http://www.skyandtelescope.com.au 55PHOTOS: NASA / LUNAR RECONNAISSANCE ORBITER; COLOUR PLOT: CHAPPAZ
ET AL
/ GEOPHYSICAL RESEARCH LETTERS
found by Chappaz’s group appears to
connect the southern ends of Rima
Sharp and Rima Mairan in Sinus Roris.
Then it continues to the north end of a
bizarre ‘braided rille’ (likely a complex
lava tube) due south of the 40-km-wide
crater Mairan.
It seems strange that one tube would
connect all of these lava channels
around Sinus Roris, for this kind of
linking doesn’t happen on Earth.
It’s also perplexing to find tubes so
much wider than the surface rille. In
terrestrial cases, the tube is just a roofed
channel — so they’re the same width.
The Japanese lunar orbiter Kaguya
has provided confirmation of tubes
in the Marius Hills. Kaguya’s Lunar
Radar Sounder generates radio-wave
pulses that penetrate tens to hundreds
of metres into mare lavas, and then
records echoes reflected from subsurface
layers. Tesuya Kaku (Japan Aerospace
Exploration Agency) and colleagues
have detected numerous linear voids
near the Marius Hills rilles that they
believe come from the floors and
ceilings of lava tubes.
Would these make suitable habitats
for astronauts? Perhaps not. All of
the lava tubes that I’ve entered had
floors littered with collapsed roof
blocks and stalactites hanging from
their ceilings. Such sharp rocks and
surfaces could make walking difficult
or easily puncture an inflatable habitat
or spacesuit. And access through a
partially collapsed skylight could
be tricky. It might be necessary touse a winch to get astronauts and
their equipment into and out of an
underlying tube.Elusive quarry
Usually less than 1 km wide, lunar
rilles are challenging to observe. Those
with roofs (lava tubes) are even more
difficult to detect, unless they have
skylights — but even then it will be astruggle to glimpse them.
Still, the areas where they occur are
readily observable. Easiest is the western
end of Schröter’s Valley, the widest
lunar rille. You’ll need a telescope with
at least 75 mm of aperture; look 12 or
13 days after new Moon.
The nearby Mairan and Sharp rilles
are both narrower than 1 km and
would be nearly impossible to visually
observe — except that parts of their
lengths run nearly north to south. This
means that, after local sunrise (likewise
13 days after new Moon), their eastern
walls cast shadows onto their floors
that appear like narrow black lines in
the maria. Use 150× to 200× and a
scope with a 15-cm or larger aperture to
explore the area.
On the same night that you search
for Schröter’s Valley and the rilles in
Sinus Roris, slide farther south along
the terminator to the Marius Hills, a
collection of nearly 200 volcanic cones,
domes and rilles. Although too narrow
to resolve visually, the rille and its
associated skylight and lava tubes occur
near the middle of this mass of mounds.
Not all rilles are in the maria.
Using Lunar Reconnaissance Orbiter
images, Pascal Lee (SETI Institute,
Mars Institute) found that Philolaus, a
young, 70-km-wide crater poleward of
the western end of Mare Frigoris, has
many small rilles and some skylights
on its floor. These formed in impact
melt that must have flowed after
being ejected upward and then falling
back to the surface. The rilles are too
small to be observed, but the nearly
20-km-long patch of smooth impact
melt can be glimpsed on the eastern
floor of the crater.
As you observe the Marius Hills and
the Sharp and Mairan rilles, imagine
what it would be like to live in one of
their tubes — with a skylight providing
night views of an inky-black sky littered
with stars.■ CHUCK WOOD singed his eyebrows
in 1971 while photographing a lava-tube
skylight in an active low on Kilauea in
Hawai’i.SLeft: The ‘mega-rille’ Vallis Schröteri (Schröter’s Valley) snakes across the lunar surface for
hundreds of kilometres. Right: New results from GRAIL spacecraft data suggest that a hollow
tube (red band left of centre) extends below the surface beyond the rille’s southwestern end.SThe lunar rilles Rima Sharp and Rima
Mairan snake along the eastern shore of Sinus
Roris and might be part of an interconnected
regional network of ancient lava tubes.50 kmHerodotus
Aristarchus“Cobra
Head”Vallis
Schröteri30 kmVallis
SchröteriHerodotusRima
SharpMairanRima
Mairan“Braided Rille”30 kmSINUS RORIS