The shape and location of density anomalies inside the Moon provide insights into processes that produced them and their subsequent evolution. Gravity measurements provide the most complete data set to infer these anomalies on the Moon [1]. However, gravity inversions suffer from inherent non-uniqueness. To circumvent this issue, it is often assumed that the Bouguer gravity anomalies are produced by the relief of the crust-mantle or other internal interface [2]. This approach limits the recovery of 3D density anomalies or any anomaly at different depths. In this work, we develop an algorithm that provides a set of likely three-dimensional models consistent with the observed gravity data with no need to constrain the depth of anomalies a priori.

Henning2, R. Maguire2,3, V. Lekic2, N. Schmerr2, M. P. Panning4, V.J. Bray5, M. Manga6, S.A. Kattenhorn7, L.C. Quick1, and A.R. Rhoden8, 1NASA Goddard Space Flight Center, Greenbelt, MD 20771 (Terry. A. Hurford@nasa.gov), 2University of Maryland, College Park, MD 20742, 3University of New Mexico, Albuquerque, NM, 87131, 4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, 5University of Arizona, Tucson Az, 85721, 6University of California, Berkeley, Berkeley, CA 94720, 7University of Alaska Anchorage, Anchorage, AK 99508, 8SouthWest Research Institute, Boulder, CO 80302.

S U M M A R Y Inversions of planetary gravity are aimed at constraining the mass distribution within a planet or moon. In many cases, constraints on the interior structure of the planet, such as the depth of density anomalies, must be assumed a priori, to reduce the non-uniqueness inherent in gravity inversions. Here, we propose an alternative approach that embraces the non-uniqueness of gravity inversions and provides a more complete view of related uncertainties. We developed a Transdimensional Hierarchical Bayesian (THB) inversion algorithm that provides an ensemble of mass distribution models compatible with the gravitational field of the body. Using this ensemble of models instead of only one, it is possible to quantify the range of interior parameters that produce a good fit to the gravity acceleration data. To represent the interior structure of the planet or moon, we parametrize mass excess or deficits with point masses. We test this method with synthetic data and, in each test, the algorithm is able to find models that fit the gravity data of the body very well. Three of the target or test models used contain only point mass anomalies. When all the point mass anomalies in the target model produce gravity anomalies of similar magnitudes and the signals from each anomaly are well separated, the algorithm recovers the correct location, number and magnitude of the point mass anomalies. When the gravity acceleration data of a model is produced mostly by a subset of the point mass anomalies in the target model, the algorithm only recovers the dominant anomalies. The fourth target model is composed of spherical caps representing lunar mass concentration (mascons) under major impact basins. The algorithm finds the correct location of the centre of the mascons but fails to find their correct outline or shape. Although the inversion results appear less sharp than the ones obtained by classical inversion methods, our THB algorithm provides an objective way to analyse the interior of planetary bodies that includes epistemic uncertainty.

