Lecture Notes for the Final
Amazonian surfaces: Youngest
- The Amazonian began with the (contested) date of the ending of the
Hesperian era, and it continues until the present day
- The Hesperian-Amazonian transition is far less distinct and much more
transitional than the more dramatic Noachian-Hesperian transition, with its
defining fall-off in the arrival of large impactors, the end of the Late Heavy
Bombardment.
- The transition between the Hesperian and the Amazonian is marked by
inherently less sharply bounded changes in dominant processes, so that
uncertainty leads to some pretty divergent estimates of the transition.
- These range from as early as 3.5 Ga ago to as late as 1.8 Ga ago
- The most commonly cited date for this gradual transition is around 2.9
to 3.0 Ga
- Things are much more peaceful in the Amazonian, as compared with
the
bombardment and flood volcanism that characterized the Noachian and the
edifice and shield volcanism
and massive outflow flooding seen throughout the Hesperian. That said,
however, the Amazonian is scarcely uneventful:
- Volcanic eruptions were still pretty intense at the beginning of the
Amazonian, as volcanic activity increasingly concentrated on Tharsis and
Elysium and the construction of the great montes.
-
It probably still goes on (there is evidence of relatively recent volcanic
eruption
activity within the last 2 million years, based on crater-counting) but
not at
the clip of Late Noachian, Hesperian, and Early Amazonian times
- Water or brine continues to be released from the subsurface somehow,
creating those fresh looking gullies we've seen in MOC, HRSC, and
HiRISE imagery on the sides of
craters and the animation of damp or wet flows I showed you a while ago.
- Some gullying may be the result of small avalanches of granular dry
ice/dust mixes.
- Massive outflows may occasionally still occur, judging from crater
counting studies in channels leading into Amazonis Planitia, which show
outflows as recently as 10-100 million years ago, though this is not
universally accepted (lava often flows down pre-existing outflow channels and
the crater counts record the ages of the recent lavas more than the underlying
channels they exploit).
- Massive long-runout landslides may also still occur right up to
the present, and smaller ones have been documented in flagrante
delicto!
- Meteorites still smack down from time to time, even being spotted
by
repeated imagery of the same sites.
- There may even be ice ages in the Amazonian. Glaciation may
sporadically affect the mid-latitudes, which coïncides with
periods of exaggerated obliquity in the martian axis of rotation. The polar
ice caps show layering of dust-rich and ice-rice materials, which demonstrates
rhythmic alterations in precipitation/frost deposition and windiness.
- Dust devils and planet-covering dust storms kick up, mixing
dust from all over Mars into
wind deposits of globally homogenized composition. You saw that in Lab 7 in
the trend for undisturbed rock surfaces to show a trend toward soil surfaces
in most of the oxides and elements you graphed from Opportunity's APXS.
- Martian geochemistry during the Amazonian is dominated by
oxidation of
iron into anhydrous iron oxides (including hæmatite or rust, magnetite,
maghemite, and ilmenite), giving the ubiquitous planetary dust that light
reddish tint. This anhydrous oxide chemistry is the basis for the Bibring
et al. team's proposal of the Siderikian (iron-loving) as their third
era in Mars geochemistry, which includes all of the Amazonian and parts of the
later Hesperian.
- Tour of Amazonian regions:
- Despite being by far the longest time division on Mars, Amazonian-
dominated
surfaces cover the least surface on Mars, approximately 26% of it
(Barlow
2010). You can view
Barlow's map of martian age distibutions here: http://bulletin.geoscienceworld.org/content/122/5-6/644/F8.large.jpg.
- Our travel itinerary for Amazonian Mars starts with the type region,
Amazonis Planitia, west of Olympus Mons. From there, we'll move west into
Elysium Planitia, up onto the lavas of Elysium Rise, and then down onto the
surface of Utopia Planitia.
We'll
pause in Acidalia Planitia and the North Polar Deposits, then visit Arcadia
Planitia and return near our starting point among the lava flows mantling
Tharsis Rise.
