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Naomi Murdoch

Research

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 The InSight Mission

The InSight mission, selected under the NASA Discovery program, landed on Mars in 2018. Since landing InSight has been performing the first comprehensive surface-based geophysical investigation of Mars. The objectives of the InSight mission are to advance our understanding of the formation and evolution of terrestrial planets and to determine the current level of tectonic activity and meteorite impact flux on Mars. SEIS (Seismic Experiment for Internal Structures) is the critical instrument for delineating the deep interior structure of Mars, including the thickness and structure of the crust, the composition and structure of the mantle, and the size of the core. SEIS consists of two independent, 3-axis seismometers : an ultra-sensitive very broad band (VBB) oblique seismometer ; and a miniature, short-period (SP) seismometer that provides partial measurement redundancy and extends the high-frequency measurement capability.

I am a collaborator of the InSight mission and have been involved in both the project team and science teams since 2013. As part of the instrument development, my contributions included the seismic sensor noise model development, instrument testing, and deployment preparations.

Here are details of some of my research topics related to InSight :

Development of a noise model for the SEIS instrument

Variations in wind (Murdoch et al. 2017), atmospheric pressure (Murdoch et al., 2018, Garcia et al., 2020), magnetic field or even temperature are natural phenomena that create noise on the SEIS instrument. Understanding and predicting such sources of noise (Mimoun et al., 2017) was important in the design of the mission, the definition of specifications and in the exploitation of the data. To help NASA and the InSight team choose the best possible location for the SEIS instrument, we also made performance and noise maps of each point around the lander. They identify all the noises likely to disturb the measurements of the instrument and take into account all the known characteristics of the Martian environment.

Estimating the properties of Martian regolith

Determining the structure of near surface provides constraints on the geologic history of a planet. The pressure fluctuations of the Mars’ atmosphere induce tiny deformations of the ground, that can be measured by the SEIS seismometer of the InSight mission. These deformations depend on the elastic properties of the regolith. From the records of pressure, wind, and ground deformation data, the elastic parameters below the lander can be estimated in the first 20 m below the surface (Lognonné et al., 2019, Kenda et al., 2020, Garcia et al., 2020, Murdoch et al., 2021). The first results show the presence of a transition between the regolith at the surface and a stiffer rock, which could be a blocky layer ejected during a meteoritic impact (Lognonné et al., 2020).

Studying convective vortices (dust devils)

InSight measures wind speed, direction and air pressure nearly continuously, offering more data than pervious landed missions. These meteorological sensors have detected hundreds of passing convective vortices (whirlwinds), which are called dust devils when they pick up dust and become visible. The InSight site actually has more whirlwinds than any other place we’ve landed on Mars while carrying weather sensors (Banfield, Spiga et al., 2020). SEIS can feel these vortices pulling on the surface like a giant vacuum cleaner (Lorenz et al., 2015). In addition to being able to calculate just how elastic the Martian surface is by measuring the tug of passing dust devils (Lognonné et al., 2020 ; Banerdt et al., 2020), combined seismic and pressure measurements allow constraints to be made on vortex properties and trajectories (Murdoch et al., 2021) and can indicate heterogeneities in the sub-surface structure around the InSight lander (Golombek et al., 2020).


Top Left : The InSight Lander during integration. Top Right : Setting up performance tests of SEIS at CNES. Bottom Left : Simulations of the wind-induced mechanical noise on the seismometers. Bottom Right : Three images from a typical Spirit Navigation camera sequence showing a Martian dust devil


 SuperCam Microphone (Perseverance - Mars 2020 rover)

The SuperCam instrument suite onboard the Mars 2020 rover includes the Mars Microphone (provided by our SSPA team here at ISAE-SUPAERO). The primary objective of the microphone is to support the Laser Induced Breakdown Spectroscopy (LIBS) investigation of soils and rocks on Mars. The LIBS instrument investigates, at remote distances, the elemental composition of Martian rocks. The Mars Microphone can complement these investigatations by providing information about the hardness and other mechanical properties of the rocks that are otherwise unknown at remote distances (Murdoch et al., 2019, Chide et al., 2019). Although the primary science objective of the SuperCam Microphone is to support the LIBS investigations, the microphone also has the capacity to provide data for new and important atmospheric investigations. With a bandwidth from 100 Hz to 10 kHz, the overarching atmospheric science goal of the microphone is to characterise the Martian atmospheric dynamics at high frequency, including the diurnal and seasonal evolution.

