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Spheres coming from bedrock?



 
 
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  #21  
Old February 15th 04, 04:37 AM
TheonFrm4
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Default Spheres coming from bedrock?

i call that the mirror sindrome

  #22  
Old February 15th 04, 04:48 AM
TheonFrm4
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Default Spheres coming from bedrock?

the spluerukes you mention come from forces iam still try to figure ou but are
more Powerful then we would to ube iwas veiwing shot i took scared the----out
of me cause i think i know were they came look at faces
  #23  
Old February 16th 04, 05:01 PM
Timothy Demko
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Default Spheres coming from bedrock?

Joe Knapp wrote:
"Timothy Demko" wrote

http://www.jpl.nasa.gov/mer2004/rove...s/image-1.html

suggests that even if it is an ash, it's been transported by some sort
of traction currents and is not a simple air fall.



Cross beds I take it simply means that the layers are not always
parallel--they diverge and converge sometimes? If so, how about this photo
of the effect of "volcanic bombs":
http://www.geo.hunter.cuny.edu/~cwang/impacts.jpg Wouldn't the layers in
that photo be a simple air fall?


The presence of cross beds means something more than the beds just not
being parallel. The beds in the link you provided would be called
deformed beds rather than cross beds. Cross beds formed by the migration
of bedforms (ripples, dunes, etc.) and are a primary featu they form
during, and are linked to, deposition. These bedforms may be subaerial
(eolian) or subaqueous. In deformed beds, the deformation event, in your
example the impact of a falling volcanic bomb, happens after the
deposition of the beds.

It's interesting that last week Squyres said there were three main theories
for the origin of the spherules: lapilli, solidified volcanic material, or
concretions. He said the lapilli idea was "fading fast" on the logic that
they look to be of a different material than the matrix. There has not been
a word in the press conferences since about any progress along those lines.

However, an interview with Ronald Greeley, Ph.D., Regents Prof. of Geology,
Arizona State University Dept. of Geological Sciences, and one of the 7
Chairs of the Science Operations Working Group (SOWG) for Spirit and
Opportunity, Tempe, Arizona this Wednesday was a bit surprising. When asked
about the spherules, Prof. Greeley said:

"The general consensus seems to be centering around two possibilities:

"First, that they are what are called 'accretionary lapillae' * that is a
volcanic feature that forms in certain kinds of eruptions in which material
is added to a small nucleus like a little dust grain and these form these
spherule masses."

"Secondly, they could be some kind of precipitate * perhaps from water. In
additional findings, there are some minerals that have been identified in
the infrared that would suggest the presence of water since they form in
water. In particular, if this is a volcanic terrain that we are looking at,
the presence of these minerals might suggest this was a hydrothermal area."

What--lapilli are fading back in and the other two theories (magma &
concretions) are gone, to be replaced with "some kind of precipitate"?


No, I think he is talking about concretions or nodules. The confusion is
in that most people are assuming that the precipitation was happening in
a water column or on a lake/sea bed. I think what Greeley means here is
that he thinks they formed by prepitation from water in the subsurface
(pore waters), and he is leaning towards a hydrothermal origin for the
water, and maybe mineralization too.

snip

AND ALL THIS IS ADDING UP TO A QUESTION ABOUT WHETHER THE PLACE WHERE
OPPORTUNITY CAME DOWN ORIGINALLY MIGHT HAVE BEEN HOT MAGMA COMING UP TO THE
SURFACE IN SOME KIND OF VOLCANIC ACTIVITY? OR AT LEAST HOT SPRINGS?

"Yes. The evidence is sort of pointing towards the presence of a hot
environment * hot for some reason and that's under debate right now.
Volcanic activity, magma close to the surface, is one possibility."

...

THE BIGGEST SURPRISE RIGHT NOW HAS BEEN THE HIGH AMOUNT OF SULFUR IN THE
BEDROCK?

"That was a surprise, yes. The measurements that have been made there
suggest there is a very high percentage of sulfur present in the material
and that there are these spherules that have been seen and reported earlier,
those are still very curious features."

