MoonWatch
October 1998
Kepler, a lunar impact crater
Clementine image of Kepler (NASA)
The precise locations of only a handful of lunar craters may be identified with any certainty using the naked eye alone. Among this elite group is Copernicus, Aristarchus and Kepler, three impact craters surrounded by bright ray systems, making a triangle in the dusky Oceanus Procellarum in the western hemisphere of the Moon.
With a diameter of just 30 km, Kepler is a small crater by lunar standards. Viewed through binoculars and small telescopes at low power, the crater is not nearly as grand or imposing as its extensive ray system would indicate. Had Kepler been formed in an earlier epoch among the southern lunar uplands or amid a continental setting, it would have blended somewhat anonymously into its surroundings.
Kepler was formed about the same time as Copernicus, 900 million years ago. Copernicus and Kepler are separated by about 500 km of hummocky plain on which the ray systems of both craters intermingle. Viewed at low power Kepler's rays appear pretty uniform in tone, whilst Copernicus' rays are rather patchy. Both of these old ray systems are overlain with the fresher ejecta of Aristarchus to the northwest, deposited perhaps as recently as 300 million years ago.
Kepler's rays occupy a total area of 40,000 square kilometres, and some spindly ray fingers can be traced to distances up to 300 km or more. There is little telescopic evidence for secondary cratering by Keplerian ejecta, as there is with Copernicus, although there are numerous bright craterlets in the vicinity which may have been produced in this manner. Under a high Sun there is clear evidence of a dark collar around Kepler's rim - this is probably a ballistic shadow zone that escaped being smothered by ejecta material. This ballistic shadow zone is also evident in other impact craters, most notably Tycho in the southern uplands.
Under a low Sun numerous low ridges and hills can be observed in Kepler's vicinity. Some of these features are aligned radially to Kepler, giving the appearance of being huge mounds of deposited ejecta material, or hills that were perhaps severely scoured and shaped by the blast of the Kepler impact. However, these relief features predate Kepler by billions of years, remnants of the early lunar crust that escaped being covered by the Procellarum basalt lava flows that erupted more than 3 billion years ago.
Although Kepler has a Copernican ray system in miniature, on close telescopic scrutiny Kepler is by no means a mini-Copernicus, as it lacks a big central mountain massif. Its rim is sharp, the walls rising some 2,300 metres above the floor. The outer walls are concentric and layered to a certain extent, and there is some internal terracing. From above, Kepler is very slightly polygonal in form. Through a 60 mm refractor or small reflector, the features on Kepler's floor are not prominent, nor is the internal terracing obvious. A high power view of the floor through a 150 mm reflector will show numerous low rounded hills.
Clementine probe images show intricate detail within and around Kepler. Along the rim there appear to be narrow gullies, just like those created by erosion at the rim of the Barringer crater in Arizona. However, the cause of such lunar features is not weather but the action of landslides, slumping and moonquakes.
Kepler is visible on 1 October, having emerged from the morning lunar terminator. On the days around full Moon on 5 October Kepler's ray system will be visible. Kepler nears the evening terminator in the early hours of 16 October, and emerges once more into the sunlight a fortnight later on the evening of 30 October.
