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Mystery Ray in Serenitatis: lunar geologists still can't identify the Bessel ray's source crater.

Splashes of bright rays are among the most conspicuous lunar features. When the Moon is nearly full, the vast ray system from Tycho seems to girdle the nearside like the segment boundaries of a peeled orange. Copernicus likewise exhibits an extensive ray family, with streamers stretching halfway across Mare Imbrium. A similar but smaller system radiates from Kepler.

A characteristic of all these familiar families is that we can readily identify their source craters. But many shorter and fainter rays have ambiguous parentage. Most famous and controversial is the so-called Bessel ray, a straight, bright streak in the southern half of Mare Serenitatis that lies tangent to the 16-km-wide crater Bessel. The brightest segment of the ray is 150 km long, though less certain extensions likely double this length.

A little background is in order here. Rays are long trails of debris that get ejected radially during the formation of impact craters. Their brightness can arise from either or both of two mechanisms. First, fragmentation of any lunar rock creates highly reflective rubble that over time will become "space weathered" by radiation to darker tones after about a billion years of exposure to space. (Even craters that formed in dark mare lavas, such as Messier, have bright rays.) The second way is to slam into light-toned highland rocks, which then spray out intrinsically bright ejecta. Some of this includes coherent chunks large enough to create secondary craters.

No matter the cause, you can identify the source crater for a given splash of rays by looking to find a relatively fresh, large crater from which these streaks fan out radially. (Also, spectral studies can determine if the rays contain highland or mare material, which also aids in identifying their source.) The Bessel ray is widely assumed to come from Tycho, about 2,000 km distant, both because the ray points toward Tycho and because it contains highland rock fragments. Situated deep in the lunar highlands, Tycho's impact surely excavated an abundance of bright material.

Yet I offer two cautions against concluding that the Tycho origin is definite. Observe during full Moon, and you'll see that most of the continuous Tycho rays, such as the one that reaches Mare Nectaris to its northwest, extend only 1,000 to 1,500 km--well short of Bessel's distance. Additionally, there are other ray craters besides Tycho that formed in the highlands and ejected fragments of bright-toned rocks.

So what other craters could be the source of the Bessel ray? Your first thought might be to pick Bessel itself. But rays typically extend in multiple radial directions from their source, not in a straight line alongside it. And Bessel formed in deep Serenitatis lavas, not in highland material. So scratch that idea.

The next nearest possible source crater is Menelaus, a young, 27-km-wide crater just 75 km from the middle of the Bessel ray. The projectile that formed Menelaus slammed half into the highland material of the rugged Montes Haemus and half into the adjacent mare.

Menelaus does have a strange ray system that fans out about 100 km to the southwest, about the same distance to the east, and possibly also to the west. A single, faint possible ray segment runs northward, exactly in line with the Bessel ray. But if the entire Bessel ray came from Menelaus, then it would extend about 285 km--much longer than other parts of Menelaus' weak ray system. This seems unlikely.

In 2010, lunar scientists Gwen Bart and Jay Melosh, then at the University of Arizona, proposed that the Bessel ray and four other short ray streaks on Mare Serenitatis all diverge from the crater Burg, about 750 km away in Lacus Mortis. This is a surprising hypothesis, because it's not obvious that Burg even has a ray system. But this 41-km-wide crater sits amid a halo of spectrally immature soils, ones that have not yet been space-weathered sufficiently to darken. No rays can be traced in any direction other than the putative distant ones in Serenitatis. Additionally, the Biirg impactor slammed into the mare lavas of Lacus Mortis, and I don't know if it dug deeply enough to excavate highland materials.

A final possible source for the Bessel ray is Thales. With a diameter of 31 km, this young, rarely noticed ray crater sits amid bright, jumbled highlands and basin ejecta at 50[degrees] north, near Humboldtianum basin. Thales formed from a strongly oblique impact that left a broad "zone of avoidance" (lacking rays) to the north, with bright rays streaming out in other directions; in this sense it's a larger version of Produs. One of Thales' bright rays extends 750 km, passing east and south of Burg. Perhaps you can convince yourself that it aligns with the Bessel ray.

If you're keeping score, I've listed five possible source craters for the Bessel ray. Bessel itself is least likely, and I'd also rank Menelaus as low probability. If the five short bright lines in Serenitatis are indeed all rays, their convergence on Burg is hard to refute. But Biirg has no other conspicuous rays. Both Tycho and Thales do have major ray systems, and both were blasted into bright-toned highland rocks. But the Bessel ray is a geometric long shot for both of them.

The early 20th-century German selenographer Philipp Fauth wrote, "The crowns of rays spread out over the face of the full moon, and seem to mock at all explanation." Despite more than 100 years of astronomical progress since Fauth made this observation, we still have an incomplete understanding of rays. Perhaps the recent series of advanced lunar orbiters has collected data that can resolve the Bessel ray mystery. I, for one, am tired of not knowing the answer.

The Moon December 2016


FIRST QUARTER December 7,9:03 UT

FULL MOON December 14,0:05 UT

LAST QUARTER December 21,1:56 UT

A NEW MOON December 29,6:53 UT


Perigee        December 12,23h UT
358,461 km     diameter: 33' 20"
Apogee         December 25,6h UT
405,870 km     diameter: 29' 26"

Favorable Librations

Mouchez (crater)   December 11
Byrd (crater)      December 15
Bailly (crater)    December 24

Visit Contributing Editor Charles A. Wood's Lunar Picture of the Day website at


Please note: Some tables or figures were omitted from this article.
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Title Annotation:OBSERVING: Exploring the Moon; Mare Serenitatis
Author:Wood, Charles A.
Publication:Sky & Telescope
Date:Dec 1, 2016
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