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The energy provided in the radioactive decay of thorium (Th) and uranium (U) isotopes, embedded in planetary mantles, sustains geodynamics important for surface habitability such as the generation of a planetary magnetic dynamo. In order to better understand the thermal evolution of nearby exoplanets, stellar photospheric abundances can be used to infer the material composition of orbiting planets. Here we constrain the intrinsic dispersion of the r-process element europium (Eu) (measured in relative abundance [Eu/H]) as a proxy for Th and U in local F, G, and K type dwarf stars. Adopting stellar-chemical data from two high quality spectroscopic surveys, we have determined a small intrinsic scatter of 0.025 dex in [Eu/H] within the disk. We further investigate the stellar anti-correlation in [Eu/ α] vs [ α/H] at late metallicities to probe in what regimes planetary radiogenic heating may lead to periods of extended dynamo collapse. We find that only near-solar metallicity stars in the disk have Eu inventories supportive of a persistent dynamo in attendant planets, supporting the notion of a ``metallicity Goldilocks zone'' in the galactic disk. The observed anti-correlation further provides novel evidence regarding the nature of r-processes injection by substantiating α element production is decoupled from Eu injection. This suggests either a metallicity-dependent r-process in massive core-collapse supernovae, or that neutron-star merger events dominate r-process production in the recent universe. 

Abstract The bulk of Uranus consists of a rock–ice core, but the relative proportions of rock and ice are unknown. Radioactive decay of potassium in the silicates produces⁴⁰Ar. If transport of argon from the core to the gaseous envelope is efficient, a measurement of⁴⁰Ar in the envelope will provide a direct constraint on the rock mass present (assuming a chondritic rock composition). The expected⁴⁰Ar concentrations in this case would be readily detectable by a mass spectrometer carried by a future atmospheric probe. For a given envelope concentration there is a trade-off between the rock mass present and the transport efficiency; this degeneracy could be overcome by making independent determinations of the rock mass (e.g., by gravity and seismology). Primordial⁴⁰Ar is a potential confounding factor, especially if Ar/H₂is significantly enhanced above solar or if degassing of radiogenic⁴⁰Ar were inefficient. Unfortunately, the primordial⁴⁰Ar/³⁶Ar ratio is very uncertain; better constraints on this ratio through measurement or theory would be very helpful. Pollution of the envelope by silicates is another confounding factor but can be overcome by a measurement of the alkali metals in the envelope. 

Abstract Single crystal paleointensity (SCP) reveals that the Moon lacked a long-lived core dynamo, though mysteries remain. An episodic dynamo, seemingly recorded by some Apollo basalts, is temporally and energetically problematic. We evaluate this enigma through study of ~3.7 billion-year-old (Ga) Apollo basalts 70035 and 75035. Whole rock analyses show unrealistically high nominal magnetizations, whereas SCP indicate null fields, illustrating that the former do not record an episodic dynamo. However, deep crustal magnetic anomalies might record an early lunar dynamo. SCP studies of 3.97 Ga Apollo breccia 61016 and 4.36 Ga ferroan anorthosite 60025 also yield null values, constraining any core dynamo to the Moon’s first 140 million years. These findings suggest that traces of Earth’s Hadean atmosphere, transferred to the Moon lacking a magnetosphere, could be trapped in the buried lunar regolith, presenting an exceptional target for future exploration. 

Jupiter’s moon Io hosts extensive volcanism, driven by tidal heating. The isotopic composition of Io’s inventory of volatile chemical elements, including sulfur and chlorine, reflects its outgassing and mass-loss history and thus records information about its evolution. We used submillimeter observations of Io’s atmosphere to measure sulfur isotopes in gaseous sulfur dioxide and sulfur monoxide, and chlorine isotopes in gaseous sodium chloride and potassium chloride. We find³⁴S/³²S = 0.0595 ± 0.0038 (equivalent to δ³⁴S = +347 ± 86‰), which is highly enriched compared to average Solar System values and indicates that Io has lost 94 to 99% of its available sulfur. Our measurement of³⁷Cl/³⁵Cl = 0.403 ± 0.028 (δ³⁷Cl = +263 ± 88‰) shows that chlorine is similarly enriched. These results indicate that Io has been volcanically active for most (or all) of its history, with potentially higher outgassing and mass-loss rates at earlier times. 

