Lecture Notes for the Final
Second order of relief: gigantic features and the dominant processes
shaping the martian surface
- Previously, we discussed the great impact craters of Mars.
- These are the ultimate in exogenous processes: Off-planet intrusions
- The next second order features are endogenic in character, originating
from tectonic processes (if not plate tectonics), that is, processes internal
to the planet, which tend to increase topographic contrast:
- Volcanic processes: The other great volcanic rise (Elysium)
- Rifting: Valles Marineris
- See Viewgraphs:
"2nd order: Elysium Rise, Valles Marineris"
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The other great volcanic rise: Elysium Rise
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Another huge rise, dwarfed only by the sheer scale of Tharsis
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"Only" 2,000 km
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"Only" 6 km thick
Also houses multiple volcanoes:
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Elysium Mons on the west central side of the rise (12.5 km high)
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Albor Tholus to the southeast (4.5 km high, with a 3 km deep caldera!)
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Hecates Tholus to the northeast
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Hecates Tholus, the northeastermost of the group:
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may have been volcanically active at least as recently as 350
million BP and this
looks like an explosive event creating a flank caldera on the northwest side
of the volcano
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There are signs of glaciation that may date to about 5-24 million years
ago, judging from crater counting:
- An article
by a team led by Ernst Hauber, based on Mars Express HRSC
data, discusses an elongated depression running NE to SW at the bottom of the
northwest slope of the volcano (~45 km by 20 km)
- It contains some 50 m wide ridges that look like terminal moraines
on
Earth, great unsorted heaps of debris bulldozed out by the glacier and piled
up at its snout.
- Another, shorter depression is completely full of striated
materials
running downslope, and these have some cracks perpendicular to them that look
like material that would be deposited in crevasses and then exposed as
the glacier melted or sublimed back
- There are steep sided valleys pouring out onto the top of the bigger
depression: Could these be hanging valleys carrying materials onto the
top of
the "valley glacier"? On Earth, Yosemite Valley was occupied by a
valley glacier and is bordered by such hanging valleys, from which its
spectacular waterfalls issue.
- These features have few craters on them, implying an age of
something
like 100-ish million BP
- Similar young-surface features have been reported on the
northwest flanks of Olympus, Arsia, Pavonis, and Ascraeus, too
- Ice age? Note the mid-latitude or even "sub-tropical" latitudes of these
potentially glacial features, far from the polar regions. Changes in
obliquity or axial tilt are strong drivers of climate change on Mars, with
high obliquity creating conditions for the sublimation of the polar ice caps,
increase in atmospheric density, and precipitation of water vapor as snow in
the mid-latitudes, even to the point of creating glaciers.
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Elysium Mons, the west-central volcano of the group, may have erupted as
recently as 20 million BP, meaning it might be an active
volcano (error bars on this date X 4: 80 million BP to 5 million BP)
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The "recent" vulcanism has put dust in the eye of the traditional theory that
Mars, being a dead planet with a cooled core, stopped being volcanically
active two billion or more years ago!
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As with Hecates, Elysium may have been glaciated, but 5-24 million BP, even
more recently than around Hecatoes Tholus, judging
from glacial deposit features and crater counting on the Hecates flank caldera
and nearby depressions:
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Again, there is (and cannot be) stable ice at these low latitudes now, though it may be possible for glaciers to perist if they are covered in enough dust to protect them from sublimation at these low temperatures and pressures.
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Such glaciation suggests climate change on Mars, particularly associated with changes in obliquity. NASA has a nice graphic explaining the distribution of ice and snow at different tilt angles of the planet's axis of rotation: https://www.nasa.gov/mission_pages/msl/multimedia/pia15095.html.
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Again, tantalizing suggestions of an ice age on Mars depositing ice, snow, and glaciers in the mid-latitudes.
- Albor Tholus is the southeasternmost of the group.
- It shows signs, along with Hecates Tholus, of initiating v olcanic activity back in Late
Noachian times during the transition to the Hesperian, perhaps 3.8 Ga ago.
- Of the three, it has the oldest caldera floor, so it was probably the
first to enter dormancy.
- Indeed, its northwest flanks have been partially covered by lavas that
come from Elysium Mons.
- It does show a number of collapsed lava tubes, especially on its south
flanks.
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Rifting on a grand scale: Valles Marineris
- Overview:
- Extensional rifting, related to the extensional stresses on the
Tharsis
Rise
- Pitting, which is another indicator of extensional strain -- thought to
reflect dilational faulting, which creates voids below, into which
unconsolidated surface regolith collapses, creating subsidence pits
- Water or water mixtures in subsoil or, in Nick Hoffman's "White Mars"
argument, carbon
dioxide ices or mixtures
- Landslides, many of them very long-runout events (see Jessica Watkins'
discussion of the dependence of these on lubrication provided by the clay
minerals created by the wetter conditions of the Noachian at the Planetary
Geomorphology blog).
- Massive outflows, like jökulhaups on Earth when vulcanism-related
warming
hits a glacier or ground ice or when an ice dam or moraine dam liberates a
huge lake (for Earth examples, see the HUGE
floods website).
- Not quite a canyon in the Earth sense, since the eastern end is higher
than the center (as those of you who profiled Valles Marineris in the Gridview
lab found out).
- Subsidiary chasmata
- Ius Chasma in the west on the south side (note the alcove-headed
short
tributaries, so like groundwater-fed networks in arid regions in the American
Southwest)
- Melas Chasma in the middle on the south side, some 9 km below the
edge of
the surrounding plains, shows some sulfates on its floor and sides, which
could indicate the presence of a lake here.
