Houghton called astronomer Brian Warner and told him what to
look for. Warner too saw the flashes and called them "so
conspicuous that they were seen immediately." The other peaks in
the vicinity remained normal.
On the night of November 2-3, 1958, Soviet astronomer
Nikolai A. Kozyrev witnessed a strange phenomenon while making
spectrograms of the crater Alphonsus with the Crimean
Astrophysical Observatory's 50-inch reflector. As he watched
through the telescope's guiding eyepiece, he saw the crater's
central peak blur and turn an unusual reddish color. The
spectrograms confirmed his visual impressions of a volcanic
event; they showed an emission spectrum of carbon vapor (S&T:
February, 1959, page 184).
On July 19, 1969, the Apollo 11 command module had just
achieved orbit around the Moon when the Mission Control Center in
Houston, Texas, received word that amateur astronomers reported
transient phenomena in the vicinity of the crater Aristarchus.
Asked to check out the situation, astronaut Neil Armstrong looked
out his window toward the earthlit region and observed an "area
that is considerably more illuminated than the surrounding area.
It just has -- seems to have a slight amount of fluorescence to
it." Although he wasn't sure, Armstrong believed the region was
Aristarchus.
Accounts of lunar transient phenomena (LTPs) are not new.
Over the past 30 years, I have collected close to 2,000
observations dating from as far back as 557 A.D. Most are visual
reports of bright spots, flashes, hazes, and curious temporary
colorations of the lunar soil. Reputable observers such as
William Herschel, Wilhelm Struve, and E. E. Barnard have seen
them. Some LTP's have even been photographed, as well as recorded
polarimetrically, photometrically, and spectroscopically. Yet,
despite a profusion of observations and six Apollo missions to
the Moon, the nature of LTPs remains elusive and their origin an
enigma.
About 200 of some 30,000 lunar features visible in
telescopes have been recorded as LTP sources. Half have shown
activity only once. Of the remainder, a mere dozen features
contribute three-fourths of all reports. One area, Aristarchus-
Herodotus-Schroters Valley, is responsible for fully one-third of
the total number sighted.
Most LTP activity occurs along the edges of the maria, near
volcanic features, like domes, sinuous rilles, and craters with
dark halos or floors. But these regions, like the rest of the
Moon, have long been considered geologically dead. Circular maria
are large, primordial impact basins that were filled with lava
about 3 billion years ago. There is evidence, however, that
volcanism has occurred in some craters that are perhaps only a
million years old. Could the bright flashes, hazes, and colors
reported at these sites be proof that the Moon is still active?
Some LTP phenomena may be caused by sunlight interacting
with the lunar soil. On October 30, 1963, James Greenacre and
Edward Barr observed red spots sparkling on the southwest wall of
the crater Aristarchus, the east wall of Schroter's Valley, and a
hill between them (S&T: December, 1963, page 316). The phenomena
was observed visually by others and recorded spectroscopically as
well. At the same lunar phase a month later, Greenacre and Barr
saw a similar event. Since sunrise on these features occurs when
the Moon is about 11 days old, Greenacre thought that the low
lunar Sun was somehow responsible. Indeed, thermoluminesence mat
be the cause. Gases in the lunar soil, frozen during the night,
could heat up and escape near sunrise.
Could high-energy solar particles impacting the Moon also
trigger LTP activity? Shortly after a large flare erupted on the
Sun in 1963, Zdenek Kopal and Thomas Rackham at Pic du Midi
Observatory in southern France photographed a local brightening
around the craters Copernicus, Kepler, and Aristarchus. Kopal
proposed that energetic particles from the flare caused lunar
rocks to fluorescence. Such activity might be expected especially
at full phase when the Moon passes through the Earth's
magnetosphere, where solar wind particles become trapped.
When LTP phenomena are plotted against the Moon's phases, it
appears that the most phenomena occur around the time of full
Moon (though LTP's have been observed throughout the lunar
cycle). Also, more are seen near the sunrise line than the
sunset line, though that might be simply because far more people
observe the waxing Moon in the evening than the waning Moon after
midnight. Gaseous phenomena and anomalistic brightenings seem to
peak when the Moon is a waxing crescent.
