Excerpts from The National Academies Press
VENUS IN SITU EXPLORER
A Venus In Situ Explorer mission would address fundamental unanswered questions of the history and current state of Venus through a characterization of the chemical composition and dynamics of the atmosphere of Venus, and/or measure surface composition and rock textures. While it is unlikely that all of the objectives delineated in the decadal survey could be addressed within the New Frontiers cost constraints, a mission that addresses a subset of these objectives would provide critical information about the present state and history of Venus. The current European Space Agency (ESA) Venus Express mission has greatly expanded knowledge of the upper atmosphere and exosphere of Venus, and has contributed to understanding of regions of the atmosphere nearer to the planet’s surface. However, characterization of the noble-gas and isotopic signatures of the well-mixed lower atmosphere would greatly expand understanding of the formation and evolution of the atmosphere of Venus, illuminate important elements of the current climate, including the drivers for the Venus greenhouse effect, and potentially provide insight on the early tectonic evolution of the planet. Prior landed missions of Soviet Venera and Vega spacecraft (Figure 2.2) have provided some information on crustal compositions and textures, but they have been confined to lowland areas composed of basaltic lava flows. Landed missions in highland regions or on older terrains could answer questions related to presence of silicic rock compositions or earlier phases of tectonism, but they present significant technological challenges.
FIGURE 2.2 Image taken of the surface of Venus by Venera 13 in 1982. The Soviet Union successfully conducted several Venus lander missions with 1980s era technology. These images depict the distorting effects of the thick Venusian atmosphere. SOURCE: C.M. Pieters, J.W. Head, W. Patterson, S. Pratt, J.B. Garvin, V.L. Barsukov, A.T. Basilevsky, I.L. Khodakovsky, A.S. Selivanov, A.S. Panfilov, Y.M. Getkin, and Y.M. Narayeva, The color of the surface of Venus, Science 234:1379-1383, 1986. Reprinted with permission of AAAS.
Volatiles and Organics: The Stuff of Life
The Origin and Evolution of Habitable Worlds
Processes: How Planetary Systems Work
Science mission objectives for VISE are as follows:
Atmospheric Science Objectives:
The composition of the lower atmosphere of Venus is unknown. Without this knowledge, comparisons of the factors that affect climate on Earth and on Venus, including photochemistry, clouds, volcanism, surface-atmosphere interactions, and the loss of light gases to space, are impossible. VISE will measure the abundance of trace gas species in the lower atmosphere of Venus to parts per million accuracy, enabling an understanding of how these processes affect terrestrial planetary climates. A fundamental quest is to understand how and why Venus, roughly the same size, composition, and distance from the Sun as Earth, has evolved to such a different state. The record of planetary atmospheres is contained in the isotope ratios of the most inert gases—xenon, krypton, argon, and neon. Are planetary atmospheres the remnants of gases that were originally solar in composition but then suffered massive hydrodynamic escape, or did they require atmospheres from volatiles that had already been differentiated? What was the role of impacts on the ultimate compositions and evolution of the terrestrial planets? Discrimination between these events for each of the inner planets is possible if noble gas isotopic ratios can be measured with a state-of-the-art neutral mass spectrometer. Previous spacecraft measurements have been inadequate to address these issues. VISE will determine the noble gas abundances and isotope ratios to sufficient accuracy to distinguish between hypotheses of the origin and evolution of Venus’s atmosphere. A meteorological package will measure atmospheric pressure and temperature profiles down to the surface, and pressure, temperature, and winds at the surface. Cloud-level winds will be determined by tracking the ascent balloon during its 3.5-day lifetime, providing improved data on atmospheric dynamics and the origin of Venus’s mysterious atmospheric superrotation.
