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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 (~450C, and ~100 bars)

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- V~ ~ _ _ _ _ lo_ _ _ _ _ _ K ~ - a ~ ~ Baa _ ~ nn ~ ns _ ~ ~ _ J 15 ~ Be . _ _ _ 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