There seems to have been a recent resurgence in the theory that some 65 million years ago a large asteroid sized body collided with the Earth, causing long lasting climatic changes, and the ultimate demise of the Dinosaurs on our planet.
Following new gravitational studies a possible site of impact has been further studied and, it seems, is much larger than originally thought. The crater lies buried beneath sedimentary rocks in Mexico's Yucatan Peninsula, and is known as the Chicxulub crater. Recent data suggests the outer "ring" is some 300km diameter.
This crater is no "oddball", in fact the feature is of a similar nature to a vast number of craters of this type throughout the solar system known as multi-ring impact basins. The Chicxulub crater has ring diameters of 104, 150 and 300km and is the largest of around 17 other similar basins to be found on the earth.
Multi-ring impact basins are very important features of our solar system they formed in the very early history of the planets and their satellites and critically effect the evolution of the impacted body. They are the fundamental cause of the current configuration of some planetary surfaces , they churn up vast quantities of material from below the crust and encourage subsequent volcanic and surface modification processes.
Current investigations suggest Mercury has 23 such structures, with outer rings approaching 4000km (but obviously at these sort of diameters the full extent of rings can be difficult to trace). Mars also has a similar number of candidates, the Utopia basin having a main topographical rim around 4700km in diameter. Basins are also present on the icy satellites of Jupiter and Saturn, Valhalla basin on Callisto being a prime example. Thanks to current mapping techniques used on the Magellan mission, basin candidates have now been identified on Venus, although the total number is somewhat meagre. However the size range of basins on Venus do correspond closely to those on Earth. These observations, and relative crater densities suggest that cratering records of Earth and Venus are very similar.
It is no wonder that our own satellite, the Moon, also has its fair share of Multi-ring impact basins. The Moon has 18 true Multi-ring basins, of which the Mare Orientale is the prime example, a further 27 basins have less than three rings.
Since the Moon is so close to us, astronomically speaking, there is chance for the amateur astronomer to study these fascinating features in great detail with only moderate equipment.
So what did happen to the dinosaurs on the Moon? Well we all know there never were dinosaurs on the Moon, but the feature on the lunar surface we will concern ourselves with in the following paragraphs is of a comparable size to the Chicxulub crater on Earth.
Our lunar basin is very much older, dating back to the lunar pre Nectarian era, 3.92 billion years ago! It has the dubious honour of being one of the most neglected ring basins to be observed by amateurs. The feature has no accepted nomenclature, is situated on the near side of the Moon and occupies most of the area between Bailly, Phocylides and Schiller.
PLATE 2: Contact of projectile and target: Shock wave propagates in both target and projectile, causing very high speed jetting of melt and vapour at target/projectile contact (time +3seconds)
PLATE 3: Projectile largely consumed. Shock wave continues its propagation into target while setting up initial crater flow field (time +5seconds)
PLATE 4: Excavation of basin. Lateral growth of the cavity is preceded at its extremities by surface spill (time +10 mins)
PLATE 5 : Stage of maximum growth. Zone of intense fracturing surrounds transient crater. (time +50 mins)
PLATE 6: In response to crustal unloading of material, floor of cavity rapidly rebounds, while a sheet of impact melt covers the basin floor. Material in the ejecta curtain begins to be deposited on the targets surface. Ejecta curtain is followed by a ground surge of secondary crater ejecta (time +100 mins)
PLATE 7: Basin floor shows maximum amount of rebound, possibly in excess of the original targets curvature. This configuration is unstable and begins to collapse. Material continues to be deposited from the ejecta curtain, though now at greater distances, followed by the surge of secondary crater ejecta. (time +200 mins)
PLATE 8: Collapse largely complete. Major interior and exterior rings have now been formed. Sheet of impact melt is nearly completely molten, and lines the basin floor. The last portions of the ejecta curtain are deposited at extreme ranges but only in a discontinuous and fragmentary manner. (time +400 mins)
CONTINUED STRUCTURAL ADJUSTMENTS HAY BE EXPECTED FOR MANY YEARS.
It has been referred to in the past as the "Schiller annular plain" (S.A.P.) and will again be so in this paper.Nigel Longshaw
This is a typical ringed basin and Plates 1-8 are a diagrammatical representation of the current theories on ring basin formation.
Now that we have a background knowledge of how these basins form, let us venture outside to the telescope and take a trip round the Schiller annular plain.
The inner ring is a much less well defined feature, formed by a range of hills with a few scarps. There seems to have been a measure of modification to this inner area in the form of craters and secondary impacts from larger formations. The inner ring is basically bounded by the craters Weigel to the East, Segner to the Southwest and the smaller craters of the Noggerath/Schiller group to the North.
Between the rings the surface has quite a high albedo, the inner area bounded by the inner ring has an intermediate albedo, with a darker area in the centre. This region has a very slightly raised rim, and is seen as a very delicate third inner ring under grazing illumination. In areas where the floor is unmodified it takes on a smooth mare like appearance, indicative of an impact melt sheet or the subsequent intrusion of basaltic material.
Inside and around the S.A.P. lie many smaller interesting features, some of which are subsequent modification features others which are consequences of original basin formation. These features would take many pages to describe fully, and are beyond this introductory paper.
In a few short paragraphs and diagrams we have only "glossed over" the topic in general, but the S.A.P. potentially gives many nights of pleasurable observing for the owner of any optical instrument, and I strongly advise the bibliography is consulted by interested readers.
When we consider the great age of the Schiller annular plain, other ringed basins, and their importance in the formation of planetary bodies, and possibly the destruction of life, it makes time spent at the eyepiece infinitely more enjoyable.
2: Spudis Paul.D. :The Geology of Multi-Ring Impact Basins: Cambridge Planetary Science Series.
3: Kitt Michael.T. :The Moon, An Observing Guide for Backyard Telescopes Kalmbach (Astronomy Magazine Linrary Number 1)
4: Rogers John.H. :The Largest Crater on the Face of the Moon: Journal of the BAA 1976 October VOL.86 Number 6.
5: University of Arizona Lunar and Planetary Laboratory: Lunar Quadrant Maps.