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Lecture Notes for the Midterm
First order of relief: Features covering at least a quarter of the
planetary surface
- The great crustal dichotomy
- See Viewgraphs: "First order dichotomy: Endogenous or
exogenous?"
- So, which processes could have created the great crustal dichotomy
in the first place? Are they endogenous (the result of processes
internal to Mars) or exogenous (the results of something external to
Mars)?
- Here is a summary of the endogenous arguments:
- On Earth, the first order of relief is endogenous, the result of
plate tectonics:
- Earth, like the other terrestrial planets, accreted from dust and
gas in the disk surrounding the developing sun.
- Like them, it accumulated heat:
- from constant impacts (accretion meant the extreme deceleration of
extremely fast moving chunks, converting the kinetic energy of their movement
into a lot of thermal energy, some of which went towards building up the heat
inside the early earth)
- from the decay of certain radioactive elements (e.g., uranium,
thorium,
and 40K),
- and gravitational compression.
- Like them, it is believed to have undergone melting once the
endogenous heat approached ~1,800° at 500 km depth within a few hundred
million years and ~2,200° at 1,800 km depth by 1.2 billion years . At
these temperatures and depths, iron melted and blobs of it began dropping
toward the center of the earth to form an iron or nickel-iron core ("the
iron
event").
- This differentiation or separation of minerals began to push
lighter elements and minerals toward the surface.
- Within a few hundred million years of its formation, heat accumulation
formed a magma ocean in the mantle and surface.
- This then cooled from the surface inward to form a thin crust,
perhaps unevenly ("rockbergs," as John Longhi called the early solid bits) on
the mantle below.
- Minerals began to crystallize, following the
Bowen's Reaction Series. Each
mineral has its "freezing" point temperature, which varies with
pressure (higher pressure results in higher temperatures required for the
liquid/solid phase shift; a reduction in pressure, holding temperature
constant, can result in melting).
- Crystals forming in the magma ocean drift downward to accumulate in great
stacks, the "cumulate pile."
- A plot complication is that a gravitational instability develops
out of a kind of "mismatch" between the temperature of crystallization and the
density of the mineral involved.
-
That is, olivines
are among the earliest minerals to crystallize and drift toward the bottom of
the cumulate pile: (Mg,Fe)2SiO4.
- Olivines with magnesium form first, then those with a mixture of
magnesium and iron, and then those with iron.
-
Magnesium-dominated olivine (aka forsterite),
however, is 3.2 times as dense as water, but iron-dominated olivine (aka fayalite) is 4.3
times as dense as water.
-
So, differentiation of the magnesium olivine from
the iron olivine creates a situation where the lighter version of the mineral
occupies a lower depth than the heavier version, which is an unstable
situation.
-
The same sort of effect goes on with the next mineral to emerge
out of the cooling magma and reactions with the olivine: pyroxene's various subtypes.
- Eventually, this instability leads to an overturn of the mantle,
pushing the heavier parts of the crystal pile toward the interior and the
lighter parts can get then get past the heavier ones above them and move
upward, which then creates the familiar gravitationally stratified iron
core/ultramafic mantle/mafic lower crust/increasingly silicic upper crust that
you first learned about in some GE course.
- The heavier minerals that had been on the exterior of the cumulate pile
are, therefore, cooler, so, when they sink down to the mantle-core boundary,
they create an intensified temperature gradient between the mantle and the
core and, thus, between the outermost core and the lower part of the liquid
core. This causes churning in the liquid core, which is believed to
initiate the planetary magnetic field. Earth's is still going; Mars'
isn't.
- On Earth, mantle overturn was followed by the initiation of tectonic
processes, that first movement of a single upward rise of material and a
complementary downward sinking of material elsewhere ("degree-1
convection").
- There's been a lot of work done to figure out whether there's a
degree-1 convection system on Mars.
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If so, perhaps it can account for the crustal dichotomy:
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That is, the crust
can be stretched and thinned above the upwelling plume (as under Earth's oceans). This is related to
the tension developing in the crust above the plume and also ablation
(erosion, scraping away) of the lower crust by its outward flow in the
æsthenosphere under the Northern Lowlands.
-
This could also build up great crustal depth under the Southern Highlands
above the downwelling branch (Zhong and Zuber, 2001, Earth and Planetary
Science Letters). Crustal materials are compressed and bunched up there, as we see on Earth (e.g.,m the Himalayas and the Andes).
-
Some versions completely reverse the
argument, saying that the zone of upwelling would be topped by higher
elevation country and a thicker crust, such as perhaps under Tharsis, which
might account for what is holding that monumental lava pile up without
depressing the crust below it (isostatic depression).