Abstract Stable isotope fractionation of sulfur offers a window into Io's tidal heating history, which is difficult to constrain because Io's dynamic atmosphere and high resurfacing rates leave it with a young surface. We constructed a numerical model to describe the fluxes in Io's sulfur cycle using literature constraints on rates and isotopic fractionations of relevant processes. Combining our numerical model with measurements of the³⁴S/³²S ratio in Io's atmosphere, we constrain the rates for the processes that move sulfur between reservoirs and model the evolution of sulfur isotopes over time. Gravitational stratification of SO₂in the upper atmosphere, leading to a decrease in³⁴S/³²S with increasing altitude, is the main cause of sulfur isotopic fractionation associated with loss to space. Efficient recycling of the atmospheric escape residue into the interior is required to explain the³⁴S/³²S enrichment magnitude measured in the modern atmosphere. We hypothesize this recycling occurs by SO₂surface frost burial and SO₂reaction with crustal rocks, which founder into the mantle and/or mix with mantle‐derived magmas as they ascend. Therefore, we predict that magmatic SO₂plumes vented from the mantle to the atmosphere will have lower³⁴S/³²S than the ambient atmosphere, yet are still significantly enriched compared to solar‐system average sulfur. Observations of atmospheric variations in³⁴S/³²S with time and/or location could reveal the average mantle melting rate and hence whether the current tidal heating rate is anomalous compared to Io's long‐term average. Our modeling suggests that tides have heated Io for >1.6 Gyr if Io today is representative of past Io. 

Plate tectonics is a fundamental factor in the sustained habitability of Earth, but its time of onset is unknown, with ages ranging from the Hadaean to Proterozoic eons1–3. Plate motion is a key diagnostic to distinguish between plate and stagnant-lid tectonics, but palaeomagnetic tests have been thwarted because the planet’s oldest extant rocks have been metamorphosed and/or deformed4. Herein, we report palaeointensity data from Hadaean-age to Mesoarchaean-age single detrital zircons bearing primary magnetite inclusions from the Barberton Greenstone Belt of South Africa5. These reveal a pattern of palaeointensities from the Eoarchaean (about 3.9 billion years ago (Ga)) to Mesoarchaean (about 3.3 Ga) eras that is nearly identical to that defined by primary magnetizations from the Jack Hills (JH; Western Australia)6,7, further demonstrating the recording fidelity of select detrital zircons. Moreover, palaeofield values are nearly constant between about 3.9 Ga and about 3.4 Ga. This indicates unvarying latitudes, an observation distinct from plate tectonics of the past 600 million years (Myr) but predicted by stagnant-lid convection. If life originated by the Eoarchaean8, and persisted to the occurrence of stromatolites half a billion years later9, it did so when Earth was in a stagnant-lid regime, without plate-tectonics-driven geochemical cycling. 

Abstract Earth’s magnetic field was in a highly unusual state when macroscopic animals of the Ediacara Fauna diversified and thrived. Any connection between these events is tantalizing but unclear. Here, we present single crystal paleointensity data from 2054 and 591 Ma pyroxenites and gabbros that define a dramatic intensity decline, from a strong Proterozoic field like that of today, to an Ediacaran value 30 times weaker. The latter is the weakest time-averaged value known to date and together with other robust paleointensity estimates indicate that Ediacaran ultra-low field strengths lasted for at least 26 million years. This interval of ultra-weak magnetic fields overlaps temporally with atmospheric and oceanic oxygenation inferred from numerous geochemical proxies. This concurrence raises the question of whether enhanced H ion loss in a reduced magnetic field contributed to the oxygenation, ultimately allowing diversification of macroscopic and mobile animals of the Ediacara Fauna.