No doubt some apparent LTP's are caused by atmospheric
effects. One is the "ashen glow." Here, sunlight scattered by
Earth's clouds is cast onto the Moon's night surface, resulting
in LTP's that simply reflect changes in the level of
illumination. Another pseudo-LTP concerns bright features
fringed with blue (north) and red (south) seen against dark
backgrounds. These probably are aberrational effects, namely
atmospheric dispersion near the observer, perhaps enhanced by a
lingering temperature inversion.
Sightings of a starlike point on the Moon may also be
disregarded as an LTP. This is the only transient phenomenon I
have ever observed myself. But I suspect it is merely a
reflection effect from flat facets on areas of large rocky
outcrops when the Sun and observer are at just the correct
angles. (High magnifications spread the light into an area
instead of a point.)
Even if we eliminate the three types of non-LTP's discussed
here, that still leaves more than 40 percent of the reports
unexplained.
There is evidence that the remaining LTP's are of lunar
origin. a substantial number of sightings were independently
confirmed. Professional astronomers have recorded them on film
and spectrograms, as well as with photoelectric photometers and
polarization equipment. Experiments on the Apollo missions
detected trace outgassings of the radioactive elements radon an
polonium, suggesting that more substantial amounts of commoner
substances were released at the same time. One experiment
possibly detected water vapor during the largest moonquake on
record (Richter 4). the epicenter of that quake was near or in
the large, fractured crater Gauss north of Mare Crisium. To me,
this is the one lunar feature that looks as if it had been
covered with a thin crust of glass subsequently shattered by an
impact.
While in lunar orbit, Harrison Schmitt of Apollo 17
witnessed a flash near the crater Grimaldi west of Oceanus
Procellarum. Since he was dark adapted, it's possible he saw a
cosmic-ray flash within his own eyeball. But it's also possible
he saw a lunar event. In the past, Grimaldi had been responsible
for more than a dozen reports of flashes. The crater Plato near
Mare Imbrium is another source of flashes. Although many craters
responsible for LTP sightings have central peaks with summit
craters, Plato has none.
So the Moon may not be such a cold, lifeless neighbor after
all. It still breathes through the action of LTP's, which in my
opinion are probably gentle outgassings of less-than-volcanic
proportions. Whatever they are, thanks to the LTP's, the Moon
remains a curious place.
The author directs the lunar transient phenomena section of the
Association of Lunar and Planetary Observers.
At least 11 possible causes of lunar transient phenomena
(LTP's) have been discussed. As a stimulus to further
discussion, it may be helpful to summarize and comment on these.
1. Tidal. There is a greater stress by the Earth when the
Moon is at perigee than at other parts of its orbit. The tidal
pull may release strains in the crust and permit the release of
trapped gases. The tidal effect of Earth on the Moon is 32.5
times greater than the effect of the Moon on the Earth.
2. Albedo changes due to dust movement. There is
essentially no atmosphere on the Moon to raise dust, so this does
not seem likely as an explanation.
3. Thermal shock. The lunar surface temperature varies
from 125 degrees to -80 degrees Celsius during a two-hour period
at both sunrise and sunset, and most LTP's occur within three
days of local sunrise or sunset. LTP's could be related to the
fact that dissimilar materials expand and contract at differing
rates. Although the maria heat and cool more rapidly than other
parts of the Moon's surface, at a depth of just 10 cm the rock
temperature is constant. Thermal conditions may be regarded as
incidental to LTP's rather than the main cause.
4. Magnetic. Solar plasma bombards the lunar surface, even
when the Moon is in the Earth's magnetic tail, and the movement
of the Moon in and out of the magnetic tail changes the field
strength. But charged particles penetrate the lunar surface to
only one-third the depth reached by the Sun's thermal rays. If
thermal shock is considered to be insufficient to cause LTP
phenomena, then the much weaker electromagnetic effect below the
surface must also be discounted.
5. Ultraviolet radiation from the Sun might cause
fluorescence at visible wavelengths, because there is virtually
no atmosphere to shield the lunar surface. I doubt that the
effect is strong enough to produce visible reactions as bright,
or as large, as the usual LTP's.
