- Subject: More handwaving?
- Date: 3-9-01
- From: Dave Pieri
- To: rcollins
- Dear Robert
- This is a professional interpretation based on having
labored over literally thousands of Viking Mars Orbiter Images, hundreds
of Mars Observer images, many, many, many Landsat, ASTER, and airphotos
of the earth, having studied aeolian and fluvial geomorphology at the graduate
level at two universities, and well over 20 years of aerial and orbital
photointerpretation and optical and radar remote sensing here at JPL/Caltech--and
years of field experience in a range of climatic zones from desert to arctic.
Also, the subject of my Ph.D. thesis was (the first) systematic global
study of martian valley networks, in which I looked at thousands of features,
such as the one in the MGS image, over a period of years.
- Realize one critical fact--these are not "tubes"...the
positive relief is an ILLUSION--the area is crossed by concave valleys.
The transverse dune trains are emplaced on the valley floors--this is not
new, either or Mars or the Earth. It even occurred at very small scale
in the Viking Lander 1 site in Chryse Planitia...not all that far away
from where this MGS picture was taken.
- Look carefully at how the craters in the larger reference
image are lit. Then look at the valley wall shadows. Seeing inverted relief
is an easy mistake to make, even by an experienced photointerpreter--at
first glance. That's why it's always important to check one's first impression
against obvious features in the landscape to get oriented properly.
- Also, the picture on the "Enterprise" web site
- Way overstretched, saturated, and doesn't show other
relevant parts of the scene. I downloaded the original data at full resolution
from Mike Malin's web site in La Jolla, then used a square-root stretch
to even out the contrast. It's clear that these crescentic dune-lets populate
the landscape--and occur in all the low troughs or valleys throughout the
image (take a look at the subscene attachment). They are a variety of sief
and nearly barchan dunes--dead ringer analogs to those in terrestrial deserts.
Turn the image 180 degrees (vs. the "Enterprise" picture) so
that valleys look like valleys and craters look like craters, not bumps.
The 3D striated "glass worm" illusion vanishes, and we're left
with a fairly mundane image, exciting only to those of us who have studied
the morphology of the valley networks on Mars.
- Dunes follow aerodynamic principles--therefore they DO
often line up in really precise, repetitive ways related to aero/hydrodynamic
streamlines and standing wave resonances.
- ****** David C. Pieri, Ph.D. Earth and Space Sciences
Division Mail Stop 183-501 Jet Propulsion Laboratory 4800 Oak Grove Drive
Pasadena, CA 91109 USA FAX: 818-354-0966 VOICE: 818-354-6299 firstname.lastname@example.org