Probing the Interior Structure of Venus

Abstract

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. The coupling efficiency is intermediate between that for the Earth's atmosphere and the ocean. Signals propagating from directly above the epicenter or from a surface wave propagating out from the quake epicenter both travel up into the atmosphere. Because the atmosphere is primarily carbon dioxide, attenuation is higher than it would be in an atmosphere with non-polar molecules. The attenuation is frequency dependent and only impacts frequencies well above 10 Hz at the altitude of a floating platform (54 km). For observations from a space platform, it may be important at much lower frequencies to 1 mHz. Detection from a Floating Platform: Infrasonic pressure signals emanating either directly above the epicenter of a seismic event or from the (surface) Rayleigh wave can be picked up by microbarometers deployed from a balloon floating in the favorable environment of the middle atmosphere of Venus atmosphere. Two or more microbarometers deployed on a tether beneath the balloon will be needed to discriminate pressure variations caused by an upwardly propagating surface wave resulting from the effects of altitude changes (updrafts and downdrafts) and changes in buoyancy of the balloon. The platform will circumnavigate Venus every few days enabling a survey of Venus seismicity. Orbital Detection: Observations from a spacecraft in orbit around Venus enable a broad range of techniques for investigating the perturbations of the neutral atmosphere and ionosphere by seismic waves. Our initial analyses confirm that non-local thermodynamic equilibrium CO_2 emissions on the day side (at 4.3 µm) will present variations induced by adiabatic pressure and density variations and energy deposition created by both acoustic and gravity waves. For detection purposes, the advantage of this emission compared to other ones considered during the study (O_2 night side airglow at 1.27 µm or ultraviolet [UV] day side emission at 220 nm) is a smoothly varying background with solar zenith angle, because of a strong CO_2 absorption at this wavelength below 110 km. Surface Detection: While important seismic measurements can be made from both balloon altitudes and from orbit, the measurement of all three dimensions of the ground motion can only be made by a sensor on the surface of Venus. However, at present, the technology for seismic experiments on the surface of Venus does not exist. Development of a seismic measurement capability equivalent to the Seismic and Interior Structure (SEIS) for the Mars InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) spacecraft is many years if not decades away. However, useful measurements of the ambient noise on the surface of Venus are feasible with existing technology and would be vital for both the design of a future seismic station with high sensitivity for teleseismic events and a pair or network of stations that could probe the interior using ambient noise tomography. Synergistic Observations in All Three Modes: The synoptic orbital view for a remote sensing spacecraft in a high orbit would enable not only sensitive detection and localization of Venus quakes with excellent background discrimination but potentially precise measurements of the propagation of the seismic surface wave counterpart in the higher atmosphere. Complementary observations of the same event at the much higher frequencies that are possible from in situ platforms on the surface and in the middle atmosphere would greatly enhance the ability to survey seismicity and probe the Venus interior. The Path Forward: The first step going forward is to develop the detailed requirements of the proposed payloads and to carry out related technology developments and laboratory or field demonstrations. In undertaking this process, we need to know more about the properties of potential Venus crustal and mantle rocks through laboratory studies and the potential of ambient noise tomography at Venus through analysis. Once this is done, our strategy for investigating the internal structure of Venus is built around programmatic realities—the missions that NASA, European Space Agency (ESA), Japan Aerospace Exploration Agency (JAXA), and the Russian Federal Space Agency (RFSA) are currently flying, are under development, or are being planned. A primary goal should be technology demonstration experiments on Venus missions where seismology is not currently an objective. These include infrasonic background measurements from a Venus balloon and infrared and visible signatures from an orbiter that might be implemented under NASA's Discovery program or as an ESA M-series mission. It would also include seismic background signals and a potential active seismic experiment from a short duration lander such as NASA's proposed New Frontiers Venus In Situ Explorer (VISE) mission. This would be followed with a much more capable mission equipped to investigate seismicity and interior structure. The orbital and balloon platforms needed for such a mission are also features of the Venus Climate Mission (VCM), a Flagship mission endorsed by the Planetary Science Decadal Survey in 2011. The study team recommends study of a Venus Climate and Interior Mission (VCIM), which could benefit from commonalities in spacecraft systems, and secure the support of the broad planetary science community for its Flagship mission for the next decade.

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