Apr 27, 2006
Luminous Crater Rims
Do the rims of the Martian craters in the
picture above appear to be glowing electrically? They are not
glowing, but they look that way for a good electrical reason.
Our picture for today comes from a high-resolution photographic strip
taken by the Mars Global Surveyor. It is from the region of
Meridiani Planum, on which the rover Opportunity landed in January
2004. (For convenience, we’ve rotated the picture 90 degrees
clockwise; north is to the right).
visited at least one of the craters in the region, and found that
the bright rim is lighter surface material beneath a dark layer of
fine sand, intermixed with millimeter-sized granules. In ways not
yet understood by NASA scientists, the dark material is created and
distributed by Everest-sized “dust devils” and by powerful
landscape exhibited here throws additional light on the electrical
nature of the Martian “winds”. We have already observed that the
dust devils on Mars “burn” the soil electrically, leaving dark
tracks meandering across the Martian surface. The dark material
resulting from electrical storms has covered large regions of Mars.
Later, as new dust devils move across the darkened regions, they
will often leave a lighter track by removing the darker surface
regional dust storm develops, its leading edge reveals a large
tornado-like vortices. In contrast to a single
dust devil, a regional dust storm—sometimes growing to
proportions—can remove much more of the superficial surface
material and deposit the darkened dust and grains over neighboring
regions. And just as a Martian dust devil moving across a darker
surface can create a lighter track, it appears that the more
energetic dust storms can create eerie effects on such darkened
areas as Meridiani Planum.
more than common sense scientifically is needed to see these effects
in electrical terms. It is inconceivable that that a mere “wind”, in
an atmosphere only one percent as dense as the atmosphere of the
Earth, could remove dust and grainy material, then elevate them in
the vertical fashion implied by pictures of the rope-like tornado
columns on the edge of powerful dust storms.
In our Picture of the Day for March 24, we
suggested, “Closer examination should show that these tornadoes form
preferentially on high points and the sharp edges of craters or
escarpments”. The effect is clear in the picture above. Material has
been removed from the rims of craters in ways that would not be
typical of the mechanical effects of wind alone. Note, for example,
that the rims exhibit radial “rays” created by the removal of
material. The rays extend in every direction around the craters—not
in the one direction expected of a mechanical wind’s path. The
result is a photographic image recording the electric discharge of
the dust storm vortices—imitating the “glow” of the air-to-ground
discharge in the contrasting light and dark material left behind.
Also significant is the “tangential”
component of the darker streaks left by removal of dust from the
rims. Many years ago, Ralph Juergens noted this function of a
rotating electrical arc. The pattern suggests a counterclockwise
rotation of an electrical vortex as it spins off the crater in the
direction of the “wind streak” left behind. The familiar winds known
to meteorologists do not create selective displacement of downwind
material in this way.
scientists will examine these features objectively and in closer
detail, they will see the signature of electrical discharge. Though
electrical events today certainly cannot compare to the
planet-altering events of the past, the tools now available should
allow for definitive answers if planetary scientists will consider
the electrical phenomena occurring episodically on the planet today.
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