Lecture Notes for the Final
Hesperian surfaces: Intermediate
- See Viewgraphs:
- "3rd order: Hesperian regions, Part A"
- "3rd order: Hesperian regions, Part B"
- The Hesperian is the shortest of the three conventional divisions of
martian geological time.
- Persistent uncertainties in absolute surface dating (the transformation
of lunar age-crater relationships into martian ones and the question of
secondary cratering) are especially evident in this time period.
-
So, it's said to begin at the end of the Noachian, somewhere between 3.9
and 3.5 billion years ago. That transition is less contentious, and the most
commonly cited date is converging on 3.7 or 3.8 Ga.
- The transition from the Hesperian to the Amazonian, however, is more
divergent in the community, ranging from as early as 3.55 billion years ago to
1.8 Ga. The most commonly cited boundary is 2.9-3.0 Ga.
- The Hesperian was an era of fundamental change in the way the martian
system
operated:
- The impact flux dropped off and the truly large impactors came no
more as the solar system was cleared of the larger objects.
- The Hesperian was also a time of widespread volcanism, following
on the increase in volcanic activity in the Late Noachian, when
volcanic flows began to build up Tharsis, with shield vulcanism beginning to
create edifices there, probably starting with Alba Mons.
- One consequence of all this was a dramatic increase in the sulfur
dioxide (SO2) content of
the dwindling atmosphere and the sulfuric acid
(H2SO4) content of the dwindling water. So, all that
volcanic activity in the transition from the Noachian to the Hesperian would
have resulted in much more acidic water in oceans, in groundwater, and
in any precipitation. Here are some (TMI?) details:
- Sulfur dioxide oxidizes into sulfur trioxide (SO3, which can
combine with water (H2O) to form sulfuric acid, making water
acidic.
- Sulfuric acid itself can dissociate readily in water to create hydrogen
sulfate ions (HSO4- and, very importantly, hydronium
ions or H3O+. Concentrations of hydronium ions are the
essence of the pH scale of acidity/alkalinity: pH is the negative common
logarithm of the hydronium ion concentration. As hydronium ions increase, pH
drops, that is, becomes more acidic.
- Hydrogen sulfate ions in water can also dissociate into more hydronium
ions and sulfate ions (SO42-). On Earth, there are
certain bacteria and archæa that can reduce sulfate ions (park
electrons/energy onto them, taken from hydrogen and hydrogen ions) turning the
sulfate into water and hydrogen sulfide or H2S (in order to oxidize
organic compounds or hydrogen for anærobic respiration). Chemosynthetic
"critters" tend to be extremophiles on Earth and may have been among the very
earliest life-forms on Earth. So, if life got going on Mars as chemosynthetic
microörganisms, they might have been able to survive in certain
situations in the sulfur-enhanced world of the Early Hesperian. Life on Earth
may have arisen during the Eoarchæan, which roughly parallels the Early
Hesperian on Mars, and it probably began as chemosynthetic
microörganisms.
-
Planetary geochemistry really changes in response to this acidification,
favoring the production and deposition
of sulfates and evaporites within and on top of rocks, such as those in
the Columbia Hills of Gusev Crater that some of you may have "met" in GEOG
400/500. This is a marked change from the
phyllosilicate (sheet-forming) clay formation and deposition seen in
the early Noachian. Water alteration of volcanic rock leads to different
minerals, now, such as jarosite, goethite, gypsum, and hæmatite. The
change is so drastic that it led the Bibring team to propose a
resequencing of geological time on Mars based on geochemistry (the Phyllocian
and the Theiikian -- drastically different chemistries).
- Jarosite is a hydrous sulfate of potassium:
KFe3+(OH)6(SO4)2
associated with very low pH water (pH of 1-3) and low water to rock ratios: It
tends to dissolve and allow formation of goethite and gypsum in less acidic
water
- Goethite is a hydrous iron oxide, FeO(OH).H2 (it often
forms from jarosite as a water body begins to evaporate or as pH becomes less
acidic)
- Gypsum is a hydrous calcium sulfate (CaSO4) (it tends
to dissolve in very acidic water, making the formation of jarosite likely).
It was found by the Opportunity rover in veins in bedrock where water had
deposited gypsum in cracks in the rock.
- Hæmatite is anhydrous ferric oxide or anhydrous iron (III) oxide or
Fe2O3. It was found by the Opportunity rover as
spherules formed within sedimentary bedrocks rich in sulfates, such as
jarosite, probably as iron-rich groundwater concentrated ferric oxide in
concretions within the evaporite beds.