- Much of our discussion will focus on the interactions among lava, outflow
debris, and periglacial phenomena.
- Amazonis Planitia to the west of Tharsis and east of Elysium
- This is the type region for the Amazonian time division, the youngest of
the three.
- It has one of the most spectacularly smooth surfaces on Mars,
which you could see for yourself doing a few transects across it in Gridview.
Many of you found yourselves frustrated in the crater-counting lab, because
this was a natural spot to try to count craters in a very young area, except
the Robbins database is confined to craters at least 1 km in diameter, and
those are very scarce in Amazonis Planitia!
- The surface is covered with evidence of many, often very recent
effusive
basaltic lava flows, the kinds of low viscosity lavas that can flow
great
distances in sheets and maintain an extremely flat or low slope surface.
- Now, Amazonis Planitia was used as the type region for all the younger
plains of the Northern Lowlands, but it turns out that Amazonis Planitia may
be a rather eccentric representative of Amazonian times, being kind of a
dammed in basin allowing all kinds of flows to pond:
- There's apparently a huge Noachian era crater basin buried under it
toward the northwest, and that badly degraded rim has acted as a partial dam.
- There's a large Late Hesperian era lava flow that's been traced to
Olympus Mons from before the time it developed that weird aureole feature.
It's about 100 m thick and also acted like a barrier to flow movement
northward out of the Amazonis basin. It's a classic Hesperian ridged plain,
like the ones we've seen in such places as Lunæ Planum and Solis Planum,
but the ridges are considerably lower than the 100-150 m height seen in those
other regions. It's as though something has partially buried the ridges.
- Then, the Olympus aureole formed, perhaps from a megaslide, and that
acted as yet another dam toward the eastern side of Amazonis.
- So, there are these three damming features to the northwest, north, and
east, which caused lava flows and water or water/brine outflows and even some
marine incursions to back up and pond in Amazonis Planitia, producing very
smooth surfaces.
- Recent lava flows from Tharsis Montes and from Elysium Montes have
gotten into Amazonis Planitia. Some of these may have been less than 10
million yeas ago (Fuller and Head 2002).
- There have also been extensive sedimentary deposits from massive
outflows, which emanated from the Mangala Vallis region to the
south and, via Marte Vallis to the southwest, from the Elysium Planitia
region.
- Subsequent lava flows have exploited the channels created by these
outflow events, so you sometimes see lava flowing almost like water down one
of these pre-existing outflow channels.
- The Parker et al. ocean Contact 2 (-3,760 m) covered Amazonis
Planitia, too, so the region could have been under water back in Noachian
times, also helping smooth the surface over which the later lava and outflow
deposits would be laid.
- So, we have a landscape that, because of the ponding created by
the Noachian crater rim, the probable Noachian ocean, the Late Hesperian
Olympus Mons flow, and the later Olympus Mons aureole collapse event, contains
an extremely smooth but very complex mix of lavas and outflow deposits.
- Elysium Planitia
- Elysium Planitia is a broad wedge-shaped region, about 3,000 km from
west to east and 1,000 km from north to south. It is directly south and to
the southeast of Elysium Rise and to the southwest of Amazonis Planitia. It
abuts the crustal dichotomy border.
- Nomenclature is a little inconsistent: Sometimes the Elysium Rise is
called Elysium Planitia, while more contemporary usage confines the term to
that flat area south of the Rise.
- It is not as smooth in texture as Amazonis, nor is it as low in
elevation.
- Like Amazonis Planitia, however, it represents a similar mix of
volcanic flows and fluvial deposits, with a similar pattern of lava and
dikes interacting with volatiles in the regolith to create outflows and using
channels earlier carved by fluvial processes.
- At roughly -3,000 m, it lies above the Parker et al. Deuteronilus
(lower, -3,760 m) contact but below the Arabia (higher, -2,000 m) contact, so
it may have been covered by a frozen ocean in Noachian times.
