Results from the Lunar Magnetotelluric Sounder on Blue Ghost Mission 1

Blue Ghost Mission 1 (BGM1) landed on the Moon in Mare Crisium (18.562 °N, 61.810 °E) on March 3, 2025.  It deployed the lunar magnetotelluric sounder (LMS), the first extraterrestrial MT experiment, designed to investigate upper mantle electrical conductivity and temperature, outside the Moon’s Procellarum KREEP Terrane (PKT).  The PKT exhibits extensive mare volcanism and surficial heat-producing elements (HPE), but their causal relationship remains unclear.  Specificially, the amount and depth distribution of HPE elements beneath the PKT is unknown and various models have different implications for mantle temperature.  Mantle electrical conductivity has previously been investigated at the Apollo 12 (A12) site, and new data acquired from BGM1 provide the opportunity to compare electrical conductivity profiles and inferred mantle temperatures beneath sites inside (A12) and outside (BGM1) the PKT.

LMS operated until March 12, 2025. Comparison of vector magnetic field data from LMS and the ARTEMIS THEMIS-B orbiting spacecraft show the transit through the solar wind, the magnetosheath and the magnetotail, with bow shock crossings and the magnetotail current sheet crossing clearly observed in LMS data.

A landing site with small crustal fields was desirable for the electrical conductivity experiment to minimize plasma interactions.  Satellite-based models predict surface fields of less than 10 nT at BGM1.  Although measurement of crustal fields was not a science requirement or objective, determination of the static field has been possible and it can be demonstrated to be of primarily crustal (not spacecraft) origin.  The resulting surface field of ~65 nT reflects only modest additional contributions from magnetizations not observable from orbit.

The magnetotelluric (MT) method uses orthogonal horizontal components of local time-varying electric and magnetic fields to determine subsurface electrical conductivity. However, a combination of plasma conductivity 10x higher than expected and magnetometer placement relatively far from the surface resulted in a frequency-dependent attenuation of the induction signal. Although MT produces plausible results, we focus on electrical conductivity results obtained using the magnetic Transfer Function (TF) approach, that compares fields measured at the surface to those measured at distance from the Moon.  We compare LMS measurements at BGM1 with reference magnetic fields measured by THEMIS-B to obtain TF at BGM1, and invert these for electrical conductivity.  We also reinvert TFs computed using A12 surface fields and those measured simultaneously by the distant Explorer 35 orbiter. We find that the temperature difference between A12 and BGM1 derived from electrical conductivity is <100 K (+1-sigma level) at 200-km depth. This is incompatible with excess HPE abundances required for PKT-centric partial melting throughout lunar history. We suggest that the thin crust at PKT led to preferential eruption of mare basalts, and preferential excavation of globally distributed urKREEP. We conclude that regional volcanism and surficial incompatible elements in PKT are not genetically related.

LMS Team: R. Grimm (PI), G. Delory, J. Espley, I. Garrick-Bethel, J. Gruesbeck, S. Howard, C. Johnson, R. Maxwell, C. Neal, T. Nguyen, R. Nolan, M. Phillips, M. Purucker, D. Sheppard, F. Simpson, C. Smith, T. Smith, D. Stillman, T. Taylor, P. Turin.