- Volcanism continues its shift in eruption style from low viscosity
flood eruptions (effusive eruptions) into vented shield volcanoes and a more
viscous or sticky/gassy style of eruption (often explosive).
- Too, there's a spatial concentration in volcanic activity from
more diffuse fissure-and-intercrater plains flows to distinct vent focussed
eruptions and the construction of large edifices around large calderas and
increasing spatial confinement of volcanic activity to the Tharsis and Elysium
bulges.
- Wrinkle ridges are common on Hesperian plains, especially those
ringing Tharsis to the east, which indicates compressional stress and strain
outward from the Tharsis pile.
- Faulting and the formation of linear grabens (fossæ) in
areas of evident extensional tension (parts of Tharsis, for example)
- The Hesperian is when the majority of the gigantic outwash floods
flowed, a fluvial behavior drastically different from the smaller groundwater
and possibly precipitation fed valley networks of Noachian areas.
-
The connection between these outflows and chaos terrain and the association of
chaos terrain with explosive subtraction of subterranean fluid and subsidence
are often linked with volcanism.
-
Magma may have intruded into or near subsurface or surface ice, often in the
form of dike swarms, which heated the undersides of ice-rich beds and/or the
ice-plugged slope walls of craters and grabens, creating confined aquifers.
-
These liquids would pour through broken megaregolith or any break in the
cryospheric layers capping or plugging them and into channels, sometimes
quietly at a small scale and sometimes explosively and massively.
- Another fundamental change in the martian system during the Hesperian was
the loss of most of the atmosphere and the drop in air pressure
below
the triple-point of water at martian temperatures. Here is a phase
diagram for water showing the triple point. Mars now averages about 6 mb
or hPa, and its temperatures rarely get above 0° C or 273K, very close to
the triple point. At that pressure, water goes from ice to vapor with a
warming of temperature, though there may be very low elevation places that
transiently experience higher pressures and, thus, potentially brief episodes
of liquid water.
-
Along with the bulk of the other gasses in the atmosphere, Mars lost as
much
as 95% of its water, as water vapor in the atmosphere dissociated under
the
barrage of the solar wind and its more extreme pulses, and the very light H+
ions easily escaped (and still escape) into space. The remaining water on
Mars sank underground into permafrost and, perhaps, liquid water under that.
-
As a consequence, rates of erosion declined dramatically on Mars, other
than those spectacular, if sporadic, outwash floods that peaked during the
Hesperian.
-
The Hesperian was the time, then, during which
martian climate began to dry out and become really dusty and water began to
concentrate in permafrost, soil ice, the polar ice caps, and very tenuous
cloud activity.
- Hesperian surfaces cover about 34% of Mars (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.
- Tour of Hesperian regions:
-
The Hesperian landscapes cover a
smaller area than the Noachian regions, and most of the regions are, on
average, considerably smaller than the Noachian units.
-
The itinerary of
discussion below starts with the type region, Hesperia Planum, to the
northeast of Hellas Planitia, and then moves to the southwest through Hellas
Planitia's surface deposits to Malea Planum. From there, it moves southwest
into the circum-polar regions around the South Pole to review Sisyphi Planum
and then, farther west, Argentea Planum. Then, we continue north for a look
at Argyre Planitia floor desposits. After that we head northwest up onto the
Thaumasia block. There, we'll look at four plana that form subregions within
Thaumasia and the sequence of volcanic flow ages. In eastern Tharsis north of
Thaumasia and across Valles Marineris, we'll move over
to Lunæ Planum and then down the dichotomy boundary into Chryse
Planitia. From there, we'll examine Vasitas Borealis, returning to the edge
of the Southern Highlands through the
Isidis Planitia floor deposits and the surface of Syrtis Major Planum to the
northwest
of Hesperia Planum.
- Hesperia Planum to the northeast of Hellas and southeast of Isidis
Planitia.
- Hesperia Planum is the type region for the Hesperian Period.
- What distinguishes Hesperia Planum visually is the fairly sharp
drop-off in cratering, both in size and in density, when compared with
the adjacent Noachian regions of Terra Tyrrhena to the west, Terra Cimmeria to
the east, and
Promethei Terra to the south.
- Closer examination brings out a large, low angle volcano in the west
central part of the region, Tyrrhena Patera. Possible point of confusion:
Tyrrhena Patera (or Tyrrhenus Mons) is not in Tyrrhena Terra but is the
volcano associated with
the younger lavas of Hesperia Planum.
- This volcano is typical of the highland pateræ, with slope
angles so low that they were named for saucers rather than shields.
