Search for: All records

An internally generated magnetic field once existed on the Moon. This field reached high intensities (∼10–100μT, perhaps intermittently) from ∼4.3 to 3.6 Gyr ago and then weakened to ≲5μT before dissipating by ∼1.9–0.8 Gyr ago. While the Moon’s metallic core could have generated a magnetic field via a dynamo powered by vigorous convection, models of a core dynamo often fail to explain the observed characteristics of the lunar magnetic field. In particular, the core alone may not contain sufficient thermal, chemical, or radiogenic energy to sustain the high-intensity fields for >100 Myr. A recent study by Scheinberg et al. suggested that a dynamo hosted in electrically conductive, molten silicates in a basal magma ocean (BMO) may have produced a strong early field. However, that study did not fully explore the BMO’s coupled evolution with the core. Here we show that a coupled BMO–core dynamo driven primarily by inner core growth can explain the timing and staged decline of the lunar magnetic field. We compute the thermochemical evolution of the lunar core with a 1D parameterized model tied to extant simulations of mantle evolution and BMO solidification. Our models are most sensitive to four parameters: the abundances of sulfur and potassium in the core, the core’s thermal conductivity, and the present-day heat flow across the core–mantle boundary. Our models best match the Moon’s magnetic history if the bulk core contains ∼6.5–8.5 wt% sulfur, in agreement with seismic structure models.

 

Observations from Cassini have identified nanometer-sized silica grains in Saturn’s E-ring although their origin is unclear. Tidal deformation within Enceladus’ silicate core has been predicted to generate hot hydrothermal fluids that rise from the core-ocean boundary and traverse the subsurface ocean. This raises the possibility that the particles observed by Cassini could have been produced by hydrothermal alteration and ejected via the south polar plumes. Here, we use an analytical model to quantify potential for particle entrainment in Enceladus’ ocean. We use scaling relations to characterize ocean convection and define a parameter space that enables particle entrainment. We find that both the core-ocean heat fluxes and the transport timescale necessary to drive oceanic convection and entrain particles of the observed sizes are consistent with observations and predictions from existing thermal models. We conclude that hydrothermal alteration at Enceladus’ seafloor could indeed be the source of silica particles in Saturn’s E-ring.

 

We use three‐dimensional numerical experiments of thin shell convection to explore what effects an expected latitudinal variation in solar insolation may have on a convection. We find that a global flow pattern of upwelling equatorial regions and downwelling polar regions, linked to higher and lower surface temperatures (T_s), respectively, is preferred. Due to the gradient inT_s, boundary layer thicknesses vary from equatorial lows to polar highs, and polar oriented flow fields are established. AHadley cell‐type configuration with two hemispheric‐scale convective cells emerges with heat flow enhanced along the equator and suppressed poleward. The poleward transport pattern appears robust under a range of basal and mixed heating, isoviscous and temperature‐dependent viscosity, vigor of convection, and different degrees ofT_svariations. Our findings suggest that a latitudinal variation inT_sis an important effect for convection within the thin ice shells of the outer satellites, becoming increasingly important as solar luminosity increases. VariableT_smodels predict lower heat flow and a more compressional regime near downwellings at higher latitudes, and higher heat flow and a more extensional regime near the equator. Within the ice shell, Hadley style flow could lead to large‐scale anisotropic ice properties that might be detectable with future seismic or electro‐magnetic observations. 

The Neptune Odyssey mission concept is a Flagship-class orbiter and atmospheric probe to the Neptune–Triton system. This bold mission of exploration would orbit an ice-giant planet to study the planet, its rings, small satellites, space environment, and the planet-sized moon Triton. Triton is a captured dwarf planet from the Kuiper Belt, twin of Pluto, and likely ocean world. Odyssey addresses Neptune system-level science, with equal priorities placed on Neptune, its rings, moons, space environment, and Triton. Between Uranus and Neptune, the latter is unique in providing simultaneous access to both an ice giant and a Kuiper Belt dwarf planet. The spacecraft—in a class equivalent to the NASA/ESA/ASI Cassini spacecraft—would launch by 2031 on a Space Launch System or equivalent launch vehicle and utilize a Jupiter gravity assist for a 12 yr cruise to Neptune and a 4 yr prime orbital mission; alternatively a launch after 2031 would have a 16 yr direct-to-Neptune cruise phase. Our solution provides annual launch opportunities and allows for an easy upgrade to the shorter (12 yr) cruise. Odyssey would orbit Neptune retrograde (prograde with respect to Triton), using the moon's gravity to shape the orbital tour and allow coverage of Triton, Neptune, and the space environment. The atmospheric entry probe would descend in ∼37 minutes to the 10 bar pressure level in Neptune's atmosphere just before Odyssey's orbit-insertion engine burn. Odyssey's mission would end by conducting a Cassini-like “Grand Finale,” passing inside the rings and ultimately taking a final great plunge into Neptune's atmosphere.