Oblique Impact Craters

Oblique Impact Craters

(glossary entry)

Description

Projectiles strike the lunar surface at all angles, yet most impact craters are circular. In the 1920s two scientists on opposite sides of the world figured out why. Ernst Opik in Estonia and Algernon Gifford in New Zealand both realized that at the very high energies of cosmic impacts, craters are produced by explosions, not gouging. In the early 1970s NASA scientists experimenting with hypervelocity impacts determined that craters remain circular until impact angles less than about 15°. A more sensitive mark of oblique impact than shape is the distribution of crater ejecta, especially rays. Low angle impacts result in rays that are not uniform in their distribution. Because there is a forward momentum to ejecta, there is commonly a zone of avoidance in the direction the projectile came from. For example, the crater Proclus lacks ejecta on its west side. For very low angle impacts, and Messier is the type example, ejecta is tightly collimated in the down range direction. And there is also wings of ejecta at right angles.
 
  I know of no list of oblique impact craters so I invite everyone to help make one by adding names of oblique impacts that you find.
  A question mark means confirmation is needed that the feature is an oblique impact crater.
 
 

List of Oblique Impact Craters

Latitude	Longitude	Name	Image	Notes
61.8°N	50.3°E	Thales
31.3°N	73.6°E	Hahn		crater on rim, dark ejecta?
28.6°N	176.3°E	Larmor Q	image
27.7°N	148.4°W	Joule T?	image	rays from smaller crater on rim?
26.6°N	16.4°E	Banting		a bright 270° (three-fourths) ejectablanket around a tiny craterlet southeast of Banting (not Banting itself!)
22.4°N	163.1°W	Jackson
20.8°N	174.5°E	Buys-Ballot
18.4°N	113.5°W	Ohm
16.1°N	46.8°E	Proclus
11.7°N	65.8°E	Condorcet T	image
8.4°N	77.6°W	Glushko
5.0°N	120.5°E	King ?
4.9°N	166.0°E	Mandel'shtam F	image
1.9°S	47.6°E	Messier and Messier A
5.9°S	157.8°E	Ventris M	image
7.8°N	177.2°W	unnamed	image	Clementine Atlas, plate 68 (east-northeast of Stein, between Hayford and Krasovsky).
9.8°S	153.3°W	unnamed	image	Clementine Atlas, plate 87 (on the southeastern rim of Korolev).
11.9°S	33.6°E	Daguerre		a bright 270° (three-fourths) ejectablanket around a tiny craterlet on Daguerre's floor
12.5°S	48.4°E	Bellot	image
12.7°S	113.6°W	unnamed	image	Clementine Atlas, plate 89 (southwest of Kearons).
17.6°S	19.2°E	unnamed	image	Clementine Atlas, plate 96 (the "half ejectablanket" immediately south of Tacitus).
20.0°S	56.9°E	Petavius B	image	(once called Orus by H.P.Wilkins).
49.2°S	6.2°E	Heraclitus ?
51.9°S	39.0°W	Schiller ?

Possible Oblique Impact Craters

Cauchy (a crater in Mare Tranquillitatis) and Kies A (a crater in Mare Nubium) could be some sort of oblique impact craters too. Both craters have chevron-shaped "wrinkle ridges" radiating outward at the western parts of their rims, looking a bit like the "wake" of a slowly moving ship on calm water. During local sunrise (the westerly moving sunrise terminator), crater Cauchy exhibits a curious "interrupted" shadow, looking very much like a couple of silhouetted rabbit's ears pointing to the west. Crater Kies A shows the same phenomenon. A remarkable photograph of the curious shadow-effect at Kies A was made by Paolo Lazzarotti (29-12-2006).
Research: Danny Caes

Hermann A

This small bowl shaped crater northwest of Hermann in Oceanus Procellarum shows a remarkable eccentric and slightly elevated ring on its inner slopes, as if the original crater is occupied by some sort of "nonchalantly disturbing new crater" or "pseudo crater". Could this odd combination of "original crater vs pseudo crater" show us a less-known example of oblique impact craters?
  The coordinates of Hermann A are: (for those who want to see this crater in the ACT-REACT Quick Map of the LROC).
  LAT: 0.40
  LON: -58.26
  Research: Danny Caes
 
 

Schrodinger W

This odd crater northwest of the large basin Schrodinger looks a bit like one of the craters of the Messier pair in Mare Fecunditatis. A large crater is overlapping a little one, or are both the result of one single oblique impact?
  Research: Danny Caes
 
 

A possible ricochet on the lunar surface?

Gartner M, a peculiar "rugbyball"-shaped depression south-southeast of Gartner (in the eastern part of Mare Frigoris), looks almost as if its the result of a lunar ricochet, made by an object which impacted the site obliquely from the east-northeast (or west-southwest?). See also LAC 13 in B.Bussey's and P.Spudis's Clementine Atlas of the Moon.
Research: Danny Caes

The "Bottle" near Niepce

LAC 9 (page 18) of the Clementine Atlas shows crater Niepce and a curious "bottle"-shaped depression at its southwestern rim. Could this be an oblique impact crater, or is it correlated to the system of east-northeast/ west-southwest oriented valleys which are scattered all over this region?
  Research: Danny Caes
 
 

Elongated crater near Wyld

Right edge of Apollo 15's ITEK-camera PAN strip AS15-P-9089.
 
 

The "Mini-Schiller" between Piazzi Smyth and Kirch

About halfway between craters Piazzi Smyth and Kirch (in Mare Imbrium) is the location of satellite crater Piazzi Smyth V, which has a remarkable elongated "Schiller"-like appearance with a curious central "spine" on its floor (see the LRO's HiRes close-up photographs of it, and LPOD Mini Schiller).
  Research: Danny Caes
 
 

LPOD Articles

Oblique Fireworks
A Dark Oblique Ray
Mini Schiller (Piazzi Smyth V)

Bibliography

• Forsberg, Herrick, and Bussey (1998): The Effects of Impact Angle on the Shape of Lunar Craters.
• NASA SP-362 APOLLO OVER THE MOON; A View From Orbit, Chapter 5; Craters, Part 2 (oblique ray-craters and partial ejectablankets).

---