Probing the Interior Structure of Venus

摘要

The formation, evolution, and structure of Venus remain a mystery more than 50 years after the first visit by a robotic spacecraft. Radar images have revealed a surface that is much younger than those of the Moon, Mercury, and Mars as well as a variety of enigmatic volcanic and tectonic features quite unlike those we are familiar with on Earth. What are the dynamic processes that shape these features, in the absence of any plate tectonics? What is their relationship with the dense Venus atmosphere, which envelops Venus like an ocean? To understand how Venus works as a planet, we now need to probe its interior. Conventional seismology for probing the interior of a planet employs extremely sensitive motion or speed detectors in contact with the planetary surface. For Venus, these sensors must be deployed on the surface and must tolerate the Venus environment (460 degrees C and 90 bars) for up to a year. The dense atmosphere of Venus, which efficiently couples seismic energy into the atmosphere as infrasonic waves, enables two alternatives: detection of these infrasonic waves in the middle atmosphere using a string of two or more microbarometers suspended from a floating platform or detection with an orbiting spacecraft of electromagnetic signatures produced by interactions of infrasonic waves in the Venus upper atmosphere and ionosphere. This report, describing the findings of a workshop, sponsored by the Keck Institute of Space Studies (KISS), concludes that seismic investigations can be successful conducted from all three vantage points—surface, middle atmosphere, and space. Separately or, better still, together, these measurements from these vantage points can be used to transform knowledge of Venus seismicity and the interior structure of Venus. Under the auspices of KISS, a multidisciplinary study team was formed to explore the feasibility of investigating the interior of the planet with seismological techniques. Most of the team’s work was conducted in a five-day workshop held at the KISS facility at the California Institute of Technology (Caltech) campus from June 2–6, 2014. This report contains the key findings of that workshop and recommendations for future work. Seismicity of Venus: The study team first performed an assessment of the seismicity of Venus and the likelihood that the planet experiences active seismic activity. The morphology of the structural features as well as the youthfulness of the planet surface testifies to the potential for seismic activity. There is plenty of evidence that the crust of Venus has experienced stress since the relief of stress is expressed in a wide range of structural features. However, the contemporary rate of stress release is unknown and it is possible that, as on Earth, much of that stress release is aseismic. Two competing conditions on Venus will influence the likelihood of stress release. On the one hand, the lack of water would result in a larger fraction of seismic energy release; on the other hand, the higher temperatures would limit the magnitude of stress release events. Experimental measurements on candidate Venus crustal and mantle materials may help define which effect is more important. Other Sources of Seismic Energy: Volcanic events are also a potential source of seismic waves on Venus. Unlike Mars, where volcanic activity appears to have ended, infrared orbital measurements may indicate that some volcanoes on Venus are still active. Disturbances due to large bolides impacting the atmosphere may also be recorded but are unlikely to be useful for probing the planetary interior. More useful than these point sources of energy will be energy injected into the subsurface from the dynamic atmosphere by atmosphere-surface coupling. This distributed source may be useful for probing the subsurface using the methods of ambient noise tomography. Atmospheric Propagation: Acoustic waves from a seismic event are coupled much more efficiently into the atmosphere than on Earth. Th