-
Tyrrhena Patera may be the earliest central vent volcano on Mars and,
thus, a marker from the extremely low viscosity flood basalt/fissure eruptions
of the Noachian toward the slightly higher viscosity, vented shield style
common in the Hesperian, including the vast shields along the spine of Tharsis
and those of Elysium.
-
Highland pateræ all over Mars seem to date from the middle of the
Hesperian, in this case on top of earlier Hesperian flood basalts that had
been worked over by impact gardening into badly brecciated megaregolith.
-
This megaregolith would have permitted the accumulation of subsurface water or
brines and ices, which would be available to interact with rising magma and
produce phreatic eruptions and pyroclastic flows, scoring
channels into the sides of such pateræ, as seen on Tyrrhena Patera.
The style of eruption is, thus, characterized as consistent with
hydrovolcanism (Greeley and Crown 1990).
-
The construction of Tyrrhena Patera then seems to have generated the kinds of
loading stresses needed to produce the network of wrinkle ridges found
in Hesperia Planum, the Hr or Hesperian ridged terrain
- Here's something kind of weird: though the planetary magnetic field is
believed to have shut down by 3.9 or 4.0 Ga, and the highland pateræ
date from after that time, ~ 3.85-3.65 Ga, they yet show strong remanent
magnetization, kind of like those strange bands in Terra Sirenum and Terra
Cimmeria. These highland pateræ include Tyrrhena Patera. Lillis et al.
(2005) argue that the anomaly suggests that the planetary dynamo might have
gotten a second wind before shutting down permanently and leaving no
magnetic trace in the later Tharsis and Elysium volcanoes.
- Hellas Planitia floor deposits
- Hellas contains the lowest point on the planet at -8,200 m!
- As such, it is the ultimate base elevation for a large watershed, which
would have included much of the Southern Highlands.
- There are two possible strandlines ringing the basin, one at
-5,800 m and the other at -3,100 m and the lower one can be traced most of the
way around Hellas, except where some younger volcanics buried it to the south.
They are well developed, suggesting the lakes/seas they bounded lasted for a
long time.
- Dao Vallis and Harmakhis Vallis end at the elevation of the lower
strandline, and Niger Vallis seems to debouch at the upper one, above Dao.
- There are no outlets from Hellas, so how would these two sea levels
equilibrate like that for so long?
- So, the floor of Hellas Planitia contains layered deposits
consistent with deposition of sediments in quiet waters.
- It is possible that Hellas was filled with a variety of materials to
quite a depth, including Noachian lacustrine deposits, Hesperian lava and
ash deposits
from Tyrrhena Patera and the nearby Hadriaca Patera in southern Tyrrena Terra
or from vents in Malea Planum to the southwest, and wind deposits in
Hesperian times.
- Then, these Noachian and largely Hesperian materials were etched out
by wind erosion during Amazonian times, creating a peculiarly complex
terrain.
- There may have been glaciation, too, and permafrost, as
well as collapse of ice-filled soils due to the subtraction of subterranean
support, as with thermokarst:
-
Signs of these include chaos terrain: Hellas Chaos
-
Hummocky, blocky terrain in Alpheus Colles
- Malea Planum south and southwest of Hellas Planitia
- Malea Planum lies to the south of Hellas Planitia, along the south and
southwestern rim of the crater where it shows a smooth and gradual slope of
younger mantling materials.
- The region contains four volcanoes believed to be of the
patera variety: Amphitrites Patera (~61° E at ~ -59°), Peneus
Patera
(~53° E at ~ -58°), Malea Patera (~52° E at ~ -63°), and
Pityusa Patera (~37°
E at ~ -68°), all believed to date to ~3.8 Ga, except for Amphitrites
Patera
at ~3.6 Ga (Williams et al. 2009b), in other words, all after the Hellas
impact around 4.0 Ga.
-
In 1978, it was suggested that these then odd volcanoes were sited on ring
fractures caused by the Hellas impact, which provided surface access for
magma intrusion Peterson 1978b; Wichman and Schultz 1988).
-
These four pateræ, together with the Tyrrhena and Hadriaca pateræ
northeast of Hellas Planitia, have been described as the product of the
earliest edifice-building volcanism on Mars, the first non-rift flows,
with magma extrusion for the first time confined to particular vents. As
such, these pateræ mark the earliest departure from the extremely low
viscosity flood basalts, akin to lunar maria, that characterized the earliest
volcanic activity on Mars (and Earth, for that matter).
- The overall regional surface is predominantly flat and made of volcanic
materials, and it is the flow of lavas and ash that poured into southern
Hellas Planitia from Malea Planum.
