Lecture Notes for the Final
Fourth order of relief
- This refers to landscape-scale features smaller than and nested within
third order terræ, plana, and planitiæ.
- Up through the third order of relief, my intent was to go over every
single major region of Mars, and I think we have.
- At this point, as we move into the much smaller fourth order features,
the number of landscapes that could be considered here goes up exponentially,
so I no longer aspire to a complete travelogue: Instead, I'll pick landscapes
to discuss that seem interesting in terms of the processes shaping them.
- Discussion, then, will be process-led (kind of like the second order),
each major process illustrated by a selection of landscapes.
-
Fluvial processes
- Noachian valley networks: Some valleys seem to show the kind of
dendritic structure and fine
dissection you'd expect from a precipitation-fed surface flow and
channelization system on Earth:
- Channels in Echus Chasma, south of Kasei Valles, originating to the
northwest of Valles Marineris
- Dendritic pattern collecting flow in progressively higher order channels
- Eventually poured over a 4,000 m cliff
-
Channels in Terra Sirenum: Viking imagery
-
Channels around Warrego Vallis in Thaumasia
- Nanedi Vallis in Xanthe Terra (Viking)
- Deltas in such places as:
- Melas Chasma
- Eberswalde Crater near Holden Crater north of Argyre Planitia
- Gusev Crater
- Jezero Crater in Syrtis Major Planum
- Groundwater sapping-fed systems akin to stream systems in arid
lands on Earth, most of the larger ones probably Noachian in age:
- These feature long main trunks.
- Relatively few, short tributaries
- Tributaries originate in theatre-headed alcoves.
- Examples:
- Ma'adim Vallis in Terra Sirenum, debouching into Gusev Crater
- Nanedi Vallis in Xanthe Terra has some of these, too
- Non-equilibrium systems: Some fluvial systems seem to carry the
overflow of water into or out of a crater or system of craters and over a
series of sharp transitions in the landscape, with little evidence of the
smoothing and construction of a graded profile (hence "non-
equilibrium").
- Ma'adim Vallis flows into Gusev Crater, which is where Spirit landed
- Ma'adim Vallis is southeast of the Elysium rise and southwest of Tharsis
- Seems to collect fluids from a series of possible lakes to the
south to empty north into Gusev, astride the crustal dichotomy.
- Lakes identified by R.P. Irwin III and G.A. Franz at the Smithsonian from
what look like deltas and terraces all at the same elevation around a surface
that does not show channelization itself, perhaps protected from fluvial
erosion by the deposition activities of a lake bottom.
- A channel descends from them, winding sinuously for some 900 km down into
Gusev
- Gusev Crater was chosen for Spirit's landing site partly because it
looked so clear that Ma'adim Vallis was carrying water from a wide watershed
into the crater.
- The hope was the crater would yield water-altered minerals.
- Spirit did not find water-altered minerals at first, the way Opportunity
did in Meridiani, but, after a year and a half, bedrock was reached and it was
dramatically altered (not coatings and veins of water-deposited minerals but
pretty wholesale alteration).
- It's bromine, sulfur and chlorine found inside the rock "Clovis."
- Then there's that enormous chain of craters and valleys linking the melt
from under the south polar cap through Argyre and Holden craters, around Aram
Crater, and through Ares Vallis into Chryse Planitia, some 8,000 km long!
- Catastrophic, jökulhlaup-like outflows
- Kasei Vallis and Ares Valles you met earlier in the semester and saw how
much of a flow they could have carried in a single event or
series of massive events
- Ares Vallis might have its source in collapsed terrain, such as Aram
Chaos, where water or other fluids are suddenly liberated by heat or
mechanical failure breaking a dam of it, which undermines the still-frozen
surface terrain, producing the chaos landform
- Earlier I mentioned Dao Vallis, Niger Vallis, and Harmakhis Vallis in
eastern Hellas Planitia, with their odd "upside down" structure with wide,
alcoved head"waters" and narrow V-shaped channels farther down
- A very interesting case is Chasma Borealis, which might be a
volcano-glacio-fluvial source of immense water volumes and catastrophic
flooding
- Kathryn Fishbaugh and James Head, III, have created a topographic map and
profiles and used them to estimate volume of a catastrophic melt (perhaps
subsurface vulcanism): 26,000 km3!
