Geology Reference
In-Depth Information
include the following. (a) What is the present state of the
planet? (b) What was the past state of the planet? (c) How
do the present and past states compare with those of other
objects in the Solar System?
The question dealing with the present state seeks to
determine the composition, distribution, and ages of
rocks on the surface, identify active geologic processes,
and characterize the interior.
Determining the past state of a planet, including Earth, is
a fundamental aspect of geology and involves determining
its geologic history. For example, is the present state repre-
sentative of previous conditions on the planet, or has there
been a change or evolution in the surface or interior?
Answering these questions is typically accomplished
through geologic mapping, coupled with the derivation of
a stratigraphic framework and geologic time scale.
Comparative planetology addresses the third aspect in
the geologic study of planets. Once the present and past
states have been assessed, the results are then compared
among all of the planets to determine their differences and
similarities. This comparision enables a more complete
understanding of geologic processes in general and of the
evolution of all solid-surface objects in the Solar System.
questions should be addressed. For example, when tar-
geting speci c planetary objects for astrobiology explo-
ration, at least three factors should be considered: the
presence of water (preferably in the liquid state), a source
of suf cient energy to support biological processes, and
the availability of organic chemistry and other elements
essential for life processes (primarily carbon, nitrogen,
hydrogen, oxygen, phosphorus, and sulfur). With current
data, the search narrows to Mars, Jupiter ' s moon Europa,
and possibly Saturn ' s moons Enceladus and Titan. If the
search is expanded to include potential past environ-
ments, objects such as Jupiter
'
s moon Ganymede might
be included.
In 1996 a meteorite (designated ALH84001) found in
Antarctica was thought to have been ejected from Mars
and was suggested to show evidence for biology.
Although much of this evidence has been rejected, interest
in astrobiology increased substantially, especially as
related to the exploration of Mars. The current search
strategy focuses on identifying the present and past envi-
ronments conducive for biology and is a
approach. Obviously, if life or the signs of life (e.g.,
fossils) are found, the result would be truly profound
( Fig. 1.8 ). However, a negative result is equally intrigu-
ing; if present or past environments are found that are
amenable for life, but life is not found, then one must
ask why not, and what is it about Earth that would make
our planet unique for life if indeed we are truly alone?
As the field of astrobiology has moved forward, life has
been found to be much more pervasive on Earth than had
previously been suspected. In recent years life-forms have
been found in extreme conditions of temperature, pres-
sure, pH, and other environmental parameters, showing
that biology can occur in a much greater range of settings
than previously suspected, thus widening the search for
life throughout the Solar System.
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1.2.2 Astrobiology
Are we alone? That fundamental question has been posed
in various forms throughout humankind
s history and
constitutes one of the key motivations in the exploration
of space. The term astrobiology was coined to encompass
all aspects of the search for present and past life, including
research on the conditions for the origin of life and study
of the environments conducive for biological processes.
The NASA Astrobiology Institute (NAI), which was
formed in 1998 and is headquartered at the NASA-Ames
Research Center in California, consists of an international
consortium of universities and institutions conducting a
wide variety of research projects in astrobiology. The NAI
organizes annual spring meetings to review the latest
results in astrobiology ( http://nai.nasa.gov) ; these meet-
ings are well attended and open to the public.
The Viking mission to Mars in the mid 1970s was the
first project to search for life beyond Earth. Experiments
for the two Vi k ing landers ( Fig. 1.7 ) were developed to
search speci cally for life-forms and to assess possible
biological processes. The results from these experiments
were negative, and the general search for life was out of
vogue for some 20 years. However, during this period,
careful considerations were given as to how astrobiology
'
1.3 Strategy for Solar System exploration
Determining the present and past states of planets and
comparative planetology requires observations and meas-
urements from orbit, placement of instruments on plane-
tary surfaces, and the return of samples to Earth. Thus, the
general exploration of the Solar System by spacecraft
follows a strategy involving a series of missions of
increasing capabilities. However, even before spacecraft
are launched, Earth-based telescopic observations are
made to determine the fundamental characteristics of
 
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