The Kuiper Belt is a distant region of the Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU69, a Cold Classical Kuiper Belt Object, a class of objects that have never been heated by the Sun and are therefore well preserved since their formation. Here we describe initial results from these encounter observations. MU69 is a bi-lobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color and compositional heterogeneity. No evidence for satellites, ring or dust structures, gas coma, or solar wind interactions was detected. By origin MU69 appears consistent with pebble cloud collapse followed by a low velocity merger of its two lobes.
The New Horizons spacecraft returned images and compositional data showing that terrains on Pluto span a variety of ages, ranging from relatively ancient, heavily cratered areas to very young surfaces with few-to-no impact craters. One of the regions with very few impact craters is dominated by enormous rises with hummocky flanks. Similar features do not exist anywhere else in the imaged solar system. Here we analyze the geomorphology and composition of the features and conclude this region was resurfaced by cryovolcanic processes, of a type and scale so far unique to Pluto. Creation of this terrain requires multiple eruption sites and a large volume of material (>104 km3) to form what we propose are multiple, several-km-high domes, some of which merge to form more complex planforms. The existence of these massive features suggests Pluto's interior structure and evolution allows for either enhanced retention of heat or more heat overall than was anticipated before New Horizons, which permitted mobilization of water-ice-rich materials late in Pluto's history.
Abstract The preponderance of impact craters and the associated crater ejecta facies are leading agents of geomorphic change across the Solar System. Interpretation of planetary landscape evolution, sample provenance, and regolith gardening all benefit from a thorough understanding of ejecta emplacement dynamics. Constraining the type and range of these dynamics has eluded experimentation even as the effects of primary impacts have become well-constrained from experiments and numerical simulations and have been shown to follow power law scaling relationships. To address the knowledge gap surrounding granular ejecta emplacement, we built and characterized a novel ejecta emplacement catapult and showed it to accurately reproduce the ejecta mass and velocity profiles predicted for in-flight natural ejecta curtains. Based on this dynamic similarity to larger, natural systems, we proceeded with a preliminary exploratory suite of experiments to constrain runout and erosion efficiencies of flowing ejecta. Our quantitative results at low speeds may suggest a new set of scaling relationships for erosion via ballistic ejecta versus crater formation and erosion from hypervelocity clustered projectiles. Our results also show significant ejecta runout efficiencies of ∼1–2 (only slightly below the efficiencies of terrestrial debris flows 12 decades more voluminous) with important erosive efficiencies of ∼2–4%. Our qualitative results reveal a stochastic and heterogeneous system: ejecta “saltation” and implantation, and regolith exhumation, erosion, and shearing. Together with the initial results showing ejecta emplacement to be violently dynamic, the development of this new laboratory technique will enable more detailed studies to better inform interpretations of sample provenance, ejecta stratigraphy, and geochemical boundaries.
The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, are primitive objects preserving information about Solar System formation. The New Horizons spacecraft flew past one of these objects, the 36 km long contact binary (486958) Arrokoth (2014 MU69), in January 2019. Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters diameter) within a radius of 8000 km, and has a lightly-cratered smooth surface with complex geological features, unlike those on previously visited Solar System bodies. The density of impact craters indicates the surface dates from the formation of the Solar System. The two lobes of the contact binary have closely aligned poles and equators, constraining their accretion mechanism.
We argue that taxonomical concept development is vital for planetary science as in all branches of science, but its importance has been obscured by unique historical developments. The literature shows that the concept of planet developed by scientists during the Copernican Revolution was theory-laden and pragmatic for science. It included both primaries and satellites as planets due to their common intrinsic, geological characteristics. About two centuries later the non-scientific public had just adopted heliocentrism and was motivated to preserve elements of geocentrism including teleology and the assumptions of astrology. This motivated development of a folk concept of planet that contradicted the scientific view. The folk taxonomy was based on what an object orbits, making satellites out to be non-planets and ignoring most asteroids. Astronomers continued to keep primaries and moons classed together as planets and continued teaching that taxonomy until the 1920s. The astronomical community lost interest in planets ca. 1910 to 1955 and during that period complacently accepted the folk concept. Enough time has now elapsed so that modern astronomers forgot this history and rewrote it to claim that the folk taxonomy is the one that was created by the Copernican scientists. Starting ca. 1960 when spacecraft missions were developed to send back detailed new data, there was an explosion of publishing about planets including the satellites, leading to revival of the Copernican planet concept. We present evidence that taxonomical alignment with geological complexity is the most useful scientific taxonomy for planets. It is this complexity of both primary and secondary planets that is a key part of the chain of origins for life in the cosmos.
New Horizons' views of Pluto The flyby of Pluto and its moon Charon by the New Horizons spacecraft generated news coverage around the world. Now Stern et al. report the first scientific results from the high-speed encounter. The surface of Pluto is surprisingly diverse, with large regions of differing brightness and composition. There is ample evidence for ongoing rich geological processes that act to sculpt its surface. Charon's surface is similarly complex, with numerous relief structures and varied coloration. Pluto's atmosphere is extensive but less dense than expected, whereas Charon has no detectable atmosphere. Science , this issue p. 10.1126/science.aad1815
