Geology Reference
In-Depth Information
CHAPTER 2
Planetary geomorphology methods
2.1 Introduction
disadvantages but collectively provides a powerful means
to decipher present and past planetary surface histories.
The starting point is the analysis of planetary data,
typically in the form of images. From these studies, the
overall terrains and varieties of landforms are identi ed
and characterized. Various hypotheses are proposed to
explain the possible formation and evolution of the land-
forms observed. With further study and new data, the
number of hypotheses can be reduced, or new ideas
emerge. The history of the study of craters on the Moon
is a good case to review. Beginning with telescopic views,
the origin of lunar craters was debated for centuries, lead-
ing to the time of the Space Age. Even with the return of
data from spacecraft sent to the Moon in preparation for
the Apollo program, there were two primary competing
ideas for craters, impact versus volcanic origins. Images
of lunar craters showed features that were used to support
both ideas. While the characteristics of volcanic craters on
Earth were fairly well understood, little thought had been
given to extrapolation of volcanic processes to the low-
gravity, airless environment of the Moon. In the early days
of the Space Age, impact cratering as a process was little
appreciated in the geologic context, and there was no
understanding of the physics of the process. At about the
same time as robotic missions were returning new, close-
up data for the Moon, experiments to study the physics of
impact events were initiated. Although similar work had
been conducted for decades by the military to understand
how projectiles could penetrate armor, much of this work
was classi ed; moreover, the work was more applicable to
man-made targets than to natural, rocky material. It is
interesting to note that in the late 1880s the American
geologist G. K. Gilbert dropped small cannon balls into
mud targets ( Fig. 2.1 ) to see what might happen. Gilbert
was very interested in the origin of lunar craters, and
proposed that the Imbrium feature on the Moon was the
result of an impact, as discussed in Chapter 4 .
For many years, the study of the geomorphology of the
Earth was primarily descriptive. In the middle of the
twentieth century, the emphasis shifted to a more process-
oriented approach, with the goal of understanding the
reasons behind a landform ' s appearance. The analysis of
planetary surfaces has gone through a similar history.
When the first close-up images of the Moon and planets
were obtained, their surfaces were described, and some
attempts were made to interpret their origin and evolution.
Unfortunately, some of these attempts were rather imma-
ture. Planetary scientists with a geology background drew
on their experiences with Earth, taking a simpli ed ana-
log
approach; i.e., if it looks like a volcanic crater, it must
be of volcanic origin. Scaling the sizes of features and
considerations of planetary environments took a back seat
to the simple look alike answer.
As the Apollo program drew to a close in the early 1970s
and the exploration of the full Solar System emerged, plan-
etary geomorphology became more process-oriented, with
attempts to take differences in planetary environments into
account, while maintaining fundamental geologic principles.
In this chapter, the following question will be addressed:
how can one study the geology of a planet or satellite without
actually going there? This will include the approaches used
in planetary geomorphology and the types of data that are
commonly available for the study of planetary surfaces.
2.2 Approach
The general approach in planetary geomorphology involves
three elements: (a) analysis of spacecraft data, (b) laboratory
and computer simulations of key geologic processes in
different planetary environments, and (c) the study of ter-
restrial analogs. Each element has its advantages and
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