[meteorite-list] Mars Odyssey THEMIS Images - April 15-19, 2002

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:28 2004
Message-ID: <200204191713.KAA28167_at_zagami.jpl.nasa.gov>

MARS ODYSSEY THEMIS IMAGES
April 15-19, 2002

o Eastern Floor of Holden Crater (Released 15 April 2002)
  http://themis.la.asu.edu/zoom-20020415a.html

o Medusae Fossae Formation (Released 16 April 2002)
  http://themis.la.asu.edu/zoom-20020416a.html

o Holden Crater/Uzboi Valles (Released 17 April 2002)
  http://themis.la.asu.edu/zoom-20020417a.html

o Bosporus Planum (Released 18 April 2002)
  http://themis.la.asu.edu/zoom-20020418a.html

o White Rock (Released 19 April 2002)
  http://themis.la.asu.edu/zoom-20020419a.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)
Eastern Floor of Holden Crater (Released 15 April 2002)

The Story

With its beautiful symmetry and gullies radially streaming
down to the floor, the dominant crater in this image is an
impressive focal point. Yet, it is really just a small crater
within a much larger one named Holden Crater. Take a look at
the context image to the right to see just how much bigger
Holden Crater is. Then come back to the image strip that
shows the mottled surface of Holden Crater's eastern floor in
greater detail, and count how many hills, ridges, channels, and
small impact craters can be seen. No perfectly smooth terrain
abounds there, that's for sure.

The textured terrain of Holden Crater has been particularly
intriguing ever since the Mars Orbital Camera on the Mars
Global Surveyor spacecraft found evidence of sedimentary rock
layers there that might have formed in lakes or shallow seas
in Mars' ancient past. This finding suggests that Mars may
have been more like Earth long ago, with water on its surface.
Holden Crater might even have held a lake long ago. No one
knows for sure, but it's an exciting possibility. Why?

If water was once on the surface of Mars long enough to form
sedimentary materials, maybe it was there long enough for
microbial life to have developed too. (Life as we know it just
isn't possible without the long-term presence of liquid
water.) The question of life on the red planet is certainly
tantalizing, but scientists will need to engage in a huge
amount of further investigation to begin to know the answer.
That's why orbital images of Holden Crater like this one are
so important. They continue to help scientists piece together
the answers to their fundamental questions about the planet's
environment and its potential as a past or present habitat for
life.

The Science

Today's THEMIS image covers territory on the
eastern floor of Holden Crater, which is located
in region of the southern hemisphere called
Noachis Terra. Holden Crater is 154 km in
diameter and named after American Astronomer
Edward Holden (1846-1914). This image shows
a mottled surface with channels, hills, ridges and
impact craters. The largest crater seen in this
image is 5 km in diameter. This crater has
gullies and what appears to be horizontal layers
in its walls.

This image is the 14th image in a series of daily
images released by the THEMIS Team.

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Mars 2001 Odyssey
Thermal Emission Imaging System (THEMIS)
Medusae Fossae Formation (Released 16 April 2002)
                      
The Story

"Yardang!"

Now, that may seem like a peculiar-sounding curse word, but
nobody would get in trouble for using it. A yardang is one of the
very cool-sounding words geologists use to describe long,
irregular features like the ones seen in this image. Yardangs
are grooved, furrowed ridges that form as the wind erodes
away weakly cemented material in the region. Rippling across
the surface, yardangs tell the story of how the powerful
Martian wind carved the surface into such a gorgeous pattern
over time. (Don't miss clicking on the above image to see a
detailed view, in which the beauty and almost dance-like
symmetry of the waving terrain pops out in highly compelling,
three-dimensional texture.)

It may be easy to see which way the wind blows in this area,
since these streamlined features point in the direction of
prevailing winds. But how can geologists understand the
various kinds of terrain seen here? First, they have to study
the different patterns of erosion, looking closely at how the
wind has stripped off certain layers and not others.