-
Picked out deposits from such an event: could fill lowest portion of north
polar basin to a few 10s of m!
-
Small sapping alcoves, channels, aprons
- These structures form on Earth in layered terrain with a more resistant
caprock as groundwater sapping from a subsurface layer exposed in a scarp
(fault? landslide?) erodes scarp face material and undermines the caprock,
which then collapses down the scarp
- MGS' Mars Orbital Camera has recorded many smaller scale versions of this
on the walls of craters and caught a few of them in the act of having recently
flowed
- Especially parallel is the imagery from Houghton Crater on Devon Island,
Canada, north of Baffin Island: The only Earth impact crater on a Mars-like
polar desert landscape
- Especially interesting is the geography of such gullies: They are found
on poleward-facing slopes, especially in the southern hemisphere, at latitudes
with absolute values >30°
- This is odd, given that poleward slopes are colder than equatorward
slopes and Mars is such a cold place with such low atmospheric pressure!
- Mars' air pressure is typically between 500 and 600 Pa (5-6 mb), where
Earth's is about 101,320 Pa!
- Water has a triple point pressure, or pressure at which solid, liquid,
and gas phases co-exist, of 610 Pa or 6.1 mb, at 0.01° C. If you hold
pressure constant, warming it turns it into a gas. If you held the
temperature constant and upped the pressure, you could cause it to change to
liquid. Below the triple point, increasing the pressure would only cause it
to become solid ice.
- Sooo, at this low pressure, warming ice in the soil will only make it
sublimate directly into vapor. Most of the time, Mars' temperatures are below
the triple point, so increasing pressure would, perversely, keep it solid!
- And yet you see these gullies and they aren't antiques: Malin's team has
found new ones popping up between MOC imaging passes!
- Some possible explanations for this maddening martian phenomenon:
- Maybe it isn't water: Brine or some other such non-pure water mix might
work. If you add salt to water, you depress the triple point, which allows
the liquid phase to exist at martian temperatures and surface pressures.
Various salts have been detected on the martian surface, so there might be
something to this approach. Salts can be picked up from volcanic fines in the
atmosphere that would interact with water or from groundwater's interactions
with various rocks.
- Maybe it's a signature from an earlier era a few thousand years ago when
Mars' axis had greater obliquity. Martian obliquity swings from ~15° from
the vertical of the plane of ecliptic to ~35° over a cycle of
approximately 124,000 years (Earth's varies by only about 4°, with its
rotation and axis stabilized by the gravitational tug of the Moon).
- At a time of greater obliquity, Mars' polar regions would receive the
concentration of solar energy, not the equatorial regions, as today and as on
Earth. So, it would make sense that water might liquify on the warmer slopes,
which would, paradoxically, be the poleward-facing slopes, an effect more
pronounced as you approached the equator.
- A French team led by F. Costard published a geographical analysis of 213
gullies that the MGS MOC camera had found: Almost all gullies from -28°
to -40° faced south toward the pole, with 4% facing east; from -40° to
-60° 55% faced south, 33% faced east or west, and 11% faced north toward
the equator; in polar regions, 58 still face south, but 35% face north. This
is consistent with the production of average daily temperatures above 0°C
during times of high obliquity.
-
Adding more credibility to this approach,
Mars at high obliquity happens to have higher atmospheric pressure as more
water vapor enters the atmosphere, and the sun-facing hemisphere will have
higher temperatures, a combination that allows more plain water to remain
liquid on the surface.
- Detracting from this is the obvious freshness of so many of these
gullies, many caught with new activity or newly created between passes of MGS
and the MOC imager.
-
Linear fossæ and catenæ
- There are linear graben type structures in several regions of Mars:
- Cerberus Fossæ running from the southern parts of the Elysium rise
to the southern parts of the Tharsis rise
- Claritas Fossæ, the rough terrain south of Noctis Labyrinthus, west
of Solis Planum, southeast of Tharsis and clearly subject to tension from its
development, and northwest of the Thaumasia mountains
- Alba Fossæ, scoring the entire old volcano and continuing on
south-southwest toward the center of Tharsis: It's as though Alba Mons came
up first and then the continuing uplift of Tharsis exerted tension on it, too,
cutting it with these grabens
- These often seem to track off radially from the center of Tharsis or, to
a lesser extent, Elysium Planum
- They probably reflect the extensional stress of the material coming up
from the hot spots under them, leading to rock failure and faulting, and down
dropping of terrain between faults, much as we see in the Basin and Range
Province of east central California and Nevada with the tension exerted by the
insertion of the Farallon Plate under
western North America. Death Valley and Panamint Valley come to mind.
