What Happened to Dinosaurs on the Moon

By Nigel Longshaw


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.


Descriptive Notes, Plates 1-8


It has been referred to in the past as the "Schiller annular plain" (S.A.P.) and will again be so in this paper.

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.

Following Figure A (which can be used as a guide chart at the telescope) We start at the crater Schiller. This unusual elongated feature actually interrupts the outer ring of the S.A.P. on its Eastern rampart. From here to the South and West stretches the flat narrow plain enclosed between two arcing scarps. The outer ring is mainly defined by a steep inward facing escarpment which is typical of basins of this type. The ramparts are most prominent on its Southern arc where it is bounded at its apparent tangent points by the craters Rost (to the East) and Zucchius (to the West). The outer ring then continues along its Southern extremities (although less well defined) from Zucchius West to the large crater Phocylides. The large enclosure of Phocylides seems to have disturbed the ring at this point, and from here the outer ring is less conspicuous but can be traced as it wraps round and back to meet Schiller.

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.

Nigel Longshaw


Bibliography


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Page modified 18 January, 2005