A document by Samuel Lihn. Click on the document to view its contents.

In 2007, the National Academies designated “understanding the structure & composition of the lunar interior” (to provide fundamental information on the evolution of a differentiated planetary body) as the second highest lunar science priority that needed to be addressed. Here we present the current status of the planned response of the Lunar Geophysical Network (LGN) team to the upcoming New Frontiers-5 AO. The Moon represents an end-member in the differentiation of rocky planetary bodies. Its small size (and heat budget) means that the early stages of differentiation have been frozen in time. But despite the success of the Apollo Lunar Surface Experiment Package (ALSEP), significant unresolved questions remain regarding the nature of the lunar interior and tectonic activity. General models of the processes that formed the present-day lunar interior are currently being challenged. While reinterpretation of the Apollo seismic data has led to the identification of a lunar core, it has also produced a thinning of the nearside lunar crust from 60-65 km in 1974 to 30-38 km today. With regard to the deep mantle, Apollo seismic data have been used to infer the presence of garnet below ~500 km, but the same data have also been used to identify Mg-rich olivine. A long-lived global lunar geophysical network (seismometer, heat flow probe, magnetometer, laser retro-reflector) is essential to defining the nature of the lunar interior and exploring the early stages of terrestrial planet evolution, add tremendous value to the GRAIL and SELENE gravity data, and allow other nodes to be added over time (ie, deliver the International Lunar Network). Identification of lateral and vertical heterogeneities, if present within the Moon, will yield important information about the early presence of a global lunar magma ocean (LMO) as well as exploring LMO cumulate overturn. LGN would also provide new constraints on seismicity, including shallow moonquakes (the largest type identified by ALSEP with magnitudes between 5-6) that have been linked to young thrust fault scarps, suggesting current tectonic activity. Advancing our understanding of the Moon’s interior is critical for addressing these and many other important lunar and Solar System science and exploration questions, including protection of astronauts from the strong shallow moonquakes.

The Moon and Mars have strong regional crustal magnetic fields as shown by the observations of orbiting spacecraft. These crustal magnetic fields indicate that a core dynamo was present in the past, and it is believed that rocks formed from cooling lava retain a memory of the strength and orientation of the magnetic field that existed during their solidification. However, a major hurdle in understanding the crustal magnetic field structure is the low spatial resolution limited by the altitude of the orbiting spacecraft. Rovers are well suited for exploring the detailed structure of the crustal magnetic fields, but they also present a technical challenge due to the necessity that sources of magnetic interference from the rover be removed from the measurements. We designed a compact rover dedicated to measuring the detailed spatial distribution of the surface magnetic field. The rover is equipped with an array of magnetic sensors mounted in a rectangular grid that aims to differentiate the rover components’ own magnetic fields from the ambient magnetic field. Another magnetic sensor is placed on top of an extendable arm to provide additional measurements for verifying the ambient magnetic field inferred from grid-point measurements. The rover consists of four wheels, a Raspberry Pi single-board computer, control circuitry, and a chassis that holds the magnetometers in a rectangular array. In the prototype of the rover, commercial small-scale microelectromechanical systems (MEMS) magnetometers are used for magnetic field measurements. The test results from these design features can provide a useful reference for future lunar or Martian surface missions with rover-based experiments that measure crustal magnetic fields.

With both the Mars Atmosphere and Volatile Evolution (MAVEN) mission and the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission concurrently operating at Mars, we are able to make two point comparisons of the vector magnetic field at Mars for the first time. During MAVEN overflights of the InSight landing site, we compared deviations in the ionospheric magnetic field to variations in the surface level magnetic field. We find significant orbit to orbit variability in the magnitude and direction of the ionospheric magnetic field as well as significant day to day variability of the surface level magnetic field. We attribute this variability to time varying ionospheric currents. However, when analyzing the ensemble of 16 individual MAVEN overflights of the InSight landing location, we see no clear correlation between the magnitudes or directions of the ionospheric magnetic field and the surface magnetic field as might be expected. If the presumed ionospheric currents have a small scale size, then the ionospheric magnetic field will display increased variability as MAVEN flies through the current structure. Whereas the present analysis is restricted to mostly nightside MAVEN overflights where current are expected to be weak, future analyses should incorporate dayside overflights where current are expected to be stronger and current signatures more clear.

We investigate the timing and relative influence of VLF in the chorus frequency range observed by the DEMETER spacecraft and ULF wave activity from ground stations on daily changes in electron flux (0.23 to over 2.9 MeV) observed by the HEO-3 spacecraft. At each L shell, we use multiple regression to investigate the effects of each wave type and each daily lag independent of the others. We find that reduction and enhancement of electrons occur at different time scales. Chorus power spectral density and ULF wave power are associated with immediate electron decreases on the same day but with flux enhancement 1-2 days later. ULF is nearly always more influential than chorus on both increases and decreases of flux, although chorus is often a significant factor. There was virtually no difference in correlations of ULF Pc3, Pc4, or Pc5 with electron flux. A synergistic interaction between chorus and ULF waves means that enhancement is most effective when both waves are present, pointing to a two-step process where local acceleration by chorus waves first energizes electrons which are then brought to even higher energies by inward radial diffusion due to ULF waves. However, decreases in flux due to these waves act additively. Chorus and ULF waves combined are most effective at describing changes in electron flux at >1.5 MeV. At lower L (2-3), correlations between ULF and VLF (likely hiss) with electron flux were low. The most successful models, over L=4-6, explained up to 47.1% of the variation in the data.