- There are weird surfaces in Elysium Planitia, which consist of large,
dark plates, with light-colored material in between. In some places,
it's possible to re-arrange them in such a way that they fit together pretty
well. They have been the subject of competing hypotheses:
- Might these be lava plates? Some flood basalts on Earth show
patterns like that, perhaps from crusts forming on the flow, fracturing,
rotating in the flow, crashing into one another, creating smaller slabs, or
pushing up ridges between one another.
- Another Earth process that can create this kind of slabbing, rotation,
pulling apart, and pushing up is ice, like the pack ice that forms around the
Arctic Ocean or in lakes and rivers when surface ice starts to break up in
spring. Maybe this stuff is pack ice or pack ice covered with the
ubiquitous martian dust.
- Crater counts indicate that the surfaces of Elysium Planitia are in the
tens to hundreds of millions of years old, on the younger end of the Amazonian
time frame. The pack ice/lava raft stuff is more like a few million years
old, really young stuff.
- Elysium Planitia, like Amazonis Planitia "next door," shows signs of
ponding against low hills to the northeast, which look like outcrops of
older materials, possibly Noachian in age, complete with valley networks, such
as Rahway Vallis. A large channel, Marte Vallis, cuts through
these and
connects Elysium Planitia with Amazonis Planitia.
- The whole thing looks as though Elysium Planitia and Amazonis Planitia
were sitting there, minding their own business, each with permafrost
saturating their regoliths, maybe even with liquid water under that cryosphere
material.
- The Elysium volcanic system, or Apollinaris Mons just south of
Elysium
Planitia, would have built up magma chambers and these may have been fringed
with dike swarms.
- The dikes, on contacting the ice, water, or other volatiles in the
regolith, would trigger massive flooding, which may have poured out of
Elysium Planitia through Marte Vallis down into Amazonis Planitia.
- The waters or brines would eventually sublime or work their way back
into the regolith and freeze.
- Then, if any of the dikes actually made it to the surface, perhaps
through such fissures as the Cerberus Fossæ toward the north, you would
have lava floods and flows, many of which would seek out the
pre-existing fluvial channels and, where these generated enough heat
penetrating downward to the permafrost, you might get the production of
rootless cones through such phreatomagmatic interactions. There are
quite a few of these.
- Speaking of Cerberus Fossæ, these are extraordinarily long
and narrow trenches in the surface, which run from Elysium Rise southeastward
toward the middle of the Tharsis Rise.
-
There has been speculation that these might be an incipent rift, like a
Valles Marineris system in the making, a great system of cracks due to an
underlying extensional stress field, perhaps associated with the Elysium Rise.
-
They are pretty new in the sense that they crack through
the relatively new lava flows of Elysium Planitia (rather than having these
newer lavas flooding over their sides).
-
In some places, they seem to be the source of lavas flooding out over
the surrounding countryside. In other places, they seem to be the source
of great water flows. Again, we see that intimate interaction between
lava and fluvial processes. Maybe this is because of magma diking creating
the extensional stresses/faulting, the catastrophic release of subterranean
water, and the occasional flood basalt lava flow.
-
In one case, a Cerberus Fossa is crossed at an angle by Athabasca
Vallis,
which is a channel for massive outflows, which is linear like a fossa because
it runs along a wrinkle ridge from an older surface, which supported it.
- Utopia Planitia to the northwest of the Elysium rise
- We already discussed the huge impact crater that formed this basin in
the second order of relief: Here the focus is on the mantling deposits.
- The Noachian impact basin was covered by subsequent materials back in
the Noachian, which continued to receive impacts: There are many buried
craters
in Utopia, which have been revealed by Mars Global Surveyor's MOLA, the
SHARAD radar sounder on the Mars Reconnaissance Orbiter, and the MARSIS radar
instrument on Mars Express.
- These ancient surfaces, however, are covered by materials from the
Hesperian and, mostly, the Amazonian:
- the Late Hesperian Vastitas Borealis Formation lavas, which, when
exposed here, has a knobby and polygonally cracked appearance
- smooth lobate flow terrain, interpreted as a more recent lava flow,
early Amazonian, probably of the pahoehoe type
- rough lobate flow terrain, interpreted as lahars triggered by the
interaction of lavas with groundwater or ice, generally on top of the smooth
lava flows
- There is also etched terrain, or landscape features showing the
effects of wind erosion during the long, dry, windy Amazonian time.