- There are a number of oddly linear patterns of pits, too, called
catenæ. The term, "catena," is neutral, without a cause implied.
-
So, they sometimes describe impact craters lined up in a row, which you
sometimes see when there have been secondary impacts from materials ejected
from the primary impact or when an impactor has already broken up before
impact, creating a streak of smaller craters.
-
The term can also be applied to strings of pits that are clearly not impact
craters, not having the tilted up rim structures: These are just
sharp-bordered lines of circular depressions.
- We talked about them earlier, in discussing features found in the
Valles Marineris system of chasmata:
- Pits in Tithonium Chasma
- Coprates Catenæ
- Catenæ may be areas related to faulting and graben construction or
even the collapse of lava tubes, in which a surface crust is undermined by
extensional stresses and perhaps the extraction of some subsurface fluid and
subsidence, leading to cave ins and pitting.
-
Secondary cratering issue
- We've encountered this issue before while discussing crater-counting as
a system to constrain relative and absolute surface ages.
- The basic idea of crater-counting is the power law relationship between
crater size and frequency.
- We saw that this is by no means a perfect power law pattern.
- https://home.csulb.edu/~rodrigue/geog441541/isochrontemplate08.jpg
-
When you
straighten the relationship out by logging both crater diameters and
frequencies, you find the "straight" line turns down above about 64 km
(probably a reflection of the end of the Late Heavy Bombardment of the solar
system and the resulting reduction in the number of big bombs still out
there).
- The far left side, below about 32 m, faintly turns down, too, possibly
because little craters are readily buried or eroded on a planet as
geologically active as Mars.
- The really problematic stretch of this "straight" line bends upward
sharply below roughly 1 km in size and above the very tiniest craters.
- We saw that this is the subject of all kinds of debates about there
being, perhaps, an actual increase in the supply of smaller objects out there,
maybe from collisions among them in outer space or?
- Another possibility is secondary cratering.
- We don't know what percentage of craters in the 32 m to 1 km size range
are secondaries, so there is a real premium on trying to find ways of
detecting them and differentiating them from small primary impact craters.
- The issue is important, because it affects the ages we assign to
landscapes, and that can affect processual analysis of those landscapes.
- Some of the attempts to detect secondaries:
- Looking for odd-shaped craters, on the assumption that secondary
impacts aren't at the hypersonic velocities of most primary impacts, so they
don't detonate in a nearly perfect hemispheric transient crater.
- Nearly all primary craters are formed by objects coming in so fast that,
no matter which angle they hit, they tend to produce a hemispherical transient
crater.
- Now there's a plot complication: It turns out that some oblique primary
impacts CAN produce irregular craters!
-
Some experiments were done by Gault
and Wedekind back in 1978 involving shooting various targets with various
bullets at various angles. Indeed, incohesive targets hit at anything higher
than about 7° produced circular craters; more cohesive targets would
produce circular craters above about 30°.
- Irregular craters seem to open out in the direction of arrival of the
impact.
- That experimental possibility seems to be reflected by certain craters
or crater-like basins, such as Orcus Patera that one of your lab projects
examined, and a few other craters on Mars, Mercury, and the Moon.
- So, we now have a possible explanations for such weird landscapes as
Orcus Patera, but we have less help in figuring out which craters are
primaries and which are secondaries, and that is an important question. Mars,
the "yes, but ..." planet.
- Looking for clusters of small craters, especially linearities
- For reasons not fully understood, secondaries tend to be aligned in
roughly linear patterns.
- This may reflect dynamics within the ejecta curtain, as fragments from
the impactor and the target, including a lot of dust, interfere with one
another while in motion. Perhaps this interference results in sorting of the
curtain into ejecta-rich regions and ejecta-poor regions, which creates a
splat of materials arranged in rays.
