[meteorite-list] Mars Odyssey Themis Images - April 22-26, 2002

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:29 2004
Message-ID: <200204270406.VAA07576_at_zagami.jpl.nasa.gov>

MARS ODYSSEY THEMIS IMAGES
April 22-26, 2002

o Alba Patera (Released 22 April 2002)
  http://themis.la.asu.edu/zoom-20020422a.html

o Eastern Floor of Holden Crater (Released 15 April 2002)
  http://themis.la.asu.edu/zoom-200204.html

o Cerberus (Released 24 April 2002)
  http://themis.la.asu.edu/zoom-20020424a.html

o Knobby terrain in Northern Arabia Terra (Released 25 April 2002)
  http://themis.la.asu.edu/zoom-20020425a.html

o Noctis Labyrinthus (Released 26 April 2002)
  http://themis.la.asu.edu/zoom-20020426a.html

All of the THEMIS images are archived here:

http://themis.la.asu.edu/latest.html

NASA's Jet Propulsion Laboratory manages the 2001 Mars Odyssey mission
for NASA's Office of Space Science, Washington, D.C. The Thermal Emission
Imaging System (THEMIS) was developed by Arizona State University,
Tempe, in collaboration with Raytheon Santa Barbara Remote Sensing.
The THEMIS investigation is led by Dr. Philip Christensen at Arizona State
University. Lockheed Martin Astronautics, Denver, is the prime contractor
for the Odyssey project, and developed and built the orbiter. Mission
operations are conducted jointly from Lockheed Martin and from JPL, a
division of the California Institute of Technology in Pasadena.

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Mars 2001 Odyssey
Thermal Emission Imaging System (THEMIS)
Alba Patera (Released 22 April 2002)
              
The Science

This image, centered near 46.5 N and 119.3 W
(240.7 E), is on the northwestern flank of a
large, broad shield volcano called Alba Patera.
This region of Mars has a number of unique
valley features that at first glance look dendritic
much in the same pattern that rivers and
tributaries form on Earth. A closer look reveals
that the valleys are quite discontinuous and
must form through a different process than
surface runoff of liquid water that is common on
Earth. A number of processes might have taken
place at some point in the Martian past to form
these features. Some of the broad valley
features bear some resemblance to karst
topography, where material is removed
underground by melting or dissolving in
groundwater causing the collapse of the surface
above it. The long narrow valleys resemble
surfaces where groundwater sapping has
occurred. Sapping happens when groundwater
reaches the surface and causes headward
erosion, forming long valleys with fewer
tributaries than is seen with valleys formed by
surface water runoff. The volcano itself might
have been a source of heat and energy, which
played a role in producing surfaces that indicate
an active groundwater system.

The Story

Fluid, oozing lava poured somewhat lazily over this area long
ago. It happened perhaps thousands of times, over hundreds of
thousands of Martian years, creating the nearly smooth,
plaster-of-Paris-looking terrain seen today. (Small craters
also dent the area, though they may deceive you and look like
raised bumps instead. That's just a trick of the eye and the
lighting - tilt your head to your left shoulder, and you should
see the craters pit the surface as expected.)

The lava flows came from a Martian "shield" volcano named
Alba Patera. Shield volcanoes get their name from their
appearance: from above, they look like large battle shields lying
face up to the sky as if a giant, geological warrior had lain them
down. Perhaps one did - if you think of a volcano as a "geologic
warrior," that is. These volcanoes aren't too fierce, however.
Because of the gentle layering of lava over time, they don't
stand tall and angry against the horizon, but instead have
relatively gentle slopes and are spread out over large areas.
(On Earth, the Hawaiian Islands are examples of shield
volcanoes, but you can't see much of their expanse, since they
rise almost three miles from the ocean floor before popping out
above the water's surface.)