Here are details of some of my research topics related to the SuperCam microphone :

SuperCam microphone testing and characterisation
Testing in Mars conditions is essential given the strong acoustic attenuation at high frequencies due to the low surface pressure. In order to test the Mars Microphone in a fully representative environment before flight, we use the Aarhus Wind Tunnel Simulator II (AWTSII) in Denmark.


Left : The Mars Microphone attached to the structural model of SuperCam. Right : Testing the Mars Microphone in the Aarhus Wind Tunnel.

LIBS science with the SuperCam microphone

Prior to sending the microphone to Mars, we studied the acoustic signal associated with the plasma formation during Laser-Induced Breakdown Spectroscopy (LIBS) experiment on SuperCam. Our research shows that listening to LIBS sparks provides information about the target hardness/density ; results that are independent from, and complementary to, the LIBS spectrum (Murdoch et al., 2017, Chide et al., 2019).

The SuperCam microphone as a high frequency wind sensor

Pre-mission, in tests performed in the Aarhus Martian wind tunnel, we have shown shown that the microphone can be used to estimate the wind speed – a quadratic relationship exists between wind speed and the microphone RMS signal (Chide et al., 2021). However, the relationship between the microphone signal and wind speed may be more complex than this simple relationship, and the sensitivity of the microphone to wind and other atmospheric effects is currently being studied.

Studying the turbulent Martian atmosphere with the SuperCam microphone

Pressure fluctuations in the atmosphere tell us about boundary layer convection, convective cells and vortices, and the inertial and dissipative regimes. Wind gustiness, convective vortex activity and the spectral slope of pressure, wind and temperature measurements can be used as indicators of turbulent motion in the atmosphere. With its high sampling frequency, we can use the SuperCam microphone to study Martian turbulence on new, previously inaccessible, scales. From recordings at different local times and over different seasons, the SuperCam microphone can complement the lower frequency MEDA wind speed measurements and provide a window into previously unexplored regimes of Martian atmospheric science.


 Small body missions : Hera, DART, MMX

My involvement in small body missions ranges from the development of instruments specifically for the exploration of small bodies, to the physical properties and internal structure of asteroids and their geophysical evolution with a strong focus on regolith dynamics.

The AIDA mission : Asteroid Impact and Deflection Assessment (DART + Hera)

The NASA Double Asteroid Redirection Test (DART) mission will reach Didymos in September 2022 and steer itself into Dimorphos at a speed of approximately 6.6 km/s. The last thing DART will transmit back to Earth in advance of the collision will a close-up of Dimorphos’s surface features. Post-impact investigations will be performed initially from Earth and then by the other component of AIDA, ESA’s Hera mission (planned for launch in 2024). Hera’s detailed post-impact survey will turn the large-scale impact experiment into a well-understood and repeatable planetary defence technique.

Overall results from the two missions should provide a baseline for planning any future planetary defence strategies, offering insight into the kind of force needed to shift the orbit of any incoming asteroid, and better understand how the technique could be applied if a real threat were to occur.
DART and Hera are self-standing ; each mission on its own will provide valuable knowledge. However, when combined together this knowledge will be multiplied considerably.

I am a co-investigator of the Hera mission, a collaborator for the DART mission, and have been contributing to the AIDA mission concept since 2007 when I was working as a YGT at ESA.

My current involvement in these mission is two-fold : as a co-lead of the Hera Data Analysis and Exploitation Working Group, I am contributing to the preparation the scientific analyses of the DART and Hera missions, specifically with regards to the geophysical properties of Didymoon (internal structure and surface mechanical properties). In addition, our research is helping to inform the close proximity operations of the Hera mission, specifically with regards to the landing and operational strategy of the CubeSats.

The MMX rover to Phobos

The Japan Aerospace Exploration Agency, JAXA, Martians Moons eXploration (MMX) mission will investigate the Martian Moons Phobos and Deimos, and return samples from Phobos to Earth. As part of this mission a small ( 25 kg) rover, contributed by the Centre National d’Etudes Spatiales (CNES) and the German Aerospace Center (DLR), with additional contributions from INTA (Spain) and JAXA, will be delivered to the surface of Phobos. The rover will demonstrate the technology of locomotion on a regolith-covered, low gravity planetary surface. In addition, the rover will provide scientific data on the regolith properties (mechanical, mineralogical and thermal), provide ground truth for the MMX orbiter instruments, give context information for the returned samples, and contribute to reducing the risk of the landing and sampling operations of the MMX mission.