Surface Science Objectives:
The former Soviet Union’s Venera landers returned basic elemental chemistry and images of four sites on the surface, and Magellan data provided evidence of possible evolved volcanic deposits. However, we lack sufficient information on surface elemental abundances and mineralogy to determine the degree of crustal evolution on Venus. The VISE mission would measure elemental compositions at a surface site complementary to those of the Veneras. Mineralogy of a surface sample core will be obtained for the first time, allowing analysis of any weathered layer and testing for depth of alteration and occurrence of unaltered material. Textural analysis of the sample using a microscope imaging system would provide information on the formation and nature of surface rocks. These data will be used to constrain questions outlined above. Despite global radar coverage of Venus by Magellan, little is known of the surface morphology at scales of 1 to 10 m. Without such information, it is difficult to determine how the plains formed and to understand the nature of mobile materials on the surface. A descent camera on the lander will provide the first broadscale visible images of the surface, with images returned from about 10 km altitude to the surface. These images will enhance interpretation of the Magellan radar images by providing ground-truth data on the surface texture of the lava flows that make up Venus’s plains. The morphology and texture of these flows can be related to emplacement rate, volatile content, and rheology, which are needed in order to understand the role of volcanism in shaping the atmosphere and surface of Venus. Images of Venus’s surface will also be returned from the lander, with filters chosen to provide compositional information. These images will help to determine the recent geological history of Venus and will resolve differences in the interpretation of Venus’s resurfacing history.
Developments Since the Decadal Survey
Goal 1: Origin and Early Evolution of Venus: How did Venus originate and evolve?
The highest priority objectives are:
Determine the elemental and isotopic composition of the atmosphere to identify earlier epochs of Venus’s history, and clues to Venus’s origin, formation and evolution.
Map the mineralogy and chemical composition of Venus’s surface on the planetary scale for evidence of past environmental conditions and for constraints on the evolution of Venus’s atmosphere.
Characterize the history of volatiles in the interior, surface and atmosphere of Venus, including volatile additions due to cometary impacts, degassing and atmospheric escape, to understand the planet’s geologic and atmospheric evolution.
Goal 2: Venus as a Terrestrial Planet: What are the processes that have shaped and still shape the planet?
The highest priority objectives are:
Constrain the coupling of thermochemical, photochemical and dynamical processes in Venus’s atmosphere and between the surface and atmosphere to understand radiative balance, climate, dynamics, and chemical cycles.
Constrain the resurfacing history of Venus, and the nature of the resurfacing processes, including the role of tectonism, volcanism, impacts of asteroids or comets, sedimentation/erosion, and chemical weathering.
Constrain the nature and timing of volcanic activity on Venus, including thermal evolution, current and past rates of volcanic activity, and the effects of outgassing on atmospheric and interior processes.
Goal 3: What does Venus tell us about the fate of Earth’s environment?
The highest priority objectives are:
Search for evidence of past global-climate changes on Venus, including chemical-and-isotope evidence in the atmosphere, as well as rock chemistry and characteristics of surface weathering. In particular, seek evidence for the presence or absence of past oceans.
Search for evidence of past changes in interior dynamics, volcanics and tectonics, including possible evolution from plate tectonics to stagnant-lid tectonics, which may have resulted in significant changes in the global climate pattern.
Characterize the Venus greenhouse effect, including the interplay of chemistry, dynamics, meteorology, and radiative physics in the atmosphere, especially in the clouds.
