The formation, evolution, and structure of Venus remain a mystery more than 50 years after theudfirst visit by a robotic spacecraft. Radar images have revealed a surface that is much youngerudthan those of the Moon, Mercury, and Mars as well as a variety of enigmatic volcanic andudtectonic features quite unlike those we are familiar with on Earth. What are the dynamicudprocesses that shape these features, in the absence of any plate tectonics? What is theirudrelationship with the dense Venus atmosphere, which envelops Venus like an ocean? Toudunderstand how Venus works as a planet, we now need to probe its interior.udConventional seismology for probing the interior of a planet employs extremely sensitiveudmotion or speed detectors in contact with the planetary surface. For Venus, these sensors must beuddeployed on the surface and must tolerate the Venus environment (460 degrees udC and 90 bars) for up to audyear. The dense atmosphere of Venus, which efficiently couples seismic energy into theudatmosphere as infrasonic waves, enables two alternatives: detection of these infrasonic waves inudthe middle atmosphere using a string of two or more microbarometers suspended from a floatingudplatform or detection with an orbiting spacecraft of electromagnetic signatures produced byudinteractions of infrasonic waves in the Venus upper atmosphere and ionosphere. This report,uddescribing the findings of a workshop, sponsored by the Keck Institute of Space Studies (KISS),udconcludes that seismic investigations can be successful conducted from all three vantageudpoints—surface, middle atmosphere, and space. Separately or, better still, together, theseudmeasurements from these vantage points can be used to transform knowledge of Venusudseismicity and the interior structure of Venus.udUnder the auspices of KISS, a multidisciplinary study team was formed to explore theudfeasibility of investigating the interior of the planet with seismological techniques. Most of theudteam’s work was conducted in a five-day workshop held at the KISS facility at the CaliforniaudInstitute of Technology (Caltech) campus from June 2–6, 2014. This report contains the keyudfindings of that workshop and recommendations for future work.udSeismicity of Venus: The study team first performed an assessment of the seismicity ofudVenus and the likelihood that the planet experiences active seismic activity. The morphology ofudthe structural features as well as the youthfulness of the planet surface testifies to the potentialudfor seismic activity. There is plenty of evidence that the crust of Venus has experienced stressudsince the relief of stress is expressed in a wide range of structural features. However, theudcontemporary rate of stress release is unknown and it is possible that, as on Earth, much of thatudstress release is aseismic. Two competing conditions on Venus will influence the likelihood ofudstress release. On the one hand, the lack of water would result in a larger fraction of seismicudenergy release; on the other hand, the higher temperatures would limit the magnitude of stressudrelease events. Experimental measurements on candidate Venus crustal and mantle materialsudmay help define which effect is more important.udOther Sources of Seismic Energy: Volcanic events are also a potential source of seismicudwaves on Venus. Unlike Mars, where volcanic activity appears to have ended, infrared orbitaludmeasurements may indicate that some volcanoes on Venus are still active. Disturbances due toudlarge bolides impacting the atmosphere may also be recorded but are unlikely to be useful forudprobing the planetary interior. More useful than these point sources of energy will be energyudinjected into the subsurface from the dynamic atmosphere by atmosphere-surface coupling. Thisuddistributed source may be useful for probing the subsurface using the methods of ambient noiseudtomography. udAtmospheric Propagation: Acoustic waves from a seismic event are coupled much moreudefficiently into the atmosphere than on Earth. The coupling efficiency is intermediate betweenudthat for the Earth’s atmosphere and the ocean. Signals propagating from directly above theudepicenter or from a surface wave propagating out from the quake epicenter both travel up into theudatmosphere. Because the atmosphere is primarily carbon dioxide, attenuation is higher than itudwould be in an atmosphere with non-polar molecules. The attenuation is frequency dependentudand only impacts frequencies well above 10 Hz at the altitude of a floating platform (54 km). Forudobservations from a space platform, it may be important at much lower frequencies to 1 mHz.udDetection from a Floating Platform: Infrasonic pressure signals emanating either directlyudabove the epicenter of a