6. Solar-wind plasma impacting the surface could produce an
electric discharge, but I doubt that the energy involved is great
enough to cause an effect visible from Earth. LTP's could, however, be caused by the explosion of
chemically reactive molecules and free radicals in small hollows;
and solar wind plasma may produce such reactive molecules in
rocks. But solar-wind plasma hardly looks like a prime cause.
7. Spectral diffraction from surface grains or
irregularities too small for telescopic resolution may cause
color. The lunar surface presents many angels to the observer,
and these vary with libration in both latitude and longitude as
well as with diurnal libration. With the varying lunar surface
slopes, many simultaneous diffraction angles should be presented.
But if this were the cause of LTP's, the effects should show all
over the disk from time to time, whereas they in fact show local
preferences.
8. Meteor strikes. These have been claimed frequently.
One of the best observed was reported by the Smithsonian
Institution near the Apollo 14 site on May 13, 1972, when a
meteor impact released an energy equal to that of 1,000 tons of
TNT. But meteor strikes cover only a small area on the lunar
surface when compared to the size of LTP's.
9. Moonquakes are often deep seated but very weak, with a
preference for times of perigee or apogee. There seems to be no
obvious connection between the frequency of moonquakes and
sightings of LTP phenomena.
10. False color is a regular feature in larger telescopes
due to terrestrial atmospheric conditions. This can be guarded
against by comparing a suspected LTP event with the appearance of
other formations on the lunar surface at the same time, and is
well recognized by practiced observers.
11. The piezoelectric effect is well known on Earth, for
example when rock strain generates a strong electric field that
ionizes the air above the rock, causing a glow. The field moves
with the strain source. There are Soviet reports that the
magnetic field drops suddenly when underground tension is
released.
Conclusion. The foregoing indicates courses that our
thinking and research might follow. I am of the opinion that
tidal strain or thermal shock causing outgassing and producing a
piezoelectric effect might be the most plausible explanation.
More on Lunar Transient Phenomena
On January 24, 1956 amateur lunar observer R. Houghton was
drawing the crater Liebig on the edge of Mare Humorum when
something bright flashed in the field of his 7-inch telescope.
The flare came from the nearby crater Cavendish, which was just
emerging from the lunar night. Closer inspection revealed that a
peak on the crater's eastern wall was repeatedly flashing.Thoughts on the Origin of LTPs
Possible explanations for LTPs are not lacking. One of the
earliest proposals was made by Jack Green of Douglas Advanced
Research Laboratories in Huntington Beach, California. While
studying the standing levels of water and oil in deep wells, he
found that the levels varied in concert with the Moon's
anomalistic month (27.55 days, from perigee to perigee), as if
the strength of the Moon's tidal force affected the tiny cracks
in the bedrock through which oil and water move. Based on this
idea, he suggested that LTPs are degassing phenomena brought
about by the Earth's tidal effects on the Moon. Maximum
degassing, he believed, would occur at the Moon's most eccentric
apogees and a minimum at the least eccentric perigees. After
analyzing 1,200 observations, however, I could not find such a
relationship.
Analysis
LTP sightings fall into five categories: brightenings,
darkenings, reddish colorations, bluish colorations, and
obscurations. When plotted against the lunar anomalistic month,
the data show that LTP activity peaks somewhat when the Moon is
moving from apogee to perigee, especially about halfway between
these points when the Moon is approaching Earth the most rapidly.
When the Moon is opposite that point in its orbit, LTP activity
is at a deep minimum. Since tidal stressed build from lunar
apogee to perigee, one might expect such a pattern.
Are They Real?
Some astronomers dismiss all LTP's as either aberrational
effects in Earth's atmosphere, changes in lunar lighting
conditions, or outright illusions. Such skepticism, however,
flies in the face of those who have devoted decades to
familiarizing themselves with the Moon, and who very well know
these common observational effects. * LTP's are localized
phenomena. They are regions or features that experience change
while the rest of the Moon remains normal.
Some Possible Causes of LTPs
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