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Not only angels have halos: hunt for elusive dark halo craters.

THE ORIGIN OF lunar craters used to be a controversial subject. It wasn't until the 1960s that studies of impact and nuclear-bomb craters on Earth, measurements of lunar crater dimensions, and samples returned by the Apollo missions provided compelling evidence for impacts. We now know that nearly all lunar craters, from 1,000-kilometer-wide basins to microscopic zap pits, resulted from the hypervelocity collisions of particles, boulders, and mountain-sized asteroids with the Moon.

But some craters are clearly volcanic, such as lava tubes with collapsed roofs, and pits on the summits of volcanic domes. There are also a small number of others whose origin seems uncertain. In this category are dark halo craters, which have two different origins. The most famous dark halo craters are those on the floor of Alphonsus. These craters have long been thought (correctly) to be small volcanoes, because most lie on narrow rilles. Such rilles are surface cracks produced by vertical sheets of magma that pushed upward. Dark halo craters exist where gas-rich pockets of magma erupted, violently spewing volcanic ash around them. The Alphonsus dark halo craters themselves are only about two or three kilometers in diameter, making them difficult to observe from Earth. Their halos, however, are typically three times as large and are thus much easier to spot.


Another classic dark halo crater is just southeast of Copernicus. When seen with the Sun high overhead, the 5-km-wide bright-rimmed crater Copernicus H is surrounded by a roughly 15-km dark halo, but there are curiously no nearby rilles or other volcanic landforms. Spectral investigations show that this halo is actually pulverized mare basalts. Looking at the larger scene reveals that Copernicus H was formed when an object impacted into bright ray material ejected by Copernicus. So Copernicus H is a different type of dark halo crater--a normal impact crater that excavated deep enough to churn up the underlying dark lavas. Look around Copernicus, and you'll see more dark halo craters, most so small that the impact craters themselves are invisible from Earth.

The fact that dark halo craters were formed by two completely different mechanisms creates an observing challenge. Can you identify the origin of each dark halo crater you happen to see? Let's look at a few and find out. Start in northern Mare Nectaris. Two lie to the southeast of Theophilus. To correctly infer their origins, you'll need to look for other nearby volcanic landforms such as rilles and domes, or a thin layer of bright ejecta covering mare lavas. If you observe this area under low lighting you won't find any volcanic features, but the high Sun view confirms that the dark haloed craters sit on a broad apron of bright ejecta from Theophilus.

Two more dark halos occur on the floor of Atlas. This area is near the northeast limb, so foreshortening makes it difficult to see its floor. A broad dark patch on the southern rim area and a smaller deposit occurs near the north wall. There are irregular shaped craters at the center of these halos and a family of rilles passes near them, providing evidence that these are indeed little volcanos.

To start your own search, observe far from the terminator to maximize the contrast between normal bright terrain and dark halos. Most dark halo craters are small, so use magnifications of roughly 150 to 200x to clearly detect them. Avoid searching the lunar highlands, because both types of dark halo craters ultimately rely on volcanic materials, and most highland areas have little volcanism. Once you find a dark halo crater, look for evidence to decide if it is of volcanic or impact origin. Good hunting!


To get a daily lunar fix, visit contributing editor Charles Wood's Lunar Photo of the Day website:
The Moon * June 2010

Highlighted feature   Size (miles)   Description

(A) Alphonsus         71 miles       Large crater with floor rilles
                                     and dark areas

(B) Theophilus        61 miles       Large crater with irregular
                                     central peak

(C) Atlas             53 miles       Large crater with floor rilles
                                     and dark areas


Last quarter         June 4,22:13 UT
New Moon             June 12,11:15 UT
First quarter        June 19,4:29 UT
Full Moon            June 26,11:30 UT


Apogee               June3,17h UT
251,199 miles        diam. 29' 26"

Perigee              June 15,15h UT
227,379 miles        diam. 32'39"


Cabeus (crater)      June 3
Vallis Baade         June 7
Mare Humboldtianum   June 20
Mare Marginis        June 26
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Title Annotation:Exploring the Moon; lunar craters
Author:Wood, Charles A.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jun 1, 2010
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