-
These are often deformed with ridges. These may mark compressional
stresses, like the wrinkle ridges around Tyrrhena Patera or the Tharsis
complex, perhaps similarly associated with volcano build up.
-
Alternatively, some of these ridges could be exposed remnants of dikes
that could have been responsible for ancient fissure eruptions of flood
basalts.
-
With the basalts, there are also ash flows, which would result from
pyroclastic activity set off by the phreatomagmatic style of
eruption produced when magma interacts with volatile-rich materials, a wet
mantle or ice-rich surface materials.
- Sisyphi Planum
- Sisyphi Planum is located about midway between Argyre Planitia and Hellas
Planitia, roughly halfway between them and the South Polar Ice Cap. It is
centered roughly 5° E and -70°
- This is countryside strongly marked by its high latitude loccation, with
abundant evidence of permafrost, seasonal ground ice, as well as the frost and
snow cover of the South Polar Ice Cap's seasonal hood.
- Western Sisyphi Planum contains an odd area called Sisyphi Cavi. This is
a terrain pockmarked by irregular pits. All sorts of explanations have been
put forward for these.
- Perhaps æolian deflation, as when wind blows out a depression in
the landscape?
- Perhaps the area had once been covered by the glaciers of the South Polar
Ice Cap and then something caused basal melting, which created these pits, and
then these were exposed as the glacier receded?
- Perhaps there had been an ice sheet here and the basal melting mechanism
was magma and lava from subglacial volcanoes? Subglacial eruptions would have
resulted in explosive cratering of the ice and terrain under the ice, and this
may have been preserved as the glaciers finally receded.
- In some ways, these things resemble those explosive sublimation pits on
the polar ice cap produced by the summer warming of the upper ice, causing the
carbon dioxide veneer to explode, carrying dust to the surface as it explodes.
- Other interesting details in Sisyphi Planum include the familiar polygon
patterning of the ground produced by the freeze-thaw cycles of polar regions
(on Earth as well as on Mars).
- Another surface feature of interest here is the presence of the
mantling unit. This is a deep layer of deposited dust, like lœss
on Earth. In periglacial environments, this lœss-like material can
accumulate quite a bit of ice in the interstices between soil particles. Over
long cycles of cold climates, this can gradually build up to the near
saturation of the soil. At greater depths, this stuff becomes permafrost.
- Closer to the surface, however, the soil ice can sublimate within the
soil spaces when climate warms (perhaps with increasing obliquity) and make
its way to the atmosphere.
- The dust covering then becomes more and more "spongy" as the ice
evaporates out of all those tiny soil spaces. It starts to look pretty pitted
at the surface and, in fact, may sag and settle down. This is the mantling
unit, and it creates a distinctive softening of the surface, a masking of
sharp-edged features, kind of like a soft blanket draped over the underlying
landscape. It is common in high latitudes that have thick accumulations of
dust.
- The images shown in the viewgraphs for this lecture show striking
examples of the mantling unit, in this area draped over what looks like
patterned ground below, softening the patterns. So, we see a landscape here
that is clearly ice-rich, in the form of permafrost evidenced by the polygonal
patterns and the mantling unit, which represents æolian deposition and
then frost and ice filling of the interstitial spaces between the dust grains.
That subsurface frost and ice then sublimates, leaving behind a visually
distinctive covering.
-
Another interesting sort of feature in these images of Sisyphi Planum is the
abundant evidence of small-scale gullying. These are common on steeper
slopes (such as crater rims) in mid-latitudes on Mars between about 30 and
60°, especially in the Southern Hemisphere. Those in Sisyphi Planum are a
litle unusual, being found well poleward of the main distribution of gullies.
- Like many other gullies, they are more common on poleward-facing
slopes.
- This may reflect the ubac effect, the coldness of shadier poleward-facing
slopes preserving frost, snow, and soil ice longer into warm seasons as a
source of flows.
- These also tend to be at lower elevations, and Sisyphi Planum is
relatively low in the Southern Highlands. This may produce higher air
pressure, allowing some liquid water or brine to form and persist for a while
after summer and daytime warmth causes the frost or soil ice to change state,
thus enabling gullying.
- The equatorward-facing slopes may warm too intensely and too fast,
causing faster direct sublimation of ice into vapor.
- You can see that adret/ubac dynamic in the viewgraphs for Sisyphi Planum,
showing a crater rim with the sunward slope dark and the poleward slope still
covered with frost and ice.
