Walter S. Kiefer

Troctolite sample 76535, collected in Serenitatis basin during Apollo 17, formed at least 50 km deep, experienced maximum shock pressures of 6 GPa, and has a ⁴⁰ Ar/ ³⁹ Ar excavation age of 4.25 Ga. Previous work attributed 76535 to the South Pole‐Aitken (SPA) basin, presumably dating the SPA‐impact and constraining lunar bombardment history. Here we use the iSALE‐2D shock‐physics code and gravity inversion modeling to determine if instead the Serenitatis impact event excavated 76535. We find nearly 140,000 km ³ of material (∼2% of near‐surface ejecta) matching the depth and pressure constraints of 76535 is displaced to the surface during crater collapse of a Serenitatis‐like impact event. We conclude that the Serenitatis impact event possibly excavated 76535, redefining its age to 4.25 Ga, 300 My older than the consensus age based on Apollo 17 samples. This finding would provide an important anchor point where lunar chronology where bombardment flux is especially uncertain.

NASA’s Europa Clipper flagship mission launched on October 14, 2024 and will arrive at Jupiter in April 2030. There, it will investigate the habitability of Jupiter’s moon Europa. Gravity and Radio Science (G/RS) is one of ten complementary investigations devoted to understanding Europa through studies of its ice shell and ocean, its composition, and its geology. G/RS makes use of the Europa Clipper telecommunications system to obtain radiometric tracking data during the Europa flybys.Unlike past missions, the primary raw data for the G/RS investigation are obtained from Open-Loop Receivers (OLR) at the Deep Space Network (DSN) ground stations. Indeed, the flyby geometry, spacecraft attitude, and spacecraft antenna configuration are such that the signal-to-noise ratio of the return radio signals will be small (

EnVision is ESA’s next mission to Venus, in partnership with NASA, where NASA provides the Synthetic Aperture Radar payload and mission support. The ESA mission adoption is scheduled for January 2024, and the launch for 2031. The start of the science operations at Venus is early 2035 following the mission cruise, and aerobraking phase around Venus to achieve a low Venus polar orbit. The scientific objective of EnVision is to provide a holistic view of the planet from its inner core to its upper atmosphere, studying the planets history, activity and climate. EnVision aims to establish the nature and current state of Venus’ geological evolution and its relationship with the atmosphere. EnVision’s overall science objectives are to: (i) characterize the sequence of events that formed the regional and global surface features of Venus, as well as the geodynamic framework that has controlled the release of internal heat over Venus history; (ii) determine how geologically active the planet is today; (iii) establish the interactions between the planet and its atmosphere at present and through time. Furthermore, EnVision will look for evidence of past liquid water on its surface.The nominal science phase of the mission will last six Venus cycles (~four Earth years), and ~210 Tbits of science data will be downlinked using a Ka-/X-band communication system. The science objectives will be addressed by five instruments and one experiment, provided by ESA memberstates and NASA. The VenSAR S-band radar will perform targeted surface imaging as well as polarimetric and stereo imaging, radiometry, and altimetry. The high-frequency Subsurface Radar Sounder (SRS) will sound the upper crust in search of material boundaries. Three spectrometers, VenSpec-U, VenSpec-H and VenSpec-M, operating in the UV and Near- and Short Wave-IR, respectively, will map trace gases, search for volcanic gas plumes above and below the clouds, and map surface emissivity and composition. A Radio Science Experiment (RSE) investigation will exploit the spacecraft Telemetry Tracking and Command (TT&C in Ka-/X bands) system to determine the planet’s gravity field and to sound the structure and composition of the middle atmosphere and the cloud layer in radio occultation. All instruments have substantial heritage and robust margins relative to the requirements, with designs suitable for operation in the Venus environment, and were chosen to meet the broad range of measurement requirements needed to support the EnVision scientific objectives. The EnVision science teams will adopt an open data policy, with public release of the scientific data after verification and validation. Public calibrated data availability is

