Dr Tristram Warren

Abstract:

Seismic observations involve signals that can be easily masked by noise injection. For the NASA Mars lander InSight, the atmosphere is a significant noise contributor, impeding the identification of seismic events for two-thirds of a Martian day. While the noise is below that seen at even the quietest sites on Earth, the amplitude of seismic signals on Mars is also considerably lower, requiring an understanding and quantification of environmental injection at unprecedented levels. Mars’ ground and atmosphere are a continuously coupled seismic system, and although atmospheric functions are of distinct origins, the superposition of these noise contributions is poorly understood, making separation a challenging task. We present a novel method for partitioning the observed signal into seismic and environmental contributions. Atmospheric pressure and wind fluctuations are shown to exhibit temporal cross-frequency coupling across multiple bands, injecting noise that is neither random nor coherent. We investigate this through comodulation, quantifying the synchrony of the seismic motion, wind and pressure signals. By working in the time-frequency domain, we discriminate between the different origins of underlying processes and determine the site's environmental sensitivity. Our method aims to create a virtual vault at InSight's landing site on Mars, shielding the seismometers with effective postprocessing in lieu of a physical vault. This allows us to describe the environmental and seismic signals over a sequence of sols, to quantify the wind and pressure injection and estimate the seismic content of possible marsquakes with a signal-to-noise ratio that can be quantified in terms of environmental independence. Finally, we exploit the relationship between the comodulated signals to identify their sources.

Abstract:

Understanding how the surfaces of airless planetary bodies—such as the Moon—scatter visible light enables constraints to be placed on their surface properties and top boundary layer inputs to be set within thermal models. Remote sensing instruments—such as Diviner onboard the Lunar Reconnaissance Orbiter—measure thermal emission and visible light scattering functions across visible (∼0.38–0.7 µm) to thermal infrared (TIR) wavelengths (∼0.7–350 μm). To provide ground support measurements for such instruments, the Oxford Space Environment Goniometer (OSEG) was built. Initially, the OSEG focused on measuring TIR directional emissivity functions for regolith and regolith simulant samples in a simulated space environment, but it has recently been modified to measure visible wavelength Bidirectional Reflectance Distribution Functions (BRDFs) of samples in ambient conditions. Laboratory-measured BRDFs can be used (1) to test and to help interpret models—such as the Hapke photometric model—and (2) as visible scattering function inputs for thermal models. This paper describes the modifications to and initial calibration measurements taken by the Visible Oxford Space Environment Goniometer with a 532 nm laser, and details how this setup can be used to measure BRDFs of regolith and regolith simulant samples of airless planetary bodies.