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Comparative Planetology and Climate Change t the most fundamental level, evolved to its present state and how Analysis of diverse surface materi- A Earth is unique; it is the only planet in the solar system where the conditions support a thriving bio- the escape of atmospheric gases affects the chemical composition of its atmosphere and surface will pro- als of the inner planets, determination of their ages, and assessment of the processes that have affected them are sphere. Through the study of other vide insight into similar processes needed to understand how important objects in the inner solar system, it is on Earth. elements have evolved differently on now understood that habitability is Climate change over long periods each of the planets. Data on oxygen, the result of a series of events that of time seems to be an inherent fea- hydrogen, and other atmospheric occurred over its 4.6 billion-year histo- ture of the terrestrial planets. Earthâs gases provide clues to planetary com- ry. The solar system provides us with climatic record illustrates that there positions and atmospheres, early two additional laboratories for study- are wide swings in regionally and solar-system processes, and environ- ing terrestrial planets. The first, Mars, globally averaged surface tempera- ments relevant to the origin of life. is a small, frozen world, whose surface tures. Mars may once have had liquid Such data from Earth and Mars sug- is hostile to life because of the planetâs water on its surface, even at a time gest that their initial atmospheres thin atmosphere and harsh radiation when the Sun was less bright than it is were lost and later replaced by gas environment. By contrast, Venus has today. There is evidence that Venusâs emitted in volcanic eruptions and a dense atmosphere that traps radia- climate has varied significantly within added by cometary impacts. By con- tion so efficiently that its surface is as the last billion years. These environ- trast, the (incomplete) measurements hot as a kiln. ments are produced and sustained by of the atmosphere of Venus are consis- Given these two extremes, and the complex interactions among the sur- tent with what would be expected of a awareness that humans are altering face, atmosphere, and interior. primordial atmosphere. However, the Earthâs climate, what clues do Mars Despite the considerable efforts of pre- state of the interactions between the and Venus hold for the eventual fate vious space missions, these processes surface and atmosphere is unknown. of Earthâs environment? Can we inad- are poorly understood. Global moni- Measurements of these interactions vertently cause Earth to evolve into a toring of Venusâs atmosphere and cli- will enable scientists to answer many state similar to that of either Mars or mate, in situ measurements of the questions about Venusâs atmosphere Venus, or some other inhospitable composition of the planetâs surface, and how it relates to Earth. regime? Part of the key to answering and detailed data on the types of gases The Pioneer Venus and Magellan these questions lies in the lower in the atmosphere are necessary to spacecraft mapped and measured the atmosphere and surface of Venus. expand our understanding of the cli- surface of Venus and, although these Determining how its atmosphere mates of the terrestrial planets. 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 Venusâs surface indicate that a major, possibly global resurfacing may have occurredâpossibly 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 Earthâs 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 Venusâs atmosphere and surface materials. Such a mission has been contemplat- ed in the past, but the technical chal- lenges are daunting. Venusâs extremely high surface temperatures A computer-generated view of a portion of Venusâs western Eistla Regio. and pressures (~450Â°C, and ~100 bars) 18 New Frontiers in Solar System Exploration
Venus In Situ Explorer Profile Venus In Situ Explorer Mission Type: Lander Cost Class: Medium Priority Measurements: â¢ Determine elemental and mineralog- ical surface compositions. â¢ Measure the composition of the atmospheres, especially trace gases and their isotopes. â¢ Undertake high-precision measure- ments of noble gases and light sta- ble isotopes. â¢ Assess processes and rates of atmosphere-surface interaction. â¢ Search for evidence of volcanic gases in inner-planet atmospheres. Artistâs 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- Guiding Themes Addressed Important Planetary Science Questions Addressed craft inoperable in a matter of hours. What global mechanisms affect the evolution of volatiles Venera 7, a Russian mission that land- Volatiles and on planetary bodies? ed on Venus in 1970âthe first space- Organics What is the history of water on the inner planets? craft to return data from the surface The Stuff of Life of another planetâsurvived for How did the atmospheres of the inner planets evolve? approximately 23 minutes (subse- quent Russian Venus landers survived The Origin Why have the terrestrial planets differed so dramatically in their evolution? for up to 2 hours). and Evolution What kinds of minerals are the inner planets made of, To survive long enough on Venusâs of Habitable and does this vary depending on a planetâs distance from hellish surface to make key scientific Worlds the Sun? measurements requires a creative approach. The Venus In Situ Explorer concept envisages a spacecraft that Processes How do the processes that shape the contemporary char- acter of planetary bodies operate and interact? descends through the atmosphere and How Planetary lands just long enough to collect a What processes stabilize climate? Systems Work sample of the surface material. The How do planetsâ varied geological histories enable predic- Explorer will study the surface for the tions of volacanic and tectonic activity? short time it touches down, but once the sample is acquired, a balloon will composition, and other measurements nologies required for survival of a inflate and carry the spacecraft up to a will be obtained during descent and spacecraft in Venusâs extreme environ- cooler region in the atmosphere, ascent. This set of experiments mental conditions. The development where the sample can be studied by should provide researchers with work done for this mission will pave onboard instruments for a much enough data to meaningfully compare the way for a mission to return a longer period of time. In addition, Venus, Mars, and Earth. Venus sample to Earth in the follow- the Explorer will make measurements A further benefit of this mission ing decade, and possibly further mis- of winds and atmospheric chemical would be the development of tech- sions in the future. 19