Seminars

List of seminars in Ile-de-France: April-May 2023

Please consult the online calendar, as cancelations can occur!

3 April 2023, 10:00

Seismic attenuation in the Martian lithosphere from InSight-SEIS data: separation and stratification of scattering and absorption processes (Ph.D. defense)
Sabrina Menina (Institut de Physique du Globe de Paris)
IPGP amphithéâtre, 1 rue Jussieu, 75005, Paris

Connection details
Zoom : https://u-paris.zoom.us/j/7568613007?pwd=MExoWDA2VDAvOVMrR1h2QzZJSVNVdz09

Summary
Since its deployment at the surface of Mars in 2019, the SEIS seismometer of the InSight mission has detected hundreds of Very high Frequency (VF; > 1 Hz) seismic events. They are characterized by emergent P and S arrivals, a broad maximum and a very slow energy decay known as “coda” that can last up to 1 hour. The objective of this thesis is to use these seismic data to constrain a 1-D model of the regional attenuation properties of the Martian lithosphere in order to estimate the characteristics of the heterogeneities and the volatile content as a function of depth.

To achieve this goal, we first examined the statistical and energetic characteristics of the VF signals. In addition to the energy equipartition observed through the stabilization of the vertical/horizontal kinetic energy ratio in the coda, our analysis shows that these signals are in general depolarized and follow a Gaussian probability distribution. We interpret these observations as direct consequences of multiple scattering. In order to characterize the shape of the envelopes of the VF signals, we measured the delay time (td) between the arrival of the direct S-wave and the maximum of the energy and the quality factor of the coda (Qc), which characterizes the rate of energy decay. We observed that the td measurements are almost frequency-independent, a characteristic which distinguishes Mars from other planets. We found that Qc increases linearly with frequency and shows intermediate values between the Earth and the Moon.

We subsequently estimated the attenuation properties of the Martian lithosphere. For this, we quantified the scattering quality factor (Qsc), which contains information about the heterogeneities of the medium and the extent of fractures at depth, as well as the absorption quality factor (Qμ) which is related to the presence of volatiles in the medium. To separate the two processes, we modeled the energy envelopes of the VF seismograms using a multiple-scattering approach. The direct problem is based on elastic radiative transfer theory, while the inversion combines a grid search approach with an optimization by the Levenberg-Marquardt algorithm. We first applied our method to the distant VF events by considering a statistically uniform medium (half-space). This first study revealed very high values for Qμ, which are characteristic of a dry environment. Moreover, we observed a rapid decrease of the attenuation with the epicentral distance, which we interpreted as an indication of the stratification of the medium properties. To further investigate this hypothesis, we applied our method not only to distant VFs but also to nearby impacts, in the case of a medium stratified in both velocity and attenuation. In this model, we assume that a strongly heterogeneous layer lies on top of a transparent medium. Our inversions indicate that a strongly diffusive and globally dry layer (Qsc and Qμ of the order of 50 and 3500 at 7.5 Hz) with a minimum thickness of 20 km under the InSight landing site (northern plains) and with high S-wave velocities (3 km/s) can explain the shape of the energetic envelopes of the near impacts. However, the thickness of this layer has to be extended to at least 60 km to recover the shape of the envelopes of the distant VF. The correlation between the thickness of the diffusive layer at the surface of Mars and the depth of the Moho suggests that the main source of heterogeneity is the crust itself and not a very thick megaregolith as is the case on the Moon. In this framework, the numerical simulations suggest that the sources of distant VF events are essentially superficial and localized on the southern highlands of Mars.

4 April 2023, 15:00

Turbulence in the atmosphere of Mars and Titan
Aymeric Spiga (Laboratoire de Météorologie Dynamique, Sorbonne Université)
IPGP Planetary and Space Sciences, Bâtiment Lamarck B, 35 rue Hélène Brion, Paris

Summary
Mars and Titan have both vastly distinct environments from the Earth’s, yet they exhibit atmospheric turbulent phenomena akin to the ones we experience at the surface of our planet. What is particularly interesting on Mars and Titan is that turbulence is somewhat a distant exotic cousin of Earth’s atmospheric turbulence, with many different aspects that allows for a rich comparative study. This study can be placed in a context of planetary exploration and multi-disciplinary planetary science.

Connection details
Zoom link
Meeting ID: 876 7331 0489
Passcode: 856642

In-person directions
The IPGP Planetary and Space Sciences seminar series is held in the building Lamarck B that is part of the Université Paris Cité - Campus des Grands Moulins near the BnF (Bibliothèque Nationale de France). You do not need a badge to enter this building. You only need to go to 35 rue Hélène Brion, and once in the lobby of the main entrance, take the elevators on the right to the 5th floor. Exit the elevator and take the corridor to the right. The seminar room is number 522.

11 April 2023, 15:00

Insights into the Origin of Volatile Elements in Terrestrial Planets through Zinc and Copper Isotopic Analyses of Ryugu Samples
Frédéric Moynier (Institut de Physique du Globe de Paris)
IPGP Planetary and Space Sciences, Bâtiment Lamarck B, 35 rue Hélène Brion, Paris

Summary
The samples collected by the Hayabusa2 spaceship from Ryugu are incredibly valuable due to their pristine nature, potentially escape terrestrial contamination, and therefore are providing a unique opportunity to study planetary building blocks. In this presentation, I will share the first stable isotopic composition data obtained from Ryugu’s samples for Zn and Cu, which are moderately volatile elements. Through this analysis, we have discovered that materials similar to Ryugu contributed approximately 30% of the budget of Zn and Cu to Earth, and approximately 5% of the total mass of the Earth. This shed light on the origins of these essential elements and provide insight into the formation and evolution of our planet.

Connection details
Zoom link
Meeting ID: 876 7331 0489
Passcode: 856642

23 May 2023, 15:00

Ring moon morphology and libration measurements
Nicolas Rambaux (Observatoire de Paris, IMCEE)
IPGP Planetary and Space Sciences, Bâtiment Lamarck B, 35 rue Hélène Brion, Paris

Summary
The Cassini-Huygens space mission made a series of observations of Saturn’s small satellites during its grand finale stage. This talk will discuss the shape analysis of the small Saturnian satellites in order to describe their global figure and large-scale topography, as well as to deduce fundamental quantities, gravity field, and amplitude of the diurnal libration by assuming that the bodies are homogeneous. These predicted librations are then compared with recent measurements obtained by astrometry to discuss the internal structure of such bodies.

Connection details
Zoom link
Meeting ID: 876 7331 0489
Passcode: 856642