- There's also been some consideration of the way that long-term climate
change induced by changes in Mars' axis obliquity might affect the orientation
and distribution of gullying. At high obliquity, the poles would get much
more sunlight, causing loss of the polar ice caps and an increase in air
pressure as all that carbon dioxide and water joined the atmosphere. Liquid
water or, especially, brines would form and persist longer under those
conditions, and then the poleward-facing slopes would be warmer and you'd see
gullying on the sunny slopes, too.
- Argentea Planum
- This is a similar terrain to the neighboring Sisyphi Planum, but further
west. It lies south and southwest of Argyre Planitia.
- It, too, is widely covered by the mantling unit.
- Gullying is not common here, however.
- It is notable for the numbers of sinuous ridges running across it and the
dendritic pattern many of these form. These ridges have been interpreted as
eskers (e.g., Fastook et al. 2012; Kress and Head 2015),
or the beds of streams that form under glaciers from basal meltwater. The
beds fill with debris, the way normal, subæreal stream beds do, and may
be partly cemented by compounds in the water. When the glaciers recede, these
are left perched above the exposed landscape. This implies that the South
Polar Ice Cap once extended much farther from the pole than now seen.
- Argyre Planitia floor deposits
- The floor of Argyre Planitia is complex, much as Hellas' floor deposits
are. A variety of explanations have been put out there to account for one or
another geological unit there:
- Parker has argued that a Noachian polar ice cap and a large watershed
contributed enough water to fill Argyre as an ancient ocean/sea. The
sea overtopped the northern rim and formed Uzboi Valles and the upper Chryse
Trough hydrological drainage. Others (e.g., Hiesinger and Head 2001)
have looked for evidence of sea still-stands/shorelines and found that any
such features are too low to have topped the northern rim. Hiesinger and Head
(2001) report calculating the amount of water that could be released by the
much larger Southern Polar Ice Cap of Noachian and Hesperian times and didn't
think there was enough to fill the crater to the brim. So, the crater may
well have had ponding of water from the valles pouring into it over its
southern rim, but that water probably did not head the Chryse Trough drainage
system.
- There are Hesperian ridged units in Argyre or at least units that
look like the Hesperian volcanic wrinkle ridged surfaces, so, if that's what
they are, that would be a resemblance to the Hellas floor. It's unclear where
the lava would have come from, though, and fine-scale analysis of the ridges
don't show the same asymmetries (the stair step leading down analogy), the
sharp ridges usually seen at the crest of wrinkle ridges, and the height of
other wrinkle ridges. They also seem to form sinuous, braided, and somewhat
dendritic patterns, which is reminiscent of eskers, akin to the eskers
discussed under Argentea Planum. So, the waters that poured into Argyre may
have formed an icy sea in Noachian times, rather like our own Arctic
Ocean, and then froze all the way down, creating a glacier by Hesperian
times. Glaciers may have flowed into Argyre, too, from the south. Basal
melting could have created eskers, which were then exposed when the ice
finally sublimed away or the waters sank into the regolith.
- Frost still forms here each winter, the crater typically covered
by the seasonal hood of the South Polar Ice cap.
- On top of the potential volcanic layers, lacustrine/marine sediments, and
glacial/periglacial features, the increasing dustiness of Mars during the
later Hesperian and Amazonian deposited dust and sand on the floor, which
produces dunes here and there.
- Thaumasia block plana
- This complex, originally discussed in the second order of relief,
comprises several subregions:
- Syria Planum to the west and northwest, encircled by Noctis
Labyrinthus and the northern reaches of Claritas Fossæ
- Sinai Planum to the north, bordering the western part of
Valles Marineris proper.
- Claritas Fossæ to the west, a horst-like block of
Noachian materials, fractured by a series of north-south striking normal
faults and grabens, some of them offset laterally
- Thaumasia Highlands to the south, a concentration of folded and
faulted mountains, the only major collection of folded/faulted mountains
on Mars.
- Solis Planum, the large flat surface, somewhat concave, in the
middle of the block, characterized by northeast-southwest trending wrinkle
ridges.
- Thaumasia Planum aka Thaumasia Minor, south of eastern Valles
Marineris, a circular planum partly surrounded by folded/faulted hills to the
east, called Nectaris Fossæ or the Coprates Rise. It has
been suggested that this circular feature might be a large crater buried by
Tharsis lava flows, such as those wrinkle ridge plains covering Solis Planum.
Thaumasia Planum, too, is covered with wrinkle ridges.
- The entire Thaumasia block seems to be tilted as a unit from the high
point of Syria Planum, around 8 km above the geoid, to the folded and
thrust-faulted perimeter highlands, themselves buckled up to 4 - 6 km high,
with the plateau lands between descending to 2.5 to 3 km behind the highlands.
