Alluvial fans and sinuous ridges are both important records of the history of fluvial activity on Mars, and they often occur together. We present observations of alluvial fans, many of which exhibit inverted relief, in five craters in the region north of Hellas basin. The observed fans ranged in size from-10 to 820 km2. We identified three primary fan surface morphology classes (chute, degraded, and Inverted) as well as many instances where the morphology transitions from proximal chutes (or, rarely, a cratered degraded surface) to distal ridges cor-responding to increasing thermal inertia. Clear superposition relationships at contacts between adjacent fans are rarely observed, suggesting interfingered deposits and concurrent fan development across the region. Localized factors appear to influence fan development as there is no systematic trend in the azimuth range of fan location, size of fan or catchment, as well as the degree of crater filling. Water and sediment availability may be controlled by lithology differences and weather patterns. Many of the fans had a mismatch between catchment and fan volume, corresponding to significant amounts of erosion perhaps due to windblown stripping of fine sediment. However, several notable fans exhibited volumes greater than their corresponding catchments. This may reflect uncertainty in the accuracy of the estimated paleosurface, or it may indicate sediment contributions to the fan from outside the mapped catchment. Ridges, inferred to be the resistant remnants of fluvially transported de-posits, were used to estimate flow magnitude in fan construction with computed discharges of 60-400 m3/s and corresponding supply rate runoff values-1-20 mm/h. Acknowledging that width-derived discharge values may overestimate flow conditions due to the likelihood of amalgamated channel deposits, this quantification provides important climate constraints. The upper range of runoff values and discharge rates are quite high, and would require either intense rain storms to generate immediate runoff, or longer-term snow accumulation and subsequent melt-runoff, potentially enhanced by rain-on-snow events. Minimum continuous formation time scales of less than a century are computed, but are incompatible with fan morphology (e.g., superposition relationships, embedded craters) and mechanisms to sustain flows. More realistic lower-limit fan construction times, accounting for modeled pre-cipitation rates from the literature, are tens to hundreds of thousands of years. Fans were active in multiple events spanning the Hesperian to Amazonian periods, requiring transient climate conditions to support the fan aggradation.

How to build a legacy of scientific leadership: the HR formula PROF. JULIA HAMMER, PHD1, LESLIE BAKER2, JENNI BARCLAY3, MICHAEL R. CARROLL4, MICHELLE COOMBS5, ELIZABETH COTTRELL6, NICHOLAS J DYGERT7, LINDA ELKINS-TANTON8, EMILY FIRST9, JAMES GARDNER10, DAVID GOLDSBY11, JAMES GREENWOOD12, MARIE JOHNSON13, MIKE KRAWCZYNSKI14, CHARLES MANDEVILLE15, MOLLY MCCANTA16, MICHELLE E. MINITTI17, WILLIAM NELSON18, TABB PRISSEL19, DINA VENEZKY20, CATHERINE WEITZ21 AND DIANE WOODRUFF22 1University of Hawaiʻi 2University of Idaho 3University of East Anglia 4Camerino University 5U.S. Geological Survey 6National Museum of Natural History, Smithsonian Institution 7University of Tennessee, Knoxville 8Arizona State University 9Cornell University 10University of Texas at Austin 11University of Pennsylvania 12Wesleyan University 13Cal State Fullerton 14Washington University in St. Louis 15US Geological Survey 16University of Tennessee at Knoxville 17Framework, Silver Spring 18University of Hawaii at Manoa 19NASA 20Smith College 21Planetary Science Institute 22Anadarko Petroleum Company Presenting Author: jhammer@hawaii.edu

The magma ocean concept was first conceived to explain the geology of the Moon, but hemispherical or global oceans of silicate melt could be a widespread "lava world" phase of rocky planet accretion, and could persist on planets on short-period orbits around other stars. The formation and crystallization of magma oceans could be a defining stage in the assembly of a core, origin of a crust, initiation of tectonics, and formation of an atmosphere. The last decade has seen significant advances in our understanding of this phenomenon through analysis of terrestrial and extraterrestrial samples, planetary missions, and astronomical observations of exoplanets. This review describes the energetic basis of magma oceans and lava worlds and the lava lake analogs available for study on Earth and Io. It provides an overview of evidence for magma oceans throughout the Solar System and considers the factors that control the rocks these magma oceans leave behind. It describes research on theoretical and observed exoplanets that could host extant magma oceans and summarizes efforts to detect and characterize them. It reviews modeling of the evolution of magma oceans as a result of crystallization and evaporation, the interaction with the underlying solid mantle, and the effects of planetary rotation. The review also considers theoretical investigations on the formation of an atmosphere in concert with the magma ocean and in response to irradiation from the host star, and possible end-states. Finally, it describes needs and gaps in our knowledge and points to future opportunities with new planetary missions and space telescopes to identify and better characterize lava worlds around nearby stars.

