Geoscience Reference
In-Depth Information
Extraterrestrial arid surface processes
Jonathan Clarke
5.1
Introduction
On Earth 'aridity' is defined by moisture deficits mostly
strongly evidenced in vegetation distribution (Chapter 1).
It is the presence or absence of vegetation that determines
the nature of many of the features seen as characteristic
of arid environments, such as wind erosion and depo-
sition, or the formation of arroyos and braided stream
deposits. Vegetation is therefore widely used as a proxy
for evaporation in the absence of direct climatic data.
However, this criterion cannot be used where it is absent,
not only on other planets but for 90 % of Earth's his-
tory prior to the evolution of terrestrial vegetation (Davis
and Gibling, 2010). On Earth areas of dunes will form
wherever there is sufficient sand supply and vegetation
cover is minimal, even in high rainfall. Much the same
applies for the formation of arroyos and braided stream
deposits.
Other proxies must therefore be used for moisture
deficits, the most useful of which is the accumulation of
salts in soils and in the terminal parts of drainage systems.
Another proxy is the evidence for the role of ephemeral
moisture and salt growth in the breakdown of rock sur-
faces, although on Earth these are not exclusive to arid
environments. This allows aridity in the terrestrial sense
to be applied to landscapes on Mars where water moisture
is at least ephemerally present and may have been more
widespread in the past.
However, the absence of moisture is not in itself a suf-
ficient criterion. While the surfaces of the Moon or Mer-
cury, for example, may be lacking in moisture, they are
also lacking in atmosphere (though perhaps not traces
of water in other states; see Clark, 2009, and Slade,
Butler and Muhleman, 1992), and the application of a
climate-defined term such as aridity in such an environ-
ment stretches it past breaking point. What are we to
make of bodies such as Titan, which, despite average
Planetary geomorphology is one of the emerging frontiers
of arid geomorphological research (Tooth, 2009). In ear-
lier editions of this work (Wells and Zimbelman, 1989,
1997) extraterrestrial aridity could only be discussed with
reference to Mars, although the nature and implications of
extraterrestrial aridity had long exercised the imaginations
of some writers (e.g. Frank Herbert's 1965 novel
Dune
).
More recent data allow the question to be considered over
a much wider range of solar system bodies than just Earth
and Mars (Figure 5.1). This chapter provides a review
of the nature of 'arid geomorphology' in a solar system
context, what it might mean in environments quite differ-
ent from those of Earth, why planetary scientists need to
be cognisant with terrestrial geomorphology and terres-
trial geomorphologists familiar with the surfaces of other
bodies in the solar system. It will then briefly review the
landscapes of Mars, Titan and Venus and the features and
processes relevant to each that can be considered compa-
rable to terrestrial arid landforms.
5.2
What does 'aridity' mean
beyond Earth?
Even the most cursory glance at images from space explo-
ration missions reveals scenes that are strongly reminis-
cent of terrestrial arid landscapes, be they dry gullies on
Mars or dune fields on Titan. These features occur on bod-
ies whose surface temperatures, atmospheric pressures,
gravity and composition are very different from that of
Earth, which raises the question as to what 'aridity' might
even mean on these bodies.
