The Moon

1. The Moon • The Moon has 1/80 the mass of the Earth and 1/6 its gravity • The Moon has no atmosphere because of its low gravity • The density of the moon is 3.3 g/cm3 compared with Earth’s 5.5 g/cm3 • The Moon rotates on its axis every 27.3 days which is exactly equal to the time it takes to orbit Earth • Views of the moon from Redshift Lecture 10

2. Exploration of the Moon • The Russian Luna 9 was the first spacecraft to land on the Moon in 1966 • On July 20, 1969, Neil Armstrong stepped out on the surface of the Moon • 9 Apollo missions were flown (list given in Table 8.2) • 3 did not land and 6 landed on the surface • The last mission was flown in December, 1972 • Here is Jack Schmidt during Apollo 17 • Here is the lunar module from Apollo 17 leaving the surface of the moon Lecture 10

3. Top 10 Discoveries from Apollo • The Moon is not a primordial object; it is an evolved terrestrial planet with internal zoning similar to that of Earth. • The Moon is ancient and still preserves an early history (the first billion years) that must be common to all terrestrial planets. • The youngest Moon rocks are virtually as old as the oldest Earth rocks. The earliest processes and events that probably affected both planetary bodies can now only be found on the Moon. • The Moon and Earth are genetically related and formed from different proportions of a common reservoir of materials. • The Moon is lifeless; it contains no living organisms, fossils, or native organic compounds. Lecture 10

4. Top 10 Discoveries from Apollo, cont. • All Moon rocks originated through high-temperature processes with little or no involvement with water. They are roughly divisible into three types: basalts, anorthosites, and breccias. • Early in its history, the Moon was melted to great depths to form a "magma ocean." The lunar highlands contain the remnants of early, low density rocks that floated to the surface of the magma ocean. • The lunar magma ocean was followed by a series of huge asteroid impacts that created basins which were later filled by lava flows. • The Moon is slightly asymmetrical in bulk form, possibly as a consequence of its evolution under Earth's gravitational influence. Its crust is thicker on the far side, while most volcanic basins -- and unusual mass concentrations -- occur on the near side. • The surface of the Moon is covered by a rubble pile of rock fragments and dust, called the lunar regolith, that contains a unique radiation history of the Sun which is of importance to understanding climate changes on Earth. Lecture 10

5. Composition of Moon • The Moon is made of lighter silicate rock • The Moon has no iron core like Earth • The Moon has no current seismic activity • The moon has no magnetic field • The Moon has no volatile materials • It is as if the Moon were composed of the same basic materials as the Earth’s crust with the volatile material and the core metals missing Lecture 10

6. General Appearance of the Moon • To the naked eye, the Moon has seas and continents • Craters are not visible • Features are named for seas • Maria are younger • Dark, round plains much less cratered • Sea of Tranquility, Sea of Clouds, etc. • Continents or plains are not named • Highlands are older • Low density rock formed early, severe cratering • Since the invention of the telescope, craters have been observed and named Lecture 10

7. Impact Craters Terrestrial Volcano Lunar Impact Crater • The moon provides an important benchmark for understanding the history of the solar system • The Moon preserves the history of the solar system through its uneroded record of impact craters • The observed craters on the moon were once thought to be volcanic in origin • Now the craters are understood to be all from the impact of meteors Lecture 10

8. The Cratering Process • Any object striking moon does so at least escape velocity (2.4 km/h or 5400 mi/h) • This impact creates an explosion the produces a round crater Lecture 10

9. Using Crater Counts • We can use the number of impact craters on the Moon’s surface to estimate its age • By looking at the current population of asteroids and comets one can deduce that • 1 km crater produced every 200,000 years • 10 km crater produced every million years • 1 or 2 100 km craters produced every billion years • This calculation gives 3.3 to 3.8 billions years for the age of the Moon • Compares well with radioactive dating • There is evidence that the cratering rate was much higher just after the formation of the solar system 4.5 billion years ago • Highlands have 10 times more craters than maria Lecture 10

10. Deducing Information from Craters Ptolemaeus Crater taken by Apollo 16 • If a crater forms on top of another crater, that crater is newer • Looking at the edges of a crater one can say • Old craters have smoother edges from meteor erosion • Young craters have sharp edges Lecture 10

11. Theories for the Origin of the Moon • Theories for the origin of the Moon fall into three categories • The fission theory • The Moon was once part of the Earth but separated early in its history • The sister theory • The Moon formed at the same time as the Earth but independently • The capture theory • The Moon was formed elsewhere in the solar system and later captured by Earth Lecture 10

12. Evidence Against These Theories • The fission theory • Modern calculations show that fission cannot occur • Recently discovered chemical differences rule out this theory • The sister theory • Moon should have much larger iron core like Earth • The capture theory • A body can only be captured into orbit if there is a mechanism to slow the body - there is none • Orbit would be very elliptical - it is circular Lecture 10

13. The Giant Impact Theory • Moon was created by collision of Earth with an object about 1/10 the size of Earth • The Mars-like object might have stuck the Earth and ejected a large chuck of the crust into orbit around the Earth that formed the Moon • Explains lack of volatiles and chemical composition Lecture 10

14. The Planet Mercury • Mercury is the closest planet to the Sun • Mercury orbits the Sun in 88 days • Mercury has an eccentric orbit (e = 0.206) with a semimajor axis of 58 million km (0.39 AU) • The plane of Mercury’s orbit is tilted 7 degrees from the ecliptic • Interactive Solar System Lecture 10

15. Mercury’s Strange Rotation • Mercury was once thought to rotate once per orbit • Radar observations showed that Mercury does not keep one face toward the Sun • Takes 59 days to rotate on it axis • Takes 88 days to orbit Sun Lecture 10

16. Viewing Mercury in the Sky • Position of Mercury in the sky at 6 pm, Oct. 3 looking SW • Mercury Sky View.RS3 Lecture 10

17. Composition and Structure • Mercury’s mass is 1/18 that of the Earth • 2nd smallest • Mercury’s diameter is 4880 km • 2nd smallest • Mercury’s density is 5.4 g/cm3 • 2nd densest • Mercury is thought to consist of an iron-nickel core surrounded by a silicate crust • Core is roughly the size of the Moon • Probably molten because Mercury has a magnetic field Lecture 10

18. The Surface of Mercury • Mercury strongly resembles the Moon in appearance • Mercury is covered with thousands of craters and larger basins up to 1300 km in diameter • Some of the brighter craters are rayed • Mercury’s basins resemble the Moon’s maria • There is no evidence of plate tectonics on Mercury • Long scarps are visible cutting across craters Lecture 10

19. Fly-by of Mercury • Here is a series of photos of Mercury shot by Mariner 10 Lecture 10

20. Origin of Mercury • The problem with understanding the origin of Mercury is the opposite of understanding the origin of the Moon • Mercury has too little silicates and too much iron • Mercury may have suffered a catastrophe early in its history that tore away a fraction of its mantle • Both the Moon and Mercury have strange compositions that tell astronomers that the early solar system was chaotic Lecture 10