Want to be a geologist yourself? Start at the bottom of the
image and scroll upward, and see how the relatively smooth,
higher terrain toward the south gradually becomes more and
more eroded. Moving up the image, at first you'll see only a
few, isolated regions of parallel ridges and knolls. Go a little
farther north with your eyes (toward the center of the image),
and you'll see how these linear knobs really get going! Once
you get to the top of the image, only patches of these grooved
ridges remain, leaving an incredibly smooth, wind-scrubbed
surface behind. You know this layer has to be made of pretty
hard material, because it seems impervious to further erosion.

Geologists studying Mars can compare these Martian
yardangs to examples found on Earth, such as those in the Lut
desert of Iran. Humans have even been known to use the wind
as their inspiration, sculpting the shape of yardangs
themselves. The famous sphynx at Giza in Egypt is thought to
be a yardang that's been whittled down a little more by ancient
human chiselers.

The Science

This THEMIS visible image was acquired near
11° N, 159° W and shows examples of the
remarkable variations that can be seen in the
erosion of the Medusae Fossae Formation.
This Formation is a soft, easily eroded deposit
that extends for nearly 1,000 km along the
equator of Mars. In this region, like many others
throughout the Medusae Fossae Formation, the
surface has been eroded by the wind into a
series of linear ridges called yardangs. These
ridges generally point in direction of the
prevailing winds that carved them, and
demonstrate the power of martian winds to
erode the landscape of Mars. The easily eroded
nature of the Medusae Fossae Formation
suggests that it is composed of weakly
cemented particles, and was most likely formed
by the deposition of wind-blown dust or
volcanic ash. Within this single image it is
possible to see differing amounts of erosion and
stripping of layers in the Medusae Fossae
Formation. Near the bottom (southern) edge of
the image a rock layer with a relatively smooth
upper surface covers much of the image. Moving
upwards (north) in the image this layer
becomes more and more eroded. At first there
are isolated regions where the smooth unit has
been eroded to produce sets of parallel ridges
and knobs. Further north these linear knobs
increase in number, and only small, isolated
patches of the smooth upper surface remain.
Finally, at the top of the image, even the ridges
have been removed, exposing the remarkably
smooth top of hard, resistant layer below. This
sequence of layers with differing hardness and
resistance to erosion is common on Earth and
on Mars, and suggests significant variations in
the physical properties, composition, particle
size, and/or cementation of these martian
layers. As is common throughout the Medusae
Fossae Formation, very few impact craters are
visible, indicating that the surface exposed is
relatively young, and that the process of erosion
may be active today.

This image is the 15th image in a series of daily
images released by the THEMIS team.

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Mars 2001 Odyssey
Thermal Emission Imaging System (THEMIS)
Holden Crater/Uzboi Valles (Released 17 April 2002)

The Story

Mars doesn't have a shortage of rugged terrain, and this area
is no exception. While things look pretty quiet now, this
cratered region was once the scene of some tremendous
action. Long ago in Martian history, an incoming meteroid
probably smashed into the planet and produced a giant impact
crater named Holden Crater, which stretches 88 miles across
the Martian surface. The history of the area around Holden
Crater doesn't stop there. At some point, a catastrophic flood
burst forth on the surface, forming an impressive outflow
channel called Uzboi Valles. No one knows exactly how that
happened, or whether the water might even have rushed into
Holden Crater at some point, forming a long-ago lake. What
we do know is that there is a lot of sedimentary material that
could have formed in two hypothesized ways: in an ancient
lake environment or as volcanic-ash deposits.

Scientists are searching for the answers by studying the
region where Uzboi Valles meets the crater. You can see the
rough edge of Holden Crater running diagonally down in a
sharply edged swath (from the top left-hand corner of this
image to the center right-hand side). Just below it, running
almost smoothly down the right-hand side of the image is an
intriguing channel where water may once have flowed. Much of
the terrain in the bottom half of the image, in fact, seems to be
cut into a swish-swash of dissected sedimentary terrain.
Sliced through in such a way, the terrain ends up carrying
bunches of small, rounded hills called "hummocks." Earth can
boast of its own rolling, hummocky terrain too, such as that
found in the ravine-cut Missouri Hills and High Plains areas
of South Dakota.