"Such a high percentage of sulfur (in the bedrock) is unusual and this is
what leads some folks to consider the volcanic origin (at the Martian
bedrock), or at least volcanic processes to be involved. There are some
minerals that have been identified in the infrared that would suggest the
presence of water since they form in water. In particular, if this is a
volcanic terrain that we are looking at, the presence of these minerals
might suggest this was a hydrothermal area and sulfur is a component of such
environments."


If you look through the older Mars literature, the origin of soil
minerals (including sulfur minerals) seen at the Viking and Pathfinder
sites if often related to something called "palagonitization". This a
garbage can term for various geochemical processes involved in the
weathering and divitrification of basaltic glass (palagonite is an
amorphous to poorly crystalline, almost clay-like substance).
Palagonitization does involve hydration and it is common in hydrothermal
areas where hot thermal waters are circulating through volcanic and
volcaniclastic rocks.

If we read between lines then, it would seem that the MER project
scientists are leaning towards a palagonitized, volcaniclastic
(re-sedimented tuff/ash) origin for the bedrock, and a subsequent
diagenetic/hydrothermal alteration origin for the sulfur anomaly and
hematite-rich spherules.

Note that these processes may all have been related to the same event of
series of events. Maybe something like: 1) rise of magma through the
mantle and crust of Mars; 2) eruption of vents, tuff rings, and maybe
maar volcanoes; 3) melting of permafrost due to higher crustal heat flow
and creation of standing water surface water (lakes, ponds, streams); 4)
deposition of glass-rich, basaltic tuff in lakes and ponds; 5)reworking
of tuff by lacustrine shoreline and fluvial processes; 6) hydrothermal
alteration (palagonitization) of tuffs; 7) leaching of iron-rich
minerals in reworked tuff and re-precipitation as hematite in
spherules/concretions/nodules; 8) heat flow subsides, everything dries
out, winds return and erode lake bottom, various impacts; 9) Opportunity
lands (bunny comes to investigate); 10) speculation ensues.

What will be interesting is to see how the spherules are concentrated in
the residual soil above the light-colored bedrock. Comparing the
density of spherules in the soil to that in the bedrock may give a good
indication of how much of the bedrock has been eroded to create the soil.
--
Tim Demko
http://www.d.umn.edu/~tdemko


  #24  
Old February 18th 04, 01:40 PM
Joe Knapp
external usenet poster
 
Posts: n/a
Default Spheres coming from bedrock?


"Timothy Demko" wrote
Cross beds I take it simply means that the layers are not always
parallel--they diverge and converge sometimes? If so, how about this

photo
of the effect of "volcanic bombs":
http://www.geo.hunter.cuny.edu/~cwang/impacts.jpg Wouldn't the layers in
that photo be a simple air fall?


The presence of cross beds means something more than the beds just not
being parallel. The beds in the link you provided would be called
deformed beds rather than cross beds. Cross beds formed by the migration
of bedforms (ripples, dunes, etc.) and are a primary featu they form
during, and are linked to, deposition. These bedforms may be subaerial
(eolian) or subaqueous. In deformed beds, the deformation event, in your
example the impact of a falling volcanic bomb, happens after the
deposition of the beds.


Thanks for clarifying that. I managed to find this short USGS video which
helped me to visualize it:

http://tinyurl.com/ysoda


"Secondly, they could be some kind of precipitate * perhaps from water.

In
additional findings, there are some minerals that have been identified

in
the infrared that would suggest the presence of water since they form in
water. In particular, if this is a volcanic terrain that we are looking

at,
the presence of these minerals might suggest this was a hydrothermal

area."

What--lapilli are fading back in and the other two theories (magma &
concretions) are gone, to be replaced with "some kind of precipitate"?


No, I think he is talking about concretions or nodules. The confusion is
in that most people are assuming that the precipitation was happening in
a water column or on a lake/sea bed. I think what Greeley means here is
that he thinks they formed by prepitation from water in the subsurface
(pore waters), and he is leaning towards a hydrothermal origin for the
water, and maybe mineralization too.