- On the Moon, the rays are often well-preserved, fines and all,
punctuated by impact craters, some of which are oddly shaped. It is easy,
then, to follow the rays back to the guilty primary crater.
- Mars is so active geologically in comparison with the Moon, that the
fines are removed by the wind, leaving just the craters and no ray structure
to link them and point back towards the guilty primary crater.
- Linking them, then, entails a lot of guesswork and individual
inspection.
- I came up with a system to detect lineations of craters as potential
candidates for secondary chains.
- This entailed selection of a badly battered landscape (I picked a MOC
image in Terra Sabæa), manually recording their centers and diameters,
entering them in OpenOffice Calc, measuring their distances from one another,
calculating for each of them (n=149) its nearest neighbor and then its
next-nearest neighbor up to the 6th order, and then calculating the azimuths
from each crater to each of its six nearest neighbors. That done, I could
compare the second nearest through sixth nearest neighbors' azimuths with the
azimuth of the original crater and nearest neighbor pair.
- I then looked for at least four neighboring craters per crater that were
aligned closely.
- Now, humans are pattern-spotting critters, so just random processes can
create "alignments." So, I used a Chi-square goodness-of-fit test to compare
my distribution of various groups of "aligned" craters with the binomial
probability distribution. That told me that azimuths should be ≤ 15°
to
be considered a real, non-random alignment, so that's what I used.
- I wound up with 32 chains of 4 or more craters, suggesting that as many
as 71 of the original 149 craters were secondaries, which is about what others
have found doing manual classification of craters elsewhere.
- I then "mapped" them, which was rather funny, given that I am not a GIS
person and am a local legend for crappy cartography! I used OpenOffice Calc's
graphing function, putting the original image in as the chartwall and then
doing a scatterplot of the X and Y coördinates.
- Much to my surprise, I found patterns among these short alignments: The
chains formed longer chains or, even more interestingly, several short chains
converged on a common location just off image!
- Map didn't come out too badly, much to the amusement of Drs. Wechsler,
Dallman, Ban, and Tyner!
- Here's a link to my LPSC paper: http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1014.pdf
Stuart Robbins and Bryan Hynek mapped tight clusters of craters all over Mars
and then fitted them onto great circle routes. Many of the various great
circles converged at one point: Lyot Crater in the Northern Lowlands just
north of Arabia Terra/Deuteronilus Mensæ.
- Some of these were more than 5,200 km away from Lyot!
- And some of the craters in the clusters were fairly large, nearly
a km across.
- This upends the idea that secondaries are little bitty guys and that
they fall within a few radii of the primary.
- It also brings out that, while secondaries impact at lower velocities
than primaries, the lower velocities are closest in to the crater, which is
where you likelier find oddly shaped craters, and are greatest for those that
fall farthest out (something lobbed very high up on a ballistic trajectory
will pick up a lot of velocity coming back to the ground far away).
- So, how many of the global population of craters are secondaries, given
that secondaries can fall so far from their primary parent, some of them are
pretty large, and the farther and faster secondaries may well produce circular
craters? This is a threat to the ability to assign absolute age ranges to
given terrains, if not a threat to relative aging.
- Here's a link to their LPSC paper: http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1330.pdf
-
Lava tubes
- These form, as on Earth, when lava flows rapidly along a surface, while
the top of the flow is cooling and solidifying, forming a crust
protecting the
fast flow underneath. This eventually drains out, leaving a tunnel in the
lava deposit, which may cave in subsequently, exposing the tube system.
- There are lava tubes all over northeasternmost California on the Modoc
Plateau, where Kintpuash or
"Captain Jack" held off the U.S. Army for a long campaign of
guerrilla raids followed by disappearance into the lava tubes that the Modocs
knew all about but the army didn't.
- An example on Mars is the ESA Mars Express image of such tubes on the
side of
Pavonis Mons.
-
Layered mesas or mensæ
- These are all over the transition zone between the highlands of Arabia
Terra, Sabæa Terra, and Terra Cimmeria with the northern lowlands:
Cydonia Mensæ, Deuteronilus Mensæ, Protonilus Mensæ north of
Arabia Terra; Nilosyrtis Mensæ north of Terra Sabæa and northwest
of Isidis Planitia; Nepenthes Mensæ north of Amenthes Planum and west of
Elysium Planitia; Zephyria Mensæ north of Gusev Crater and south of
Elysium Planitia;.
