Dave Pieri of JPL suggests that the ubiquitous "tubes" on Mars are
sand dunes. [<http://www.sightings.com/general9/jpl.htm>]
Let's look at this in two parts:
(1) The white bands. Pieri is really trying to explain only the white
bands, not the glassy tubes, as dunes. We see these in many places -- there must
be thousands of examples by now. SPSR geologists ruled out dunes early-on as a
viable explanation because of considerations such as these:
(a) "Winds" on Mars are insufficient to build large dunes. The
surface atmosphere on Mars is roughly 1% that on Earth, and Earth's is in turn
only about 1% that on Venus. The atmosphere on Venus is so thick that a 7-mph
wind has hurricane force. Analogously, winds on Mars would have to reach
hurricane speeds to be felt as the gentlest of breezes. They would require
substantially greater speeds to develop enough lift to perform large
dune-building on otherwise flat terrain.
Jet-stream speeds might arise at higher altitudes, as they do on Earth; but are
least likely at the surface.
(b) Below-surface winds require channeling. The white bands are often
seen in cracks and fissures. These ought to be sheltered from surface winds. The
exception would be a case where a canyon had a wide mouth at one end that could
collect a large volume of wind and channel it into a much narrower canyon.
However, the cracks and fissures containing white bands are not of that
(c) The white bands are perpendicular to adjacent features. Where the
white bands are seen on the surface of Mars (the "crenulations"; e.g.,
Fig. A), they
often align perpendicular to adjacent surface features. It is easy so see how
sand might be blown up against a surface feature and pile up there, but such
piles would be elongated along the surface feature. The surface feature might
perform a one-sided channeling of the winds, but that is inconsistent with
individual white bands showing no width, height, or spacing variation with
distance from the surface feature.
(d) The white bands have appreciably higher albedo than the surface
material around them. Dunes ought to be made of the same material as the
nearby surface, but wind-blown and deposited in wave-like patterns. However, the
same material should cover all nearby terrain. But the actual white bands are
sharply brighter than the material between them, and in some cases brighter than
anything on the surrounding terrain.
(2) The glassy tubes. In a few places where the surface is cracked or
where erosion has exposed what was previously buried, we see the white bands as
markings on, or bands around, glassy tube-like structures (e.g., Fig. B). The "dunes"
hypothesis does not explain these features.
(a) The glassy tubes have distinct outlines. Nothing
about the "dunes" hypothesis requires that the extremities of the
dunes be connected by an outline, yet the "flat view" interpretation
of the glassy tubes is that they are outlines paralleling or connecting the
extremities of the dunes.