- In fact, related to this, the Viking 2 lander showed a lot of perched
rocks, that is, rocks that seemed to have had a lot of the soil support
around them blown away
- There are also extensive fluvial deposits and the kinds of shattered and
pulverized debris from magma and volatile interactions (dikes moving through
ground ice, lava flowing over surfaces with a lot of water, brine, or ice in
them).
- These are associated with great outflows, rather than valley networks,
which seem to come out of a series of outflow channels and grabens on the
western side of the Elysium Rise.
- Granicus Vallis
- Tinjar Vallis
- Hrad Vallis
- Acidalia Planitia to the north/northwest of Arabia Terra and north
of Chrys Planitia
- It is a large region, mostly very flat and smooth, and it has a low
albedo, giving it a rather dark color and making it visible from Earth back in
the 19th century.
- The few craters that are found here have that "wet splat" look to them,
an indicator of subsurface volatiles and ground ice.
- Further indicating the presence of subterranean ice is the large
polygonal structure on much of the surface.
-
These are large polygons, perhaps 5 or 10 km wide, which is large
enough and far enough out of the norm for polygon-patterned ground on Earth to
cause some skepticism about the permafrost analogy.
- Surrounding the polygons are sometimes sharp canyons.
- The polygons increase in height toward the Arabia Terra borderland,
shading into that mensæ territory, where the Face on Mars is found.
- On top of the polygon surfaces can often be found the pimply signal of
those odd little depression-tipped cones we've seen in other places, which may
be phreatomagmatic rootless cones or, in a newer interpretation,
possibly mud volcanoes.
- Mud volcanoes are found on Earth in situations where hot water
infiltrates fine soil materials and, if the water is under pressure, the
water, now dirty with mud, burps up onto the surface, forming a small cone,
complete with a vent on top.
- Mud volcanoes are connected with actual volcanism in the sense that magma
intrusions are what heats subterranean water or ice and the movement of magma,
as well as the thermal expansion of the water, creates the pressure that leads
to mud eruptions.
- So, you tend to find them in subduction zones on Earth, places you would
also find true igneous volcanism.
- One of the wilder ideas about Acidalia Planitia is that it may
have once
contained a natural fission reactor! This was presented at the 2011
refereed Lunar and Planetary Science Conference by J.E. Brandenberg.
- The basis of his argument is analogy with an actual natural fission
reactor that developed in the Oklo region of Gabon, Africa, about 1.7
billion years ago (Cowan, George A. 1976, A natural fission reactor.
Scientific American 235, 36 (July): 36-47, available http://brendans-island.com/blogsource/20101015ff/A-Natural-Fission-Reactor.pdf)
- Scientific American has an article on this: Meshik, Alex P. 2005.
The workings of an ancient nuclear reactor. Scientific American 293,5
(November): 82-96, 88, 90-91 https://www.scientificamerican.com/article/ancient-nuclear-
reactor/.
-
About 2.3 billion years ago, oxygen concentrations on Earth had gotten just
high enough to allow uranium in rocks to dissolve in water when
cyanobacteria's photosynthetic production of oxygen outstripped the ability of
iron-rich rocks to oxidize.
-
On Earth, this build-up in oxygen as metabolic
waste from photosynthetic cyanobacteria was actually a disaster for many
bacterial and archæal lifeforms at the time, which were obligate
anæobes. This is called the Great Oxygenation Event or, more
bluntly, the Oxygen Catastrophe.
-
For respiration, they used other electron acceptors than
oxygen, such as sulfate, iron, or manganese or relied on fermentation and were
actually poisoned by exposure to molecular oxygen.