OK, my misunderstanding as a google-ogist rather than a geologist. When I
hear precipitate I think of something falling, like through said column of
water.


snip

AND ALL THIS IS ADDING UP TO A QUESTION ABOUT WHETHER THE PLACE WHERE
OPPORTUNITY CAME DOWN ORIGINALLY MIGHT HAVE BEEN HOT MAGMA COMING UP TO

THE
SURFACE IN SOME KIND OF VOLCANIC ACTIVITY? OR AT LEAST HOT SPRINGS?

"Yes. The evidence is sort of pointing towards the presence of a hot
environment * hot for some reason and that's under debate right now.
Volcanic activity, magma close to the surface, is one possibility."

...

THE BIGGEST SURPRISE RIGHT NOW HAS BEEN THE HIGH AMOUNT OF SULFUR IN THE
BEDROCK?

"That was a surprise, yes. The measurements that have been made there
suggest there is a very high percentage of sulfur present in the

material
and that there are these spherules that have been seen and reported

earlier,
those are still very curious features."

"Such a high percentage of sulfur (in the bedrock) is unusual and this

is
what leads some folks to consider the volcanic origin (at the Martian
bedrock), or at least volcanic processes to be involved. There are some
minerals that have been identified in the infrared that would suggest

the
presence of water since they form in water. In particular, if this is a
volcanic terrain that we are looking at, the presence of these minerals
might suggest this was a hydrothermal area and sulfur is a component of

such
environments."


If you look through the older Mars literature, the origin of soil
minerals (including sulfur minerals) seen at the Viking and Pathfinder
sites if often related to something called "palagonitization". This a
garbage can term for various geochemical processes involved in the
weathering and divitrification of basaltic glass (palagonite is an
amorphous to poorly crystalline, almost clay-like substance).
Palagonitization does involve hydration and it is common in hydrothermal
areas where hot thermal waters are circulating through volcanic and
volcaniclastic rocks.

If we read between lines then, it would seem that the MER project
scientists are leaning towards a palagonitized, volcaniclastic
(re-sedimented tuff/ash) origin for the bedrock, and a subsequent
diagenetic/hydrothermal alteration origin for the sulfur anomaly and
hematite-rich spherules.


OK, had to look that up:

diagenesis: The physical, chemical or biological alteration of sediments
into sedimentary rock at relatively low temperatures and pressures that can
result in changes to the rock's original mineralogy and texture. After
deposition, sediments are compacted as they are buried beneath successive
layers of sediment and cemented by minerals that precipitate from solution.
Grains of sediment, rock fragments and fossils can be replaced by other
minerals during diagenesis.

Note that these processes may all have been related to the same event of
series of events. Maybe something like: 1) rise of magma through the
mantle and crust of Mars; 2) eruption of vents, tuff rings, and maybe
maar volcanoes; 3) melting of permafrost due to higher crustal heat flow
and creation of standing water surface water (lakes, ponds, streams); 4)
deposition of glass-rich, basaltic tuff in lakes and ponds; 5)reworking
of tuff by lacustrine shoreline and fluvial processes; 6) hydrothermal
alteration (palagonitization) of tuffs; 7) leaching of iron-rich
minerals in reworked tuff and re-precipitation as hematite in
spherules/concretions/nodules; 8) heat flow subsides, everything dries
out, winds return and erode lake bottom, various impacts; 9) Opportunity
lands (bunny comes to investigate); 10) speculation ensues.


By george, you may have it... Water not persistent in that scenario though,
just a flash in the pan.

Joe

  #25  
Old February 18th 04, 03:35 PM
Jo Schaper
external usenet poster
 
Posts: n/a
Default Spheres coming from bedrock?



Timothy Demko wrote:


If you look through the older Mars literature, the origin of soil
minerals (including sulfur minerals) seen at the Viking and Pathfinder
sites if often related to something called "palagonitization". This a
garbage can term for various geochemical processes involved in the
weathering and divitrification of basaltic glass (palagonite is an
amorphous to poorly crystalline, almost clay-like substance).
Palagonitization does involve hydration and it is common in hydrothermal
areas where hot thermal waters are circulating through volcanic and
volcaniclastic rocks.