The Science

This image, located near 27.0S and 35.5W,
displays the intersection of Holden Crater with
Uzboi Valles. This region of Mars contains a
number of features that could be related to liquid
water on the surface in the Martian past. Holden
Crater contains finely layered sedimentary units
that have been subsequently dissected. The
hummucky terrain in the bottom half of the image
is the remnants of this terrain, though the fine
layers are not visible in this image at this
resolution. The sedimentary units could have
formed through deposition of material in a
lacustrine type environment. Alternately, these
layers could also be volcanic ash deposits.
Uzboi Valles, which enters the crater from the
southwest, is a catastrophic outflow channel
that formed in the Martian past. The streamlined
nature of the topographic features at the
intersection of the crater with Uzboi Valles
record the erosional pattern of flowing liquid
water on the surface of Mars during the episodic
outflow event.

This image is the 16th image in a series of daily
images released by the THEMIS team.

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Mars 2001 Odyssey
Thermal Emission Imaging System (THEMIS)
Bosporus Planum (Released 18 April 2002)

The Story

Splat! Take a look at the lumpy edge of the large crater half
(left-hand side of the image) and compare it to the much
neater rims of other craters in the region. Why is there such a
difference? Scientists believe that when something hit the
surface of Mars long ago, ice may have been present in the
subsurface and was "regurgitated" upward into the Martian air
along wih dirt and rock, "splooshing" outward. When that
happened, the mixed-up, ejected material created a wavering,
batter-like edge that is not typical for most (ice-free) craters.
More ejected material from this same impact radiates much
farther out from the crater, giving it a vague, sun-like
appearance.

Many of the small craters in this image appear much fainter
and more subdued than the others. Their ghostly appearance
may be due to a lava flow that smoothed out most of the
terrain in this image, partially burying them . . . . Or???? Maybe
it was a layer of dust that settled in this region to accomplish
the same concealed look.

And what about that scar-like trek that cuts through the upper
third of the image? It's an elongated fault created when a
crust-breaking, tectonic force ripped apart the Martian terrain,
leaving a long depression on the surface. This feature is called
a graben, and we find them on Earth too (think of Death
Valley, the lowest dry land in the United States, or the Jordan
Dead Sea depression). The graben's rumpled, scar-like
appearance is only enhanced by the stitchy-looking sand
dunes that run down its sides. This dune pattern shows that
the Martian wind probably blew down through the graben
canyon to create their ruffled appearance.

The wind doesn't have its way everywhere, though. The
brighter surface material on the western side of the two
diagonally positioned smaller craters is probably a layer of
dust that has been shielded from removal by the craters'
higher rims. Dark streaks (possibly dark sand) on the
opposite side of these craters reveal that the wind has been
blowing to no avail in the opposite direction too.

So, think that explains everything in this image? Here's a
quick geology quiz! Which features happened first? The
dunes, the lava plains, the big crater, or the linear
depression called a graben? To find out if you're right, check
out the last paragraph in The Science caption. Hint! Whatever
happened later has to be on top of whatever came before.

The Science

This THEMIS image is of Bosporus Planum,
located in a region of smooth plains that appear
to have formed from lava flows. A crater, ~7 km
in diameter, on the left edge of the image has
produced an ejecta blanket that can be seen
radiating from the crater. Lobes of ejecta such as
those seen close to the crater rim are not formed
at most typical craters and may indicate that
there was a ice component in the sub-surface
material when the impact occurred. A linear
depression trending from the northwest to
southeast along the top of the image is about 1
to 2 km wide. This may be a tectonic feature,
known as a graben, that forms when a region is
under stresses that are pulling it apart. There
are numerous small bright dunes or ripples along
the margins of the floor of this linear feature that
have formed perpendicular to the sides of the
graben. This pattern of ripples suggests that the
wind was blowing down the graben canyon.
Similar small bright dunes can be faintly seen on
top of the crater ejecta along ridges (most
apparent directly to the east of the crater) and
along the southern margin of the interior
deposits in the crater. Bright wind streaks are
also apparent in this area to the west (right) of
several large craters. These streaks likely
formed when very small particle size materials
(like dust) is deposited on the surface and then
protected from removal by the wind shadow
produced by the crater's rim. Shorter dark
streaks, possible deposits of dark sand, have
formed to the east side of the smaller craters.
These streaks on opposite sides of craters may
indicate that there have been different wind
patterns in the area, blowing in opposite
directions. Subtle ridges near the south end of
the image hint that there may have been other
graben that have been nearly filled in. Many of
the craters in this image have a subdued, buried
appearance and may have been partially filled by
lava flows or mantled by dust.