What's most interesting in this picture are all of the branching
features that lightly texture the terrain. The patterns may look
like those caused by rivers here on Earth, but geologists say
that no surface streams on Mars were responsible. That's no
disappointment, however, to those who'd like to find water on
Mars, because there are still intriguing water-related
possibilities here.

Some of the broad valley features in this image look like karsts,
a terrain found on Earth in Karst, a limestone area on the
Adriatic Sea in modern-day Croatia, and in other world regions
including France, China, the American Midwest, Kentucky, and
Florida. Karst terrain on Earth is barren land with all kinds of
caves, sinkholes, and underground rivers that excavate the
subsurface, causing the surface above it to collapse. So,
perhaps it's like that in this region on Mars as well. Future
Martian spelunkers should be excited, because most caves on
Earth are in karst areas.

Other suggestions of water here are some long, narrow valleys
that resemble Earth surfaces where groundwater has sapped
away the terrain. Sapping occurs when groundwater erodes
slopes, creating valleys. Water action can be concentrated at
valley heads, leading to what is called their "headward growth."
That may be what has happened here on Alba Patera as well.

All of these features suggest the action of liquid water, but
Mars is so cold, you might wonder if any water would have to
be as frozen as the world it is on. Well . . . that depends!
Remember that this area is part of a volcano, and volcanoes can
put out enough heat and energy below the surface to keep
water warm enough to flow - if not now, then at least in the
past when the volcano was more active.


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http://themis.la.asu.edu/zoom-20020423a.html

Mars 2001 Odyssey
Thermal Emission Imaging System (THEMIS)
A Cloudy Day on Mars (Released 23 April 2002)
 
              
The Science

This image, centered near 49.7 N and 43.0 W
(317.0 E), displays splotchy water ice clouds
that obscure the surface. Most of Mars was in
a relatively clear period when this image was
acquired, which is why many of the other
THEMIS images acquired during the same
period do not have obvious signs of
atmospheric dust or water ice clouds. This
image is far enough north to catch the edge of
the north polar hood that develops during the
northern winter. This is a cap of water ice and
CO2 ice clouds that form over the Martian
north pole. Mars has a number of interesting
atmospheric phenomena which THEMIS will be
able to view in addition to water ice clouds,
including dust devils, dust storms, and tracking
atmospheric temperatures with the infrared
camera.

The Story

Anyone who's been on an airplane in a storm knows how
clouds on Earth can block the view below. The thin water ice
clouds on Mars might make things slightly blurry, but at least
we can still see the surface.

While the surface features may not be as clear in this image,
it's actually kind of fascinating to see clouds at work, because
we can get a sense of how the north pole on Mars influences
the weather and the climate. In this image, the north pole is
responsible for the presence of the clouds. Made of water ice
and carbon dioxide, these clouds "mist out" in a atmospheric
"hood" that caps the surface during the northern Martian
winter, hiding it from full view of eager observers here on Earth.

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Mars 2001 Odyssey
Thermal Emission Imaging System (THEMIS)
Cerberus (Released 24 April 2002)

The Science
                      
The Cerberus feature is a relatively dark region
at the southeastern edge of the huge Elysium
Mons volcanic complex. It was visible to early
astronomers of Mars because it was a
distinctive dark spot on a large bright region of
the planet. Today we recognize that the
Cerberus region encompasses a range of
geologic terrains from relatively young and
smooth lava flows to the very rugged, ancient
eroded landscape seen in this THEMIS image.
The Cerberus feature has also proven to be
ephemeral. Compared to just 20 years ago when
the Viking orbiter instruments viewed the
planet, the Cerberus feature has shrunk down
from its original length of roughly 1000
kilometers to just a few isolated dark splotches
of just a few 100 kilometers. This is testament
to the active eolian environment on Mars where
global dust storms can lift and then later deposit
significant amounts of dust, brightening formerly
dark surfaces. The THEMIS image occurs in a
portion of Cerberus that remains relatively dark
and dust-free although in the bottommost
portion of the image are faint, criss-crossing
lines that likely are dust devil tracks. The
abundant dune-like features covering many of
the low, smooth surfaces are similar to those
found in many places across the planet. They
are evidence of the interaction of wind and
movable particles at the surface but not
necessarily in today's environment. In many
other places on Mars they are clearly inactive;
relicts of a different climate.

