1. The Solar System Chapter 9 “Cratered Worlds”
Astronomy 04
The Solar System
Chapter 9
“Cratered Worlds”
Please pick up your Test 1 Scantron on front table
if you haven’t already.

2. The diameter of the Moon is 3476 km (2160 miles) with a mean distance from the Earth of 384,000 km (239, 000 miles).
The distance of the Moon from Earth varies from about …
220,000 to 260,000 miles.
The Moon has a gravity 1/6 that of Earth.
The Moon has no atmosphere.

3. The combination of the Moon's lower mass and its smaller radius means that its surface gravity is 1/6 the Earth's surface gravity.
Walking on the Moon is much different from walking on the Earth, as is falling on the Moon.

4. The Moon/Earth mass ratio is 1/80.
The Moon is made mostly of rock with little metallic material.
The average density of the Moon is 3.37 g/cm3 which is consistent with basaltic silicates and not consistent with a large iron (Fe)/nickel (Ni) core like the Earth.
This relates to the Giant Impact Theory of the Moon’s formation.

6. The lunar surface is a grey-tan color with a low albedo of 0.07

7. General Properties of the Moon
The Moon has 1/80th mass of Earth and 1/6th the gravity – not enough gravity to retain an atmosphere.
Any gases expelled or outgassed during the Moon’s early history have escaped.
Luna 3 was a Russian spacecraft which returned first photos of Moon’s far side in 1959 and Luna 9 first spacecraft to land on Moon in 1966.
It was the U.S. Apollo Program which used 9 spacecraft during the period to put 12 astronauts on the Moon’s surface.

8. The U.S. Apollo Program accomplished three major objects for lunar science:
It collected 837 lbs of moon rock which was returned to Earth.
It deployed ALSEP (Apollo Lunar Surface Experiments Packages)
The Apollo Command Module which orbited Moon photographed and analyzed the lunar surface.
  Today no nation has the technical ability to return to the Moon. It would take a ten year committed effort to do so.

11. Moon’s surface is covered with fine-grained soil or dust
a few cm deep.

12. The Moon has a cold solid interior and little seismic activity.
The Moon is geologically dead and has experienced little change over the last 3.5 billion years.

13. The Moon also lacks a magnetic field which would be generated by motions in liquid metallic core.
Moon rock has been dated from 3.3 to 4.4 billion years. The oldest earth rock is dated to 3.8 billion years.

14. The Major Surface Features:
Maria,
Highlands
Craters
Rays
Impact Basins
Mountains
Rilles (Valleys)

15. The dark areas are called maria or seas.
The Moon’s surface shows light rough cratered areas and dark relatively smooth areas with fewer craters.
The dark areas are called maria or seas.

16. Maria Maria cover 17% of Moon’s surface and are much less cratered.
They consist of volcanic plains and impact basins. They are
composed of basaltic rocks dated to 3.8 billion years.

17. Maria:
Dark-colored regions which turn out to be smooth plains of basaltic lava.
They are the remnants of large impact events that cracked the crust and allowed the lava from the mantle to flow upward and erase early cratering.
Note that the impacts must have occurred after the initial phase of cratering.

18. The Moon – Part 2
Highlands:
The lighter colored, heavily cratered regions are called the lunar highlands.
The bare, chaotic terrain indicates that these regions are primordial and one would expect the oldest rocks in these regions.

19. Lunar Highlands
The lunar highlands are the oldest geologically. They date
to 4.4 billion years and are composed of anorthosites
(silicate rocks). The lunar highlands are heavily cratered.

21. Normal, round craters are due to impacts from objects up to a couple thousand meters.
Object larger (asteroids) will typically crack the crust and form impact basins.

22. Lunar mountains have low rounded profiles and are the result of early impacts.

23. Mare Oriental: The Moon’s Largest Impact Basin
The Moon – Part 2

24. Geological History of the Moon:
The weight of the evidence is that the Moon was active geologically in its early history, but the general evidence suggests that the Moon has been essentially dead geologically for more than 3 billion years.
Based on that evidence, we believe the chronology of Lunar geology was as follows:
The Moon was formed about 4.5 billion years. The surface was subjected continuously to an intense meteor bombardment associated with debris left over from the formation of the Solar System.
As the intense meteor bombardment associated with debris left over from the formation of the Solar System continued, most of the craters that we now see on the surface of the Moon were formed.