If we read between lines then, it would seem that the MER project
scientists are leaning towards a palagonitized, volcaniclastic
(re-sedimented tuff/ash) origin for the bedrock, and a subsequent
diagenetic/hydrothermal alteration origin for the sulfur anomaly and
hematite-rich spherules.

Note that these processes may all have been related to the same event of
series of events. Maybe something like: 1) rise of magma through the
mantle and crust of Mars; 2) eruption of vents, tuff rings, and maybe
maar volcanoes; 3) melting of permafrost due to higher crustal heat flow
and creation of standing water surface water (lakes, ponds, streams); 4)
deposition of glass-rich, basaltic tuff in lakes and ponds; 5)reworking
of tuff by lacustrine shoreline and fluvial processes; 6) hydrothermal
alteration (palagonitization) of tuffs; 7) leaching of iron-rich
minerals in reworked tuff and re-precipitation as hematite in
spherules/concretions/nodules; 8) heat flow subsides, everything dries
out, winds return and erode lake bottom, various impacts; 9) Opportunity
lands (bunny comes to investigate); 10) speculation ensues.

--
Tim Demko
http://www.d.umn.edu/~tdemko


The description above reads like a textbook description of the formation
of the St. Francois Mountains-- in the anorogenic rhyolite-granite
terrane of the US Midwest. Hence the reason I posted the photos under
the "Earth rock Mars rock" geology thread. Include caldera rise and
collapse, and by jove, you've got it.
Mars would be expected to be slightly different, assuming that it has
always had a high CO2 atmosphere, since atmospheric effects on all this
would be different. The analogy to Yellowstone some have drawn is
incomplete, since much of the rhyolite there is too recent to have been
extensively reworked.
Jo



  #26  
Old February 18th 04, 04:04 PM
Jo Schaper
external usenet poster
 
Posts: n/a
Default Spheres coming from bedrock?



Timothy Demko wrote:


If you look through the older Mars literature, the origin of soil
minerals (including sulfur minerals) seen at the Viking and Pathfinder
sites if often related to something called "palagonitization". This a
garbage can term for various geochemical processes involved in the
weathering and divitrification of basaltic glass (palagonite is an
amorphous to poorly crystalline, almost clay-like substance).
Palagonitization does involve hydration and it is common in hydrothermal
areas where hot thermal waters are circulating through volcanic and
volcaniclastic rocks.

If we read between lines then, it would seem that the MER project
scientists are leaning towards a palagonitized, volcaniclastic
(re-sedimented tuff/ash) origin for the bedrock, and a subsequent
diagenetic/hydrothermal alteration origin for the sulfur anomaly and
hematite-rich spherules.

Note that these processes may all have been related to the same event of
series of events. Maybe something like: 1) rise of magma through the
mantle and crust of Mars; 2) eruption of vents, tuff rings, and maybe
maar volcanoes; 3) melting of permafrost due to higher crustal heat flow
and creation of standing water surface water (lakes, ponds, streams); 4)
deposition of glass-rich, basaltic tuff in lakes and ponds; 5)reworking
of tuff by lacustrine shoreline and fluvial processes; 6) hydrothermal
alteration (palagonitization) of tuffs; 7) leaching of iron-rich
minerals in reworked tuff and re-precipitation as hematite in
spherules/concretions/nodules; 8) heat flow subsides, everything dries
out, winds return and erode lake bottom, various impacts; 9) Opportunity
lands (bunny comes to investigate); 10) speculation ensues.

