It is uncertain whether the Moon ever formed a metallic core or generated a core dynamo. The lunar crust and returned samples are magnetized, but the source of this magnetization could be meteoroid impacts rather than a dynamo. Here, we report magnetic measurements and 40Ar/39Ar thermochronological calculations for the oldest known unshocked lunar rock, troctolite 76535. These data imply that there was a long-lived field on the Moon of at least 1 microtesla approximately 4.2 billion years ago. The early age, substantial intensity, and long lifetime of this field support the hypothesis of an ancient lunar core dynamo.

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We derive the depth of the water ice table on Mars by fitting seasonal surface temperature trends acquired by the Mars Climate Sounder and Thermal Emission Imaging System with a two-layer regolith model assuming frozen H 2 O as the lower material. Our results are consistent with widespread water ice at latitudes as low as 35°N/45°S buried sometimes a few centimeters below sand-like material, with high lateral ice depth variability, and correlated with periglacial features. While several investigations have already predicted, identified, and characterized some properties of near-surface ice on Mars, our results constitute a significant advance in the context of the upcoming crewed exploration because (1) they focus on very shallow depths accessible with limited equipment, (2) they provide continuous regional coverage including the midlatitudes, and (3) they yield moderate spatial resolution maps (3 ppd) relevant to landing site selection studies. Plain Language SummaryFrozen water is a very strong heat conductor compared to typical Martian regolith. As a result, near-surface ice measurably influences seasonal surface temperature trends, and the depth of the H 2 O table controls the amplitude of this effect. We leverage this influence on orbital temperature observations using a numerical heat transfer model to derive regional and local maps of the ice depth on Mars, at much higher spatial resolution than previously available. We show that water ice is present sometimes just a few centimeters below the surface, at locations where future landing is realistic, under mobile material that could easily be moved around. This ice could be exploited on-site for drinking water, breathable oxygen, etc., at a much lower cost than if brought from Earth. Key Points: • Shallow subsurface water ice on Mars influences seasonal surface temperatures in a measurable manner with MCS and THEMIS • We leverage this effect to map the depth to the water ice table at middle and high latitudes • Large continuous units of shallow ice are found~35°N and~45°S and could be exploited for future crewed missions Supporting Information: • Supporting Information S1

A 500 km long network of valleys extends from Herschel crater to Gale, Knobel, and Sharp craters.The mineralogy and timing of fluvial activity in these watersheds provide a regional framework for deciphering the origin of sediments of Gale crater's Mount Sharp, an exploration target for the Curiosity rover. Olivine-bearing bedrock is exposed throughout the region, and its erosion contributed to widespread olivine-bearing sand dunes. Fe/Mg phyllosilicates are found in both bedrock and sediments, implying that materials deposited in Gale crater may have inherited clay minerals, transported from the watershed. While some topographic lows of the Sharp-Knobel watershed host chloride salts, the only salts detected in the Gale watershed are sulfates within Mount Sharp, implying regional or temporal differences in water chemistry. Crater counts indicate progressively more spatially localized aqueous activity: large-scale valley network activity ceased by the early Hesperian, though later Hesperian/Amazonian fluvial activity continued near Gale and Sharp craters.