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Hints of vents: track down the volcanic sources of the lunar maria.

Dark lava flows of the lunar maria cover about 30% of the Moon's nearside. Geologists know from investigating volcanism on Earth that such lavas come from magma that melts at the top of a planet's mantle, which then rises to erupt at the surface. The process on the Moon is thought to be similar, and the lunar mantle is about the same depth as many places on Earth, perhaps 35 to 45 kilometers (22 to 28 miles) or more below the surface. Despite the vast extent of lunar lava flows, there are few easily observed vents where these lavas erupted.

On Earth, volcanic eruptions pile lavas and ash around their source vents, building conspicuous volcanic cones and mountains. The existence of such mountains requires a substantial flow of magma, often occurring over tens of thousands to a few million years. The buildup occurs because terrestrial lavas and ash don't spread far beyond the vents, so many small eruptions concentrate volcanic materials around their source.

We know from studying lunar rocks collected by Apollo astronauts that lunar lavas were much more fluid than typical terrestrial analogs. This means that lunar lava flows traveled far from their vent sources, building few, if any, volcanic cones. Although the Moon lacks large volcanic mountains, many small hills and depressions mark the location of vents, a few of which are visible in a mid-sized amateur telescope.

One of the Moon's richest volcanic areas is the Aristarchus Plateau, an elevated rectangle about 180 km on a side with the Aristarchus impact crater on its southeastern corner. The plateau appears to have been uplifted, perhaps due to a huge underground intrusion of magma.

That suggestion is likely correct because the plateau is the source of a number of rilles, including Schroter's Valley (Vallis Schroteri), the largest on the Moon. The flow of lava away from a vent formed this valley. The lava built a channel that we see today as the valley's walls, and its source region is a 2-km-high broad volcanic mountain with a deep pit on its western flank. The 12-km-wide pit is known as the Cobra Head, because it's an entrance to a narrower, sinuous channel. Smaller rilles have their vents along the serrated eastern edge of the Plateau, and more appear north of the nearby ruined crater Prinz. These rilles start in vent areas 3 to 6 km wide that form oval, rimless depressions that are just visible telescopically.

In fact, the vents of most sinuous rilles are depressions, as if the eruption caused the roof to collapse as the source magma chamber was emptied to form the rilles. At Rima Hadley and other sinuous rilles, the vent area is a sickle-shaped trough, though we don't know why rille vents commonly appear this way. Rima Hadley's sickle is difficult to spot; look for it at the southwest end of the rille in the foothills of Montes Apenninus.

All of the vents described so far carried molten lava downslope, creating lava tubes, channels, and flows. Other vents hosted more explosive eruptions that scattered volcanic ash (known as pyroclastics) and rock fragments around them. Telescopic observers can see two kinds of ash deposits and associated vents. The more familiar ones are small circular collapse pits surrounded by haloes of dark ash, such as the famous dark spots on the floor of Alphonsus. Similar volcanic, dark halo craters also appear on the floor of Atlas. The best time to spot these dark halo craters is when the solar illumination is high, producing the greatest contrast between the dark ash and the brighter surrounding crater floor.

The second kind of pyroclastic eruption is less familiar to most observers. In these cases, ash erupted from a rille or a V-shaped depression with tapered ends. Two of the largest of these pyroclastic deposits appear as dark mantles over low hills east and south of Sinus Aestuum. The V-shaped vent that produced the eastern deposit is 7 by 3 km across, just northwest of the small crater Bode E; it is another challenging target for observers. Most other pyroclastic deposits are smaller, but two others are easy to find because of their darkness--the Taurus-Littrow landing site of Apollo 17, and Rimae Sulpicius Gallus, across the southern margin of Mare Serenitatis.

RELATED ARTICLE: The Moon * April 2013

Phases

LAST QUARTER

April 3, 4:37 UT

NEW MOON

April 10, 9:35 UT

FIRST QUARTER

April 18, 12:31 UT

FULL MOON

April 25, 19:57 UT

For key dates, yellow dots indicate which part of the Moon's limb is tipped the most toward Earth by libration under favorable illumination.

S&T: DENNIS DI CICCO

Distances

Apogee          April 15, 22h UT
251,568 miles   diam. 29' 30"
Perigee         April 27, 20h UT
225,103 miles   diam. 32' 59"

Librations

Cabeus (crater)      April 1
La Perouse (crater)  April 12
Mare Humboldtianum   April 14
Eddington (crater)   April 25


Charles Wood (lpod.wikispaces.com) is coauthor of the new book 21st Century Atlas of the Moon.
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Title Annotation:Exploring the Moon
Author:Wood, Charles A.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Apr 1, 2013
Words:831
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