A short geologic history of the area in this image
can be created using the basic principles of
geology, such as the principle of superposition
(deposits that lie on top of other materials are
younger). The linear depression must have
formed after the deposition of the lava plains
since it is a feature that would not have been
otherwise preserved. Ejecta from the large
crater has been deposited inside and over the
edges of the linear depression, thus the crater
must have formed after the linear depression.
Finally, the bright dunes and dust streaks
formed last because they have been deposited
on top of all of these different features.

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Mars 2001 Odyssey
Thermal Emission Imaging System (THEMIS)
White Rock (Released 19 April 2002)
                     
The Story
Fingers of hard, white rock seem to jut out like icy daggers
across a moody Martian surface, but appearances can be
deceiving. These bright, jagged features are neither white, nor
icy, nor even hard and rocky!

So what are they, and why are they so different from the
surrounding terrain?

Scientists know that you can't always trust what your eyes
see alone. You have to use other kinds of science instruments
to measure things that our eyes can't see . . . things like
information about what kinds of minerals make up the
landforms. Mars scientists once thought, for instance, that
these unusual features might be vast hills of salt, the dried up
remains of a long-ago, evaporated lake.

Not so, said an instrument on the Mars Global Surveyor
spacecraft, which revealed that the bright material is probably
made up of volcanic ash or windblown dust instead. And talk
about a cyclical "ashes to ashes, dust to dust" story! Particles
of this material fell and fell until they built up quite a
sedimentary deposit, which was then only eroded away again
by the wind over time, leaving the spiky terrain seen today. It
looks white, but its apparent brightness arises from the fact
that the surrounding material is so dark.

Of course, good eyesight always helps in understanding. A
camera on Mars Global Surveyor with close-up capabilities
revealed that sand dunes are responsible for the smudgy dark
material in the bright sediment and around it. But that's not
all. The THEMIS camera on the Mars Odyssey spacecraft
that took this image reveals that this ashy or dusty deposit
once covered a much larger area than it does today. Look
yourself for two small dots of white material on the floor of a
small crater nearby (center right in this image). They preserve
a record that this bright deposit once reached much farther.
Since so little of it remains, you can figure that the material
probably isn't very hard, and simply blows away.

One thing's for sure. No one looking at this image could ever
think that Mars is a boring place. With all of its bright and
dark contrasts, this picture would be perfect for anyone who
loves Ansel Adams and his black-and-white photography.

The Science

"White Rock" is the unofficial name for this
unusual landform which was first observed during
the Mariner 9 mission in the early 1970's. As
later analysis of additional data sets would
show, White Rock is neither white nor dense
rock. Its apparent brightness arises from the fact
that the material surrounding it is so dark.
Images from the Mars Global Surveyor MOC
camera revealed dark sand dunes surrounding
White Rock and on the floor of the troughs within
it. Some of these dunes are just apparent in the
THEMIS image. Although there was speculation
that the material composing White Rock could be
salts from an ancient dry lakebed, spectral data
from the MGS TES instrument did not support
this claim. Instead, the White Rock deposit may
be the erosional remnant of a previously more
continuous occurrence of air fall sediments, either
volcanic ash or windblown dust. The THEMIS
image offers new evidence for the idea that the
original deposit covered a larger area.
Approximately 10 kilometers to the southeast of
the main deposit are some tiny knobs of similarly
bright material preserved on the floor of a small
crater. Given that the eolian erosion of the main
White Rock deposit has produced isolated knobs
at its edges, it is reasonable to suspect that the
more distant outliers are the remnants of a once
continuous deposit that stretched at least to this
location. The fact that so little remains of the
larger deposit suggests that the material is very
easily eroded and simply blows away.
Received on Fri 19 Apr 2002 01:13:38 PM PDT