The New Horizons spacecraft's encounter with the cold classical Kuiper Belt object (486958) Arrokoth (provisional designation 2014 MU
) revealed a contact-binary planetesimal. We investigated how ...Arrokoth formed and found that 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 cloud of solid particles. The geometric alignment of the lobes indicates that they were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly because of 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 therefore informs the accretion processes that operated in the early Solar System.
The vast, deep, volatile-ice-filled basin informally named Sputnik Planum is central to Pluto's vigorous geological activity. Composed of molecular nitrogen, methane, and carbon monoxide ices, but ...dominated by nitrogen ice, this layer is organized into cells or polygons, typically about 10 to 40 kilometres across, that resemble the surface manifestation of solid-state convection. Here we report, on the basis of available rheological measurements, that solid layers of nitrogen ice with a thickness in excess of about one kilometre should undergo convection for estimated present-day heat-flow conditions on Pluto. More importantly, we show numerically that convective overturn in a several-kilometre-thick layer of solid nitrogen can explain the great lateral width of the cells. The temperature dependence of nitrogen-ice viscosity implies that the ice layer convects in the so-called sluggish lid regime, a unique convective mode not previously definitively observed in the Solar System. Average surface horizontal velocities of a few centimetres a year imply surface transport or renewal times of about 500,000 years, well under the ten-million-year upper-limit crater retention age for Sputnik Planum. Similar convective surface renewal may also occur on other dwarf planets in the Kuiper belt, which may help to explain the high albedos shown by some of these bodies.
Interstellar Pickup Ion Observations to 38 au McComas, D. J.; Zirnstein, E. J.; Bzowski, M. ...
The Astrophysical journal. Supplement series,
11/2017, Letnik:
233, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We provide the first direct observations of interstellar H+ and He+ pickup ions in the solar wind from 22 to 38 au. We use the Vasyliunas and Siscoe model functional form to quantify the pickup ion ...distributions, and while the fit parameters generally lie outside their physically expected ranges, this form allows fits that quantify variations in the pickup H+ properties with distance. By ∼20 au, the pickup ions already provide the dominant internal pressure in the solar wind. We determine the radial trends and extrapolate them to the termination shock at ∼90 au, where the pickup H+ to core solar wind density reaches ∼0.14. The pickup H+ temperature and thermal pressure increase from 22 to 38 au, indicating additional heating of the pickup ions. This produces very large extrapolated ratios of pickup H+ to solar wind temperature and pressure, and an extrapolated ratio of the pickup ion pressure to the solar wind dynamic pressure at the termination shock of ∼0.16. Such a large ratio has profound implications for moderating the termination shock and the overall outer heliospheric interaction. We also identify suprathermal tails in the H+ spectra and complex features in the He+ spectra, likely indicating variations in the pickup ion history and processing. Finally, we discover enhancements in both H+ and He+ populations just below their cutoff energies, which may be associated with enhanced local pickup. This study serves to document the release and serves as a citable reference of these pickup ion data for broad community use and analysis.
Nonthermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed ...that PUIs should be preferentially heated at quasiperpendicular shocks compared to thermal SWIs. We present in situ observations of interstellar hydrogen (H^{+}) PUIs at an interplanetary shock by the New Horizons' Solar Wind Around Pluto (SWAP) instrument at ∼34 au from the Sun. At this shock, H^{+} PUIs are only a few percent of the total proton density but contain most of the internal particle pressure. A gradual reduction in SW flow speed and simultaneous heating of H^{+} SWIs is observed ahead of the shock, suggesting an upstream energetic particle pressure gradient. H^{+} SWIs lose ∼85% of their energy flux across the shock and H^{+} PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy flux of all H^{+} PUIs is approximately six times that of H^{+} SWIs. We find that H^{+} PUIs, including their suprathermal tail, contain almost half of the total downstream energy flux in the shock frame.
Interstellar neutral atoms, unlike charged particles, freely penetrate the heliosphere, allowing us to sample the physical state of the interstellar matter directly. Most interstellar hydrogen atoms ...are ionized before reaching the inner heliosphere and become energetic protons picked up by the solar wind and transported away from the Sun. Consequently, observations of interstellar hydrogen atoms by missions operating within a few astronomical units from the Sun are subject to significant systematic uncertainties. We analyze observations from the Solar Wind Around Pluto instrument on New Horizons, the first experiment to provide extensive measurements of the picked-up protons far from the Sun. Analyzing the density of these protons, we find an interstellar neutral hydrogen density at the termination shock of 0.127 0.015 cm−3, i.e., ∼40% higher than previously thought. We show that the Voyager observations of the slowdown of the solar wind further support this value. This result resolves a problem of why energetic neutral atom fluxes, created from pickup ions by charge exchange with hydrogen atoms, are roughly twice that expected from numerical models. Our result also implies higher charge exchange rates at the heliospheric boundaries and, consequently, a less asymmetric shape of the heliosphere. Based on a previous study of the atom filtration in the heliospheric boundaries, we estimate the neutral hydrogen density in the unperturbed local interstellar medium of 0.195 0.033 cm−3. This value agrees with astrophysical observations of the interstellar clouds in the Sun proximity.
