Sky and Telescope - December 2016 - 52

OBSERVING

Exploring the Moon

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,

Thales
Bürg
Bessel
ray

Proclus
Menelaus

Kepler
Copernicus

NASA / LUNAR RECONNAISSANCE ORBITER

Tycho

Several craters display impressive sets of ejected rays when the Moon is
near full. But the so-called Bessel ray, splashed across Mare Serenitatis,
might be from one of several craters, including Thales, Bürg, or Menelaus.

December 2016 sky & telescope

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

Sky and Telescope - December 2016

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