- There's an age progression across the block: Wrinkle-ridge plains
material in Thaumasia Planum (Late Noachian) is older than those of Solis
Planum
(Early Hesperian), which is older than the Late Hesperian material to the west
in Sinai and Syria Planum. To the west of Noctis Labyrinthus lie Amazonian
aged volcanic materials.
- The wrinkle-ridge features imply radial compression and
contraction on the circumference of the Tharsis complex, probably of
superposed shallow strata of basalt, crater ejecta related breccias
(megaregolith), Hesperian lavas and ash flows (Mueller and Golombek 2004).
-
They have been argued to be the result of deep rooted thrust faults, and this
is supported by analysis of MOLA topographic data, which shows they form a
downward-stepping stair with offsets of the plains on either side of them in
the 50-180 m range (Golombek et al. 2001).
-
The layers crumpled into wrinkle ridges may represent the kind of surface
originally deformed by the uplift of Tharsis, which were then covered by lava
and pyroclastic flows from the Tharsis Montes.
- The whole rather tidy progression of ages across Thaumasia suggests the
history of Tharsis:
- Late Noachian commencement of Tharsis volcanism, perhaps as flood
basalts pouring layer upon layer of lava over a very extensive area, the
weight of which began to distort the crust of the planet.
- By Early Hesperian times, some of this was organized into
edifice-constructing volcanic vents, creating the patera and tholus
types of volcanoes, perhaps first at the Uranius group in northern Tharsis
Rise (Uranius Mons, Uranius Tholus, and Ceraunius Tholus), Tharsis Tholus,
and/or those west of Tharsis Montes (Jovis Tholus, Biblis Tholus, Ulysses
Tholus). Early Hesperian lavas covered a lot of terrain, but didn't get as
far as Thaumasia Planum.
- By Late Hesperian times, shield volcanism establishes itself on
Tharsis, eventually creating several very tall mountains. Alba Mons and then
Tharsis Montes had begun to build up. Lavas from Tharsis Montes only reached
to Solis, Sinai, and Syria plana.
- By Amazonian times, these and Olympus Mons spewed lava between and among
themselves west of Thaumasia.
- So, the whole series of Thaumasia plana exposes the history of lava flows
from Tharsis and the spatial confinement of Tharsis lavas even as Tharsis
Montes and Olympus Mons began to grow vertically to their current
unimaginable heights.
- Lunæ Planum
- Lunæ Planum is located between the central portions of the Valles
Marineris complex and the lowlands of Chryse Planitia. It lies west of the
Maja Vallis outflow channel and east of the massive Kasei Vallis channel.
- In many ways, it seems to be a continuation of the wrinkle-ridged plains
of Solis Planum in Thaumasia to the north of Valles
Marineris.
- The wrinkle-ridge features imply radial compression and
contraction on the circumference of the Tharsis complex.
-
Lunæ Planum may be a surviving exposure of the Early
Hesperian countryside affected by Tharsis' buildup and flood vulcanism.
- As seen in the discussion of the Syria-Thaumasia block, there is a
sequence of progressively younger surfaces from east to west, running from the
Noachian surfaces of Xanthe Terra through the Hesperian surfaces of Lunæ
Planum, to the Amazonian surfaces west of Kasei Vallis running up the Tharsis
Rise to the northern Tharsis Montes and the Uranius group of volcanoes.
- Unlike Solis Planum and the rest of Thaumasia, however, Lunæ Planum
is one of the surfaces cut by the great jökulhlaup outflows, being
bounded by Kasei Vallis on one border and Maja Vallis on the opposite border.
- Chryse Planitia
- Chryse Planitia is the semicircular embayment of the Northern Lowlands
that received the flows of the major outflow channels from Kasei Valles on the
northwest to Ares Valles on the east.
- Given its semicircular boundary with Xanthe Terra, Lunæ Terra, and
Tempe
Terra, it was long suspected that Chryse Planitia may be another gigantic
impact basin (Frey 2008), an impression supported by analysis of MOLA
topographic data, which has revealed quasi-circular depressions that may be
buried craters (Frey et al. 2002; Carr and Head 2010) and backed up by
Mars Express MARSIS data (Watters 2007). If so, this one is over 1,500 km
across!
- The surface of Chryse Planitia is overwhelmingly comprised of Hesperian
aged surface materials.
- These include Early Hesperian ridged plains (Hr1) with relatively
unaltered impact craters and sharp wrinkle ridges.