The short-lived Hf-W isotope system (t1/2 = 9 Ma) left evidence in both ancient and modern terrestrial rock record of processes that took place during the earliest stages of Earth’s accretionary and differentiation history. We report mW values (the deviation of W/W of a sample from that of laboratory standards, in parts per million) and corresponding He/He ratios for rocks from 15 different hotspots. These rocks are characterized by mW values that range from 0 to as low as 23 ± 4.5. For each volcanic system that includes rocks with negative mW values, the values tend to be negatively correlated with He/He. The W-He isotopic characteristics of all samples can be successfully modeled via mixing involving at least three mantle source reservoirs with distinct mW-He/He characteristics. One reservoir has He/He 8 R/RA and lW 0, which is indistinguishable from the convecting upper mantle. Based on high He/He, the other two reservoirs are presumed to be relatively un-degassed and likely primordial. One reservoir is characterized by mW 0, while the other is characterized by mW 23. The former reservoir likely formed from a silicate differentiation process more than 60 Myr after the origin of the solar system, but has remained partially or wholly isolated from the rest of the mantle for most of Earth history. The latter reservoir most likely includes a component that formed while Hf was extant. Mass balance constraints on the isotopic composition of the core suggest it has a strongly negative mW value of 220. Thus, it is a candidate for the origin of the negative mW in the plume sources. Mixing models show that the direct addition of outer core metal into a plume rising from the core-mantle boundary would result in collateral geochemical effects, particularly in the abundances of highly siderophile elements, which are not observed in OIB. Instead, the reservoir characterized by negative mW most likely formed in the lowermost mantle as a result of core-mantle isotopic equilibration. The envisioned equilibration process would raise the W concentration and lower the mW of the resulting silicate reservoir, relative to the rest of the mantle. The small proportion (<0.3 %) of this putative core-mantle equilibrated reservoir required to account for the mW signatures observed in OIB is insufficient to result in observable effects on most other elemental and/or isotopic compositions. The presumed primordial reservoirs may be linked to seismically distinct regions in the lower mantle. Seismically imaged mantle plumes appear to preferentially ascend from the vicinity of large low-shear velocity provinces (LLSVPs), which have been interpreted as thermochemical piles. We associate the LLSVPs with the primordial reservoir characterized by high He/He and mW = 0. Smaller, ultra-low velocity zones (ULVZs) present at the core-mantle boundary have been interpreted to https://doi.org/10.1016/j.gca.2019.12.020 0016-7037/ 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Lithospheric Research, University of Vienna, Vienna, Austria. E-mail address: andrea.mundl@univie.ac.at (A. Mundl-Petermeier). A. Mundl-Petermeier et al. /Geochimica et Cosmochimica Acta 271 (2020) 194–211 195 consist of (partially) molten lower mantle material. The negative mW signatures observed in some plume-derived lavas may result from small contributions of ULVZ material that has inherited its negative mW signature through core-mantle equilibration. 2019 Elsevier Ltd. All rights reserved.

InSight is the first planetary mission with a seismometer package, SEIS, since the Apollo Lunar Surface Experiments Package. SEIS is complimented by APSS, which has as a goal to document the atmospheric source of seismic noise and signals. Since June 2019, SEIS has been delivering 6 axis 20 sps continuous seismic data, a rate one order of magnitude larger originally planned. More than 50 events have been detected by the end of July 2019 but only three have amplitudes significantly above the SEIS instrument requirement. Two have clear and coherent arrivals of P and S waves, enabling location, diffusion/attenuation characterization and receiver function analysis. The event’s magnitudes are likely ≤ 3 and no clear surface waves nor deep interior phases have been identified. This suggests deep events with scattering along their final propagation paths and with large propagation differences as compared to Earth and Moon quakes. Most of the event’s detections are made possible due to the very low noise achieved by the instrument installation strategy and the very low VBB self-noise. Most of the SEIS signals have amplitudes of spectral densities in the 0.03-5Hz frequency bandwidth ranging from 10-10 m/s2/Hz1/2 to 5 10-9 m/s2/Hz1/2. The smallest noise levels occurs during the early night, with angstrom displacements or nano-radian tilts. This monitors the elastic and seismic interaction of a planetary surface with its atmosphere, illustrated not only by a wide range of SEIS signals correlated with pressure vortexes, dust devils or wind activity but also by modulation of resonances above 1 Hz, amplified by ultra-low velocity surface layers. After about one half of a Martian year, clear seasonal changes appear also in the noise, which will be discussed. One year after landing, the seismic noise is therefore better and better understood, and noise correction techniques begun to be implemented, either thanks to the APSS wind and pressure sensors, or by SEIS only data processing techniques. These data processing techniques open not only the possibility of better signal to noise ratio of the events, but are also used for various noise auto-correlation techniques as well as searches of long period signals. Noise and seismic signals on Mars are therefore completely different from what seismology encountered previously on Earth and Moon.

Using a 20‐year continuous broadband record and two independent single‐station techniques—ambient noise autocorrelation and receiver functions—we document a relationship between subsurface seismic response and groundwater levels (GWLs) in the Gulf Coast Aquifer System of southern Texas. We find that a surge of GWL following three consecutive hurricanes and documented at an adjacent monitoring well is accompanied with changes in receiver function power spectra and ambient noise autocorrelations. Using a simple physical model, we show that GWL changes should affect P‐ (VP) more strongly than S‐wave (VS) velocities, consistent with our observations and previous ones based on inter‐station correlations. Agreement between receiver function and ambient noise analyses shows that both can be used to reliably estimate temporal changes in subsurface properties on long timescales. Due to their sensitivity to VP, single‐station techniques respond more strongly to GWL changes, making them useful for characterizing and monitoring aquifer systems.