54 AUSTRALIAN SKY & TELESCOPE May | June 2018
EXPLORING THE MOON by Charles A. WoodDALE CRUIKSHANKLiving in the lunar shadows
Challenging to spot, some lunar rilles might someday shelter astronauts from harm.
L
unar craters have undeniable
appeal when seen through a
telescope, but eventually you’ll
want to explore the subtle details found
splayed across the Moon’s vast lava
plains. High on my list are sinuous rilles,
snakelike ‘riverbeds’ that sometimes
meander across the maria for hundreds
of kilometres. They’re worth studying
because one of them might someday
serve as a habitat for visiting astronauts.
This is an old idea, dating back to the
1960s and early 1970s, when geologists
realised that these winding features
weren’t formed by flowing water but by
flowing lava. Dale Cruikshank (now
at NASA Ames Research Center) and I
were among those who proposed this
explanation. We’d noted how fluid
eruptions from Hawai’ian volcanoes
tend to develop channels that efficiently
transport lava downslope.Splashes of lava solidify along the
edges of these channels and gradually
build levees that in some cases grow
to span the flow’s entire width.
This insulating cap keeps the lava
underneath from radiating heat away
quickly, allowing it to flow further.
Pieces of the roofs of some tubes
collapse, creating ‘skylights’ that expose
torrents of hot lava flowing below.
Future astronauts might descend
into a hollow, long-frozen lava tube on
the Moon to gain shelter from cosmic
rays and small impact events, and to
moderate the extreme temperature
swings (300°C) that the surface
endures over a 29½-day-long lunar
diurnal cycle.More common than thought
Among a comprehensive census of
195 sinuous rilles compiled by DebraHurwitz while a graduate student at
Brown University (see lpi.usra.edu/
lunar/rilles), only six are known to
have skylights. It seems that most
channels were too wide for roofs to
form and be supported.
Unfortunately, these six skylights are
relatively tiny. The largest are only about
100 m wide, far too small for even the
best telescopes to spot. Areas between
lines of skylights should be uncollapsed
lava tubes — in theory, at least.
Two teams have recently used
different geophysical measurements
to provide more direct evidence that
such tubes exist. Loïc Chappaz (Purdue
University) and six colleagues analysed
the very precise measurements obtained
by NASA’s twin Gravity Recovery and
Interior Laboratory spacecraft. Since a
lava tube is empty space, it’s detectable
as a tiny deficit of mass. The Chappaz
team delicately tweaked GRAIL’s data
to find indications of 11 lunar tubes —
some extending beyond sinuous rilles,
some between skylights, and some with
no surface expression at all.
Among the most conspicuous is
an extension beyond the end of the
‘mega-rille’ Vallis Schröteri (Schröter’s
Valley). The researchers model this tube
as being 60 km long, 3.75 km wide, and
an amazing 600 m tall. It apparently
starts near the end of Schröter’s Valley
and extends out to the southwest under
younger lavas that overlap the edges of
the Aristarchus Plateau.
They identified another lava tube
near two large rilles in the western
Marius Hills. That’s where spacecraft
imagery revealed a 65-m-wide skylight,
evidence for a relatively recent collapse.
The tube continues 60 km past the end
of the surface rille and apparently is
much broader than the 400-m-wide
channel seen on the surface.
The final large gravity anomalySAn opening or ‘skylight’ on Mauna Ulu in Hawai’i reveals molten rock lowing through a lava
tube just below the surface. Lunar rilles likely have hidden, but no-longer-active lava tubes as well.