Abstract:Opportunity has investigated in detail rocks on the rim of the Noachian age Endeavour Crater, where orbital spectral reflectance signatures indicate the presence of Fe +3 -rich smectites.The signatures are associated with fine-grained, layered rocks containing spherules of diagenetic or impact origin. The layered rocks are overlain by breccias and both units are cut by calcium sulfate veins precipitated from fluids that circulated after the Endeavour impact. Compositional data for fractures in the layered rocks suggest formation of Al-rich smectites by aqueous leaching. Evidence is thus preserved for water-rock interactions before and after the impact, with aqueous environments of slightly acidic to circum-neutral pH that would have been more favorable for prebiotic chemistry and microorganisms than those recorded by younger sulfaterich rocks at Meridiani Planum. Main Text:Introduction.

The New Horizons spacecraft's encounter with the cold classical Kuiper belt object (486958) Arrokoth (formerly 2014 MU69) revealed a contact-binary planetesimal. We investigate how it formed, finding it is the product of a gentle, low-speed merger in the early Solar System.Its two lenticular lobes suggest low-velocity accumulation of numerous smaller planetesimals within a gravitationally collapsing, solid particle cloud. The geometric alignment of the lobes indicates the lobes were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly due to dynamical friction and collisions within the cloud or later gas drag. Arrokoth's contact-binary shape was preserved by the benign dynamical and collisional environment of the cold classical Kuiper belt, and so informs the accretion processes that operated in the early Solar System.Main Text: Following its encounter with Pluto in 2015 (1), the New Horizons spacecraft continued further into the Kuiper belt (2). This included a flyby of (486958) Arrokoth (also informally known as Ultima Thule), discovered in a dedicated Hubble Space Telescope campaign (3). Arrokoth's orbit has a semimajor axis a⨀ = 44.2 astronomical units (au), Submitted Manuscript: Confidential 3 eccentricity e = 0.037, and inclination i = 2.54°, making it a member of the cold classical Kuiper belt (CCKB), a reservoir of mainly small bodies on dynamically cold orbits, i.e., those with lowto-moderate e and low i (typically i < 5°), in the outer Solar System (4). CCKB objects have a steeper size-frequency distribution, higher binary fraction, higher albedos, and redder optical colors than the dynamically hot and Neptune-resonant populations of the Kuiper belt, implying a distinct formation mechanism and/or evolutionary history (4). CCKB objects are thought to have formed in place and remained largely undisturbed by the migration of the Solar System's giant planets (4, 5, 6), making them unperturbed remnants of the original protoplanetary disk.The encounter showed Arrokoth is a bi-lobed object, consisting of two discrete, quasiellipsoidal lobes (equivalent spherical diameters 15.9 and 12.9 km, respectively) joined at a narrow contact area or "neck" (Fig. 1) (7,8). We interpret this geometric, co-joined object as a contact binary, i.e., two formerly separate objects that have gravitated towards each other until they touch. The larger lobe (hereafter LL) is more oblate than the smaller lobe (hereafter SL) (8).Arrokoth rotates with a 15.92-hr period at an obliquity of 99° (the angle between its rotation axis and heliocentric orbital plane). The short axes of both lobes are aligned to within a few degrees of each other and with the spin axis of the body as a whole (8). The average visible and nearinfrared colors of both lobes are indistinguishable (9). Near-infrared spectral absorptions on both lobes indicate the presence of methanol ice-a common, relatively thermally stable component (for an ice) of cometary bodies and extrasolar protoplanetary disks (10). The very red optical c...

Abstract:The New Horizons mission has provided resolved measurements of Pluto's moons Styx, Nix, Kerberos, and Hydra. All four are small, with equivalent spherical diameters of ~40 km for Nix and Hydra and ~10 km for Styx and Kerberos. They are also highly elongated, with maximum to minimum axis ratios of ~2. All four moons have high albedos (~50-90%)suggestive of a water-ice surface composition. Crater densities on Nix and Hydra imply surface ages ≳ 4 Ga. The small moons rotate much faster than synchronous, with rotational poles clustered nearly orthogonal to the common pole directions of Pluto and Charon. These results reinforce the hypothesis that the small moons formed in the aftermath of a collision that produced the Pluto-Charon binary. 3 Background and Context

Examination of eroded and intact earth mounds in the Clanwilliam district, South Africa, indicates that they are well-established active termitaria of the harvester termite Microhodotermes viator. Unoccupied lower portions of the mounds contain ubiquitous trace-fossil evidence of earlier inhabitation by the same species. Previous studies indicating that fossorial molerats played a major role in the formation of the mounds are not supported by the observations presented here. Calcretization of the basal parts of the earth mounds has been caused by groundwater interaction with the more alkaline mound soil. C dating of this calcrete indicates that the earth mounds have been in existence for at least 4000 years, an order of magnitude greater than any previously recorded longevity for termitarium inhabitation.

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During late 1999/early 2000, the solid state imaging experiment on the Galileo spacecraft returned more than 100 high-resolution (5 to 500 meters per pixel) images of volcanically active Io. We observed an active lava lake, an active curtain of lava, active lava flows, calderas, mountains, plateaus, and plains. Several of the sulfur dioxide-rich plumes are erupting from distal flows, rather than from the source of silicate lava (caldera or fissure, often with red pyroclastic deposits). Most of the active flows in equatorial regions are being emplaced slowly beneath insulated crust, but rapidly emplaced channelized flows are also found at all latitudes. There is no evidence for high-viscosity lava, but some bright flows may consist of sulfur rather than mafic silicates. The mountains, plateaus, and calderas are strongly influenced by tectonics and gravitational collapse. Sapping channels and scarps suggest that many portions of the upper approximately 1 kilometer are rich in volatiles.

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