Europa Clipper is the next NASA Flagship mission that will explore Jupiter’s moon Europa. It has a rich payload with ten instruments and investigations, including the Gravity and Radio Science investigation (G/RS). The synergistic science made possible will provide a synoptic view of the Europa system. The overarching goal of Europa Clipper is to Explore Europa to Investigate its Habitability with a number of science objectives and themes related to its ice shell and ocean, its composition, its geology, and its potential recent activity. The Europa Clipper spacecraft is currently undergoing assembly and testing at NASA JPL (follow live on Youtube! https://bit.ly/clippercam) and it will be shipped to KSC in early 2024. The launch window opens on October 10, 2024.With 49 planned flybys, the tour trajectory samples Europa globally, but not evenly, with gaps at 90° and 270° longitude due to the constraints of the multiple-flyby mission design strategy to limit radiation impacts. Flyby altitudes typically vary between 25 km and 100 km, providing for higher sensitivity to shorter-wavelength gravity signal. The primary raw data for the G/RS investigation are collected from DSN 70-m antennas through Open-Loop Receivers (OLRs) in the ±2h periods around each flyby, leveraging the telecom subsystem’s three fan beam (FBA) and two low-gain (LGA) antennas because the high-gain antenna (HGA) is not steerable.The highest priority for the G/RS investigation is to obtain an accurate measurement of the tidal Love number k2, which describes the amplitude of the gravitational response of Europa to the forcing tidal potential imposed by Jupiter. The measurement requirement is set at an uncertainty of 0.06 to provide an unambiguous independent assessment of the presence of an ocean. Expectations from orbit determination simulations show a robust margin of 3-4 times. Simulations of the gravity field recovery show that the low-degree gravity field can be resolved to degrees 5-10, depending on the assumptions for the level of gravity anomalies in the truth field.The interior structure of Europa will be informed by its hydrostatic equilibrium state (a current assumption as the Galileo data is not sufficient to independently estimate J2 and C22) and its moment of inertia. Given the uneven low-altitude spatial sampling, “Line-of-Sight” (LOS) analysis techniques will be important to extract the most from the signatures in the radio Doppler data. Other constraints on the ice shell, ocean, and seafloor will be possible especially in combination with the data collected by the other Europa Clipper instruments. Moreover, Europa Clipper will probe Europa’s ionosphere with radio occultations, with geographic coverage complementary to in situ instruments.The G/RS team is currently supporting Phase D efforts, developing cruise activities, and finalizing the PDS archive plans. We will report on the current G/RS science and operational plans.

A key question in the planetary sciences centers on the divergence between the sibling planets, Venus and Earth. Venus currently does not operate with plate tectonics, and its thick atmosphere has led to extreme greenhouse conditions. It is unknown if this state was set primordially or if Venus was once more Earth-like. Here, we explore Venus as an example of a planet that recently transitioned between tectonic regimes. Our results show that transitions naturally lead to substantial resurfacing and melt-generated outgassing from lithosphere-breaking events and overturns, with 3 to 10 bars of atmosphere generated per overturn over ~60–million year timescales and ~10 to 100 bars outgassed over billion-year time frames. We find that the observation of Venus with a thick greenhouse atmosphere and the inferences of currently low volcanic rates and previous prodigious volcanic rates are consistent with a planet that has undergone a transition in tectonics, suggesting that Venus once hosted clement surface conditions and was more Earth-like.

Europa Clipper is the next NASA Flagship mission that will explore Jupiter’s moon Europa. It has a rich payload with ten instruments and investigations, including the Gravity and Radio Science investigation (G/RS). The synergistic science made possible will provide a synoptic view of the Europa system. The overarching goal of Europa Clipper is to Explore Europa to Investigate its Habitability with a number of science objectives and themes related to its ice shell and ocean, its composition, its geology, and its potential recent activity. The Europa Clipper spacecraft is currently undergoing assembly and testing at NASA JPL (follow live on Youtube! https://bit.ly/clippercam) and it will be shipped to KSC in May 2024. The launch window opens on October 10, 2024.With 49 planned flybys, the tour trajectory samples Europa globally, but not evenly, with gaps at 90° and 270° longitude due to the constraints of the multiple-flyby mission design strategy to limit radiation impacts. Flyby altitudes typically vary between 25 km and 100 km, providing for higher sensitivity to shorter-wavelength gravity signal. The primary raw data for the G/RS investigation are collected from DSN 70-m antennas through Open-Loop Receivers (OLRs) in the ±2h periods around each flyby, leveraging the telecom subsystem’s three fan beam (FBA) and two low-gain (LGA) antennas because the high-gain antenna (HGA) is not steerable.The highest priority for the G/RS investigation is to obtain an accurate measurement of the tidal Love number k2, which describes the amplitude of the gravitational response of Europa to the forcing tidal potential imposed by Jupiter. The measurement requirement is set at an uncertainty of 0.06 to provide an unambiguous independent assessment of the presence of an ocean. Expectations from orbit determination simulations show a robust margin of 3-4 times. Simulations of the gravity field recovery show that the low-degree gravity field can be resolved to degrees 5-10, depending on the assumptions for the level of gravity anomalies in the truth field.The interior structure of Europa will be informed by its hydrostatic equilibrium state (a current assumption as the Galileo data is not sufficient to independently estimate J2 and C22) and its moment of inertia. Given the uneven low-altitude spatial sampling, “Line-of-Sight” (LOS) analysis techniques will be important to extract the most from the signatures in the radio Doppler data. Other constraints on the ice shell, ocean, and seafloor will be possible especially in combination with the data collected by the other Europa Clipper instruments. Moreover, Europa Clipper will probe Europa’s ionosphere with radio occultations, with geographic coverage complementary to in situ instruments.