In addition, ESA’s Venus Express has entered Venus orbit and has returned new data since the decadal survey. Venus Express has expanded understanding of the upper atmosphere and exosphere and has contributed to knowledge of the mid- to lower atmosphere. However, most of the science objectives from the decadal survey require in situ measurements that are beyond the capabilities of an orbital mission such as Venus Express.12
The committee concludes that a VISE mission remains a very scientifically important mission that should be considered for the New Frontiers Program. The VEXAG goals and objectives align well with the New Frontiers Venus mission objectives, further validating the selection process in the decadal survey. Although these objectives address fundamental science themes for Venus exploration, it is unlikely that they can be fully addressed in a single mission. Cost and technology risk factors may preclude a single VISE mission proposal from addressing all of the objectives. Consequently, a mission that addresses a major subset of the objectives would be consistent with the recommendations of the decadal survey. For example, a successful mission might not have to include a landed component if it addressed the major atmospheric objectives. In addition, an interpretation of the decadal survey’s science objectives should prescribe the important data to be collected, rather than dictate measurement techniques or mission scenarios. While no attempt is made here to prescribe or define implementation strategies, potential challenges related to the Venus environment—such as high temperatures, high pressures, and a corrosive atmosphere in the near-surface environment—may require the use of nontraditional (though previously demonstrated) mobility systems such as balloons (a technology that also has some applications on other atmospheric bodies). The committee also notes that most of the technologies required to address the decadal survey objectives have been demonstrated on prior missions. For instance, Soviet-era Venus missions not only successfully reached the surface, but also operated there for up to an hour, proving that surface missions are possible.
In the decadal survey, the VISE mission concept was discussed in terms of what it could contribute to a future flagship-class Venus sample return mission. While such an approach has significant merit, the committee warns that placing a technology demonstration for a future mission in the critical path of VISE mission success is unwise, particularly given the technical challenges for Venus sample return. Nonetheless, future Venus exploration beyond a VISE mission would require major technology development and demonstration, so that the inclusion of demonstration technologies in a VISE mission on a non-interference, non-critical-path basis is justified.
The committee concluded that a VISE mission that addresses a significant number of the decadal survey objectives is tenable. Such a mission would make use of technologies that have been successfully demonstrated in prior missions to the Venus surface and near-surface environment. The committee also concluded that several of the VEXAG goals should be included with the goals established in the decadal survey, particularly the VEXAG goals concerning understanding the thermal balance of the atmosphere and gathering global mineralogic data.
The challenges associated with landing in a region not previously sampled, collection of a sample, and lofting to a more clement altitude are the source of greatest technology and cost risk. Consequently, the New Frontiers announcement of opportunity should not preclude a mission that addresses the major goals for chemical sampling of the mid- to lower atmosphere on Venus and characterizing atmospheric dynamics, but lacks a surface sampling component. On the other hand, a mission that only addressed surface sampling would not be acceptable.
The science goals for this mission, which are not in priority order, should be to:
Understand the physics and chemistry of the atmosphere of Venus through measurement of its composition, especially the abundances of sulfur, trace gases, light-stable isotopes, and noble-gas isotopes;
Constrain the coupling of thermochemical, photochemical, and dynamical processes in the atmosphere of Venus and between the surface and atmosphere to understand radiative balance, climate, dynamics, and chemical cycles;
Understand the physics and chemistry of the crust of Venus, for example, through analysis of near-infrared descent images from below the clouds to the surface and through measurements of elemental abundances and mineralogy from a surface sample;
Understand the properties of the atmosphere of Venus down to the surface through meteorological measurements and improve understanding of zonal cloud-level winds on Venus through temporal measurements over several Earth days;
Understand the weathering environment of the crust of Venus in the context of the dynamics of the atmosphere and the composition and texture of surface materials; and
Map the mineralogy and chemical composition of the surface of Venus on the planetary scale for evidence of past hydrologic cycles, oceans, and life and constraints on the evolution of the atmosphere of Venus.
8 New Frontiers in the Solar System, p. 3, Table ES.1.
9 New Frontiers in the Solar System, p. 58.
10 New Frontiers in the Solar System, p. 59.
11 The committee has reproduced only the top three VEXAG goals but notes that the VEXAG committee has produced a valuable document that can be used as a reference on Venus science objectives. This document, Venus Exploration Goals, Objectives, Investigations, and Priorities: 2007, is available at http://www.lpi.usra.edu/vexag/vexag_goals_2007.pdf.
12 New Frontiers in the Solar System, p. 58
13 New Frontiers in the Solar System, p. 6.
March 15, 2012