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Lecture Notes for the Midterm
History of Mars exploration
- The orders of relief: Scales of topographic variation
- See Viewgraphs:
"1st order: Crustal dichotomy."
- One of my goals in this class is to give you a vivid mental map of
Mars,
something that might stick in your mind years after taking this class, the way
you have a general sense of the different regions of our own planet, or the
USA, or California.
- A mental map tends to be a nested structure: finer areal units embedded
in coarser ones (Belmont Shore inside of Long Beach, inside of the Greater Los
Angeles Area, etc.), so I wanted to come up with a regionalization scheme that
had this sense of a nested hierarchy at multiple spatial scales.
- I decided to model it on an old scheme often encountered in introductory
physical geography or world regional geography textbooks: the "orders of
relief" scheme
- When I started looking more closely at these schemes, I found that they
are highly variable, almost idiosyncratic by author, and authors may discuss
anywhere from three to seven levels.
- This got me curious about the intellectual history of the scheme, one of
those canonical constructs often used by a discipline to tell its story, the
history of which gets lost in the mists of time.
- The "orders of relief" scheme goes back to a 1916 article that appeared
in the Annals of the Association of American Geographers, titled
"Physiographic subdivision of the United States" (physiography then a common
name for physical geography). It was written by Nevin M.
Fenneman, who was a geologist who often worked with the U.S. Geological
Survey. He worked comfortably both in geology and in geography and
founded the Department of Geology and Geography at the University of
Cincinnati (where he served as chair for thirty years, from 1907 to 1937. He
did a stint as president of the Association of American Geographers and
another as president of the Geological
Society of America. He developed his regionalization scheme under contract
with the AAG.
- His scheme divided the Lower 48 into a three part nested system of:
- physiographic divisions (e.g., the Atlantic Plain or the
Pacific Mountain
System)
- geomorphic provinces (e.g., the Pacific Border Province)
- sections (e.g., the California Coast Ranges)
- The USGS uses the scheme even today in an educational web site called "A
Tapestry of
Time and Topography" -- http://tapestry.usgs.gov/physiogr/physio.html.
- The system was translated into introductory geography textbooks, being
extended in scale into a planet-organizing system.
-
It's usually presented as a descriptive scheme (as in the Robert W.
Christopherson Geosystems many of you may have used in introductory
physical geography), but sometimes these days people may try to tie it to
plate tectonics (first order being the plates, second order being the features
that develop along the margins of plates, third order being largely erosional
and depositional features at a smaller scale -- as in Michael E. Ritter's
online physical
geography textbook (now hosted by our graduate alum, Lisa Pitts, Professor of Geography at Chaffey College).
- Depending on which textbook you look at, you may see schemes such as:
- First order: oceans vs. continents or oceanic plates
vs.
continental plates (updated for the plate tectonics era)
- Second order: major mountain systems vs. great lowlands on
a
continent and, in the oceans, continental rises and slopes, mid-ocean ridges,
abyssal plains, and subduction trenches. This is often equated with
Fenneman's physiographic division level, sometimes with comments about regions
dominated by endogenous Earth processes (as opposed to exogenous,
erosion/deposition dominated processes). One author equates the second order
with great regions resulting from plate collision or divergence.
- Third order: sometimes equated with Fenneman's geomorphic
province,
sometimes described as the "local landscape" level, the local contrasts
between mountains and valleys, for example. Some authors leave off at this
level; others define it more coarsely but attributable to minor tectonic
forces.
- Fourth order (if it's used): equated with Fenneman's section, and
there
are a few who take it to finer and finer scales, one taking it down to
features under 5 sq. km in size.
- In other words, kind of a mish-mash of systems, each being used as a
pædagogical device shaped to a particular author's ends, sometimes tied
to Fenneman's original scheme.
