motion The Earth s tidal bulges are slightly tilted in the direction of Earth s rotation Gravitational force pulls the Moon slightly forward along its orbit The energy loss from friction coupled with conservation of angular momentum, causes the Moon to drift slowly away from the Earth

8 Lunar Surface Features: Near Side Two dramatically different kinds of terrain: Highlands: Mountainous terrain Scarred by craters Maria (pl. of mare): ~3 km lower than highlands Smooth surfaces Basins flooded by lava flows

9 Lunar Surface Features: Far Side The far side of the Moon has many craters but virtually no maria

10 Impact Cratering Impact craters on the Moon can be seen easily even with small telescopes. Ejecta from the impacts can be seen as bright rays originating from young craters

11 Craters Craters are typically about 10 times as wide as the meteoroid creating them, and twice as deep. Crater classes: (on the Moon) Bowl craters: D < 15 km Complex craters: 15 km < D < 300 km Basins: D > 300 km Rock is pulverized to a much greater depth forming the lunar regolith or soil. Most lunar craters are 3.9 x 10 9 years old; much lower impact rates since then.

12 Craters Bowl craters have a simple shape. Copernicus is an example of a complex crater. Notice the terraced walls caused by slumping and the central peak caused by a rebounding shock wave. Central peak Terraced walls Bowl craters Overlapping bowl craters

13 Basins are characterized by 3 rings. One is the crater rim. The others are caused by rebounding shock waves like the central peaks of complex craters. Mare Orientale shows basin structure because it is only partially flooded with lava. Near-side basins are all flooded with lava out to the third ring. Craters

14 Craters The smallest craters on the Moon, such as this one on a small glass sphere, must be viewed using scanning electron microscopes Small meteoroids are much more numerous than large ones and they are the ones that produce the regolith

15 History of Impact Cratering Rate of impacts by interplanetary bombardment decreased rapidly after the formation of the solar system. Most craters seen on the Moon s surface were formed within the first ~½ billion years.

16 Cratering as a Geological Process It is the dominant geological process on many bodies. Even though it is a minor process on Earth, there are ~180 visible craters on Earth. This is Barringer crater in Arizona which formed ~50,000 years ago. It is ~1 km in diameter.

17 Craters on Earth A comet nucleus impact produced the Chicxulub crater ~65 million years ago. It may have caused major climate change, leading to the extinction of many species, including dinosaurs. 300 km Gravity map shows the extent of the crater hidden below limestone deposited since the impact.

18 Craters Meteoroids have produced most of the visible craters in the solar system. The Earth has about 100 craters more than 0.1 km in diameter; erosion has made most of them hard to discern. One of the largest is Manicouagane in Quebec.

19 More than 3 billion years ago, the Moon was volcanically active; Hadley rille, as well as others, was formed then Lunar Volcanism

20 Moon Rocks All lunar rocks brought back to Earth are igneous (= solidified lava). No sedimentary rocks No sign that liquid water was ever present on the Moon. Different types of lunar 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 the highlands) Older rocks become pitted with small micrometeorite craters

21 The History of the Moon 1. The Moon is small; low mass rapid cooling; small escape velocity no atmosphere unprotected against meteoroid impacts. 2. The Moon must have formed in a partially molten state ( sea of lava ); 3. Partial differentiation 4. No magnetic field small core with little metallic iron. 5. Surface solidified ~ billion years ago. 6. Heavy meteorite bombardment for the next ~1/2 billion years. Alan Shepard (Apollo 14) analyzing a lunar rock, probably ejected from a distant crater.

22 Formation of Maria Impacts of heavy meteorites broke the crust and produced large basins that were flooded with lava. Flooding ended about 3.2 billion years ago.

23 Origin of Mare Imbrium Its history is typical of the history of flooded basins on the near side Terrain opposite to Mare Imbrium is jumbled by seismic waves from the impact.

24 The Origin of the Moon Early (unsuccessful) hypotheses: Fission hypothesis: Break-up of Earth during early period of fast rotation Problems: No evidence for fast rotation; the Moon s orbit is not in the equatorial plane Condensation hypothesis: Condensation at time of formation of Earth Problems: Different chemical compositions; the Moon is low in volatiles and iron Capture hypothesis: Capture of the Moon that formed elsewhere in the solar system Problem: Requires succession of very unlikely events

25 Current Theory of the Formation of the Moon The Large-Impact Hypothesis The impact heated material enough to melt it consistent with a sea of magma The collision was not head-on large angular momentum of Earth-Moon system The collision occurred after differentiation of Earth s interior; it did not penetrate to the core Moon will not have much iron