The areas of chaotic terrain, the giant chasma of the Valles Marineris region, the complex linear and circular depressions of Labyrinthus Noctis on Mars all suggest the possibility of large scale collapse of parts of the martian crust within equatorial and sub equatorial latitudes. It seems generally accepted that the above features are fossil, being perhaps, more than a billion years old. It is possible that parts of Earth's crust experienced similar episodes of large scale collapse sometime early in the evolution of the planet

Sharp, R. P.

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MEGAGEOMORPHOLOGY: MARS VISA VIS EARTH
Robert O. Sharp
California Institute of Technology
Pasadena, California
The following comments are based largely on Mars vis-a-vis
Earth. Personal knowledge of Moon, Venus, Mercury and other
solar system bodies is too scanty for them to be included herein.
Outrageous Hypotheses
One of the greatest benefits of the planetary exploration
program could be to blast us out of intellectual ruts, forcing
consideration or reconsideration of different, unusual, even
seemingly outrageous explanations for puzzling natural phenomena.
The areas of chaotic terrain, the giant chasma of the Valles
Marineris region, the complex linear and circular depressions of
Labyrinthus Noctis on Mars all suggest the possibility of large-
scale collapse of parts of the martian crust within equatorial
and sub-equatorial latitudes. It seems generally accepted that
the above features are fossil, being perhaps, more than a billion
years old. Is it possible that parts of Earth's crust
experienced similar episodes of large-scale collapse sometime
early in the evolution of the planet?
We may have failed to recognize features created by regional
collapse simply because of their scale or more likely because of
alteration and obscuration by metamorphism, erosion, weathering,
and burial, or any combination of those processes. It is
difficult to find something if you don't know what you are look-
ing for, lost car keys or trilobite fragments for example. What
would a billion- year old area of chaotic terrain look like on
Earth? Like the assemblage of rocks and structures constituting
some terrestrial metamorphic complexes, for instance? Do some of
our thick, multikilometer, localized accumulations of sedimentary
rocks occupy ancient collapsed chasma? Does the seemingly
nonsensical distribution of lithological units within some
ancient complex rock terranes make sense if viewed in the
pattern of martian labyrinth features?
An even more fundamental problem is the possible cause of
large-scale regional collapse. On Mars deterioration of massive
bodies of ground ice or the migration of magma from one area to
another, to create gigantic volcanic constructs, have been
proposed. Perhaps, it is time we looked again at the possibility
of planetary expansion for both Mars and Earth. This old idea
has not been generally welcomed to the banquet table by a
majority of earth scientists, although vigorously sponsored by
some, because of a seeming lack of adequate energy sources to
produce significant expansion. Some of the discrediting
calculations, however, have been made for an axial expansion of
1000 km, which seems an excessive requirement, or may not have
looked far enough back into planetary history.
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Planetary expansion might logically be expected to produce
planet-wide manifestations. On Mars, visible surface features of
collapse are located in equatorial and sub-equatorial areas, they
are not planet-wide. Since both martian polar and subpolar
regions, especially the north appear to have been sites of
extensive deposition, collapse features there might be buried.
It seems more likely, however, that martian collapse was actually
equatorial in distribution. This makes one wonder if equatorial
expansion of Mars may have involved a change in the figure of the
planet rather than over-all expansion. An increase in oblateness
of the spheroid, owing to an increase in rate of planetary
rotation, could cause equatorial expansion in a latitudinal
direction, a decrease in rotation rate would presumably result in
longitudinal extension. An increase in rotational rate might
have been caused by heavier elements settling to the core during
planetary differentiation, but possible causes of a decrease are
more speculative. Irregular distribution of martian collapse
features within the equatorial region could reflect inhomogen-
cities of crustal and subcrustal materials. If Mars suffered a
change in its spheroid because of a change in rotational rate,
why not also Earth, albeit perhaps earlier in its history?
Scale
The message from Mars in terms of surface features and
relationships is "Think big." Many of the landslides, flood-
scoured channels, valleys, chasms, labrinths, volcanoes, poly-
gonal structures, collapse areas, rock streams, sand dunes,
eolian deflated features, and other forms and relationships
displayed on the martian surface are gigantic by Earth standards.
This maybe, in part, a matter of viewing distance, commonly
around 1000 km on Mars and only a fraction of that on Earth.
More likely it is the result of the effective erasing and obscu-
ring processes operating on Earth, weathering, erosion, and depo-
sition, which allow us to see only the youngest features. By
terrestrial temporal standards, much martian topography is
fossil, the product of bygone processes and conditions. Could
Earth once have had features of a size corresponding to those
seen on Mars but now so modified by secondary surface processes
to be overlooked?
If so, is it possible that relationships created by those
features, mega-scars so to speak, could be identified or recog-
nized by extreme remote sensing of Earth's surface? Further, are
they worth looking for? The answer must assuredly be yes, for
the unknown is always worth exploring. Mega-features of Earth's
surface and crust already recognized and interpreted as the
product of currently favored models, plate tectonics for example,
might more satisfactorily be accounted for by some other model
derived from the concept of large, fossil features developed
perhaps by collapse of parts of the crust on a scale comparable
to that seen on Mars.
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Genetic Processes
The processes of obscuration are so effective on Earth that
we usually see or recognize only a part of the total work done by
some particular genetic activity. This is a differential matter,
and certain genetic processes are more suppressed or obscured
than others. Features on the surface of Mars suggest that mass
movements, wind, and ground-water (or ground-ice) sapping deserve
more consideration than normally given in the creation of
terrestrial landscapes. Wind is recognized as effective in local
and special terrestrial settings, but it is possibly underrated
in terrains of regional extent, except, in areas of extreme
aridity. Features of the martian surface strongly suggest that
wind is effective regionally, as well as locally, and should be
factored into considerations of regional landscapes.
Many forms of mass movement are subtle and easily obscured
by more vigorous and rapidly operating processes to the extent
that the true contribution of mass movements is not recognized.
On Mars, where the obscuring and competing processes are weaker,
it is clear that mass movements of various types, including
creep, slides, and collapse, have played a truly major role in
landscape evolution.
Thanks to large and extensive features, widely distributed
on the surface of Mars, that appear to have been created by
sapping, this much neglected and overlooked process is now
getting considerable well-deserved attention from terrestrial
geomorphologists. The role of sapping in creating landforms
merits further nourishment and cultivation.
Catstrophies
Thanks, in part, to the opportunity for viewing of other
planetary surfaces where fossil forms are preserved more fully
than on Earth, terrestrial geomorphologists and planetary scien-
tists have become more sensitive to the role and importance of
catastrophics in creating landscape features. This is a healthy
trend which deserves encouragement.
Remote _ensing and Data Treatment
I suspect we have not yet succeeded in bringing into full
play all the techniques, equipment, knowledge, and tallents
currently available for remote sensing of Earth's surface.
Astronomers, astrophysicists, and planetary scientists may be
doing a better job in that regard on distant planetary bodies.
We should be able to measure more precisely and more fully the
spectra of radiations emitted from Earth's surface.
Just as fruitful might be a more exhaustive processing of
remote sensing data already on hand. Anyone who looked at the
seemingly blank photos of Mars first returned by the Mariner 4
flyby, and then compared them with the final computer-enhanced
product revealing a heavily crater martian surface understands
the potential of these methods. Are we perhaps sitting on a mine
dump of valuable ore which has not been fully analysed?
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Mega-geomorphology: Mars vis a vis Earth

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