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OCR for page 18
t the most fundamental level,
Earth is unique; it is the only
A. planet in the solar system where
the conditions support a thriving bio-
sphere. Through the study of other
objects in the inner solar system, it is
now understood that habitability is
the result of a series of events that
occurred over its 4.6 billion-year histo-
ry. The solar system provides us with
two additional laboratories for study-
ing terrestrial planets. The first, Mars,
is a small, frozen world, whose surface
is hostile to life because of the planet's
thin atmosphere and harsh radiation
environment. By contrast, Venus has
a dense atmosphere that traps radia-
tion so efficiently that its surface is as
hot as a kiln.
Given these two extremes, and the
awareness that humans are altering
Earth's climate, what clues do Mars
and Venus hold for the eventual fate
of Earth's environment? Can we inad-
vertently cause Earth to evolve into a
state similar to that of either Mars or
Venus, or some other inhospitable
regime? Part of the key to answering
these questions lies in the lower
atmosphere and surface of Venus.
Determining how its atmosphere
evolved to its present state and how
the escape of atmospheric gases
affects the chemical composition of
its atmosphere and surface will pro-
vide insight into similar processes
on Earth.
Climate change over long periods
of time seems to be an inherent fea-
ture of the terrestrial planets. Earth's
climatic record illustrates that there
are wide swings in regionally and
globally averaged surface tempera-
tures. Mars may once have had liquid
water on its surface, even at a time
when the Sun was less bright than it is
today. There is evidence that Venus's
climate has varied significantly within
the last billion years. These environ-
ments are produced and sustained by
complex interactions among the sur-
face, atmosphere, and interior.
Despite the considerable efforts of pre-
vious space missions, these processes
are poorly understood. Global moni-
toring of Venus's atmosphere and cli-
mate, in situ measurements of the
composition of the planet's surface,
and detailed data on the types of gases
in the atmosphere are necessary to
expand our understanding of the cli-
mates of the terrestrial planets.
A computer-generated view of a portion of Venus's western Eistla Regio.
faze {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~
Analysis of diverse surface materi-
als of the inner planets, determination
of their ages, and assessment of the
processes that have affected them are
needed to understand how important
elements have evolved differently on
each of the planets. Data on oxygen,
hydrogen, and other atmospheric
gases provide clues to planetary com-
positions and atmospheres, early
solar-system processes, and environ-
ments relevant to the origin of life.
Such data from Earth and Mars sug-
gest that their initial atmospheres
were lost and later replaced by gas
emitted in volcanic eruptions and
added by cometary impacts. By con-
trast, the (incomplete) measurements
of the atmosphere of Venus are consis-
tent with what would be expected of a
primordial atmosphere. However, the
state of the interactions between the
surface and atmosphere is unknown.
Measurements of these interactions
will enable scientists to answer many
questions about Venus's atmosphere
and how it relates to Earth.
The Pioneer Venus and Magellan
spacecraft mapped and measured the
surface of Venus and, although these
data reveal extensive geological activi-
ty (such as volcanism), the expressions
of Earth-like plate tectonics are absent.
Instead, the topography and relative
youth of Venus's surface indicate that
a major, possibly global resurfacing
may have occurred possibly numer-
ous times. Although Venus appears to
have an iron core, the absence of a
magnetic field suggests that it does
not have a magnetic dynamo like that
operating in Earth's liquid core. The
slow rotation of Venus (whose day is
longer than its year) could explain the
missing dynamo, but data are needed
to test this hypothesis.
Motivated by the scientific ques-
tions discussed above, the Venus In
Situ Explorer mission is designed to
undertake a detailed exploration and
study of the composition of Venus's
atmosphere and surface materials.
Such a mission has been contemplat-
ed in the past, but the technical chal-
lenges are daunting. Venus's
extremely high surface temperatures
and pressures (~450°C, and ~100 bars)
OCR for page 19
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~ - a ~ ~ Baa _
~ nn ~ ns _ ~ ~ _
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Profile
Venus In Situ Explorer
Mission Type: Lander
Cost Class: Medium
Priority Measurements:
· Determine elemental and mineralog-
ical surface compositions.
· Measure the composition of the
atmospheres, especially trace gases
and their isotopes.
· Undertake high-precision measure-
ments of noble gases and light sta-
ble isotopes.
· Assess processes and rates of
atmosphere-surface interaction.
· Search for evidence of volcanic
gases in inner-planet atmospheres.
Artist's impression of the Venus In Situ
Explorer. One of its goals is to provide
ground truth for the Magellan radar
images used to create the three-
dimensional view on page 18.
would render the most rugged space-
craft inoperable in a matter of hours.
Venera 7, a Russian mission that land-
ed on Venus in 1970 the first space-
craft to return data from the surface
of another planet survived for
approximately 23 minutes (subse-
quent Russian Venus landers survived
for up to 2 hours).
To survive long enough on Venus's
hellish surface to make key scientific
measurements requires a creative
approach. The Venus In Situ Explorer
concept envisages a spacecraft that
descends through the atmosphere and
lands just long enough to collect a
sample of the surface material. The
Explorer will study the surface for the
short time it touches down, but once
the sample is acquired, a balloon will
inflate and carry the spacecraft up to a
cooler region in the atmosphere,
where the sample can be studied by
onboard instruments for a much
longer period of time. In addition,
the Explorer will make measurements
of winds and atmospheric chemical
Guiding Themes Addressed Important Planetary Science Questions Addressed
composition, and other measurements nologies required for survival of a
will be obtained during descent and
ascent. This set of experiments
should provide researchers with
spacecraft in Venus's extreme environ-
mental conditions. The development
work done for this mission will pave
enough data to meaningfully compare the way for a mission to return a
Venus, Mars, and Earth. Venus sample to Earth in the follow-
A further benefit of this mission ing decade, and possibly further mis-
would be the development of tech- sions in the future.
Dig
Representative terms from entire chapter:
terrestrial planets
