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Lunar Rilles of mystery: strange scars on the Moon's surface defy easy explanation.

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THE LUNAR 100 is my selection of targets to guide your Moon gazing and help you understand what you see. In this space over the last three years I described all 100 of them--or so I thought. Keen-eyed reader William Todd wrote recently to point out that I had somehow skipped five entries. So, here we go again. In this column and next month's, I'll finish up the Lunar 100. This time, for sure!

Let's resume our quest with two strange sets of lunar rilles (or rimae in Latin). Generally, these long, narrow depressions are either straight (like the Ariadaeus Rille), concentric (such as the beautiful Hippalus Rilles on the east side of Mare Humorum), or sinuous (typified by the famous Hadley Rille where Apollo 15 astronauts David Scott and James Irwin landed). And nearly all of them are found on or very near maria. However, the Janssen and de Gasparis rilles are oddball exceptions.

Aim your scope at the Janssen Rilles (L40 in the Lunar 100). This collection of scars cuts across the relatively smooth southern part of Janssen, a battered, ancient crater roughly 120 miles (190 km) across. Right away you can see that these rilles don't fit into any of the categories I just described. Not only are they situated deep in the lunar highlands, but they're neither linear, concentric, nor sinuous.

At first glance it appears that a single wide rille curves down from Janssen's hilly center (near Fabricius) and narrows as it crosses the smooth part of its floor. But look carefully, and you'll see that the wide and the narrow segments don't actually connect. The wide section ends as it meets two or three very narrow rilles that continue to

Janssen's southern rim. The Janssen Rilles are flat-floored troughs with steep walls--like miniature versions of the Ariadaeus Rille to the north. Often these kinds of rilles are associated with volcanic cones or dark pyroclastic deposits, demonstrating that they are probably manifestations of near-surface magma. And yet there don't appear to be any volcanic landforms around Janssen. So what accounts for these strange rilles? We just don't know.

West of Mare Humorum is another collection of odd rilles. The de Gasparis Rilles (L91) are focused on the 19-mile-wide crater of the same name. There are two sets of features here--one rille group trends northeast-southwest, and a second set lies nearly at right angles to the first. Like the Janssen system, these have trough-like cross sections, suggesting that they may be a consequence of magmatic activity.

The nearness of the de Gasparis Rilles to the Humorum basin suggests a possible relationship. It might seem that they are parts of the features in western Mare Humorum, but actually they're not. The northeast-trending de Gasparis Rilles are nearly tangent to Humorum and point northward to the Mersenius Rilles. The de Gasparis and Mersenius systems are basin-related, but They're actually radial to the huge Procellarum basin. They are, like the Sirsalis Rilles to their northwest, probably the surface expressions of vertical conduits of magma (called dikes), fed by the same source as the Procellarum lavas.

But we still haven't explained the other de Gasparis Rilles--the ones running northwest-southeast. The origin of these, like the ones adorning Janssen, remains a tantalizing lunar mystery.

ALL ABOUT RILLES

Lunar rilles typically are associated with volcanism. Sinuous rilles are created by lava flowing on the surface, like meandering rivers. Concentric rilles from as fractures at the edges of subsiding mare-filled basins. And linear rilles (like the Gasparis ones) usually map out the existence of underground volcanic dikes.

Charles A. Wood is author of The Modern Moon: A Personal View (available from ShopatSky.com). He also conducts the Lunar Photo of the Day website, www.lpod.org.
The Lunar 100

     Name                                Length (miles)    Description

L40 Janssen Rille                             124         Rare example
                                                          of a highland
                                                          rille

L91 de Gasparis Rilles                        580         Area with
                                                          many rilles
Moon, February 2008

New Moon         Feb. 7, 3:44 UT
First quarter    Feb. 14, 3:33 UT
Full Moon        Feb. 21, 3:30 UT
Last quarter     Feb. 29, 2:18 UT

Perigee          Feb. 14, [1.sup.h] UT
(dist. 230,043 miles; diam. 32'17")
Apogee           Feb. 28,1" UT
(dist. 251,309 miles; diam. 29'33")

Max. libration   6.9[degrees], Feb.
                   1, [15.sup.h] UT

Min. libration   4.8[degrees], Feb.
                   8, [16.sup.h] UT

Max. libration   6.8[degrees], Feb.
                   14, [11.sup.h] UT

Min. libration   4.8[degrees], Feb.
                   20, [11.sup.h] UT

Max. libration   7.0[degrees], Feb.
                   29, [21.sup.h] UT
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Article Details
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Title Annotation:Exploring the Moon
Author:Wood, Charles A.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Feb 1, 2008
Words:761
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