- There's been some cool simulation work to establish whether degree-1
convection could create a huge single upwelling plume: See http://www.pmc.ucsc.edu/~jhr/research/oneplume.gif
for a humongous animation of a model created by James H. Roberts, who was a
post-doc at Johns Hopkins Applied Physics Lab in 2010.
- Related to the question of degree-1 convection has been debate over
whether Mars ever had incipient plate tectonics and what sorts of
features would evidence it.
- No trench: Nothing analogous to a compressional zone trench can
be found.
-
Thickening of the crust has been proposed as evidence of a
compressional zone in the 1990s (Sleep,
1994, Martian plate tectonics, JGR: Planets).
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A south-dipping plate boundary was proposed for the Terra Cimmeria borderlands
with Amazonis Planitia and Elysium Planitia.
-
Alternatively and not compatibly with the first version, an east-dipping plate boundary was proposed for a zone running from
Dædalia Planum along the western edge of Tharsis, perhaps resulting in the Tharsis Montes that resemble a gigantic version of a volcanic arc on Earth above a subduction zone (e.g., the Cascades above the Nisqually subduction zone, the Indonesian archipelago between the Australian plate and the Pacific and Eurasian plates).
- There was a lot of excitement in 1999, when Mars Global Surveyor's MAG/ER
(MAGnetometer and Electron Reflectometer) showed banded magnetization of
rocks through much of the Southern Highlands, which was especially strong
in Terra Cimmeria and Terra Sirenum south and southwest of Tharsis
(Acuña et al., 1999, Science). They looked like what you
might find in a terrestrial spreading zone.
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When the changes in magnetization field strength with latitude were mapped, a
striking pattern of alternations in these changes showed up in a series of
bands about 600 km long and 100 km wide in Cimmeria and Sirenum, with
blotchier and more broken echoes of this pattern in Arabia, Noachis, and
Promethei terræ and mostly missing in the four great impact craters and
Tharsis.
- This pattern seemed to recall the famous bands of alternating magnetic
field orientation preserved in the basalts of Earth's ocean floors in both
directions on either side of a mid-oceanic ridge system.
- There were claims that, at last, martian plate tectonics had been
discovered, including some NASA press releases, which you can find online.
- Sober reconsideration has dampened this extrapolation in the decade since
then. The preserved record of the martian planetary magnetic field, while
alternating rhythmically in strength, does not show the change in magnetic
orientation associated with pole reversals recorded in Earth's oceanic
basalts. Moreover, there is no pairing of bands in such a way as to identify
a spreading zone.
- Other explanations for the banded magnetization than plate tectonic
spreading have been mentioned, such as the formation of dikes of rising
basaltic lava filling cracks in the crust and then, as they solidified,
preserving a record of the then-existing planetary magnetic field as the iron
in the basalt aligned during solidification.
- Once again, Mars pulls this "yes, but ..." thing.
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The MGS MAG/ER team has responded with analysis of fault systems they
argue are consistent with plate tectonics, showing spreading zones and
rotational transform or shear zones: Connerney et al., 2005,
PNAS, available at http://www.pnas.org/content/102/42/14970.full.
- Cerberus Rupes is a fault, part of the Cerberus Fossæ fracture
system, which runs for about 2,000 km from southeast of Elysium to west of
Dædalia Planum, parallel with the magnetic lineations in Terra Sirenum
and Terra Cimmeria. These fossæ look like grabens produced by normal
faulting in areas of tensional stress, with downdropped blocks between pair of
faults.
- Valles Marineris is an even larger and wider complex of faulting and
landsliding, which Connerney et al. cite as bounded by parallel bands
of magnetization, positive to the north and, symmetrically, negative to the
south.
- The team has picked out what may be hitherto unrecognized great fault systems that
run roughly parallel to one another about 1,400 km apart in the Noachis Terra,
Arabia Terra area between Hellas Planitia and Argyre Planitia and passing
north of Isidis Planitia. They seem centered around an axis of rotation just
northeast of Hellas Planitia. They find continuations of the magnetic banding
across these two faults, but with slight but visible offsets consistent with
lateral motion along transform faults.
- The equation of the banded magnetic anomalies as evidence of a spreading
zone has been turned on its head by Fairén et al., in a 2002
paper in Icarus. They argue that, instead, the banded magnetization
pattern reflects a convergent plate boundary, which accumulated
terranes (small plate fragments) of different magnetic properties. This is consistent with the
greater thickness of the martian crust in the Terra Cimmeria and Terra Sirenum
areas and the most common interpretation of degree-1 convection having a
downwelling region under the Southern Highlands. Their paper is available at
http://eprints.ucm.es/10428/1/6-Marte_2.pdf.
