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Left: 3mm crater electrically etched in
caliche.
Credit:
C. J. Ransom, VEMASAT Labs.
Right: 150 meter Martian "skylight." Original image credit:
NASA/JPL/University of Arizona
Jun 19, 2008
Martian Skylights in the Laboratory
Deep cylindrical pits on Mars are not easy to explain.
Small-scale plasma discharge experiments could offer some clues to
their formation.
In
previous Thunderbolts Picture of the Day articles about
unusual geological structures on Mars, we have included
craters,
canyons, dunes and many other features that do not
readily correspond to contemporary theories about their
evolution.
Dunes that do not align with the direction of the
prevailing winds and that do not show any movement after
years of observation are odd. Other anomalies include crater
rims with steep vertical walls and kilometer-deep canyons
with no outflow channels.
On the slopes of
Olympus Mons, often
mistakenly called the largest volcano in the solar
system, are several
large holes that seem to be excavated vertically for
hundreds of meters. On Earth some volcanoes have similar "pit
craters" on their flanks. They have no connection to
fumaroles or lava chambers so they are thought to be from
gas pockets that collapsed, leaving sinkholes behind.
However, with the recent awareness that
plasma activity is involved with volcanic eruptions on
Earth it should also be considered when dealing with Mars.
The same
activity may have occurred there, but if so, it must have
been billions of years ago when the last volcanic eruptions
took place there. If the "skylights"
are that old, surely they would not look like they were dug
into
the surface a short time ago.
As we have
written elsewhere, Olympus Mons has all the
characteristics of a lightning blister, or fulgamite, but on
an incredible scale. If small blisters have been found on
spark arrestors after a lightning strike on Earth, one can
imagine the size of the burst that hit Mars and formed the
Tharsis Montes volcanoes - Olympus Mons in particular.
The giant mound is covered with raised dendritic ridges
flowing down its sides like hardened
Lichtenberg figures. The caldera chiseled out of its
summit is unlike the volcanic vents found anywhere on Earth
- the same dendritic ridges outlining its foundation are
also molded into those
cliff faces.
In the image at
the top of the page, an enhanced Martian skylight is
compared to a crater blasted into caliche by Dr.
C. J. Ransom's electrical apparatus. A one-second spark from
the discharge terminal drilled into the stone, forming a
crater almost identical to the large version from Mars. The
cracks, the helical path downward into both craters, the
"rubble" on the bottom and the hemispherical nips taken out
of both rims are quite evident.
Craters and pits
such as these vary enormously but they all possess the
distinguishing features of electrical arc machining. An arc
will usually  contact the surface at a 90-degree angle and
typically will consist of one or more discharge channels
that rotate around an empty center. If the electric arc
remains stationary for a period, it will carve out a
circular crater. Most of the surface material will be lifted
away and the edge will have a sharp rim. If the current
passing through the surface varies, the depth and diameter
of the crater may vary, causing terraces to be cut into the
walls.
All of those
characteristics can be seen in both "pit craters" - C. J.
Ransom's laboratory example and the example from Mars.
Perhaps the other examples on Earth should be evaluated
differently?
By Stephen Smith
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