-
As if that weren't bad enough, the oxygen waste of cyanobacteria, having
saturated available supplies of iron-rich rock, oxidized methane in the
atmosphere (a potent greenhouse gas) into water and carbon dioxide (much
weaker greenhouse gasses). 2O2 + CH4 -->
CO2 + 2H2O.
-
The sun was much feebler back then, perhaps only 70% as energetic as today:
The Faint Young Sun. The greenhouse gasses made Earth, paradoxically,
habitable for bacteria and archæa in the face of a much cooler sun. So,
this shift in the greenhouse gasses meant that the planet froze over,
virtually all of it: Snowball Earth. This was a one-two punch hitting
along with the Great Oxygenation Event. Perhaps 99% of all life on Earth at
the time went extinct, during the Great Oxygenation Event and Snowball Earth.
-
So, the upshot of all this was that there was now, finally, enough oxygen in
the atmosphere and dissolved in water to enable uranium to dissolve in water.
Once dissolved, the uranium-235 was free to move with the water solution.
-
Uranium needed to be dissolved from rocks and
then moved by groundwater into places it could accumulate.
-
In Oklo, uranium concentrations in an aquifer were bracketed by sandstones
below and above and a
granite mass further down.
- The ore body has all sorts of radioactive substances decaying into other
daughter elements, emitting, among other things, neutrons from their nuclei.
These crash into other nuclei, which can induce fission in these nuclei. The
probability of this happening increases as their velocities decrease.
- Groundwater moving through the aquifer could get into the uranium ore,
where it turns the "fast neutrons" produced by natural radioactive
decay into "thermal neutrons" moving slowly enough that they have a
greater probability of entering the uranium nucleus and triggering a fission
reaction, releasing huge amounts of energy by so doing. Thus, criticality was
reached in the ore, which was contained by the sandstone and granite above and
below..
- There would be runaway nuclear heat production and explosive ejection of
water. This would dehydrate the aquifer, which would cool, relieving the
pressure, stopping the runaway chain reaction, and dropping the uranium down
over a couple of hours below criticality.
- Water would then be able to re-enter the now cool and dry aquifer,
triggering the chain reactions once again.
- This would cycle back and forth between criticality and release in a 3
hour cycle for
several million years, producing natural plutonium.
- Oklo is the only place on Earth where this process has been documented,
because of the unique combination of oxygen buildup, groundwater leaching and
concentration of 235U, and ore buildup in a contained aquifer with
a lot of water.
- It shut down eventually and it can't re-appear there or anywhere else
because, in the intervening 2 Ga, the supply of 235U has declined
due to radioactive decay (half-life = 0.7 Ga). This kind of reaction needs
235U to be enriched to at least 3% of the fuel; radioactive decay
has knocked that percentage well below this criticality requirement.
- Brandenberg argues that the same conditions are found in northern
Acidalia, especially in Acidalia Colles, except the reaction was bigger
and
led to a huge explosion that created concentrations of radioactive potassium
and thorium there.
- The process happened once on Earth, so it's not entirely impossible that
it would happen on Mars.
- If this intrigues you, here is the paper: http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1097.pdf.
- The proposed mechanism was received with the usual mix of neutral
curiosity, positive interest, and skepticism that is normal in the scientific
community.
- But then Brandenburg has since gone way past the slack initially cut him:
He's now going on about a nuclear war on Mars and some interstellar species
wiping out martian civilization (the Face) and we should worry about them
coming back around and nuking us on Earth. This Lowell-like trajectory is
succinctly described on the Pharyngula science blog at http://scienceblogs.com/pharyngula/2014/11/22/the-two-faces-of-je-brandenburg,
including commentary by Brandenburg.
- Planum Boreum around and under the North Polar Ice Cap
- This is the material underlying and supporting the North Polar Ice Cap.
- It also extends out past the ice cap, being exposed in Olympia
Planum
and also inside Chasma Boreale.
- It is a thick bed of material rising up above the North Polar Lowlands
as much as 3 km.
- It shows complex layering in the SHARAD radar sounder on the Mars
Reconnaissance Orbiter and MARSIS sounder on Mars Express.
