The Kuiper Belt is a broad, torus-shaped region in the outer Solar System beyond Neptune’s orbit. It contains primordial planetary building blocks and dwarf planets. NASA’s New Horizons spacecraft ...conducted a flyby of Pluto and its system of moons on 14 July 2015. New Horizons then continued farther into the Kuiper Belt, adjusting its trajectory to fly close to the small Kuiper Belt object (486958) 2014 MU69 (henceforth MU69; also informally known as Ultima Thule). Stellar occultation observations in 2017 showed that MU69 was ~25 to 35 km in diameter, and therefore smaller than the diameter of Pluto (2375 km) by a factor of ~100 and less massive than Pluto by a factor of ~106. MU69 is located about 1.6 billion kilometers farther from the Sun than Pluto was at the time of the New Horizons flyby. MU69’s orbit indicates that it is a “cold classical” Kuiper Belt object, thought to be the least dynamically evolved population in the Solar System. A major goal of flying past this target is to investigate accretion processes in the outer Solar System and how those processes led to the formation of the planets. Because no small Kuiper Belt object had previously been explored by spacecraft, we also sought to provide a close-up look at such a body’s geology and composition, and to search for satellites, rings, and evidence of present or past atmosphere. We report initial scientific results and interpretations from that flyby.
The New Horizons spacecraft flew past the Kuiper Belt object (486958) Arrokoth (also known as 2014 MU69) in January 2019. Because of the great distance to the outer Solar System and limited ...bandwidth, it will take until late 2020 to downlink all the spacecraft's observations back to Earth. Three papers in this issue analyze recently downlinked data, including the highest-resolution images taken during the encounter (see the Perspective by Jewitt). Spencer et al. examined Arrokoth's geology and geophysics using stereo imaging, dated the surface using impact craters, and produced a geomorphological map. Grundy et al. investigated the composition of the surface using color imaging and spectroscopic data and assessed Arrokoth's thermal emission using microwave radiometry. McKinnon et al. used simulations to determine how Arrokoth formed: Two gravitationally bound objects gently spiraled together during the formation of the Solar System. Together, these papers determine the age, composition, and formation process of the most pristine object yet visited by a spacecraft.
In 2015 the New Horizons spacecraft reached the Pluto system and returned unprecedentedly detailed measurements of its surface properties. These measurements have already been integrated into global ...reflectance, topography and narrow-band multispectral surface maps. However, analysis of the hyperspectral data from the Ralph/LEISA infrared spectrometer, which lets us analyse the surface composition, has until now been confined to the high-resolution encounter hemisphere of Pluto. We use an innovative technique — intensity-based registration — to co-register this high-resolution data with lower-resolution measurements taken during the spacecraft’s approach, and present the first global qualitative composition maps for CH4, N2 and H2O ice, and a tholin-like red material. We compare these maps with the other maps produced for Pluto and study the global extent of the previously-described latitudinal distribution of the surface components, which is relatively longitudinally constant with the exception of Sputnik Planitia. We also correlate these compositional components with geological features and propose physical interpretations, which include: CH4-ice-rich dissected plateaus in high northern latitudes, CH4-rich eroded terrain with N2-rich infill in medium northern latitudes, CH4-rich bladed terrain in low northern latitudes, and a red material belt overlaying H2O ice in low southern latitudes.
•Intensity-based methods allow registering low-resolution datasets.•First global quantitative composition maps of Pluto.•CH4-rich bladed and dissected terrains continue around Pluto’s far side.
The Kuiper Belt is a distant region of the outer Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU
, a cold classical Kuiper Belt object approximately 30 ...kilometers in diameter. Such objects have never been substantially heated by the Sun and are therefore well preserved since their formation. We describe initial results from these encounter observations. MU
is a bilobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color or compositional heterogeneity. No evidence for satellites, rings or other dust structures, a gas coma, or solar wind interactions was detected. MU
's origin appears consistent with pebble cloud collapse followed by a low-velocity merger of its two lobes.