- Somewhat newer are similar plains materials (Hr2), but showing
modification of craters and wrinkle ridges, many of which are eroded or partly
buried, possibly by fluvial sedimentation in standing water bodies during the
major channel outwash flood events -- or by pyroclastic or volcanic flows.
- There are also drifts (large dune-line deposits), which are often
oriented northwest to southeast, though prevailing winds today are biased
northeast to southwest. These "shifts in the drifts" suggest a change in the
æolian régime from deposition to erosion, as many of the dune-
like features are scoured.
-
Many other æolian features were revealed by Viking 1, including
deflation hollows, tails of wind deposits in the lee of rocks and
scours along the base of rocks, leading to an appreciation of the power
of æolian processes on Mars, even though the atmosphere is less than 1%
as dense as our own.
- Further complicating the picture, close examination of drifts seen in
the Viking 1 panchromatic images and their reprocessing to enable stereoscopic
analysis revealed coarse granular material embedded within the drift deposits,
which are too large to be moved by the wind and, so, probably were emplaced
by water in the drifts after their formation.
- The ambiguities about the nature of the ridged plains, whether they are
of volcanic or fluvial origin, were not settled by analysis of the results of
the Viking 1 lander or of the Pathfinder lander and its Sojourner rover.
- Soil analysis showed elements seen with alteration of basaltic materials:
iron-rich phyllosilicates, some magnesian sulfates, and small amounts of
carbonates and iron oxides.
- This is consistent with several explanations: water alteration, global
homogenizing of a basaltic source rock turned into the ubiquitous dust,
Theiikian style sulfate-aqueous alteration. So, big help.
- The surfaces at both Viking (western Chryse) and Pathfinder/Sojourner
(eastern Chryse) were scattered with angular, often pitted rocks, mostly
andesitic and drastically different in chemical composition from the soils
nearby that the soils could not have been derived from these rocks.
So, they must have been brought to Chryse from elsewhere, perhaps in outflows.
- Vastitas Borealis
- Vastitas Borealis is a part of the Northern
Lowlands, a depression surrounding the North Polar Deposits,
mostly below -4,500 m.
- Judging from MOLA profiles, this is the apparent ultimate destination for
the fluids and deposits of the circum-Chryse outflow channel floods, which may
have periodically created or recreated the ocean in the Northern Lowlands.
- Vastitas Borealis is largely covered with Late Hesperian deposits.
- These feature a set of low linear ridges running roughly parallel
with one another in a north-northwest to south-southeast direction, which Head
et al. (2002) interpret as wrinkle ridges continuing from Chryse Planitia and,
indeed, from Lunæ Planum, Solis Planum, and Syria Planum.
- So, they seem to reflect the contractional stresses in the areas around
the periphery and circumferential to the Tharsis Rise.
- They are generally less distinct than these highland versions,
softened or partially buried. But by what? Hesperian lavas and ash flows?
or Oceanus Vastitas marine deposits?
- The Hesperian ridged plains are in places covered by younger Hesperian
channel deposits of some variety, no doubt emanating from the outflow channels
of the circum-Chryse Planitia regions of Lunæ Planum, Xanthe Terra, and
Arabia Terra.
- Under the ridged plains material is evidence of a profusion of buried
craters, some so indistinct in the MOLA data as to be termed "stealth
craters" (Head et al. 2002).
- The number and size distribution of these hidden craters imply a truly
antique surface is buried under the far younger surface materials of
the Northern Lowlands, making them at least as old as the battered Southern
Highlands surfaces.
- This realization has further undermined plate tectonic interpretations
of the great crustal dichotomy.
- To summarize this as an historical narrative, Vastitas Borealis seems
to had had a primordial basaltic basement of some sort after whatever process
operated to create the crustal dichotomy. This was crater-battered, and the
hidden craters suggest it may be as old as the earliest Noachian. As
volcanism ramped up in the Late Noachian to Early Hesperian, volcanic
materials (ash and lava) were deposited in the North Polar Basin, covering the
earlier surface and its cratering. This material was subjected to the
circum-Tharsis compressional stress field as the great volcanic complex began
to build up, creating wrinkle ridges in the basin. These were then subjected
to sedimentation, possibly at times under oceanic conditions, by the
stupendous outwash floods from the circum-Chryse outwash channels. These
seemingly marine deposits eroded and partly buried the wrinkle ridge surface
and the ancient craters.
- Isidis Planitia floor deposits
- I discussed the origins of the Isidis impact feature under the second
order of relief. Here, I want to focus on the Hesperian aged surfaces that
cover most of the Isidis Planitia floor.