What will be interesting is to see how the spherules are concentrated in
the residual soil above the light-colored bedrock. Comparing the
density of spherules in the soil to that in the bedrock may give a good
indication of how much of the bedrock has been eroded to create the soil.
--
Tim Demko
http://www.d.umn.edu/~tdemko


The description above reads like a textbook description of the formation
of the St. Francois Mountains-- in the anorogenic rhyolite-granite
terrane of the US Midwest. Hence the reason I posted the photos under
the "Earth rock Mars rock" geology thread. Include caldera rise and
collapse, and by jove, you've got it.
Mars would be expected to be slightly different, assuming that it has
always had a high CO2 atmosphere, since atmospheric effects on all this
would be different. The analogy to Yellowstone some have drawn is
incomplete, since much of the rhyolite there is too recent to have been
extensively reworked.
Jo


  #27  
Old February 19th 04, 02:00 PM
Joe Knapp
external usenet poster
 
Posts: n/a
Default Spheres coming from bedrock?


"Greg Crinklaw" wrote
Joe Knapp wrote:
http://www.earthfiles.com/news/news....tegory=Science


Joe I was with you right up to that point. This web site is not a
reliable source of information. The reporter in question is a nutter
who thinks crop circles are made by visiting aliens.


That is an ad hominem fallacy. I was not recommending the web site per se.
The interviewed source is on the MER team (Dr. Greeley). It's interesting
that MER personnel (Greeley, Grove and others) are talking to and emailing
these reporters that you shun.

Puritans, a very dogmatic people, couldn't go to sleep at night worrying
that somewhere, somebody was having fun. Mars Puritans can't sleep at night
thinking that somewhere, someone might be having an idea about Mars outside
the envelope of the latest JPL press release. The horror!

Better that your nemesis should not be the people with off-beat ideas about
space, but the people who don't care one whit about space exploration on way
or the other. It seems JPL is giving "earthfiles.com" the time of day at
least, and not with supercilious condescension. Without nutty ideas based on
almost total speculation, the pages of Mars history would be blank. Or it
would be a pamphlet.

Joe

  #28  
Old February 19th 04, 02:21 PM
George
external usenet poster
 
Posts: n/a
Default Spheres coming from bedrock?


"Joe Knapp" wrote in message
.com...

"Timothy Demko" wrote
Cross beds I take it simply means that the layers are not always
parallel--they diverge and converge sometimes? If so, how about this

photo
of the effect of "volcanic bombs":
http://www.geo.hunter.cuny.edu/~cwang/impacts.jpg Wouldn't the layers

in
that photo be a simple air fall?


The presence of cross beds means something more than the beds just not
being parallel. The beds in the link you provided would be called
deformed beds rather than cross beds. Cross beds formed by the migration
of bedforms (ripples, dunes, etc.) and are a primary featu they form
during, and are linked to, deposition. These bedforms may be subaerial
(eolian) or subaqueous. In deformed beds, the deformation event, in your
example the impact of a falling volcanic bomb, happens after the
deposition of the beds.


Thanks for clarifying that. I managed to find this short USGS video which
helped me to visualize it:

http://tinyurl.com/ysoda


"Secondly, they could be some kind of precipitate * perhaps from

water.
In
additional findings, there are some minerals that have been identified

in
the infrared that would suggest the presence of water since they form

in
water. In particular, if this is a volcanic terrain that we are

looking
at,
the presence of these minerals might suggest this was a hydrothermal

area."

What--lapilli are fading back in and the other two theories (magma &
concretions) are gone, to be replaced with "some kind of precipitate"?


No, I think he is talking about concretions or nodules. The confusion is
in that most people are assuming that the precipitation was happening in
a water column or on a lake/sea bed. I think what Greeley means here is
that he thinks they formed by prepitation from water in the subsurface
(pore waters), and he is leaning towards a hydrothermal origin for the
water, and maybe mineralization too.


OK, my misunderstanding as a google-ogist rather than a geologist. When I
hear precipitate I think of something falling, like through said column of
water.


snip

AND ALL THIS IS ADDING UP TO A QUESTION ABOUT WHETHER THE PLACE WHERE
OPPORTUNITY CAME DOWN ORIGINALLY MIGHT HAVE BEEN HOT MAGMA COMING UP

TO
THE
SURFACE IN SOME KIND OF VOLCANIC ACTIVITY? OR AT LEAST HOT SPRINGS?