- They may represent mesas topped by more resistant layers that form a
caprock protecting the stack of sediments or lava flows or consolidated
wind
deposits below.
- These are attacked by erosive agents, notably wind on Mars, and sculpted
into striking etched patterns wherever certain layers are more
resistant than others.
- We see these all over the American Southwest, such as in Monument Valley,
Arizona.
- Mars' answer to such structures include the Cydonia Mensæ region
and its infamous Face on Mars. This has become the canals craze of modern
times. NASA Headquarters ordered the Mars Global Surveyor mission to drop
what it was doing on Mars and reprogram it to get an image of the Face, which
cost a lot of money and effort that would have otherwise gone to data
processing and scientific analysis of priority targets. It was like finding a
needle in a
haystack using a microscope. Here is Malin Space Science Systems' discussion
of the issue (the team running the Mars Observer Camera): http://www.msss.com/education/facepage/face_discussion.html.
- There's also now the Heart on Mars, a 255 m mesa found in the south polar
region by Mars Global Surveyor.
- Anyone who's taken one of my stats classes has heard me rant about the
human bias toward seeing pattern even in completely random phenomena (we are
the descendants of ancestors who over-reacted to perceived and imagined
predators and made asses of themselves but survived to procreate: the
rational folks who dismissed a rustling in the shrubbery as random wind
sometimes wound up dinner! So, we come by it honestly). We are especially
hard-wired to see faces. This kind of pattern-seeing in images is called
pareidolia or apophenia. Layered mensæ aren't the only
contexts for pareidolia:
- The Elephant on Mars in Elysium Planitia is currently making the rounds,
but that is actually
a superimposed lava flow structure rather than mensæ.
- As long as we're
on this tangent, there's a Happy Face on Mars, too, in Galle Crater in the
eastern rim of Argyre Crater. There's another, smaller one in a 3 km wide
crater in Nereidum Montes in the northern rim of Argyre Crater.
- Libya Montes on the south rim of Isidis Planitia has a cool-looking face
wearing a crown popping out of a degraded crater rim structure.
- We even have these on Earth: Do a search on the Badlands Guardian in
Alberta, Canada!
-
Yardangs
- Erosional features created by wind, just as on Earth
- Sandblasting of features, sometimes with intense sculpturing at
base and
often creating long, linear features where the wind blows in a consistent
direction
- Given the predominance of æolian processes on Mars, these
structures have many similarities with mensæ
- An example is seen in the HRSC (Mars Express) image of yardangs south of
Olympus Mons at ~6° at ~220° with a comparison image in the military
illustration of yardang terrain (sometimes called grooved terrain)
- Another example is seen in the viewgraphs showing a MOC image of
Æolis Mensæ (which is just south of Elysium Planum on the margins
of the southern hemisphere highlands, around +1° by ~145°), again
contrasted with a mystery military image of Earth yardangs
-
Dune fields
- Depositional features created by wind, just as on Earth
- Olympia Undæ is covered by a great erg or sand sea.
- On Earth, dunes are ominated by silica sand
- On Mars, they're dominated by basalt-derived dust, so they are sometimes
dark.
- Often found in craters or other depressions, where wind drops its
load and from which it's nearly impossible for sand to escape.
- If the wind comes from a consistent direction and the sand supply is
still relatively sparse, it will group the sand into classic barchans,
or
crescent-shaped dunes with the horns pointing downwind: The viewgraphs show
Nili Patera (Syrtis Major just west of Isidis) with its classic dark barchans
- If the wind is consistent in direction and there's a large sand supply,
barchans will merge into long ridges running at right angles to the prevailing
wind direction, rather like an ocean's waves on a sand sea. These are
transverse dunes.