We report the detection of ammonia (NH
) on Pluto's surface in spectral images obtained with the New Horizons spacecraft that show absorption bands at 1.65 and 2.2 μm. The ammonia signature is ...spatially coincident with a region of past extensional tectonic activity (Virgil Fossae) where the presence of H
O ice is prominent. Ammonia in liquid water profoundly depresses the freezing point of the mixture. Ammoniated ices are believed to be geologically short lived when irradiated with ultraviolet photons or charged particles. Thus, the presence of NH
on a planetary surface is indicative of a relatively recent deposition or possibly through exposure by some geological process. In the present case, the areal distribution is more suggestive of cryovolcanic emplacement, however, adding to the evidence for ongoing geological activity on Pluto and the possible presence of liquid water at depth today.
The Terminal Tracking Camera system (TTCam) on the NASA
Lucy
Trojan asteroid Discovery mission consists of a pair of block redundant cameras and their associated electronics that are mounted on the ...spacecraft’s Instrument Pointing Platform and co-boresighted with the rest of the mission’s science payload instruments. The primary function of the TTCams is as a navigation system designed to provide an autonomous onboard late pre-encounter update of the location of each asteroid flyby target relative to the spacecraft. However, once the terminal tracking function is complete, the TTCam system will also provide 11.0°×8.2° field of view broadband (425-675 nm) images during the close approach phase of each asteroid flyby that will be used for scientific analyses like shape modeling and assessment of each target’s geology and topography. This paper provides an overview of the TTCam cameras and electronics, the science-focused requirements that the system is designed to meet, pointers to pre-flight calibration and in-flight calibration details for the cameras, as well as a high-level summary of the kinds of science that these images will enable for the mission.
•We analyze 63 near-infrared spectra of Triton from IRTF/SpeX.•Volatile ices exhibit stronger longitudinal variability than non-volatile ices.•Triton’s southern latitudes are dominated by ...non-volatile ices.•Absorption from CH4 increased between 2002 and 2014.•The 2.405μm feature is consistent with ethane absorption.
We present the results of an investigation to determine the longitudinal (zonal) distributions and temporal evolution of ices on the surface of Triton. Between 2002 and 2014, we obtained 63 nights of near-infrared (0.67–2.55μm) spectra using the SpeX instrument at NASA’s Infrared Telescope Facility (IRTF). Triton has spectral features in this wavelength region from N2, CO, CH4, CO2, and H2O. Absorption features of ethane (C2H6) and 13CO are coincident at 2.405μm, a feature that we detect in our spectra. We calculated the integrated band area (or fractional band depth in the case of H2O) in each nightly average spectrum, constructed longitudinal distributions, and quantified temporal evolution for each of the chosen absorption bands. The volatile ices (N2, CO, CH4) show significant variability over one Triton rotation and have well-constrained longitudes of peak absorption. The non-volatile ices (CO2, H2O) show poorly-constrained peak longitudes and little variability. The longitudinal distribution of the 2.405μm band shows little variability over one Triton rotation and is 97±44° and 92±44° out of phase with the 1.58μm and 2.35μm CO bands, respectively. This evidence indicates that the 2.405μm band is due to absorption from non-volatile ethane. CH4 absorption increased over the period of the observations while absorption from all other ices showed no statistically significant change. We conclude from these results that the southern latitudes of Triton are currently dominated by non-volatile ices and as the sub-solar latitude migrates northwards, a larger quantity of volatile ice is coming into view.
Charon tectonics Beyer, Ross A.; Nimmo, Francis; McKinnon, William B. ...
Icarus,
05/2017, Letnik:
287
Journal Article
Recenzirano
Odprti dostop
•Observations of extensional features on Charons surface are discussed.•These features suggest an areal strain of 1%.•This is consistent with an ancient global ocean that is now frozen.
New Horizons ...images of Pluto’s companion Charon show a variety of terrains that display extensional tectonic features, with relief surprising for this relatively small world. These features suggest a global extensional areal strain of order 1% early in Charon’s history. Such extension is consistent with the presence of an ancient global ocean, now frozen.
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