Previous Juno mission event studies revealed powerful electron and ion acceleration, to 100s of kiloelectron volts and higher, at low altitudes over Jupiter's main aurora and polar cap (PC; poleward of the main aurora). Here we examine 30-1200 keV JEDI-instrument particle data from the first 16 Juno orbits to determine how common, persistent, repeatable, and ordered these processes are. For the PC regions, we find (1) upward electron angle beams, sometimes extending to megaelectron volt energies, are persistently present in essentially all portions of the polar cap but are generated by two distinct and spatially separable processes. (2) Particle evidence for megavolt downward electrostatic potentials are observable for 80% of the polar cap crossings and over substantial fractions of the PC area. For the main aurora, with the orbit favoring the duskside, we find that (1) three distinct zones are observed that are generally arranged from lower to higher latitudes but sometimes mixed. They are designated here as the diffuse aurora (DifA), Zone-I (ZI(D)) showing primarily downward electron acceleration, and Zone-II (ZII(B)) showing bidirectional acceleration with the upward intensities often greater than downward intensities. (2) ZI(D) and ZII(B) sometimes (but not always) contain, respectively, downward electron inverted Vs and downward proton inverted Vs, (potentials up to 400 kV) but, otherwise, have broadband distributions. (3) Surprisingly, both ZI(D) and ZII(B) can generate equally powerful auroral emissions. It is suggested but demonstrated for intense portions of only one auroral crossing, that ZI(D) and ZII(B) are associated, respectively, with upward and downward electric currents. Plain Language Summary The science objectives of the Juno mission, with its spacecraft now orbiting Jupiter in a polar orbit, include understanding the space environments of Jupiter's polar regions and generation of Jupiter's uniquely powerful aurora. In Jupiter's polar cap regions (poleward of the main auroral oval encircling the northern and southern poles), we find here that (1) beams of electrons aligned with the upward magnetic field direction are ever-present with energies extended to the 100s to 1,000s of kilo electron volts and (2) downward magnetic field-aligned electrostatic potentials reaching greater than a million volts occur over broad regions for 80% of the polar cap crossings. For the main auroral oval, we find three distinct zones: designated here as diffuse aurora (DifA), Zone-I (ZI(D)) showing downward electron acceleration to 100s of kiloelectron volts, and Zone-II (ZII(B)) showing bidirectional acceleration with the upward intensities often greater than downward intensities. ZI(D) sometimes shows upward electrostatic potentials reaching 100s of kilovolts and is associated with upward magnetic field-aligned electric currents. ZII(B) sometimes shows downward electrostatic potentials reaching 100s of kilovolts and is associated with downward electric currents. Unexpectedly from Earth studies, ZI(D) and ZII(B) ar...

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 MU69, 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. MU69 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. MU69’s origin appears consistent with pebble cloud collapse followed by a low-velocity merger of its two lobes.

Published in: ScienceLink to article, DOI: 10.1126/science.aam5928 Publication date: 2017 Document VersionPeer reviewed version Link back to DTU Orbit Citation (APA): Connerney, J. E. P., Adriani, A., Allegrini, F., Bagenal, F., Bolton, S. J., Bonfond, B., ... Waite, J. (2017). Jupiter's magnetosphere and aurorae observed by the Juno spacecraft during its first polar orbits. Science, 356(6340) Abstract:The Juno spacecraft acquired direct observations of the Jovian magnetosphere and auroral emissions from a vantage point above the poles. Juno's capture orbit spanned the Jovian magnetosphere from bow shock to the planet, providing magnetic field, charged particle, and wave phenomena context for Juno's passage over the poles and traverse of Jupiter's hazardous inner radiation belts. Juno's energetic particle and plasma detectors measured electrons precipitating in the polar regions, exciting intense aurorae, observed simultaneously by the ultraviolet and infrared imaging spectrographs. Juno transited beneath the most intense parts of the radiation belts, passed ~4,000 kilometers above the cloudtops at closest approach, well inside the Jovian rings, and recorded the electrical signatures of high velocity impacts with small particles as it traversed the equator.One Sentence Summary: Juno's instruments provide complete polar maps of Jovian UV aurorae, spatially resolved images of the IR southern aurorae, and in-situ direct measurements of precipitating charged particle populations exciting the aurora. only one bow shock upon approach suggests that the magnetosphere was expanding in size, a conclusion bolstered by the multiple BS encounters experienced outbound during the 53.5 day capture orbit at radial distances of 92-112 Rj before apojove on DOY 213 (~113 Rj), and at distances of 102-108 Rj thereafter . Apojove during the 53.5day orbits occurred at a radial distance of ~113 Rj, so Juno resides at distances of >92 Rj for little more than half of its orbital period (~29 days). Thus on the first two orbits, Juno encountered the MP boundary a great many times at radial distances of ~81-113 Rj.Juno's traverse through the well-ordered portion of the Jovian magnetosphere is illustrated in The magnetic field observed in the previously unexplored region close to the planet (radius<1.3Rj) was dramatically different from that predicted by existing spherical harmonic models, revealing a planetary magnetic field rich in spatial variation, possibly due to a relatively large dynamo radius [1]. Perhaps the most perplexing observation was one that was missing: the expected magnetic signature of intense field aligned currents (Birkeland currents) associated with the main aurora. We did not identify large magnetic perturbations associated with Juno's traverse of field lines rooted in the main auroral oval (supplementary material).Juno's Waves instrument made observations of radio and plasma wave phenomena throughout the first perijove ( Figure 2). These observations were obtained at low altitudes whilst crossing magnetic field lines...