- Coprates Chasma to the east on the south side, the location of the
subsidence pits I showed you in discussing extensional stresses.
- Eos Chasma, the southern fork on the east side, shows patches of
chaos
terrain toward the west and the kinds of braiding patterns and flow structures
that add to the impression that Valles Marineris once carried water, yet it
also contains a layer of exposed olivine toward the bottom, which weathers
rapidly in the presence of water. Perhaps Mars dried up quickly after the
olivine layer was exposed?
- Capri Chasma, the northern fork on the east side, has
hæmatite
"blueberries" like those in Meridiani that Opportunity imaged. Hæmatite
is an iron (III) oxide ((Fe2O3) that can be formed from
prolonged exposure of iron to water.
- Tithonium Chasma in the west to the north of Ius shows deep
layered
deposits of sulfates and iron oxides, suggestive of water alteration: The
layering basically goes all the way down the sides of the canyon for
kilometers. Could these indicate miles of sedimentary deposition?
- Candor Chasma in the center north of Melas and south of Ophir. It
is
itself split into two sections, East Candor and West Candor. Calcium sulfate
and kieserite (hydrated magnesium sulfate, or MgSO4-H2O)
have been identified by the OMEGA spectrometer on Mars Express, and these are
commonly products of water alteration.
- Ophir Chasma is on the north end of the main Valles Marineris
sequence of
chasmata. It features landslides on a stupendous scale.
- Ganges Chasma to the east north of Coprates/Eos/Capri, that "Rat
Fink
hotrod" shaped canyon, where Lab 1 was situated. This canyon also shows
olivine, a mineral that alters very rapidly in the presence of water, so its
presence here goes against the impression of water alteration minerals in
other canyons (unless the climate drastically dried immediately after the
olivine layer was exposed).
- Juventae Chasma off to the northeast is an almost totally boxed in
canyon, with an exit to the north, at the head of Maja Valles, a major outflow
channel forming the boundary between Xanthe Terra and Lunæ Tera. It
contains a mountain about 2.5 km high, which is made of sulfate deposits. The
canyon shows a number of water-altered minerals.
- Hebes Chasma off to the northwest shows exposures of gypsum (a
very soft
sulfate mineral, CaSO4-2H2O. It is an evaporite,
suggesting a wet phase in Mars' history.
- Echus Chasma to the immediate west of Hebes, forms the head of the
enormous Kasei Valles. It also shows a sickle-shaped dike. The vast
outpouring down Kasei Valles may have been triggered by dike formation, which
would catastrophically have liberated huge amounts of frozen groundwater.
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Emergent Rifting: Cerberus Fossæ
- Another rift system has developed far west of Tharsis in the Elysium Planitia area to the southeast of Elysium Rise. Cerberus Fossæ run for over 1,200 km a few degrees north of the equator and some 20° of longitude, from southeast of Albor Tholus, petering out south of Orcus Patera.
- They form an interrupted rift zone, often with multiple, nearly parallel graben.
- These run roughly at right angles to the wrinkle ridges on the periphery of the Elysium Rise lava flows, possibly implying association with crustal stresses associated with the uplift of Elysium Rise, perhaps Elysium's answer to Tharsis' Valles Marineris.
- This rift zone is geologically active now!
- A 2021 paper by Horvath et al. (10.1016/j.icarus.2021.114499) found evidence of pyroclastic flows from explosive fissure eruptions along Cerberus Fossæ as recently as 53,000 years ago! These are in an area with fissure-sourced effusive lava flows that are also pretty young: 0.5 to 2.5 million years ago. This kind of activity finding its way onto the surface through this rift system implies that Mars vulcanism is still a thing!
- A 2022 paper by Stäher et al. (doi: 10.1038/s41550-022-01803-y) reports on two relatively large marsquakes detected by the InSight lander's seismometer. With only one seismic station, the epicenters can't be triangulated: All you get is a distance implied by the different arrival times of primary and secondary seismic waves. But you can infer the epicentral region by searching for features at that distance that can generate quakes, and the Stäher team was able to pin these quakes on Cerberus Fossæ! Again, this rift is quite active!
- Another pretty recent activity along Cerberus Fossæ may have entailed jökulhlaup-type outflows from the rift, pouring out into Athabasca Valles, which crosses Cerberus Fossæ at roughly right angles south-southeast of Albor Tholus (see, for example, Burr et al. 2003 paper (doi:10.1006/icar.2002.6921). The materials of Athabasca Valles are young, around 20 million years, which would make the Cerberus-sourced outflow the youngest on Mars. The idea is that a buried cryosphere under hydrological pressure from below the ice was breached by a rifting episode along Cerberus Fossæ and that allowed the explosive outflow, forming and racing along Athabasca Valles.
- Not everyone is on-board with the outwash megaflood idea. An alternative interpretation of the Cerberus Fossæ and Athabasca Valles relationship posits really low viscosity, effusive, runny lavas instead of water. So, the mechanism would be a fissure eruption of flood basalt, channelized into Athabasca Valles. Some papers presenting this interpretation include Jaeger et al. in 2007 (doi: 10.1126/science.114331).
- Whether Cerberus Fossæ is the source of megaflood events or of very effusive flood lavas, explosive pyroclastic flows, or earthquakes registered by NASA InSight, this emergent rift zone has clearly shown a variety of very recent geological activity. Activity may be much less than in Noachian, Hesperian, and early Amazonian times, but the planet is still active!
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