[100-101] The deeply
textured rocks at the Lander 2 site yield attractive graphic
patterns, especially when they are accentuated by low Sun
and deep shadows (figs. 116 and 117).
The blocks are so distinctive that
one might expect that their origin could be easily
determined. However, this turns out not to be the case. The
abundant pits are similar to vesicles that occur in
terrestrial volcanic rocks derived from gas charged lavas.
As the gas rises to the surface, some of the bubbles are
essentially frozen in the solidifying lava. Although this is
the favored interpretation for the martian rocks, the pits
are larger and more widespread than is typical for
terrestrial situations. Alternate interpretations are that
the pits mark the former presence of easily eroded clasts or
crystals, or that the rock results from partial cementation
in an upper soil zone in a manner analogous to that for some
tropical regions on Earth.
All these arguments proceed by
analogy: the features on Mars resemble those on Earth that
we know to be formed by particular processes. Unfortunately,
it is difficult to quantify the likelihood of one analogous
comparison as opposed to another. In reasoning by analogy we
are constrained by our own experience. The number of
analogies at our disposal is a function of our knowledge of
terrestrial landform. If processes on Mars produce unique
forms, then dependence upon analogies may blind us to that
uniqueness.
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