Infrasounds from Venus quakes : Numerical modeling and balloon observation project R.F. Garcia, D. Mimoun, Q. Brissaud, G. Poler ISAE-SUPAERO, Toulouse, France S. Lebonnois LMD, Paris, France
Atmospheric seismology on Venus Internal structure of Venus mainly unknown (poorest estimates of planet moment of inertia in the solar system) Venus quakes expected for such a living planet Surface deployment of seismometers challenging due to high temperature Ground/Atmosphere coupling on Venus : Better coupling than on Earth due to high atmospheric density at the surface 15% of quake energy lost in atmosphere (Lognonne et al., 2007) => Observation of seismic Rayleigh surface waves dispersion curve => Seismic velocity profile in the first 0 to 500 km of planet interior
Numerical modeling tool for acoustic and gravity waves Finite difference code taking into account: Exponential density decrease and horizontal winds Attenuation by viscosity, thermal losses and molecular relaxation Acoustic and gravity waves Bottom forcing and/or point source PML (absorbing) or rigid/forcing boundary conditions Simulated acoustic and gravity wavefronts Brissaud et al., in Press, GJI Validation relative to analytical in windy attenuating C02 atmosphere Garcia et al., in Prep, JGR Planets
Venus atmosphere models LMD GCM simulations and Venus GRAM model at 12h LT : Strong zonal wind duct at cloud level, low meridional winds. Low attenuation due to CO2 molecular relaxation below 80 km because peak frequency (fr) outside infrasonic range.
Application to infrasounds generated by seismic surface waves Equatorial model at 12h LT for downwind, upwind and no wind. Bottom forcing by dispersive seismic surface wave for magnitude Ms=6.0 at 20s period : At 20 deg (~2000 km) from source A~ µm/s At 90 deg (~ 000 km) from source A~1 µm/s time 87 km 4 km/s
Effect of wind duct Wind duct has negligable effect on waveforms because almost vertical wave propagation Almost no attenuation below 80km for frequency < 1Hz.
Amplitude variations along the vertical Pressure perturbation scale as ρ½ in Pa, and as 1/ρ½ in percent of background pressure Pressure perturbation scale as freq => x at 1Hz
Principle of balloon infrasonic experiment Infrasonic sensors may be able to record the seismic signal [0.05 5 Hz] (with some air propagation delay in minutes) Filtering of upward waves to discriminate perturbations from signal Typical life duration : about 6 months Sensor 1 Upward wave Sensor 2
Investigation preliminary feasibility P re s s u r e in P a The pressure wave can be recorded by a micro-barometer 2 1 0 1 2 3 4 5 M =4 M =5 M =6 Differen al pressure @ 50 m ( 1/30 radian) Pressure sensor performance 0 20 30 40 50 E p i c e n tr a l d i s t a n c e 60 70 80 90 Pressure Signal Amplitude for surface waves, M=4,5,6 magnitude as a function of quake epicentral distance (@ 1 Hz) Already existing sensors have the capability to detect these waves
Potential noise contributors for a infrasonic measurement Venus Atmosphere is very dynamic There are several potential perturbations from the upper atmosphere Local effects : turbulence, wind, motion of the gondola and of the tether Feasibility to be checked on Earth balloon experiment (see Cutts et al) Regional effects Acoustic/gravity waves? Volcanoes, storms, thunder? However infrasonic frequency range usually much quiet than gravity wave frequency range (Taylor, 2006) Investigation complementary to atmospheric science
Other potentials observables : airglows Upper atmosphere markers benefit from wave amplification and proper characterisation by previous missions (Venus-Express) O2 nightglow at 1.27 µm (~96 km) Seismic infrasound on time scales shorter than variations induced by atmosphere dynamics Soret et al., 2012 CO2 dayglow at 4.3 µm (~130 km) Gravity waves already observed by VIRTIS on board VEX Garcia et al., 2009
Take home messages Both quakes and related infrasound signals are expected in the solid/atmosphere system of Venus. Zonal wind and attenuation effects on infrasounds generated by seismic surface waves are negligible below 80 km altitude. Infrasonic sensors within the noise requirements are available. Low noise and directional pressure inlets, and payload electronic architecture under development. Low environment background noise expected in infrasonic frequency range (to be better quantified) for a platform moving with wind. Payload complementary to atmospheric science. Upper atmosphere airglow markers are also possible both on day and night side.
Vega father of Venus Balloons Sagdeev al., 1986