Imbrium Lineaments and Davy's Weird Chain.
THE REGION SURROUNDING PTOLEMAEUS is one of the best places on the Moon to see the linear features that were named the Imbrium sculpture by Grove Karl Gilbert, the U.S. Geological Survey's first director. In addition to the furrows that cut into the rims of Ptolemaeus and Alphonsus, long troughs cross the nearby highland terrain. The longest is west of Ptolemaeus and Alphonsus, extending about 225 kilometers from the small crater Lalande C to the rim of Alphonsus. Look at this trough and you will see that it is made up mostly of coalescing craters with smooth floors. A similar furrow runs from east of Alphonsus nearly 200 km to the south-southeast. It is overlaid, and thus interrupted, by the younger crater Parrot C, just east of Arzachel. A few other shorter troughs are visible near the craters Hipparchus and Albategnius.You don't have to be a certified lunar scientist to see that these troughs are radial to the Imbrium basin, though no one noticed before Gilbert a little more than a century ago. But people weren't very interested in the Moon then nor 40 years later when French astronomers Gabriel Delmotte and Maurice Darney also discovered "le Systeme Imbrien." Astronomer Ralph Baldwin's independent recognition of the network of troughs also made little impression on lunar mappers, but Nobel Prize-winning chemist Harold Urey and lunar scientist Gerard Kuiper immediately understood the importance of these troughs as scars from the Imbrium basin impact.
The Imbrium sculpture is not the only example of straight features on the Moon. As you look at various alignments near Ptolemaeus, you may notice a small linear streak in the squared-off old crater that embraces the northeastern rim of Davy, located on the northeastern shores of Mare Nubium. The Davy crater chain (marked on some Moon maps as Catena Davy) is a 45-km-long alignment of more than a dozen small craters, most of them 1 to 2 km wide. This crater chain has been a mystery for many years. It is apparently not volcanic, since the craters don't look like the volcanic pits found along the Hyginus rille and elsewhere. And there is no obvious source crater if the feature is a secondary crater chain. Robert Wichman, then at the University of North Dakota, and I, and independently Jay Melosh and Ewen Whitaker at the University of Arizona, proposed an explanation. Suppose a comet or small asteroid passed so close to Earth that it was torn apart by our planet's gravity, with the resulting pieces dispersing into a line. If such a train of particles hit the Moon, it could produce a crater chain like the one near Davy.
This may seem like a preposterous idea, but we have actually witnessed just such an event. In 1992 Comet Shoemaker-Levy 9 passed perilously close to Jupiter and was gravitationally shredded into a "string of pearls" of 23 or more cometary fragments. These pieces sequentially collided with Jupiter in July 1994, in rapid-fire succession. The aftereffect was a series of dark splotches in Jupiter's atmosphere that persisted for many days. Crater chains on Jupiter's icy moons Callisto and Ganymede were likely formed by collisions of this kind too.
This multiple-impact process initially seemed so bizarre that no one even thought of it, but we now know it did occur on Jupiter in 1994 and apparently in the Moon's past as well.
CHARLES A. WOOD, a dedicated Moon enthusiast and planetary scientist, is director of education at Biosphere 2, near Tucson, Arizona.
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| Title Annotation: | region of the Moon |
|---|---|
| Author: | Wood, Charles A. |
| Publication: | Sky & Telescope |
| Date: | Nov 1, 2002 |
| Words: | 583 |
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