"Yes. The evidence is sort of pointing towards the presence of a hot
environment * hot for some reason and that's under debate right now.
Volcanic activity, magma close to the surface, is one possibility."

...

THE BIGGEST SURPRISE RIGHT NOW HAS BEEN THE HIGH AMOUNT OF SULFUR IN

THE
BEDROCK?

"That was a surprise, yes. The measurements that have been made there
suggest there is a very high percentage of sulfur present in the

material
and that there are these spherules that have been seen and reported

earlier,
those are still very curious features."

"Such a high percentage of sulfur (in the bedrock) is unusual and this

is
what leads some folks to consider the volcanic origin (at the Martian
bedrock), or at least volcanic processes to be involved. There are

some
minerals that have been identified in the infrared that would suggest

the
presence of water since they form in water. In particular, if this is

a
volcanic terrain that we are looking at, the presence of these

minerals
might suggest this was a hydrothermal area and sulfur is a component

of
such
environments."


If you look through the older Mars literature, the origin of soil
minerals (including sulfur minerals) seen at the Viking and Pathfinder
sites if often related to something called "palagonitization". This a
garbage can term for various geochemical processes involved in the
weathering and divitrification of basaltic glass (palagonite is an
amorphous to poorly crystalline, almost clay-like substance).
Palagonitization does involve hydration and it is common in hydrothermal
areas where hot thermal waters are circulating through volcanic and
volcaniclastic rocks.

If we read between lines then, it would seem that the MER project
scientists are leaning towards a palagonitized, volcaniclastic
(re-sedimented tuff/ash) origin for the bedrock, and a subsequent
diagenetic/hydrothermal alteration origin for the sulfur anomaly and
hematite-rich spherules.


OK, had to look that up:

diagenesis: The physical, chemical or biological alteration of sediments
into sedimentary rock at relatively low temperatures and pressures that

can
result in changes to the rock's original mineralogy and texture. After
deposition, sediments are compacted as they are buried beneath successive
layers of sediment and cemented by minerals that precipitate from

solution.
Grains of sediment, rock fragments and fossils can be replaced by other
minerals during diagenesis.

Note that these processes may all have been related to the same event of
series of events. Maybe something like: 1) rise of magma through the
mantle and crust of Mars; 2) eruption of vents, tuff rings, and maybe
maar volcanoes; 3) melting of permafrost due to higher crustal heat flow
and creation of standing water surface water (lakes, ponds, streams); 4)
deposition of glass-rich, basaltic tuff in lakes and ponds; 5)reworking
of tuff by lacustrine shoreline and fluvial processes; 6) hydrothermal
alteration (palagonitization) of tuffs; 7) leaching of iron-rich
minerals in reworked tuff and re-precipitation as hematite in
spherules/concretions/nodules; 8) heat flow subsides, everything dries
out, winds return and erode lake bottom, various impacts; 9) Opportunity
lands (bunny comes to investigate); 10) speculation ensues.


By george, you may have it... Water not persistent in that scenario

though,
just a flash in the pan.

Joe


One of the problems with this scenario (and it is a good one) is that we
have yet to see any clay beds, which you would expect to see in a lacustrine
deposit. even if the clay beds had worn away, there should still be clay in
the soil. Unfortunately for that scenario, the microscpic images taken so
far of the trench at the opportunity site appear to indicate a sandy soil.

  #29  
Old February 20th 04, 10:06 AM
George
external usenet poster
 
Posts: n/a
Default Spheres coming from bedrock?


"Joe Knapp" wrote in message
.com...

"Greg Crinklaw" wrote
Joe Knapp wrote:
http://www.earthfiles.com/news/news....tegory=Science


Joe I was with you right up to that point. This web site is not a
reliable source of information. The reporter in question is a nutter
who thinks crop circles are made by visiting aliens.


That is an ad hominem fallacy. I was not recommending the web site per se.
The interviewed source is on the MER team (Dr. Greeley). It's interesting
that MER personnel (Greeley, Grove and others) are talking to and emailing
these reporters that you shun.

