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Impact Craters and Lunar Rays

Over 99% of the existing lunar craters are impact originated. Their diameters range from about 300 km to 1 km and below. Diameters larger than 300 km are generally referred to Impact Basins.

A mid-sized impact crater is illustrated by Eratosthenes in Image T084. It is a prominent object in the central region of the Moon disc. The crater is characterized by central peaks and terraced walls. The highest peak rises 3600 m above the crater floor, and the rim of the terraced walls measures 58 km across. Both the peaks and terraced walls are natural formations from a massive impact process, in which the impactor was an asteroid-like body of several kilometers only, hitting the lunar surface at 10 km per second or so. The tremendous impact energy vaporized a portion of the impactor and melted the rocky materials of the impact site to a much larger circular cavity. The sudden release of impact pressure also induced a concentrated rebound that uplifted subsurface rocks into central peaks. Other impact melt splashed out in all directions as ejecta. Most of the ejecta deposited around the cavity as ejecta blanket, the rest might take the advantage of low surface gravity (1/6 that of Earth) to fly far away before raining down to hit the surface again as secondary craters. All secondary craters are too small to have central peaks. The right part of T084 shows a mix of secondary craters produced by various sources including the Eratosthenes impact.

The inner terraced walls of Eratosthenes formed marginally few minutes after the impact contact. At later stage when the walls could not sustain their own weights, they collapsed in segments along the steep slopes, making the terrace more profound. Usually, lunar craters with diameters less than about 20 km are lack of terraced walls; small craters with diameters less than about 15 km are lack of central peaks and are shaped like a bowl with sharp crested-rim. Craters with off-center peaks are likely due to oblique impacts.

A unique feature, known as ghost crater, is also shown in T084. It is a crater almost buried beneath the surface of the Moon or destroyed through aging, leaving only a bare hint of recognition.

In general, craters created by explosion on impact remain circular regardless of the impact direction, except for very oblique impacts (less than about 50 measured from ground level). A typical crater caused by very oblique impact is Messier A as shown below. It appears elongated, and its ejecta in the pattern of dual rays implies a grazing impactor traveling from east to west. Lunar rays are elaborated in |Map 33.

It must be aware that lunar nomenclature is not always exact. For instance, today's selenographic coordinates make Mare Orientale (Eastern Sea, Farside| map) confusingly on the western longitude. Vallis Rheita in Map 4 is not a true valley but a chain of overlapping craters. Grimaldi in Map 26| appears like a lava-filled basin more than a crater. Rupes Recta (the Straight Wall) in Map 12| is not a narrow wall but a fault where its western side slopes down by 300 m. Many lunar views in Earth-based telescopes are dramatically different from the scenes on the Moon's surface and from the images taken in space. Below are some comparisons.

Selenographic Coordinates
View in telescope, \Map14
View in telescope, \Map 13
Rima Hyg^n us from Lunar Orbiter 3 probe, 1967. This close-up shows numerous craterlets on the lunar surface not resolvable by any observatory telescopes. The central crater is Hyginus, diameter 9 km. It is rimless, has flat floor and hence appears volcanic rather than impact-originated.

View in telescope, \Map 1$. Poor seeing creates Closeup image of Promontorium Herachdes from the illusion which makes Promontorium Heraclides to Lunar Orblter mapping. Here it is rotated with south up.

resemble a maiden's face with waving hair.

View in telescope, \Map 1$. Poor seeing creates Closeup image of Promontorium Herachdes from the illusion which makes Promontorium Heraclides to Lunar Orblter mapping. Here it is rotated with south up.

resemble a maiden's face with waving hair.

This image from Apollo 11 spacecraft shows the non-oblique view of Mare Crisium Its east-west diameter is longer than the north-south by 33m,,

Two views of Mare Crisium in telescope, ¡Map 2. They differ slightly in aspect ratio (east-west : north-south diameter) due to libration. Libration is the apparent vertical or horizontal rocking motion of the Moon as it orbits around the Earth. It distorts the surface features seen near the Moon's limb, or even^ makes them temporarily out of sight. Libration is detailed in Event 1| pages.

This image from Apollo 11 spacecraft shows the non-oblique view of Mare Crisium Its east-west diameter is longer than the north-south by 33m,,

Resolving Power of Telescopes

Left: The isolated boulder on the Moon is probably a dropping of ejecta from crater impact. Although it looks big to the Apollo-17 astronaut, it is not detectable by any observatory optics. Even the Hubble Space Telescope, with its 2.4-m mirror, is unable to spot moon rocks smaller than about 80 m. At best night, a 25-cm (10-inch) telescope resolves to 0.45 arcseconds, or lunar craters of about 800 m in diameter.

Right: A 25-cm (10-inch) telescope can be tested by resolving the pit-craterlet pair in Mons g Gruithuisen Gamma, \Map 22 |.

Right: A 25-cm (10-inch) telescope can be tested by resolving the pit-craterlet pair in Mons g Gruithuisen Gamma, \Map 22 |.

In telescopes, a crater close to the lunar terminator looks like deep hollow because the shadow on it exaggerates the impression of depth. Actually the floor of a lunar crater is not deep against its diameter. For example, the cavity of Tycho (figure below) is 85 km in diameter, 4.8 km deep. The depth-diameter ratio is 1 : 18, rather shallow by terrestrial norm. During the full moon, the exaggerated depth of Tycho will vanish, and the crater looks almost flat with a dark halo.

3/4 Tycho in shadow 112 Tycho in shadow No shadow (at full moon)

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