- If the wind comes from a variety of directions, sometimes it will bunch
the sand into star dunes, which may have three or sometimes four ridges
going
out radially from the summit, as we see in the viewgraphs contrasting
Opportunity's view of a dune field in Endurance Crater with another military
mystery yardang photo
-
Patterned ground
- Polygons are often seen on Earth in periglacial environments due
to the
mechanical stresses caused by:
- Freeze-thaw expansion-contraction of water
- Expansion-contraction of other materials due to temperature changes,
which can be considerable in such environments
- Water in the active layer above permafrost is drawn toward the frozen
face of ice at the surface of the polygons, the
ground surface, and the permafrost itself. This desiccates the soil into
blocks, and this partly explains the chunky look of these patterned grounds.
Meltwater gets under these blocks seasonally and refreezes, wedging the blocks
upward with their expansion. Subsequent melting does not allow the block to
settle back down because fines/mud are pulled in under the block and prevent
that.
- Finer and coarser materials respond differently to these stresses of ice
wedging and frost heaving,
leading to segregation by particle size, which creates the sorting you see
around the edges of the polygons, with the largest clasts in the crevices
between hummocks
- Such patterned ground is found on Mars more and more frequently at higher
latitudes, which is what you would expect from a planet with little permafrost
or deeply buried permafrost in the tropics, a lot of permafrost closer to the
surface in the mid-latitudes, and sitting atop the surface at the ice caps
themselves.
-
Eskers
- These are streams that form under glaciers due to basal melting.
- Like any streams, they engage in erosion, transport, and deposition of
weathered material.
- Like any streams, a lot of this winds up as bed load on the floor of the
channel.
- Upon ablation of the glacier, the beds of these streams are exposed
along with the ground till under the glacier (boulder-studded clay), where
they look like sinuous, gravelly ridges.
- Dorsa Argentea shows good examples of what are believed to be South
Polar Ice Cap eskers.
-
Evidence of mass wasting: Landslides
- Common on crater gully walls at a small scale
- Very evident as a major mechanism expanding the chasmata of Valles
Marineris
- We've looked at Coprates Chasma
- Ganges Chasma, which was the site of your earliest lab
- Candor Chasma contains a truly spectacular example
- Noctis Labyrinthus to the west of Valles Marineris, seemingly at an
earlier stage of development and facing different stress fields
-
Chaos
- Collapsed, jumbled terrain
- May be source of massive, possibly explosive outflows
- Catastrophic melting of water or brine, possibly accelerated by carbon
dioxide outgassing, totally undermining the floors of canyons and craters
- Aram Chaos blasting a channel into the side of Aram Crater to get to Ares
Vallis looks prototypical, as does Iani Chaos on the other side of Ares Vallis
and Aram's crater walls
-
Softened craters
- Very characteristic of Mars, unlike the Moon (and yet not so far gone as
the craters, or astroblemes, of Earth).
- Crater walls eroded through fluvial and æolian processes
and,
possibly, marine or lacustrine processes (if the state of the buried craters
in the northern lowlands is an indicator).
- Crater floors fill with æolian debris, which eventually
flattens
the floor.
- Some may have been filled, too, with lakes and their bed deposits or, as
with the Moon, even lava deposits.
- Often these deposits, given their varying nature and climate change on
Mars, will vary in cohesion, density, and resistance to weathering and
erosion.
- Once filled, these flattened deposits can be eroded by wind
- As Dr. Laity's experiments showed, targets of varying strength will be
etched by wind and the sand it carries (where targets of uniform composition
and strength will not).
- So, a very distinctive landform is created by æolian processes
acting on crater fill, often intensely etched.
- As we've also seen earlier, in discussion of Promethei Terra, crater
appearances can be softened also by the visco-elastic relaxation of
ground ice over entire regions.
- Further softening the look of many martian craters is that "wet splat"
fluidized look of rampart and pedestal craters
- Mars ends up with a distinctive look to its craters, a look that is
unmistakably martian: You will always be able to look at Mars imagery and
instantly suspect it is, in fact, Mars.
|
![[ orthographic image of Mars on a black background ]](https://home.csulb.edu/~rodrigue/mars/marsblackbg.jpg)
![[ Olympus Mons seen at oblique angle that gives a 3-d sense ]](https://home.csulb.edu/~rodrigue/mars/olympusmons3d.jpg)
![[ Mars explorer ]](https://home.csulb.edu/~rodrigue/mars/marsexplorerpainting.jpg)
|
|
![[ image of Mars ]](https://home.csulb.edu/~rodrigue/mars/marsatlas.gif)