- At a coarse level, there are two main divisions of these layers under
the ice:
- Polar Layered Deposits (PLD), subdivided into:
- Upper Layered Sequences (ULS)
- Lower Layered Sequences (LLS)
- Basal Unit (BU)
- The Polar Layered Deposits show alteration between water ice and dust
layers.
- That kind of layering suggests that the polar region is responding to
climate changes, probably over the last 10-100 million years,
judging
from crater counts, which alternate between two phases:
- Accelerated deposition of frost or snow in the polar regions
- Decline in precipitation/sublimation and increase in dryer conditions
that favor the liberation and depositon of dust.
- These alternating phases may be connected with changes in the planet's
obliquity, and Mars undergoes more extreme oscillations in obliquity than does
Earth:
- As the axial tilt declines, the polar regions become more persistently
cold, persistently at very low sun angles, which would promote frost
deposition or even precipitation there.
- As the axial tilt becomes more extreme, the polar regions would
experience higher sun angles in the summers and much longer day lengths, which
would cause accelerated sublimation of the ice,
more dustiness, and an increase in the dust to ice deposition ratio.
- Below the PLD is the Basal Unit, which is an even more complexly layered
and deformed unit.
- It is easily made out from the PLD by a major unconformity in the SHARAD
profiles and is a
darker color.
- The layering seems to be cross-bedding of sandy layers.
- There's a lot of speculation about what the source of the BU is:
- Perhaps the classic channel outflow deposits carrying material
there all the way from Chryse Planitia? (but why built up here?)
- Perhaps a marine depositional system? (again, why built up only
here?)
- Maybe an older polar depositional system now decayed and eroded?
(glaciation episodes much older than the ice caps we see now, with perhaps
intervening times with little to no ice?)
- Maybe the ubiquitous martian dust so common through the Amazonian
back
before any martian glacial ages began? These could have been deposited and
eroded in complex patterns, perhaps drenched from time to time and frozen,
hardening them into layers.
- A speculation of my own here: In the circumpolar regions of Mars,
a powerful polar vortex develops in the global circulation, kind of a
cyclone on steroids. Polar vortices develop on other planets and moons, too,
including Earth, Venus, Saturn, and Titan. The polar vortex produces strong
winds, spiraling counterclockwise around the martian North Polar Ice Cap. I'm
wondering if:
- These have sufficient force, especially armed with dust and sand, to
carve the polar cap chasmata and, given their counterclockwise flow, account
for the counterclockwise orientation of Chasma Borealis and the other chasmata
there. Their orientation seems baffling in the light of clockwise katabatic
winds blasting off the polar high pressure cell atop the ice cap.
- On a much smaller scale, there is a persistent vortex in Gale Crater,
where the Curiosity rover is now operating. Kite et al. (2013) argue
that the 5 km tall "Mt. Sharp" (properly Aeolis Mons) in the center of Gale
Crater (and sticking out above the crater's rim) is a stack of wind-depositied
material shaped by strong winds running down the crater's walls and down the
side of Mount Sharp and then spiraling around the floor of Gale Crater around
the base of Mount Sharp. A 5 km stack of æolian deposits in this
argument could be created by a crater's wind vortex. Could the process be
scaled up to the level of the northern polar vortex to build up and shape the
Basal Unit platform now carrying the North Polar Layered Deposits of Planum
Boreum and the North Polar Ice Cap?
-
The article proposing a localized version
of this for Gale Crater is Kite, Edwin S.; Lewis, Kevin W.; Lamb, Michael P.;
Newman, Claire E.; and Richardson, Mark I. 2013. Growth and form of the mound
in Gale Crater, Mars: Slope wind enhanced erosion and transport.
Geology 41, 5 (May): 543-456. doi: 10.1130/G33909. This model, in
fact, might be useful in accounting for many puzzling interior layered
deposits found in craters and chasmata, sometimes taller than the
surrounding terrain, which would be hard to explain as lacustrine sediments!