I am the Principle Investigator of one of the WheelCams, one of the four scientific instruments on board the rover. These cameras will observe the properties of the regolith compaction and flow around the rover wheels, and the resulting trenches in order to characterise the mechanical properties of the regolith itself. We are studying the feasibility and the expected performance of the rover on the surface of a Phobos, in addition to preparing for the data analysis of the WheelCams.

H2020 NEO-MAPP Project

NEO-MAPP stands for Near Earth Object Modelling And Payload for Protection. This project is funded by the H2020 program of the European Commission and addresses the topic "Advanced research in Near Earth Objects (NEOs) and new payload technologies for planetary defence" (SUSPACE-23-SEC-2019). The main goal of NEO-MAPP is to support the development and data analysis of NEO missions, as Hera and provide significant advances in both our understanding of the response of NEOs to external forces (in particular a kinetic impact or a close planetary approach), and in the associated measurements by a spacecraft (including those necessary for the physical and dynamical characterization in general).

I am leading a large NEO-MAPP work package focussed on the development of innovative and synergetic measurement and data-analysis strategies that combine multiple payloads, to ensure optimal data exploitation for NEO missions. As part of the NEO-MAPP project, the SSPA team is also developing a seismic sensor specifically designed for the asteorid environment.

Here are details of some of my research topics related to small bodies :

Regolith dynamics

The dynamics of the surface material are also involved in the evolution of small bodies in our Solar System and are critical for the design and/or operations of landers, sampling devices and rovers to be included in space missions.

  • Asteroid landing : The understanding of surface-lander interactions is important for all asteroid landers as these considerations influence the deployment strategy, the mission design and operations, and even the choice of payload. To simulate landing on an asteroid we have developed a variable gravity drop tower (Sunday et al., 2016). With the tower we perform low velocity collisions in low gravity conditions. The results of our experiments indicate that the lower gravity collisions lead to a more fluidized behaviour of the grains (Murdoch et al., 2017, 2021). To see a video explanation of our drop tower experiment click here.
  • Rolling in low gravity : Using a DEM code that we have improved and validated to model the specific interactions of a wheel with the regolith of a small body (Sunday et al., 2020, 2021), we are studying how sinkage, traction and (simplified) maneuverability vary in different types of regolith, at different levels of gravity. The results will be used directly for the MMX rover operations planning, and interpretation of the wheel motion and interaction with the regolith on Phobos.

Asteroid seismology

Understanding the internal structure of an asteroid has important implications for interpreting its evolutionary history, for understanding its continuing geological evolution, and also for asteroid deflection and in-situ space resource utilisation. There is strong evidence that asteroids are seismically active (see Murdoch et al., 2015 for a review). The SSPA team studies the natural seismicity (tidal forces, impacts, thermal cracking, …) of asteroids to understand the consequences for their evolution and internal structure, and for future asteroid seismic stations (Garcia et al., 2015 ; Murdoch et al., 2017).

Instrumentation for probing the physical properties (surface and interior) of asteroids

By measuring the ground displacement due to seismic activity on the surface of asteroids a geophone can provide constraints on the layering and mechanical properties of the subsurface, in addition to the typical size of subsurface heterogeneities.

Seismology has long been considered a key technique for understanding a planetary body and its interior. However, despite the evidence that asteroids are seismically active and the obvious need to further our understanding of their internal structure, no seismic experiment has been performed on an asteroid’s surface.

To achieve the goal of performing seismology on the surface of a small body, we are developing an instrument that is adapted to the small body environment. Our low mass, low power seismometer can fit inside a small asteroid lander (e.g. a CubeSat) and function in the challenging environment of the asteroid surface.


Top Left : Dust ponds on the asteroid Eros (Credit : JHU-APL,NASA). Bottom Left : An 18 km high mountain (twice the height of Everest !) on asteroid Vesta. Middle : Performing regolith dynamics experiments during a parabolic flight. Right : A prototype facilty used for reduced gravity testing developped at ISAE-SUPAERO

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