- So, while the enthusiasm for magnetization-based theories of martian
plate tectonics reached a crescendo in the 1999-2005 time frame, the community
seems unconvinced, but the topic remains an energetic area for research.
- An alternative line of argument concerns whether the martian crust is
too thick to allow subduction, even in the face of the extensional and
compressional stresses of degree-1 convection below ("stagnant lid"
convection). Most versions of this argument place the upwelling plume
somewhere under the Northern Lowlands and the downwelling branch somewhere in
the Southern Highlands, helping to account for crustal thinning in the
Lowlands and thickening in the Highlands.
- Mars' crust is about 50 km thick on average + 12 km,
Wieczorek and Zuber, 2004, JGR: Planets, and ranging up to 75 km
thick in parts of the Southern Highlands and down to 25 km thick in the
Northern Lowland, as seen in this crustal thickness map, https://mars.nasa.gov/system/downloadable_items/39133_gravity-mars-map-topography-PIA20277.jpg.)
- Earth, by contrast, has oceanic crust less than 10 km thick,
ranging up to 40 km thick on the continents (and 70 km in the Tibetan
Plateau), as seen in this "isopach" map or map of equal crustal
thickness: http://earthquake.usgs.gov/research/structure/crust/images/topo.jpg.
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It should be noted that analyses combining MOLA topography with gravitmetric
measurements have shown discrepancies between the equation of thin
crust under the Lowlands and thick crust under the Highlands. It isn't a one-
to-one match. The thinnest crust, actually, lies under Arabia Terra, as seen
in this block diagram: http://mola.gsfc.nasa.gov/images/crustalthick.jpg.
- There's been some work, too, arguing that the proposed single-plate,
stagnant lid view of Mars does not preclude drifting motion of the
lithosphere with respect to the core and any upwelling plumes. See
Kobayashi and Sprenke, 2010, Icarus. If you are curious about this,
you can get the abstract for this article at http://www.sciencedirect.com/science/article/pii/S001910351000237X,
and CSULB has an electronic subscription to the journal.
- If, in fact, the stagnant lid argument holds, the lack of plate tectonics
offers a coherent mechanism for the collapse of Mars' planetary magnetic
field: the failure to bring cooler mantle material into proximity with the
outer core reduces the temperature contrast between mantle and core and
between the outermost core and the part of the core just interior to it.
-
This
reduces the forces causing circulation in the liquid outer core, weakening and
then collapsing the planetary field.
-
This also implies that heat loss from
the core is far less efficient on Mars than on Earth, which means that the
outer core could still be liquid.
-
Earlier, it was thought that the smaller
Mars had lost its internal heat faster than Earth and, so, the core cooled and
solidified and that's why the planetary magnetic field shut down.
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My impression of the general consensus these days is that the majority of the
community is not convinced of past or present plate tectonics on Mars, but
degree-1 convection is discussed as a possible endogenous mechanism for
thinning the Northern Lowlands crust and thickening the Southern Highlands
Crust. In this framework, the crust never failed in such a way as to develop
plates and subduction, which would make convection less vigorous than on
Earth. This less vigorous convection would reduce motion within the core,
too, perhaps leading early to the collapse of the planetary magnetic field.
The field had collapsed by ~4.1 to ~3.9 billion years ago, so that when the
great impactors of the Late Heavy Bombardment struck Hellas, Argyre, Isidis,
and Utopia, there was no field to imprint itself through iron alignment in the
resolidifying basalts.
- That said, there are a number of people who are still exploring the
possibility that plate tectonics may have initiated, if not gotten very far,
or that there are other plate tectonic-like processes operating more recently.
These focus on such features as the magnetic lineations found in Terra Sirenum
and Terra Cimmeria,
signs of strike-slip motion in central Valles Marineris (An Yin at UCLA),
shallow plate-like mass movement perhaps in the Thaumasia block, stresses
associated with the sheer size and gravitational effects of Tharsis, and
hunting
for faults. These, however, are less concerned with applying plate tectonic
theory
to an understanding of the crustal dichotomy than with analysis of features of
smaller dimensions.
- An alternative approach is exogenous: Maybe something struck the Northern Lowlands and excavated a lot of it.
- An earlier proposal for impact excavation of the Northern Lowlands was put forward in 1988 by Frey and Schultz. This earlier exogenous model is called the multiple large impacts model and held that the Northern Lowlands was the result of several large impacts, which would have produced several basins: Utopia Planitia, Chryse Planitia, Amazonis Planitia, Vastitas Borealis. This model was piled on by many critics, who raised questions about why the Northern Hemisphere would have the target of all of these lowland-producing impacts. They pointed out that there are no gravimetric anomalies or topographic highs between such surface impacts.
