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鈥淔OR two centuries and a half, her face has been scanned with the closest
possible scrutiny, her features portrayed in elaborate maps . . . but hitherto
no certain evidence has been afforded that the Moon is other than a dead and
useless waste . . . 鈥�

R. A. Proctor, The Moon, 1878.

WHEN Neil Armstrong bounced across the lunar surface in 1969, he kicked up
grey dust that had lain undisturbed for billions of years. To geologists, it was
pretty clear that nothing much had changed here for aeons. There were no signs
of recent eruptions or bubbling lava flows, just dust and rock.

Unless you count the occasional ghostly glow. For the past few centuries,
observers squinting through telescopes at the Moon鈥檚 surface have glimpsed
luminous spots, coloured glows and mists that last anything from a few seconds
to an hour or two. Apollo astronauts even reported seeing the Moon鈥檚 surface
shining with an eerie light.

Reactions to these reports are often sceptical or even angry. Many
astronomers insist that the lights are simply optical illusions. But in the past
few years, some more substantial observations have appeared. The lunar surface
might be a lot more exciting than many would have us believe鈥攁nd it could
even tell us something about the inner workings of the Moon.

Most moonglow sightings come from amateur astronomers. One such is David
Darling, who coordinates reports of lights and glows for the Association of
Lunar and Planetary Observers, an international organisation of astronomers
based in Arizona. 鈥淯sually people see something they can鈥檛 explain themselves,鈥�
he says. 鈥淭hat鈥檚 what got me started. I saw something glowing on the Moon in the
crater Aristarchus. It glowed so bright that it was like a beacon shining
through the telescope eyepiece.鈥�

Darling has caught sight of another twenty or so events since then, including
a dark 鈥渕ist鈥� that temporarily obscured the lunar crater Reiner. Over the years
he has received dozens of similar reports from other observers around the globe.
These tales of 鈥渢ransient lunar phenomena鈥�, or TLPs, gained some credibility
when professionals, such as Dinsmore Alter, director of the Griffith Observatory
in Los Angeles in the 1960s, started spotting them.

But all talk of TLPs is nothing but moonshine to Tom Dobbins, an astronomer
based in Coshocton, Ohio. 鈥淚n the vast majority of cases, these lights are
simply due to the Earth鈥檚 atmosphere,鈥� says Dobbins. Sudden turbulence can make
a bright crater wall appear to flicker, and particles of dust in the Earth鈥檚
atmosphere disperse light like a prism, creating violet, blue or reddish
glows.

Clear photos of TLPS are few and far between, so in 1965, NASA set up an
observatory at Corralitos in New Mexico with the express purpose of recording
these events. But in almost 3000 hours of surveillance, it failed to spot
anything unusual, says Dobbins. An opportunity to settle the arguments came when
the Clementine orbiter headed for the Moon in 1994. Amateur astronomers in 20
different countries looked for signs of glows or mists in the hope that
Clementine鈥檚 cameras could confirm them. But two months of careful observation
drew a blank.

Another problem for TLP enthusiasts is explaining what could cause these
strange glows. Short flashes or sparkles are put down to meteorite impacts, but
longer-lived glows are harder to fathom. One popular idea is that these glows
are caused by pockets of gas or vapour trapped just below the lunar surface,
which are released by a meteorite strike or a moonquake.

But vapour alone would be invisible, so the gas must either fluoresce under
intense solar radiation or lift bright dust off the surface. British astronomer
Patrick Moore backs the theory: 鈥淭here鈥檚 no question. It鈥檚 gases released from
below the crust.鈥� NASA has already detected gases released from the Moon, he
says. Apollo 15, flying low over the crater Aristarchus, collected traces of an
isotope of radon that had almost certainly diffused up through the lunar
crust.

How could these gases have got there in the first place? If you only look at
surface geology then it鈥檚 easy to conclude that the Moon is dead, says Maria
Zuber, a planetary geophysicist at the Massachusetts Institute of Technology.
鈥淏ut if you take the geophysical viewpoint and look at the interior, then you
get a different answer.鈥� Observations from orbiters and laser ranging
experiments suggest that the Moon has a partially molten interior. 鈥淚t鈥檚 not
dead yet, it鈥檚 hanging on,鈥� says Zuber. That could create problems for future
lunar prospectors鈥攖hat is, if the lunar lights are even real.

Night lights

But in 1999 came tantalising evidence that they are. Veteran French
astronomer Audouin Dollfus at the Meudon Observatory in Paris reported in the
journal Icarus that he had seen a strange glow coming from inside the
crater Langrenus. He even had the photos to prove it. 鈥淭he glow covered a
portion of the inner part of the crater, perhaps 30 or 40 kilometres across,鈥�
says Dollfus. 鈥淚 saw it on two nights.鈥� His measurements suggested that the
light was polarised, meaning it could have been sunlight bouncing off dust
lifted by gas emitted from the surface.

More support for this outgassing theory appeared the next year. A team led by
Bonnie Buratti from NASA鈥檚 Jet Propulsion Lab in Pasadena, California, analysed
some of the photos that Clementine took of the 11 regions where TLPs had
previously been reported. The photos showed bright, reddish deposits, perhaps
material thrown up by relatively recent volcanic activity, says Buratti. 鈥淭LPs
tend to occur in areas of the crust that are fresher, near the edges of the
maria.鈥� This seems to agree with the idea that gas collects under these large,
unbroken regions and escapes through weaknesses in the crust around their edges.
In some craters, the walls may have slumped, releasing pockets of gas from
below.

But maybe we don鈥檛 need lunar blow-offs to explain the odd light or two. In
1966, the American probe Surveyor pointed its cameras towards the Moon鈥檚 horizon
at sunset and recorded a strange glow. According to American researchers, this
was caused by particles of dust floating above the surface, held up not by gas
but by electric fields.

Solar radiation can create short-range electric fields by ionising atoms on
the surface of sunlit rocks. These fields exert a force on small, charged dust
grains, sometimes levitating them a few metres above the surface. The same
effect can create horizontal electric fields near the terminator鈥攖he
dividing line between day and night鈥攚hich would drag the levitated dust
cloud across the surface. Perhaps these are the dust clouds that create some
TLPs, which are often seen around the terminator. It has even been suggested
that these clouds might get so dense that if you were to drive into one on a
Moon buggy, you could get lost.

To Dobbins, electrically generated dust storms seem a more likely explanation
for TLPs than outgassing, but he remains unconvinced by most reports. Some
people simply want to see these things, he believes.

Darling isn鈥檛 so sure: in the 1960s, pilots flying X-15 rocket planes saw
jellyfish-like globules of red and blue light in the Earth鈥檚 sky, he says. 鈥淭hey
were afraid to report it because they didn鈥檛 want to be criticised.鈥� But 30
years later we know they鈥檙e real, and we call them sprites. 鈥淲ho knows what鈥檚
out there.鈥