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 outer Solar System object (486958) Arrokoth (provisional designation 2014 MU\(_{69}\)) has been largely undisturbed since its formation. We study its surface composition using data collected by ...the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through radiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/ or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, suggesting Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29\(\pm\)5 K.
Pluto has a heterogeneous surface, despite a global haze deposition rate of ∼1μm per orbit (Cheng et al., 2017; Grundy et al., 2018). While there could be spatial variation in the deposition rate, ...this has not yet been rigorously quantified, and naively the haze should coat the surface more uniformly than was observed. One way (among many) to explain this contradiction is for the atmospheric pressure at the surface to drop low enough to interrupt haze production and stop the deposition of particles onto part of the surface, driving heterogeneity. If the surface pressure drops to less than 10−3 - 10−4μbar and the CH4 mixing ratio remains nearly constant at the observed 2015 value, the atmosphere becomes transparent to ultraviolet radiation (Young et al., 2018), which would shut off haze production at its source. If the surface pressure falls below 0.06 μbar, the atmosphere ceases to be global, and instead is localized over only the warmest part of the surface, restricting the location of deposition (Spencer et al., 1997). In Pluto’s current atmosphere, haze monomers collect together into aggregate particles at beginning at 0.5 μbar; if the surface pressure falls below this limit, the appearance of particles deposited at different times of year and in different locations could be different. We use VT3D, an energy balance model (Young, 2017), to model the surface pressure on Pluto in current and past orbital configurations for four possible static N2 ice distributions: the observed northern hemisphere distribution with (1) a bare southern hemisphere, (2) a south polar cap, (3) a southern zonal band, and finally (4) a distribution that is bare everywhere except inside the boundary of Sputnik Planitia. We also present a sensitivity study showing the effect of mobile N2 ice. By comparing the minima of the modeled pressures to the three haze-disruption pressures, we can determine if or when haze production is disrupted. We find that Pluto’s minimum surface pressure in its current orbit is predicted to be between 0.01 and 3 μbar, and that over the past 10 million years the surface pressure has not fallen below 0.004 μbar. According to our model, southern N2 ice is required for haze aggregation to be interrupted, and southern N2 with very low thermal inertia is required for the possibility of a local atmosphere.
•Haze production is not likely to be interrupted via a decrease in surface pressure.•The atmosphere will not collapse to a local atmosphere without southern N2 ice.•Pluto’s minimum pressure is predicted to be 0.01–3 μbar in its current orbit.•Pluto’s minimum pressure over the past 10 My is predicted to be 0.004 μbar or higher.•The annual minimum pressure will occur near southern winter solstice, not aphelion.
Pluto has a heterogeneous surface, despite a global haze deposition rate of ~1 micrometer per orbit (Cheng et al., 2017; Grundy et al., 2018). While there could be spatial variation in the deposition ...rate, this has not yet been rigorously quantified, and naively the haze should coat the surface more uniformly than was observed. One way (among many) to explain this contradiction is for atmospheric pressure at the surface to drop low enough to interrupt haze production and stop the deposition of particles onto part of the surface, driving heterogeneity. If the surface pressure drops to less than 10^-3 - 10^-4 microbar and the CH4 mixing ratio remains nearly constant at the observed 2015 value, the atmosphere becomes transparent to ultraviolet radiation (Young et al., 2018), which would shut off haze production at its source. If the surface pressure falls below 0.06 microbar, the atmosphere ceases to be global, and instead is localized over only the warmest part of the surface, restricting the location of deposition (Spencer et al., 1997). In Pluto's current atmosphere, haze monomers collect together into aggregate particles at beginning at 0.5 microbar; if the surface pressure falls below this limit, the appearance of particles deposited at different times of year and in different locations could be different. We use VT3D, an energy balance model (Young, 2017), to model the surface pressure on Pluto in current and past orbital configurations for four possible static N2 ice distributions: the observed northern hemisphere distribution with (1) a bare southern hemisphere, (2) a south polar cap, (3) a southern zonal band, and finally (4) a distribution that is bare everywhere except inside the boundary of Sputnik Planitia. We also present a sensitivity study showing the effect of mobile N2 ice...(cont.)
PDF