The Juno spacecraft crossed flux tubes connected to the Io footprint tail at low Jovian altitudes on multiple occasions. The transits covered longitudinal separations of approximately 10° to 120° along the footprint tail. Juno's suite of magnetospheric instruments acquired detailed measurements of the Io footprint tail. Juno observed planetward electron energy fluxes of ~70 mW/m2 near the Io footprint and ~10 mW/m2 farther down the tail, along with correlated, intense electric and magnetic wave signatures, which also decreased down the tail. All observed electron distributions were broad in energy, suggesting a dominantly broadband acceleration process, and did not show any broad inverted‐V structure that would be indicative of acceleration by a quasi‐static, discrete, parallel potential. Observed waves were primarily below the proton cyclotron frequency, yet identification of a definitive wave mode is elusive. Beyond 40° down the footprint tail, Juno observed depleted upward loss cones, suggesting that the broadband acceleration occurred at distances beyond Juno's transit distance of 1.3 to 1.7 RJ. For all transits, Juno observed fine structure on scales of approximately tens of kilometers and confirmed independently with electron and wave measurements that a bifurcated tail can intermittently exist.

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 ± 5 K.The New Horizons spacecraft flew past (486958) Arrokoth at the beginning of 2019 (1). Arrokoth rotates with a 15.9 hour period about a spin axis inclined 99.3° to the pole of its 298 year orbit at a mean distance from the Sun of 44.2 AU (2, 3). Its near-circular orbit, with a mean eccentricity of 0.03 and inclination of 2.4° to the plane of the Solar System, makes it a Kuiper belt object (KBO) and more specifically, a member of the "kernel" sub-population of the cold classical KBOs (CCKBOs) (4). CCKBOs have distinct origins and properties from KBOs on more excited orbits, which are thought to have formed closer to the Sun before being perturbed outward by migrating giant planets early in Solar System history (5). CCKBOs still orbit where they formed in the protoplanetary nebula, the accretion disk of gas and dust around the young Sun. They have a high fraction of binary objects (6), a uniformly red color distribution (7, 8), a size-frequency distribution deficient of large objects (9, 10), and higher albedos (11,12). These properties arise from the environment at the outermost edge of the protoplanetary nebula, from a distinct history of subsequent evolution of CCKBOs compared to other KBOs, or of some combination of these two. Arrokoth provides a record of the process of forming planetesimals, the first generation of gravitationally bound bodies, that has been minimally altered by subsequent processes such as heating and impactor bombardment (3). Its distinctive bi-lobed, 35 km-long shape with few impact craters favors formation via rapid gravitational collapse, rather than scenarios involving more gradual accretion via piece-wise agglomeration of dust particles to assemble incrementally larger aggregates (13). We study Arrokoth's color, composition, and thermal environment using data from the New Horizons flyby, and discuss the resulting implications for its formation and subsequent evolution.