- A novel variant on the idea was proposed by Richard Dikau, a geographer
at the University of Bonn, in the 1980s and 1990s. He breaks down the scale
of topographic relief as:
- picorelief (area up to ~1 cm2, e.g., glacial striations
- nanorelief (area from ~1 cm2 up to ~1 m2,
e.g., erosion rills on a slope)
- microrelief (area from ~1 m2 up to ~ 1 hectare, e.g.,
gully, dune, landslide)
- mesorelief (area from 1 hectare up to ~100 km2, e.g.,
valley, hill, moraine)
- macrorelief (area from ~100 km2 up to ~1,000,000,
km2, e.g., mountain range, such as the Sierra Nevada, or
major valley, such as the Great Central Valley of California)
- megarelief (area larger than a million square kilometers, e.g.,
cratons, such as the Canadian Shield)
- kind of the same idea as the classic physiographic subdivision or orders
of relief scheme, trying to tie it to a systematic metric scale.
-
No matter the breakdown, they represent the geographical attraction to nested
hierarchical spatial schemes and reflect geographers' concerns, not only with
spatial analysis and regional synthesis, but with the scales at which regions
and processes operate and interactions across scales. This is a tendency
across the different subfields of geography:
- In spatial statistics, there's the Modifiable Areal Unit Problem (MAUP)
- In human geography, there are local cultural and political responses to
global economic and political processes
- Biogeography works with alpha, beta, and gamma measures of
biodiversity
- Geomorphologists have recently been addressing "megageomorphology" as
remote sensing technology has made the simultaneous examination of form and
process at large scale (small map scale) possible
- I think the idea of "orders of relief" is a handy descriptive scheme for
organizing the emerging geography of Mars and conveying an intelligible mental
map of the planet, to make Mars a real place to you, not just a pale orange
dot in the sky.
- In this spirit, the next section presents a five orders of relief
scheme
for the physiographic characterization of Mars.
- I'll be comprehensive in discussing the first three orders and then
selective at the far more numerous examples at the fourth and fifth levels.
-
It should be noted that there are some ways in which the proposed scheme
departs from the nesting hierarchical conceptual structure of the classic
orders of relief scheme.
- The fourth order nests tidily within the third order, and the fifth order
nests within the fourth. So far, so good.
- The third order, however, does not nest within the second order but
within the first order.
-
This is because the second order does not nest tidily
within the first order (e.g,, the great impact craters are found on
both sides of the dichotomy)
-
Third order regions sometimes contain parts of second order structures
(e.g., the Chryse Trough cuts through a giant impact crater, Argyre,
and the third order landscapes of Noachis Terra and Margaritifer Terra and
into Chryse Planitia north of the dichotomy).
- And there are two ginormous structures comprising the first order (the
great crustal dichtomy and the Tharsis bulge) and the latter sits right on top
of the former.
- And while I'm at my mea culpas, the third order is organized not
so much on a spatial basis as it is on a temporal basis: This is the order I
use to discuss deep time in the martian landscape and the geological time
scale used there. The resulting areal units, though, are fairly comparable in
scale.
-
As much as this offends the sense of the orders of relief as a nesting
hierarchy of spatial scales, the main purpose of the original scheme was to
build a mental map of Mars.
-
The second order evolved out of the need to use
the small set of visually or conceptually conspicuous markings on the martian
landscape to create a memorable network of landmarks. These, then, permit the
anchoring of references to other, lower order regions. These could be
"placed" on the progressively more detailed
mental map of Mars.
- In a manner of speaking, the first-third-fourth-fifth
orders of relief represent the spatial scale dimension of the orders of
relief, and the second
order is "orthogonal" to it, representing a dimension for
conspicuousness/memorability.
- I hope these necessary departures from tidiness don't blow the whole
point of all this, building your mental map of Mars as a place. So, with all
these caveats, let's start in on the exploration of Mars!
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![[ 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)
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![[ image of Mars ]](https://home.csulb.edu/~rodrigue/mars/marsatlas.gif)