Puritans, a very dogmatic people, couldn't go to sleep at night worrying
that somewhere, somebody was having fun. Mars Puritans can't sleep at

night
thinking that somewhere, someone might be having an idea about Mars

outside
the envelope of the latest JPL press release. The horror!

Better that your nemesis should not be the people with off-beat ideas

about
space, but the people who don't care one whit about space exploration on

way
or the other. It seems JPL is giving "earthfiles.com" the time of day at
least, and not with supercilious condescension. Without nutty ideas based

on
almost total speculation, the pages of Mars history would be blank. Or it
would be a pamphlet.

Joe


I don't know about anyone else, but despite their enthusiasm (and mind you,
I see nothing wrong with mixing it up with the ideas), I don't know if
people who espouse the wonders of crops circles are necessarily who I want
sitting behind me at a Congressional budget meeting when the decision is
made on funding for the next space mission.

  #30  
Old February 20th 04, 06:13 PM
Timothy Demko
external usenet poster
 
Posts: n/a
Default Spheres coming from bedrock?

George wrote:
"Joe Knapp" wrote in message
.com...

"Timothy Demko" wrote


Note that these processes may all have been related to the same event of
series of events. Maybe something like: 1) rise of magma through the
mantle and crust of Mars; 2) eruption of vents, tuff rings, and maybe
maar volcanoes; 3) melting of permafrost due to higher crustal heat flow
and creation of standing water surface water (lakes, ponds, streams); 4)
deposition of glass-rich, basaltic tuff in lakes and ponds; 5)reworking
of tuff by lacustrine shoreline and fluvial processes; 6) hydrothermal
alteration (palagonitization) of tuffs; 7) leaching of iron-rich
minerals in reworked tuff and re-precipitation as hematite in
spherules/concretions/nodules; 8) heat flow subsides, everything dries
out, winds return and erode lake bottom, various impacts; 9) Opportunity
lands (bunny comes to investigate); 10) speculation ensues.


By george, you may have it... Water not persistent in that scenario


though,

just a flash in the pan.

Joe



One of the problems with this scenario (and it is a good one) is that we
have yet to see any clay beds, which you would expect to see in a lacustrine
deposit. even if the clay beds had worn away, there should still be clay in
the soil. Unfortunately for that scenario, the microscpic images taken so
far of the trench at the opportunity site appear to indicate a sandy soil.


Well, a couple of things to keep in mind:

1) The trench was dug in the crater where Opportunity ended up. The
impact that created this crater happened at step #8 in my scenario
above...after the volcanic and lacustrine/fluvial events. The impact
event excavated through the residual soil and spherule-bearing bedrock.
I would expect the trench that Opportunity just made within the crater
to have been made through materials that have accumulated since the
impact (eolian and mass-wasting from the crater rim).

2) Trenches to come, especially outside the inner/lower part of the
crater, and completely outside the crater on the plains, may yet
encounter clay-sized material. However, my view is that much of this
material will be lag from wind-eroded bedrock (spherules etc.) and
eolian material from elsewhere. The key boundary will be at the very top
of the bedrock, although this, too, may end up being a saprolitic or
soil "C" horizon type of boundary.

3) Clay-rich lake deposits need a source of the clay (most of it in
terrestrial lakes is detrital and comes in as suspended load material in
feeder streams, with a minor, but variable, input from airborne dust).
The martian systems may have been much different...less chemical
weathering and therefore less detrital clay...more relative input of
airborne dust...etc.

4) Finally, the very fine-grained nature of the light-colored bedrock
may suggest that at least some of it was originally was clay-sized
material. It may have been altered, cemeneted, leached, etc. since its
original deposition. However, I do submit that some of it was originally
sand-sized material because of the presence of cross beds. It's hard to
get clay-sized material to make constructional bedforms because the
grains are sticky and don't like to move after they've been deposited.
You can get ripple-forms, gutters, and other forms, in clayey materials,
but they are due to fluid stressing and deformation rather than
transport (you would not produce cross beds).
--
Tim Demko
http://umn.edu/home/tdemko
 




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