- An article comparing martian and
terran polar vortices can be found here: http://onlinelibrary.wiley.com/doi/10.1002/qj.2376.pdf
(Mitchell, D.M.; Montabone, L.; Thomson, S.; and Read, P.L. 2015. Polar
vorticles on Earth and Mars: A comparative study of the climatology and
variability from reanalyses. Quarterly Journal of the Royal Meteorological
Society 141 (January): 550-562).
- Wherever the BU comes from, it seems to be the source of huge dune
fields that immediately surround the North Polar Ice Cap in Olympia
Undæ,
ergs of sand like in certain Earth deserts. These great dune fields
imply the strong polar vortex circulation.
- Whenever climate conditions favor ice deposition, the North Polar Ice
Cap begins to cover these outcrops; whenever conditions favor ice ablation,
the PLD is exposed.
- These deposits, then, are going to be a virtual goldmine of climate
change data over the Late Amazonian for some future geoscientists!
- Planum Australe lies around and under the South Polar Ice Cap
- This is a thick stack of layered materials comparable to those in Planum
Boreale, the South Polar Layered Deposits.
- Again, we see a pattern of alternation between ice layers and layers
containing dust, again in a rhythmic pattern suggesting climate change due to
changes in Mars' orbital parameters, especially obliquity.
- As we discussed earlier, the residual South Polar Ice Cap has turned out
to be nearly all water ice, with a few meters of carbon dioxide ice on top
(which expands like crazy in the winter to form the seasonal ice cap).
- One difference is that, for some reason, the layers in the SPLD form a
kind of "staircase" topography, like we see in, say, the Grand Canyon,
where
layers differ in terms of strength and angle of cliffs that can be sustained
(as in sandstone alternating with shale).
The NPLD are clearly layered, but there's greater uniformity in slope angle
from one layer to the next.
- Something interesting has turned up: There are pockets of carbon
dioxide ice here and there among the ice layers and the SPD in places
where
parts of the layering has collapsed due to explosive sublimation.
Apparently,
carbon dioxide builds up in these holes and freezes, and there's enough of it
in there that, if it were all released, it would double Mars' atmospheric
density!
- Faulting is found on the SPLD, too.
- Arcadia Planitia to the northwest of Tharsis
- Arcadia Planitia is another smooth Amazonian surface, just to the north
of the type region, Amazonis Planitia.
- It lies generally between 1 and 3 km below the geoid.
- The surface seems to be dominated by younger lava flows.
- Again, we see a pattern of interaction between subsurface volatiles and
lavas, which produces strings of small cinder cones, probably of the
phreatomagmatic rootless cone variety.
- Some of the lower elevation areas show a pattern of furrowed ground with
almost, vaguely parallel ridges. This looks a bit like solifluction in
Earth's Arctic periglacial areas, those weird slumps produced by occasional
thawings.
- Cratering is sparse, as with all Amazonian regions, but the walls of
these relatively few craters show gullying, as do the slip faces of now
inactive, indurated sand dunes in Arcadia.
- Many craters show signs of expansion and terracing, suggesting
thermokarstic processes going on in deep layers of ice-rich soil, kind
of like the scalloping seen in polygonal terrain in Utopia Planitia but
distorting the shape of craters here.
- Lavas of Tharsis Rise and Dædalia Planum
- Again, Tharsis Rise was discussed under the second order of relief.
Here the focus is the Amazonian lava covering much of the rise.
- Volcanism probably began in the Late Noachian, probably as flood basalt
eruptions: low viscosity flows from fissures over very extensive areas without
much edifice-building around vents.
- Much of these lavas
have been buried by subsequent events, exposed here and there on the periphery
of modern Tharsis, as on Thaumasia Minor Planum on the eastern end of the
Thaumasia block.
- The bulk of the Tharsis Rise built up in Hesperian times, but even the
Hesperian eruptions have generally been covered up by subsequent Amazonian
flows. We see the Early Hesperian flows exposed in Solis Planum and the Late
Hesperian flows in Sinai and Syria plana.
