Martian Crustal Magnetism
While Mars does not possess a currently active geodynamo, remanent crustal magnetization has been found across the planet and contains records of the origin, scale, and timing of Martian magnetization. The first in situ measurements of the Martian magnetic field on the planet’s surface, at the InSight and Zhurong landing sites, allow for better constraints on magnetization coherence and depth scales near the surface, as crustal fields are closely related to a variety of geological and topographic features. We develop Monte Carlo models of the Martian crustal magnetization near the two landing sites on small-scales to meso-scales to compute altitude profiles of the magnetic field intensity. We compare our simulations with the Langlais et al. 2019 crustal field model and surface measurements, indicating that power law distributions more accurately describe Martian altitude profiles compared to Gaussian models. Observations are best explained by fractal parameter values near 2.7 and coherence scales roughly 250 km near InSight, with larger coherence scales and possibly thicker crustal magnetization near Zhurong. Motivated by these length scales, we create additional magnetization models based on the geological units near each lander to relate them to different time periods of Martian history. Our results suggest at least one polar field reversal in Martian history based on the simulated magnetization near the North-South dichotomy boundary. Furthermore, we propose that the Martian geodynamo might have weakened or suspended during the late Noachian, followed by revitalization of the core dynamo during the Hesperian period.
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Plasma Waves Upstream Mars
The presence of plasma waves upstream from the Martian bow shock, with frequencies near the local proton cyclotron frequency in the spacecraft frame, constitutes, in principle, an indirect signature for the existence of planetary protons from the ionization of Martian exospheric hydrogen. In this study, we determine the 'proton cyclotron wave' (PCW) occurrence rate between October 2014 through February 2020, based on Magnetometer (MAG) and Solar Wind Ion Analyzer (SWIA) measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We characterize its dependence on several wave and solar wind (SW) properties, and solar longitude ranges. We confirm a previously reported long-term trend with more PCWs near perihelion, likely associated with changes in exospheric hydrogen density. Furthermore, we report for the first time a decrease in median PCW amplitude for each consecutive Martian perihelion. Such variability cannot be attributed to differences in the distribution of SW conditions. This trend could be associated with changes in solar inputs, foreshock effects, and asymmetries due to the SW convective electric field influencing newborn protons. In addition, we observe PCWs more frequently for low to intermediate interplanetary magnetic field (IMF) cone angles, slower SW speeds, and higher SW proton densities. The IMF cone angle preference likely results from the trade-off between associated linear wave growth rates, wave saturation energies, and pick-up proton densities. Moreover, the dependencies on SW speed and density indicate the importance of the characteristic SW transit timescale and the charge exchange process coupling SW protons with the hydrogen exosphere.
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Terrestrial Seismology
To better comprehend the characteristics of the Earth’s interior structure, we calculate the frequency-dependent delay times for different seismic waves during the 2013 Sea of Okhotsk Earthquake (M 8.3). Seismic waves propagate distinctively through the planet when encountering various compositions and states of matter depending on their wave type, frequency, and phase. Accounting for these parameters, Seismic stations all over the world detect and record the arrival time and direction of these waves to create models of the Earth’s structure, such as PREM (Preliminary Reference Earth Model). The observed travel times of P and PP waves of this event are cross correlated with the synthetic seismograms of the PREM model and then filtered based on varying levels of frequency. The delay times from the filtered correlograms are calculated to determine substantial deviations from the model and understand how seismic waves containing different frequencies travel through the Earth. Substantial deviations in delay times from the PREM predictions can provide new constraints on the thermo-chemical state of Earth's deep interior.