The evolution of the lunar interior is constrained by samples of the magnesian suite of rocks returned by the Apollo missions. Reconciling the paradoxical geochemical features of this suite constitutes a feasibility test of lunar differentiation models. Here we present the results of a microanalytical examination of the archetypal specimen, troctolite 76535, previously thought to have cooled slowly from a large magma body. We report a degree of intra-crystalline compositional heterogeneity (phosphorus in olivine and sodium in plagioclase) fundamentally inconsistent with prolonged residence at high temperature. Diffusion chronometry shows these heterogeneities could not have survived magmatic temperatures for >~20 My, i.e., far less than the previous estimated cooling duration of >100 My. Quantitative modeling provides a constraint on the thermal history of the lower lunar crust, and the textural evidence of dissolution and reprecipitation in olivine grains supports reactive melt infiltration as the mechanism by which the magnesian suite formed.

Interactions between fluid lava and the ocean can be explosive and thus hazardous to nearby communities. These eruptions can be difficult to study as at least half of the deposit is lost to the sea. During 2008, lava flows from episode 58 of Pu ʻ u ʻŌʻō entered the ocean near the town of Kalapana. The deposits are well-preserved, not covered by subsequent lava flows, and well-documented by photography; therefore, eruptive products can be tied to observed processes, making this event an ideal case study. We identified four different pyroclast categories on and around the preserved half-cone: Pele’s hair, Pele’s ‘locks’, fluidal spatter, and dense blocks. Photography reveals that steam generated from entrained sea water produced bubble-like bursts of juvenile material. This process is likely responsible for the first three pyroclast types, as all three require the material to still be fluid at time of formation. The blocks, however, are more ambiguous as to whether they represent fragmentation of lava tube walls or lava that drained back to the explosive vent. The fluidal spatter is crystal-poor and displays a distinct pattern between bubble texture and aspect ratio (AR; here, defined as the ratio of length to thickness) that implies a relationship between in-flight time and relative viscosity. Proximal spatter has very high ARs (max: 454), but the bubbles are small and spherical, suggesting that the spatter cooled very rapidly due to its thinness.

Fan deposits located at the mouths of Martian valleys have been interpreted as indicators of wet conditions during Mars history. However, the processes, time and amount of water needed to carve the valleys and form the fans are still debated. Here we present a detailed morphometric and mass balance analysis of valleys and fan-shaped deposits using high resolution topography, which provides new insights into past depositional environments. Based on morphometric and volumetric measurements, we found that several Martian fans previously interpreted as deltas can be categorized into two main types. Type I fans are more numerous, relatively well preserved, are associated with smaller and less mature drainage networks and were deposited on steeper gradients. The balance between eroded and deposited materials indicates that this set of fans might have been formed mainly in subaerial settings by depositional processes other than typical deltaic sedimentation (e.g. alluvial, glacial, mass-wasting in general), since a significant loss of sediment during deposition did not occur (we obtained a volume ratio of eroded vs. deposited materials of approximately 1). Therefore, we propose that the identified Type I fans were not deposited in prevailing subaqueous deltaic settings, but mainly in subaerial conditions with perhaps sporadic presence of ephemeral bodies of standing water within the basins. In contrast, Type II fans are less abundant, highly eroded, were deposited on flatter areas and formed downstream of more mature drainage networks with longer, deeper and wider valleys. The mass balance for this set of fans clearly shows that considerable amounts of sediment were not retained in the fans, implying large offshore transport of sediment during their formation (eroded vs. deposited volume ratios between 3 and 10 are probable). This evidence supports a fluvio-deltaic origin for this class of fans, indicating the existence of paleolakes over substantial periods of time. Based on the collected morphometric measurements and mass balance modeling we conclude that only a small percentage of the fans (Type II fans correspond to similar to 1/3 of the sampled areas) are consistent with the occurrence of favorable and durable conditions for life, i.e. long-lived integrated fluvial, deltaic and lacustrine environments. Whereas, the majority of fans might have been formed in subaerial settings with significant contributions of alternative processes besides fluvial transport and deposition thus not necessarily requiring the occurrence of extended epochs of clement climatic conditions. (C) 2020 Elsevier B.V. All rights reserved.