- Alba Mons, with its odd patera-like shape and the intense fracturing of
its surface, seems to be the focus of the first shield-building volcanism in
the Hesperian to Amazonian transition, while the
Tharsis Montes and Olympus Mons show newer episodes, building extremely tall
mountains. Olympus Mons is surrounded by Late Amazonian volcanic materials,
and there are aprons of Late Amazonian materials on the western slopes of the
Tharsis Montes.
- Crater counts indicate that there have been eruptions in this complex
within the last 100 million years, and some are even younger than that:
Olympus Mons shows areas as young as 2 million years at its western
scarp!
- The lava lump has bent the lithosphere, creating the Chryse Trough and
other relatively low elevation areas ringing it, while the magma's upward
movement has created extension stresses on and radial to the uppermost surface
of the Rise, creating fossæ
- Dædalia Planum is that smooth area on the southernmost part
of
Tharsis Rise, south of Arsia Mons.
- It's intriguing, because there seems to be yet another monster crater
buried under there, some 4,500 km across!!! That would dwarf Hellas Planitia!
It's thought to be among the earliest big impactors, maybe Early Noachian in
age, older than Hellas (Craddock 1990). Whatever its origins, it is now
blanketed with lavas dating from the Hesperian-Amazonian transition.
- Tharsis shows a pattern of old volcanic activity that was very low
viscosity and spread out over huge areas, followed by later episodes of more
viscous eruptions and flows that didn't sprawl out as far but, instead, built
edifices on top of the older material, eventually culminating in the great
Tharsis Montes perched on top of the lava pile and Olympus Mons perched on its
western edge. Kind of a switch from Los Angeles style land use (sprawling,
horizontal) to New York style (spatially more confined but vertical)!
- Elysium Rise
- We covered Elysium Rise under the second order of relief as one of the
large and visually conspicuous features of Mars, one produced by epic
volcanism.
- Here, I simply want to comment on the young surfaces of this feature.
- These surfaces are quite recent, with very little cratering, many under
100 million years old.
- Most of the surface is covered with lava, in some places with
pyroclastic deposits (especially around Hecatoes Tholus, the
northernmost of
the three large volcanoes). There is an exposure of a Late Hesperian volcanic
field southwest of Elysium Mons, but the majority of the Rise's surface is
covered with Hesperian-Amazonian volcanics.
- There is also evidence of lahars, especially on the western side
of the Elysium Rise. These are volcanic mudflows produced when an eruption
liquefies or sublimates glaciers or permafrost, which then saturates
pyroclastic material on the sides of the volcano and it all starts flowing
downslope.
- Elysium Rise is also characterized by fossæ or grabens,
which often are
the site of origination of great outflows, especially on the west side.
- On the east side, there are these mostly straighter channels that are
believed to be lava channels, similar to lunar rilles.
- The Amazonian is, by far, the longest of the three martian time
divisions, commencing about 2.9 or 3.0 Ga. It began as a gradual transition
away from the Hesperian, as processes of vulcanism and outflows slowed
considerably, the atmosphere dwindled until the air pressure dropped below the
triple point of water except for narrow exceptions. The loss of the atmosphere
obliterated shelter from short-wave solar radiation and cosmic rays, creating
a sterile surface that would destroy any life forms, if such had evolved on
Mars. The planet had now desiccated to such an extent that the extremely
acidic geochemistry of the Late Noachian and the Hesperian gave way to the
anhydrous oxidation of iron-bearing rocks and a pervasive wind redistributing
and homogenizing these oxides and dust grains all over the globe. Though much
quieter than the preceding Hesperian and Noachian, the Amazonian bears
evidence of regular climate change in response to orbital dynamics.
Glaciation comes to the mid-latitudes when obliquity is high and the polar ice
caps experience accelerated sublimation, which produces increases in
atmospheric pressure sufficient to support snow or frost accumulation. When
obliquity declines, ice is confined to the polar regions. The general quiet
can be disturbed by vulcanism even now, landslides large and small, occasional
outflows, gullying perhaps by brines and perhaps by granular dry ice
avalanching, and dust devils and great dust storms.
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