- Of growing stature in the debate about the first order crustal dichotomy
is another attempt at an exogenous explanation: Mars was struck obliquely by a single very large
impactor very early in its history, and the Northern Lowlands is, basically,
the largest impact crater in the solar system.
- As far back as the 1980s, there'd been speculation that maybe the crustal
dichotomy had something to do with a doozy of an impact or multiple such impacts, but the non-circular
shape of the basin raised skepticism. Nearly all high speed impacts generate
nearly perfectly circular craters, even if the object comes in at an oblique
angle. The crater is a product of detonation, which is relatively
symmetrical.
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Complicating the issue of the shape of the proposed crater is
the existence of the Tharsis bulge, which blurs the boundary between the
Northern Lowlands and the Southern Highlands, making it hard to determine the
shape of the Lowlands boundary. Chryse Planitia, too, seems to be a later
impact basin embroidering the edge of the proposed huge impact basin.
- In 2008, Jeffrey Andrews-Hanna, Maria Zuber, and W. Bruce Banerdt
re-examined this idea in an article published in Nature. The lecture viewgraphs show maps of the developing argument and you can get
to the article itself here: https://www.researchgate.net/profile/William-Banerdt/publication/5275120_The_Borealis_basin_and_the_origin_of_the_Martian_crustal_dichotomy/links/00b7d539641c638e07000000/The-Borealis-basin-and-the-origin-of-the-Martian-crustal-dichotomy.pdf
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They used the MOLA topography and the gravimetric representations of crustal
thickness to model the isostatic balance of the crust compensating for
the flexure created in the crust to provide support for the massive Tharsis
volcanic complex.
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The model shows the crustal dichotomy border continuing under Tharsis -- and
forming a nearly perfect ellipse about 10,600 km by 8,500 km, centered
around 67° N and 208° E in Vastitas Borealis (Arcadia Planitia)
northwest of Alba Mons.
- The authors also point out that such an impact would generate a
multi-ring crater structure, and they suggest that a secondary scarp in
Arabia Terra might be part of such a circumferential secondary ring structure. Arabia Terra has unusually thin crust for a Southern Highlands region, with two areas of strong change in slope: one is the crustal dichotomy and the other is set back from the dichotomy about 2,000 km.
- The upshot of it all would be that the tremendous mantle melt under the impact would rise toward the excavated surface and spread out, forming the thin crust of the Northern Lowlands. See the viewgraphs for diagrams of this process from Robert Citron (2021)
- Zuber, one of the authors, commented that this project forced her to
re-evaluate her own decades of work under the endogenic framework: She had
done a lot of the work on establishing the thickness of the martian crust,
which was part of the debate about degree-1 convection and the stagnant lid
hypothesis. She is now a convert to the exogenous framework, which is quite
something. Perhaps scientific paradigm shifts do not depend on "demographics" after all, as Thomas S. Kuhn had argued in The Structure of Scientific Revolutions!
- Zuber isn't the only one having an impact epiphany: This work has gotten
a lot of people to re-examine the origins of the first order crustal
dichotomy, and my impression is that this is now the leading account for its
formation.
- This idea didn't go too long without some counterpoint models being put forward, however!
- In 2010, Reese and Solmatov put forward a diametrically opposed giant impact model: The impactor struck right over the South Pole, striking a planet with a thin-walled crust. The energy from this event was so tremendous that a very large proportion of Mars' mantle under the South Pole melted and this melt began to rise and spread out forming an impact megadome, a build up of resolidified magma into a very thick crust. gain, see the viewgraphs for diagrams.
- Reese et al. (2011) put out a kind of matrix among the size and energy of the impactor, the thickness of the initial crust of Mars, and which hemisphere was struck from outer space. If the planet had a thick crust to begin with and was struck by a (relatively) modest impactor in the Northern Hemisphere, the rise and spreading of the melt would solidify a thin Northern Lowlands crust. If the planet had a thin initial crust and the bomb was hugely energetic and struck the South Pole, the large mantle melt would rise and spread to become an impact megadome or large expanse of very thickened crust forming the Southern Highlands. See viewgraphs for diagrams.
- If you'd like to play around with impactor trajectories and
masses on Earth and the resulting craters, do help yourself to https://impact.ese.ic.ac.uk/ImpactEarth/
(thanks to Ms. Allie Stoddard, Mars class of 2012, for sending me this link!). An expanded version can be found at http://simulator.down2earth.eu/planet.html?lang=en-US.
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