If Venus were a newly discovered exoplanet, it would be one of the most Earth-like yet identified. Its similarity in size, bulk density and cloud top temperatures give no clue to the hellish temperatures at its surface. Venus hosts an array of geological features as complex as Earth, but without its organisation, and sustains a chemically reactive atmosphere, but without life. Proposed in response to ESA’s M5 call, with enabling support from NASA, EnVision is currently in Phase A study for the next medium-class mission opportunity proceeding towards a final mission selection in summer 2021 for a launch in 2032. Here we discuss the science questions motivating the mission in more detail. Building on discoveries from Magellan, Venus Express and Akatsuki, EnVision will focus on three overarching questions:Is Venus geodynamically active? How did Venus arrive at its current state? How does the Venus climate work and how do the interior, surface and atmosphere interact? 1 – Is Venus geodynamically active?Venus should be geologically active today and its globally young surface implies extensive volcanic resurfacing, but whether this happened in episodic global events or in a continuous process of small-scale resurfacing is uncertain. Constraining the rate of volcanic and tectonic activity can reveal whether Venus occupies one of these end members, or lies somewhere on the spectrum between.Magellan radar image mosaic of a 550 km wide region of Lada Terra (47°S, 25°E) showing a system of radar-bright and dark lava flows breaching a ridge belt and ponding in a large radar-bright deposit covering 100,000 km². Ammavaru, the source caldera, lies 300 km to the west (JPL/Caltech).Understanding the tectonic regime driving resurfacing and heat loss is as important as knowing the rate of activity. The surface appears partitioned into areas of low strain bounded by narrow high strain margins. The abundance of steep slopes and landslides implies active uplift in these high strain areas, but existing data provide no constraint on the frequency of landslides or rates of tectonic movement. Are the low strain regions actively created and destroyed, like Earth’s oceanic plates, or simply mobilised locally? What is the significance of the global network of elevated rift systems and linear lowland wrinkle-ridged plains? Unique to Venus are coronae, quasi-circular volcano-tectonic features, typically 100–500 km across. Are they the surface expression of plumes, or magmatic intrusions, or subduction zones?The processes of weathering, mass wasting and aeolian transport are critical for understanding both the geological history and climatic evolution of Venus. Venus Express found anomalously high IR emissivities near suspected active volcanoes, interpreted as fresh, unweathered lava flows, but both their mineralogy and the weathering processes involved are unknown. Magellan detected several, probably impact-related, dune fields at the limit of its resolution, and found indirect evidence for globally distributed smale-scale ripples.2 – How Did Venus Arrive at its Current State? The cratering record reveals that the Venus surface is, on average, under 1 Ga old. While some areas are likely active today, other regions, e.g. the tessera highlands, may be considerably older. Magellan data imply a variety of age relationships and long-term activity, with a non-random association between geological features and elevation, e.g. the uplands are consistently more deformed than the lowlands. Impact craters themselves show alteration to dark floors, perhaps from airfall deposits or magmatic resurfacing. Densely fractured plains (right) abutting tessera (bottom); both are embayed by younger plains (dark areas). A steep-sided volcanic dome (upper left) may be the source of the plains or unrelated to them, and older or younger than the tesserae. Magellan image at 46°N, 360°E, after Basilevsky and McGill (2007).Constraining the history is critical to understanding not only when and how resurfaced, but whether that activity has changed systematically through time. Were the plains formed in a short period by massive outpourings or by many thousands of small flows over their entire history? Or did many different mechanisms – including sedimentary processes – operate at different times and places? Globally, Venus exhibits perhaps even more tectonic deformation than Earth: what is its role in resurfacing the planet and have the regimes of tectonic deformation changed over time? How did tesserae, especially, accumulate their extraordinary degree of deformation?The interior of Venus is probably Earth-like, but not the same; its core size is poorly constrained by Magellan gravity data. These data are consistent with an organised pattern of mantle convection but lack the resolution necessary to connect it with geological-scale features.3 - How does the Venus climate work and how do the Interior, Surface and Atmosphere Interact? How and why the atmospheres of two Earth-like planets evolved so differently is one of the many compelling reasons to study Venus, directly addressing the question of how our own world became habitable. Understanding its evolution depends on knowing the exchanges between its interior, surface and space.The lower/middle clouds of Venus as imaged by Akatsuki IR2 camera. Dark areas represent thicker clouds, which may represent volcanic plumes of ash or sulphate particulates. (JAXA/ISAS/DART/Damia Bouic)Models suggest that the clouds are maintained by a constant input of H2O and SO2, both of which vary considerably, perhaps because of volcanic emissions. Elevated D/H ratios indicate the loss of early oceans, but the ‘starting’ mantle D/H ratio is unknown. Identification of granites in the ancient highlands would support this inference, but the direct detection and characterisation of volcanic volatiles is key to inferring past climate...

The primary objective of the Europa Clipper mission is to assess the habitability of Europa, an overarching goal that rests on improving our understanding of Europa’s interior structure, composition, and geologic activity. Here we describe the Gravity and Radio Science (G/RS) investigation. The primary measurement, the gravitational tidal Love number $k_{2}$ k 2 , will be an independent diagnostic of the presence of a global subsurface ocean, but G/RS will make a number of other key measurements related to Europa’s deep interior, silicate mantle-ocean interface, ice shell, ionosphere, and plasma environment. Although radio science is common to many missions, Europa Clipper’s orbit and spacecraft configuration during flybys present special challenges for the design of this experiment. The information obtained through G/RS will be complementary to the measurements by the other instruments onboard Europa Clipper, and their combined analysis will refine the geophysical understanding of Europa necessary to best assess its potential habitability.