seismic event or from the (surface) Rayleigh wave can be picked up byudmicrobarometers deployed from a balloon floating in the favorable environment of the middleudatmosphere of Venus atmosphere. Two or more microbarometers deployed on a tether beneathudthe balloon will be needed to discriminate pressure variations caused by an upwardlyudpropagating surface wave resulting from the effects of altitude changes (updrafts anduddowndrafts) and changes in buoyancy of the balloon. The platform will circumnavigate Venusudevery few days enabling a survey of Venus seismicity.udOrbital Detection: Observations from a spacecraft in orbit around Venus enable a broadudrange of techniques for investigating the perturbations of the neutral atmosphere and ionosphereudby seismic waves. Our initial analyses confirm that non-local thermodynamic equilibrium CO_2udemissions on the day side (at 4.3 µm) will present variations induced by adiabatic pressure anduddensity variations and energy deposition created by both acoustic and gravity waves. Foruddetection purposes, the advantage of this emission compared to other ones considered during theudstudy (O_2 night side airglow at 1.27 µm or ultraviolet [UV] day side emission at 220 nm) is audsmoothly varying background with solar zenith angle, because of a strong CO_2 absorption at thisudwavelength below 110 km.udSurface Detection: While important seismic measurements can be made from both balloonudaltitudes and from orbit, the measurement of all three dimensions of the ground motion can onlyudbe made by a sensor on the surface of Venus. However, at present, the technology for seismicudexperiments on the surface of Venus does not exist. Development of a seismic measurementudcapability equivalent to the Seismic and Interior Structure (SEIS) for the Mars InSight (InteriorudExploration using Seismic Investigations, Geodesy and Heat Transport) spacecraft is many yearsudif not decades away. However, useful measurements of the ambient noise on the surface ofudVenus are feasible with existing technology and would be vital for both the design of a futureudseismic station with high sensitivity for teleseismic events and a pair or network of stations thatudcould probe the interior using ambient noise tomography.udSynergistic Observations in All Three Modes: The synoptic orbital view for a remoteudsensing spacecraft in a high orbit would enable not only sensitive detection and localization ofudVenus quakes with excellent background discrimination but potentially precise measurements ofudthe propagation of the seismic surface wave counterpart in the higher atmosphere.udComplementary observations of the same event at the much higher frequencies that are possibleudfrom in situ platforms on the surface and in the middle atmosphere would greatly enhance theudability to survey seismicity and probe the Venus interior.udThe Path Forward: The first step going forward is to develop the detailed requirements ofudthe proposed payloads and to carry out related technology developments and laboratory or fielduddemonstrations. In undertaking this process, we need to know more about the properties ofudpotential Venus crustal and mantle rocks through laboratory studies and the potential of ambient udnoise tomography at Venus through analysis. Once this is done, our strategy for investigating theudinternal structure of Venus is built around programmatic realities—the missions that NASA,udEuropean Space Agency (ESA), Japan Aerospace Exploration Agency (JAXA), and the RussianudFederal Space Agency (RFSA) are currently flying, are under development, or are being planned.udA primary goal should be technology demonstration experiments on Venus missions whereudseismology is not currently an objective. These include infrasonic background measurementsudfrom a Venus balloon and infrared and visible signatures from an orbiter that might beudimplemented under NASA’s Discovery program or as an ESA M-series mission. It would alsoudinclude seismic background signals and a potential active seismic experiment from a short durationudlander such as NASA’s proposed New Frontiers Venus In Situ Explorer (VISE)udmission. This would be followed with a much more capable mission equipped to investigateudseismicity and interior structure. The orbital and balloon platforms needed for such a mission areudalso features of the Venus Climate Mission (VCM), a Flagship mission endorsed by theudPlanetary Science Decadal Survey in 2011. The study team recommends study of a VenusudClimate and Interior Mission (VCIM), which could benefit from commonalities in spacecraftudsystems, and secure the support of the broad planetary science community for its Flagshipudmission for the next decade.
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