MoonWatch
September 1998
Reiner Gamma and lunar impacts
Image of Reiner Gamma by Clementine NASA
We are reminded of the sheer power of cosmic impact every time we look at the Moon. In the collision-fission (or "big whack") scenario of lunar genesis, the Moon itself becomes the by-product of a titanic impact in which a Mars-sized planetesimal careered into the embryonic Earth, splashing out a stream of debris which later coalesced to form the Moon.
Our local planetary environment used to be a hazardous place, the space lanes jammed with debris left over from the formation of the solar system. Asteroidal impact undoubtedly played a great part in shaping our own planet, but most traces of this cataclysmic epoch have been obliterated by Earth's dynamic crust and the actions of its hydrosphere, biosphere and atmosphere. But the Moon has never experienced plate tectonics, and many of its impact features are carved in the crust and appear much as they did soon after their formation.
Easily visible with the average naked eye, the dark circular lunar maria are vast impact basins that were excavated by asteroids from the young lunar crust between 3 and 4 billion years ago, and later filled with lava. Through binoculars and small telescopes hundreds of impact craters are brought into view, some of them surrounded by bright rays of ejected material that stretch for hundreds of kilometres. More than 6,000 lunar impact craters are visible through a 150 mm reflector, an instrument which can resolve craters as small as 2 km across. At high magnification the crater-crowded lunar southern uplands are a truly awe-inspiring spectacle. No less than 97 percent of the Moon's far-side resembles the southern uplands, since only a small number of the larger far-side impact basins have been flooded with lava.
In comparison to asteroids, comets are flimsy conglomerates of rock and ice, but they can still make their mark. In June 1908 a small comet is believed to have exploded in Earth's atmosphere over Siberia and devastated more than 3,500 square kilometres of tundra in the Tunguska region. This summer's blockbusting disaster movie Deep Impact portrayed the catastrophic consequences of the impact of a sizeable cometary fragment in the Atlantic Ocean off the US coast. Although the film has several technical flaws, this doesn't lessen the sense of menace posed by cosmic impact, with special effects that give the viewer a real feel for the enormity of a dire collision scenario. Comets have undoubtedly hit the Moon many times in the past, providing the lunar poles with their newly-discovered ice reserves. What kind of a blemish would a comet leave on the Moon?
Several unusual features known as swirls have been identified. They appear to be delicately shaped light coloured surface markings without much obvious relief, even at low angles of illumination. Notable near-side swirls visible through backyard telescopes lie in Mare Marginis and Oceanus Procellarum. An unusual tadpole-shaped smudge named Reiner Gamma is one such swirl. Lying 100 km west of the crater Reiner in western Procellarum, Reiner Gamma measures about 35 x 100 km, making it of comparable size to the area of Tunguska devastation. It can easily be seen through binoculars and small telescopes, and a larger telescope will enable you to see the intricate streamers within.
Studies have shown that the composition of Reiner Gamma does not differ significantly from the surrounding darker mare, so it is unlikely to be a patch of ejecta deposited by some impact further afield, nor is it a lava flow of different mineral composition. It may have been formed quite recently on the geological time scale (perhaps less than 100 million years ago) when a small comet impacted on the mare surface, exploding at or near ground-level and scouring away the upper layers of dark lunar soil, exposing lighter soil beneath. On close scrutiny of Clementine spaceprobe images there appears to be an alignment of several bright craterlets running east-west along the centre of Reiner Gamma, perhaps indicating that it was formed by an east-west moving fragmented cometary nucleus like Shoemaker-Levy 9 (but on a smaller scale). If they have been created by regolith-scouring comet explosions then swirls are bound to fade over time under the relentless action of solar baking and micrometeoritic erosion.
This month Reiner Gamma is visible for a fortnight from the evening of 3 September (near the sunrise terminator) to the morning of 17 September (near the lunar sunset terminator). If you're trying to locate the feature at low power, first identify the bright Copernicus, hop west to Kepler, and Reiner Gamma is the isolated bright spot one hop further west, near the border of Procellarum. The Moon's libration this month does not favour observation of the swirls in the vicinity of Mare Marginis on the Moon's eastern limb - the next good opportunity takes place before full Moon in late December when you will be able to see wispy bright patches running eastward, between northern Marginis and the crater Hubble, towards the limb.
