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A lunar volcanic crater: few large volcanic craters are visible on the Moon... except this one.

FOR GENERATIONS, observers have debated the origin of lunar craters. Were they formed by impact or volcanism? Since the 1960s, the evidence has become overwhelming that the vast majority of craters on the Moon and every solid body in the solar system have an impact origin. Vast fields of mare lavas clearly show that volcanism was also a major lunar process, but other than small pits on mare domes, the features we commonly call craters were not formed by volcanism. Except for one.

Even die-hard advocates of impact origins think that Hyginus is a volcanic crater. This 11-kilometer-wide rimless pit resides at the juncture of the two arms of a rille known as Rima Hyginus, in the east end of Sinus Medii. The fact that Hyginus lacks an elevated rim differentiates it from impact craters, and its association with rilles unambiguously links it to internal lunar forces. U.S. Geological Survey cartographers in the late 1960s noted another likely volcanic manifestation: a faint mantle of dark material surrounding Hyginus. Recent radar investigations of the material indicate that it's most likely a very thin pyroclastic or ash deposit. This is strong evidence for the interpretation that Hyginus is a volcanic structure.

You can see the morphological evidence--Hyginus has no rim, is at the juncture of two rilles, and is surrounded by dark material--quite easily in the eyepiece of a small telescope. Understanding the eruption mechanism requires detailed mathematical modeling of the physical processes of an eruption. Lionel Wilson and his colleagues at Lancaster University in the U.K. have now done that. Their model takes into account the 23 pits that lie mostly along the western limb of Rima Hyginus. Like Hyginus Crater, these are rimless depressions, and the fact that many have diameters wider than the average rille width of 3 km means that they must have formed after the rille.

Based on studies of small pyroclastic eruptions on Earth, and the formation of lines of pit craters in Hawaii, Wilson and his colleagues propose this sequence of events: a 240-meter-wide vertical sheet of magma called a dike moved upward and horizontally from the mantle. The dike presumably followed lines of weakness associated with the formation of Imbrium Basin. The rising dike produced upward pressure that slightly uplifted the surface above. An elongated depression known as a graben formed where surface rocks collapsed between two parallel faults, creating Rima Hyginus. The dike stalled, however, and none of its magma erupted onto the surface.

Some of the dike's magma spread laterally (becoming a mass of igneous rock known as a laccolith) about a kilometer below the surface, eventually swelling to a diameter of roughly 10 km. Gas pressure built up at the top of the laccolith, fracturing overlying rocks. This allowed carbon dioxide gas and pyroclastic debris to erupt over the surface. The explosive release of gas and ash created a vacant space underground, so the overlying material collapsed to form Hyginus Crater. The pyroclastic debris is still seen today as the dark material around Hyginus. Gas flowed along the top of the dike to enhance the eruption, and the decrease in pockets of gas along the dike led to additional collapses, creating the 23 pits along the rille.

This complex scenario might not be the only explanation for what we observe, though the group's model is generally consistent with morphological constraints and known eruption physics. The long dikes were required to account for the rille's two arms, and the laccolith was necessary to provide a void for the collapse that formed Hyginus. Yet the estimated volume of the pyroclastic deposit is about the same as the volume of Hyginus Crater, which formed by removal of the magma that made the ash. The volume of the 23 pits is nearly the same as the calculated volume of gas that escaped.

An intriguing additional consideration is that the high-resolution narrow-angle camera on NASA's Lunar Reconnaissance Orbiter has recently imaged very small depressions and smooth surfaces on the floor of Hyginus. These are thought to be very young (millions rather than billions of years old) and raise the question of whether volcanic gases are still escaping from Hyginus.

To get a daily lunar fix, visit contributing editor Charles Wood's website: Ipod.

The Moon February 2012


Full Moon          February 7, 21:54 UT
Last quarter       February 14, 17:04 UT
New Moon           February 21, 22:35 U


Perigee            February 11, 19h UT
231,619 miles      diam. 32' 4"
Apogee             February 27, 14h UT
251,345 miles      diam. 29' 32"


Volta (crater)     February 9
Pascal (crater)    February 11
Oken (crater)      February 24
Demonax (crater)   February 26
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Title Annotation:Exploring the Moon
Author:Wood, Charles A.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Feb 1, 2012
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