25. The largest meteoroids punctured the Moon’s crust and lava poured out from inside the Moon producing the Moon’s mare and impact basins.
The regions that were not covered by the lava flows are the present Highlands; thus, they are heavily cratered, and formed from different rocks than the seas.
The volcanism stopped about 3.1 billion years ago: the Moon has been largely dead geologically since then except for the occasional meteor impact or small moonquake, and micro-meteorite erosion of the surface.

26. Erosion is slow on a world without an atmosphere and is caused by:
1) slumping (gravity)
2) other impacts
3) temperature changes
4) moonquakes
The result is that young craters have sharp edges (usually less than 200 million years old) and old craters are rounded, smoother (with ages of order a billion years old).

27. Current crater-formation-rate can be estimated from the number of craters and the number of potential projectiles present in the Solar System today.
It indicates a crater of 1 km is formed every 200,000 years.
A 10 km crater is formed every few million years.
A 100 km crater every billion years.
Both this and radioactive dating of moon indicates age of 4.5 billion years.

28. Origin of the Moon
The Fission Theory suggests the Moon broke off from Earth. But this is Impossible physically. It cannot be modeled by computer. If this were then case then why are Earth and the Moon such different compositions?
The Capture Theory proposes the Moon formed elsewhere but was captured by Earth’s gravity. But this is highly improbable mechanically.

29. Origin of the Moon
The Sister Theory suggests the Moon formed together with but independent of Earth.
The Giant Impact Theory suggests the Moon formed when a giant planetesimal struck Earth and threw material off into space.

31. Mercury

32. Mercury is named after swift messenger of the Roman gods.
Mercury is very difficult to observe in the sky because it is so close to the Sun.

33. Mercury as seen through a telescope shows little detail.
iew of Mercury

34. Mercury – Viewed by Mariner 10

35. Mercury is best viewed from Earth at its time of
greatest eastern or western elongation.

36. Mercury is moonlike world.
It is the nearest planet to Sun.
It has the has the most elliptical orbit of any planet. Its distance varies from 28 to 43 million miles from Sun with average of 36 million miles.
Mercury revolves around the Sun faster than any other planet with year only 88 days long.
It rotates once in about 59 days.

37. Mercury completes
3 rotations every 2
orbits of the Sun.

38. Mercury has been visited by only two spacecraft, Mariner 10 and Messenger.
Mariner 10 flew past Mercury three times in 1974 and 1975.
Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by the HST.

39. N.A.S.A.’s Messenger Spacecraft made 3 successful passes close to Mercury since 2004 and entered into orbit around Mercury in March 2011.

40. Chapter 21.2: “Mercury” N.A.S.A.’s Messenger Spacecraft
is scheduled to be launched toward Mercury in 2004.
Messenger will fly by Venus twice, then Mercury twice before entering orbit about Mercury in April Data collection will conclude in 2010.

41. Mercury has the highest density of any planet - 5.4 g/cm3.
The planet has a dense iron (Fe) core.
Its core is probably solid rather than liquid.

42. During the long Mercurian day the temperature rises to about 425 degrees Celsius, hot enough to melt lead.
Because there is no substantial atmosphere to retain heat, during the equally long nights, the temperature drops quickly to around -180 degrees Celsius.
This range of -180 Celsius at night to 425 Celsius
in the day is the largest surface temperature variation in the Solar System.

43. In general, the surface of Mercury is similar to the Moon.
It is heavily cratered. .

44. Although Mercury is heavily cratered there are intercrater plains that have less craters.

45. Smaller regions called smooth plains resemble the maria of the Moon and appear to be ancient lava flows.

46. Long scarps or wrinkles are found on the crust and the tops of craters.

47. A Scarp on Mercury
Scarp

48. One of the best known features on Mercury is Caloris Basin – a huge impact basin over 800 miles in diameter.
Caloris Basin resembles Mare Oriental on the Moon.

49. N.A.S.A.’s Messenger Spacecraft was launched toward Mercury in 2004.
Messenger will enter into orbit around Mercury on
March 18,

50. During its two flybys of Mercury, Messenger will
map nearly the entire planet in color,
Image most of the part of the planet unseen by Mariner 10
take reconnaissance measurements of surface, atmosphere and magnetosphere composition.
The flyby results will be invaluable in planning the year long orbital mission.

56. Summary:
Mercury is a small, hot, dry, airless world whose surface resembles the Moon.
It is heavily cratered.
It has the greatest day-night temperature range of any planet or moon.
It has the greatest density of any planet or moon because of its massive metal core.
The single most visible feature is Caloris Basin.
It’s surface displays “wrinkles” known as scarps.
It possesses a weak magnetic field.