Pluto, Titan, and Triton, are all low-temperature environments with a N2/CH4/CO atmospheric composition on which solar radiation drives an intense organic photochemistry. Titan is rich in atmospheric hazes and Cassini-Huygens observations showed their formation initiates with the production of large molecules through ion-neutral reactions. New Horizons revealed that optical hazes are also ubiquitous in Pluto's atmosphere and it is thought that similar haze formation pathways are active in this atmosphere as well. However, we show here that Pluto's hazes may contain a major organic ice component (dominated by C4H2 ice) from the direct condensation of the primary photochemical products in this atmosphere. This contribution may imply that haze has a less important role in controlling Pluto's atmospheric thermal balance compared to Titan. Moreover, we expect that the haze composition of Triton is dominated by C2H4 ice. Pluto's atmosphere is the equivalent of Titan's upper atmosphere above 400 km altitude, with comparable CH4, CO, and N2 density profiles and pressure scale heights 1 . Photochemistry models for these environments demonstrate that the anticipated chemical products are similar 2,3 , therefore Pluto's hazes are thought to be of a similar nature based on molecular growth 4 (see Methods for haze nomenclature). The fraction of the mass flux generated from the photolysis of Titan's main atmospheric composition that ends in haze particles is ~30% 5,6 . Such a yield for Pluto's haze would suggest a mass flux of ~6x10 -15 gcm -2 s -1 (all reported mass fluxes are referred to Pluto's surface). However, the opacity of particles characterizing such a mass flux falls short of the available observations and a twice-higher haze formation efficiency is required to generate enough material to reproduce the UV extinction observations below 200 km 7 . As Pluto's upper atmosphere is much colder than Titan's (~70K compared to ~150K for Titan, see Fig. 1), an increased haze yield for Pluto is surprising. On the other hand, the photochemical gases produced on Pluto may condense at lower pressures than on Titan (Fig. 1). Therefore organic ices could be responsible for, or at least contribute to, the formation of the observed hazes in Pluto's atmosphere. We explore the extent of this contribution using coupled models of atmospheric photochemistry and microphysics, and following the evolution of the organic ice haze particles from their formation in the upper atmosphere to their sedimentation on Pluto's surface. The models are adapted from previous studies of photochemistry and microphysics in Titan's atmosphere, taking into account high-resolution energy

On 27 August 2016, the NASA Juno spacecraft performed its first close‐up observations of Jupiter during its perijove. Here we present the UV images and color ratio maps from the Juno‐UVS UV imaging spectrograph acquired at that time. Data were acquired during four sequences (three in the north, one in the south) from 5:00 UT to 13:00 UT. From these observations, we produced complete maps of the Jovian aurorae, including the nightside. The sequence shows the development of intense outer emission outside the main oval, first in a localized region (255°–295° System III longitude) and then all around the pole, followed by a large nightside protrusion of auroral emissions from the main emission into the polar region. Some localized features show signs of differential drift with energy, typical of plasma injections in the middle magnetosphere. Finally, the color‐ratio map in the north shows a well‐defined area in the polar region possibly linked to the polar cap.

Jupiter's ultraviolet (UV) aurorae, the most powerful and intense in the solar system, are caused by energetic electrons precipitating from the magnetosphere into the atmosphere where they excite the molecular hydrogen. Previous studies focused on case analyses and/or greater than 30-keV energy electrons. Here for the first time we provide a comprehensive evaluation of Jovian auroral electron characteristics over the entire relevant range of energies (~100 eV to~1 MeV). The focus is on the first eight perijoves providing a coarse but complete System III view of the northern and southern auroral regions with corresponding UV observations. The latest magnetic field model JRM09 with a current sheet model is used to map Juno's magnetic foot point onto the UV images and relate the electron measurements to the UV features. We find a recurring pattern where the 3-to 30-keV electron energy flux peaks in a region just equatorward of the main emission. The region corresponds to a minimum of the electron characteristic energy (<10 keV). Its polarward edge corresponds to the equatorward edge of the main oval, which is mapped at M shells of~51. A refined current sheet model will likely bring this boundary closer to the expected 20-30 R J . Outside that region, the >100-keV electrons contribute to most (>~70-80%) of the total downward energy flux and the characteristic energy is usually around 100 keV or higher. We examine the UV brightness per incident energy flux as a function of characteristic energy and compare it to expectations from a model.