- This forms a generally smooth surface interrupted by ridges of
wrinkle-ridge appearance and a scattering of mounds in the 500 m
diameter range.
- This surface has been interpreted as Late Hesperian lava flows
emanating from Syrtis Major to the west, possibly covered with sedimentary
deposits and perhaps some periglacial ice features.
- A very peculiar feature of this complex in western Isidis is
"thumbprint" terrain, which consists of strings of low, often fused,
conic structures that have circular depressions at their apices.
- The individual cones are generally about 400 or 500 m in diameter at
their bases and typically range from 10 - 50 m in height, averaging under 35 m
and rarely attaining 70 m in height.
- These strings, some of them several kilometers long, form subparallel
curves reminiscent of fingerprints, leading to the name for this kind of
landscape.
- Thumbprint terrain has been the subject of far-ranging hypotheses:
- volcanic features ("rootless cones" or phreatomagmatic
cones from the interaction of magma intrusions with soil ice and water)
and lava flow patterns
- ice-related structures, e.g., moraines (unsorted debris dumped by
a glacier), eskers (subglacier streams from basal melting), pingoes
(earth-covered ice lenses forming mounds, sometimes topped by crater-like
breaks at the top), or boundaries between ice floes.
- shorelines and sediments deposited in a body of standing water
- Syrtis Major Planum surface
- We already talked about Syrtis Major in the second order of relief, with
reference to the persistent low albedo feature that dominates it, the so-
called "Blue Scorpion" or the "Hourglass Sea."
- This turned out to be an area where the Mars global circulation features
prevailing winds that keep the surface clean of the otherwise pretty
ubiquitous bright dust, revealing a basaltic surface. This was produced by the
two calderas on the planum, Meroë Patera (the older, smaller, more
mafic of the two) and Nili Patera (larger, younger, less mafic). Both
may be vents for a single volcano or magma chamber.
- That surface includes:
- Basaltic lava flows that form a very slightly sloped surface, the kind of
very low viscosity flow seen in flood basalts and in the very early
vented edifices of Mars.
- Very interestingly, there is also some dacite on the floor of the
Nili Patera caldera, Nili Patera being the younger of the two volcanoes on
Syrtis. Dacite is an igneous rock that is somewhere between andesite and
rhyolite or granite in composition, that is, some of the most felsic and
silicic rock found on Mars, the product of advanced differentiation at
the top of a magma body. Mars' igneous rocks, as we've seen before, are
predominantly basaltic, ranging up to basaltic-andesite or andesite. So, this
is quite a find. Dacitic magmas, too, tend to be quite viscous or sticky,
loaded with gasses, which makes its eruption an explosive event.
- Very, very interestingly, there are all kinds of signs of water-altered
minerals, including phyllosilicate clays (neutral water) and
carbonates, the long-presumed but rarely seen mineral that one would
expect to form in any water interacting with a carbon dioxide atmosphere. The
next major rover, Mars2020, may be sent here to explore these minerals.
- Given the abundant evidence of water, it's odd that olivine has
also been found there, especially in Nili Fossæ (the concentric grabens
on Syrtis Major that center on Isidis Planitia). Olivine, being one of the
first minerals to crystallize at the highest temperature in a magma body, is
also one of the most susceptible to water alteration, changing to iddingsite,
serpentine, and magnesite. So, finding it here unaltered is intriguing.
- Extremely interestingly, Nili Fossæ is believed to be the major
source of the great methane plumes that have been sporadically detected
on Mars! Methane can have biotic origins. So, this area has become a top
priority for the next rover!
- So, the Hesperian was the shortest of the three martian time divisions.
It marked a drastic change in the way the martian system operates. With the
loss of the planetary magnetic field, the atmosphere becomes less and less
dense, eventually dropping below the triple point of water. Precipitation-fed
valley networks cease to flow, but the massive outflow floods begin. Volcanic
activity continues from the Noachian and volcanic activity shifts from
trap-style flood basalt eruptions to vented eruptions and the construction of
volcanic edifices, including the great shield volcanoes. Volcanic activity
also shifts from a fairly diffuse geography to one concentrated increasingly
in the Tharsis and Elysium rises. The effect of prodigious eruptions alters
planetary geochemistry, water shifting from a neutral or neutral-alkaline
state to a very strongly acidic condition. The shift in pH causes a drastic
change in the alteration products of water interacting with basalt. Even as
hydrochemistry shifts to a more acidic pattern, the planet becomes drier and
drier until, by the end of the Hesperian, the dominant geomorphic force is the
wind.
|
![[ 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)