The Story

Hellhound of Greek mythology, Cerberus was the
three-headed, dragon-tailed dog that stood guard at the
opening to the underworld. This rough-and-tumble Mars
terrain looks just as fierce and foreboding. At the edge of the
huge Elysium Mons volcano complex, the Cerberus area
appeared as a dark spot to early Mars astronomers in an
otherwise bright region of the planet. If this dark area seems
somewhat hellish to your imagination too, you'll be glad to
know that the Martian wind has been brightening up the area.

Just twenty years ago, the Viking orbiters reached Mars for the
first long-term studies of Mars up close. The Cerberus feature
was then almost 600 miles long, but has now been vanquished
down to few small splotches about 60 miles long. Call that a
triumph of lightness upon the surface, but don't think that the
force bringing back the light is gentle and kind. The Martian
wind can kick up a fierce global dust storm that lifts up the
bright Martian dust into the air and then blankets the surface
with the brighter material as it settles down again.

The ancient, eroded terrain in this image is still rather dark and
dust free, so you might say it's one area where a mythical
Cerberus still guards its shrinking territory. The wind teases it,
however, by kicking up small, whirling dust devils that leave
long, dark, scratchy tracks upon the land. Fields of dunes
wrinkle the surface in places as well, but they may be
permanently cemented upon the surface now, no longer able to
blow and drift as they did in their younger days.

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Mars 2001 Odyssey
Thermal Emission Imaging System (THEMIS)
Knobby terrain in Northern Arabia Terra (Released 25 April 2002)

The Science
              
This THEMIS visible image shows a region in
northern Arabia Terra near 44° N, 322° W (38°
E). Knobby or "scabby" plains units that mantle
and modify a pre-existing cratered surface
dominate the unusual landscape in this region.
Several large (5-8 km diameter) impact craters
seen in the upper left of the image have been
extensively modified since their initial
formation. The rims of these craters can still be
seen, but the ejecta deposits and the
surrounding plains have been buried by a layer
of material. This mantling layer has itself been
modified to produce a pitted, knobby surface.
Circular depressions of all sizes, presumably
the remnants of impact craters, are filled with
smooth deposits. In some places large regions
have been covered by this smooth material; an
example can be seen in the lower right portion
of this image. In many cases the impact craters
have been extensively modified prior to their
being filled. This modification indicates an
erosion process that has removed material from
the walls to produce shapes that vary from
circular with crisp rims, to circular with no rims,
to oblong and elliptical forms, and finally to
irregular shapes whose initial circular outline
can barely be detected. The slope of the channel
at the top of the image has an unusual deposit
of material that occurs preferentially on the
cold, north-facing slope. Similar deposits are
seen frequently at mid- northern and southern
latitudes on Mars, and have a characteristic,
rounded boundary that typically occurs at
approximately the same distance below the
ridge crest. It has been suggested that these
deposits once draped the entire surface and
have since been removed from all but the cold
north-facing slopes. The presence and removal
of ground ice may play an important role in the
formation of this layer, as well as the knobby
terrain and unusual features seen in this image.

This image is the 22nd image in a series of
daily images released by the THEMIS Team.

The Story

There's no way these impact craters are in their original,
pristine shape. Check out their strange deformities and register
the geological gross-out factor of all the "scabs" upon the land.
You can still see the rims of craters in this savaged land, but an
aggressive layer of material once spread out across it, burying
the ejected material and all the surrounding plains. This
cloaking layer didn't win the battle of dominance, however, as it
too has been battered over time, producing the pitted, knobby
surface seen today.

