Understanding Venus' interior structure is critical to determine its origin and evolution and explain why conditions on this planet are so drastically different than on Earth. On Earth, seismic waves generated by seismic events such as Earthquakes and recorded by seismometers at the ground are utilized to constrain our planet down to its very core. However, deploying seismometers on Venus is significantly more technologically challenging due to its harsh surface conditions (more than 460°C and pressure near 90 atmos.). However, Venus conditions are far more favorable in the higher atmosphere (50-60 km alt.) where they are close to Earth conditions. Could we deploy seismometers in the atmosphere to sense seismic waves? The short answer is yes! The same way a drum emits sound when it vibrates, Earth's surface generates inaudible acoustic waves, called infrasound, when an Earthquake hits. Yet, very few seismic events have been detected from a balloon and we still ignore how much we can learn from these sound waves.
Our project AIR finished its first year and already addressed two critical questions: Can we detect venusquakes with a balloon mission? and Can we determine the properties of Venus’ interior with infrasound waves? Our new results are showing yes for both! Together with our collaborators from Jet propulsion Laboratory, the Swedish Institute of Space Physics, and the California Institute of Technology, we showed that a 4 months-long mission should allow for at least one clear detection. Not only we found that detections were highly probable but we also demonstrated that a planet’s interior properties can be accurately determined using only the acoustic signature of an earthquake or a venusquake. Our work is intrinsically multi-disciplinary and combines seismic hazard frameworks, large-scale numerical simulations of seismic and acoustic, models from geochemistry, and results from geodynamics, i.e., the modelling of a planet’s interior over millions of years. Let’s now dive into the details!
First question: Can we detect venusquakes with a long-duration balloon mission? We addressed this point by computing the probability of observing a signal from a venusquake above a given amplitude threshold and over a certain observation time. The amplitude threshold is key since small amplitude signals are buried in noise, such as wind turbulence, and do not contain much information about a planet's interior. On the other hand, the larger the observation time the more likely our balloon is to detect an event. Ideally we would leave our balloons for over a decade in the Venusian sky but this is technically impossible. By considering multiple models for the number of venusquakes on the planet and by simulating the acoustic signatures of each venusquake, we determined that future missions should consider 3 to 4 months long balloon flights on Venus.
Second question: Can we determine the properties of Venus’ interior with infrasound waves? To address this question, we developed a new inversion model that attempts to iteratively retrieve the source characteristics as well the Venus interior properties only from the acoustic signature of a venusquake. We tested our approach on acoustic observations of earthquakes collected in Alaska, a very seismically active region on Earth, as a validation case for our new inversion model. We were able to retrieve multiple properties of Earth’s crust and mantle only with three acoustic signals which is very promising for balloon signals on Venus.
With the AIR project we want to bring the Venus science, seismology, and infrasound communities together to make great strides in preparing the next round of planetary missions. That’s why we organized the first workshop in 10 years dedicated to Venus seismology from NORSAR. In order to open discussions to a wider audience, all the material from this workshop, including presentations, video and text transcripts, meeting notes, and collaboration plans will be made publicly available online. Note that our results received a lot of attention by the planetary science community at international conferences and are currently being reviewed in scientific journals. With significant progress made on detectability and inversion, what are our next steps? In two words, volcanoes and uncertainties. Indeed, volcanic activity has recently been observed on Venus and volcanoes will generate both a lot of sound in the atmosphere and in the underground. Such waves could be picked up by our balloon network and tell us about underground volcanic processes as well as help us locate the source of seismic waves. Our goal now is to both model and analyze volcanic signals on Earth and extrapolate to the Venus conditions. In addition to volcanic activity, we will investigate in detail how much our uncertainties in terms of rock properties, signal processing, and noise conditions will affect the resulting models on the Venus interior.
Our understanding of Earth's internal structure comes primarily from seismic waves that provide important constraints on subsurface seismic-velocity properties. However, traditional inversion methods cannot be implemented in regions of limited seismic-station coverage, in particular on Venus due to its harsh surface conditions but also in remote Earth regions. This lack of seismic data greatly limits our understanding of Venus’ origin and evolution, but also of the Earth’s subsurface. However, the mechanical coupling between the ground and its atmosphere enables the seismic energy to be transmitted into the atmosphere as low-frequency acoustic waves carrying information about the seismic source and the subsurface properties. While infrasound is traditionally recorded at ground-based stations, which suffers from the same in-situ deployment limitations as seismic stations, recent studies have demonstrated that balloon platforms can be used to monitor seismic activity from the atmosphere at a low operational cost. Balloon-borne seismology is a new dynamic field considered to be the only way to investigate Venus' interior. However, inversion of balloon-borne infrasound data has never been reported in the literature as field data are lacking and the coupling between seismic and acoustic waves in realistic media is poorly understood. Taking advantage of balloon pressure data collected by the Jet propulsion Laboratory and the Swedish Institute of Space Physics during large-scale balloon campaigns, the current project will address these key theoretical and practical issues by analyzing and modeling these seismically-induced infrasound signatures to retrieve the source and subsurface properties. AIR will first process and model the seismo-acoustic waves to analyze the field data. Results will then be injected in a statistical Bayesian inversion framework to retrieve uncertainties on source and subsurface properties and field-data processing from balloon campaigns.