Only a few smooth deposits in the area are spared from the
scabby, scarred look of the long barraged (see lower right
portion of the image). Circular depressions, the probable
remains of impact craters, are filled with this smooth material.
Some were already well eroded prior to being filled, with
material removed from their walls used to sculpt the varying
shapes.

The dark, shadowed channel at the top of this image has an
unusual deposit of material on its cold, north-facing slope.
Since this material is found elsewhere on Mars, at
approximately the same distance below the ridge crest, could it
have draped the entire surface of Mars long ago? Why has it
been lost from all but the northern slopes? Could ice in the
ground play a role in forming and preserving this layer? And did
it craft the knobby terrain and other strange features in this
area?

These are the kinds of questions geologists are asking. As this
image proves, the more you discover, the more questions you
have. That's what keeps exploration so exciting.

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Mars 2001 Odyssey
Thermal Emission Imaging System (THEMIS)
Noctis Labyrinthus (Released 26 April 2002

The Science

This image shows a portion of Noctis
Labyrinthus, a large valley system at the
western end of the Valles Marineris canyon
system. Noctis Labyrinthus is notable for its
unusual pattern of intersecting valleys, which
give the region a maze-like appearance when
viewed from above. The walls of these valleys
are very high (~5 km) and quite steep, with
slopes approaching 35°. Dust covers most of this
region, leading to its rather uniform appearance.
At the tops of the ridgelines, small dark streaks
can be observed trailing downslope; these
streaks suggest that the sediments covering this
area occasionally become unstable and slide.
Ridges of resistant material also can be observed
in the highest terrains. In the lower half of the
image, a small linear feature appears to cut
across the generally NE/SW-trending slopes.
This feature is not continuous, indicating that
geologic activity has disrupted it since its
formation. The northeastern termination of the
feature is on a mesa, where it is joined by a less
pronounced but similar feature that trends
NE/SW. These small features may have
originated in several ways: they may be ridges
formed by compression, they may be small fault
scarps, or they may represent the edges of
ancient lava flows that have been disrupted by
the formation of the valley system.

The Story

The smoothly sculpted surface in this close-up image belies
the bizarre and twisted Martian landscape of which it is a part
(seen at a larger scale in the context image). Labyrinths have
long been in the human imagination, and it's no wonder that
this area conjured up for early viewers all of the legends of
antiquity, of a land where a Minotaur hides and a conquering
hero needs a spool of thread to guide him through an inner
maze.

As writer Jorge Luis Borges might say, this Martian region is
a real-life example of a geological "garden of forking paths," a
dangerous-seeming place where "the paths of the labyrinth
converge." Noctis Labyrinthus, as it's called, is an area of
sprawling, intersecting valleys on Mars, and like a Borgesian
story, holds within it elusive truths about the passage of time
and a multi-layered landscape of possibility.

At the western end of Valles Marineris, the largest canyon in
the solar system, Noctis Labyrinthus holds the secrets to
long-term geologic change in the area. It would be easy to
lose oneself on a wandering path through the terrain. The
walls of this Martian valley maze rise swiftly and steeply to
their three-mile heights, and a layer of long-settled dust
deceives the eye, making everything look the same. (Well,
almost everything. Look closely, and some of realities of the
labyrinth are revealed.)

>From the tops of ridge lines, small, dark streaks trail down
the sides, leaving scant but clear evidence of the sediment
that once slid downslope. A long, jagged slash cuts the land
(lower third of the image), but is broken in the middle by some
unknown geologic force that moved the land through it,
"erasing it" sometime later. And then the mysteries: what are
the features seen in this image and how did they come to be?
Ridges formed by compression? Small cliff lines ("scarps")
caused by faults? Or perhaps the edges of ancient lava flows,
disrupted by the formation of the valley system?

Whatever they are, they represent well the strange and
misleading passageways of legend and lore, where the way to
the truth of the matter and back again is hard to find.
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Received on Sat 27 Apr 2002 12:06:25 AM PDT