The Pluto System After New Horizons Stern, S. Alan; Grundy, William M; McKinnon, William B ...
Annual review of astronomy and astrophysics,
09/2018, Letnik:
56, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The
New Horizons
(NH) flyby of the Pluto-Charon binary planet and its system of four small surrounding satellites in mid-2015 revolutionized our knowledge of this distant planet and its moons. Beyond ...providing rich geo-logical, compositional, and atmospheric data sets, NH demonstrated that Pluto has been surprisingly geologically and climatologically active throughout the past 4+ Gyr and that the planet exhibits a remarkably complex range of atmospheric phenomenology and geological expressions that rival Mars in their richness. In contrast, Pluto's large, planet-sized satellite Charon, though also geologically complex, has no detected active surface volatiles, has no detectable atmosphere, has much more muted colors, has lower albedo, and exhibits only ancient terrains. Pluto's system of four small satellites orbiting outside of Charon is itself dynamically complex and geologically interesting. Here, we review both what was known about the Pluto system before NH and what it has taught us about the Pluto system specifically and, by inference, other small planets in the Kuiper Belt. We go on to examine the natural next steps in Kuiper Belt exploration.
We use published models of the early solar system evolution with a slow, long-range and grainy migration of Neptune to predict the orbital element distributions and the number of modern-day Centaurs. ...The model distributions are biased by the Outer Solar System Origins Survey (OSSOS) simulator and compared with the OSSOS Centaur detections. We find an excellent match to the observed orbital distribution, including the wide range of orbital inclinations which was the most troublesome characteristic to fit in previous models. A dynamical model, in which the original population of outer disk planetesimals was calibrated from Jupiter trojans, is used to predict that OSSOS should detect 11 4 Centaurs with semimajor axes of a < 30 au, perihelion distances of q > 7.5 au, and diameter of D > 10 km (absolute magnitude Hr < 13.7 for a 6% albedo). This is consistent with 15 actual OSSOS Centaur detections with Hr < 13.7. The population of Centaurs is estimated to be 21,000 8000 for D > 10 km. The inner scattered disk at 50 < a < 200 au should contain (2.0 0.8) × 107 D > 10 km bodies and the Oort cloud should contain (5.0 1.9) × 108 D > 10 km comets. Population estimates for different diameter cutoffs can be obtained from the size distribution of Jupiter trojans (N(>D) ∝ D−2.1 for 5 < D < 100 km). We discuss model predictions for the Large Synoptic Survey Telescope observations of Centaurs.
NMR spectroscopy is one of the most powerful and versatile analytic tools available to chemists. The discrete Fourier transform (DFT) played a seminal role in the development of modern NMR, including ...the multidimensional methods that are essential for characterizing complex biomolecules. However, it suffers from well-known limitations: chiefly the difficulty in obtaining high-resolution spectral estimates from short data records. Because the time required to perform an experiment is proportional to the number of data samples, this problem imposes a sampling burden for multidimensional NMR experiments. At high magnetic field, where spectral dispersion is greatest, the problem becomes particularly acute. Consequently multidimensional NMR experiments that rely on the DFT must either sacrifice resolution in order to be completed in reasonable time or use inordinate amounts of time to achieve the potential resolution afforded by high-field magnets. Maximum entropy (MaxEnt) reconstruction is a non-Fourier method of spectrum analysis that can provide high-resolution spectral estimates from short data records. It can also be used with nonuniformly sampled data sets. Since resolution is substantially determined by the largest evolution time sampled, nonuniform sampling enables high resolution while avoiding the need to uniformly sample at large numbers of evolution times. The Nyquist sampling theorem does not apply to nonuniformly sampled data, and artifacts that occur with the use of nonuniform sampling can be viewed as frequency-aliased signals. Strategies for suppressing nonuniform sampling artifacts include the careful design of the sampling scheme and special methods for computing the spectrum. Researchers now routinely report that they can complete an N-dimensional NMR experiment 3 N–1 times faster (a 3D experiment in one ninth of the time). As a result, high-resolution three- and four-dimensional experiments that were prohibitively time consuming are now practical. Conversely, tailored sampling in the indirect dimensions has led to improved sensitivity. Further advances in nonuniform sampling strategies could enable further reductions in sampling requirements for high resolution NMR spectra, and the combination of these strategies with robust non-Fourier methods of spectrum analysis (such as MaxEnt) represent a profound change in the way researchers conduct multidimensional experiments. The potential benefits will enable more advanced applications of multidimensional NMR spectroscopy to study biological macromolecules, metabolomics, natural products, dynamic systems, and other areas where resolution, sensitivity, or experiment time are limiting. Just as the development of multidimensional NMR methods presaged multidimensional methods in other areas of spectroscopy, we anticipate that nonuniform sampling approaches will find applications in other forms of spectroscopy.
The Cosmic Origins Spectrograph Green, James C; Froning, Cynthia S; Osterman, Steve ...
The Astrophysical journal,
01/2012, Letnik:
744, Številka:
1
Journal Article
Recenzirano
The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing ...Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F lambda 1.0 X 10 super(-14) erg cm super(-2) s super(-1) A super(-1), COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Ly alpha absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.
Aims. The Alice far-ultraviolet spectrograph onboard Rosetta is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/ Churyumov-Gerasimenko and to ...determine their spatial distribution and evolution with time and heliocentric distance. Methods. Following orbit insertion in August 2014, Alice made observations of the inner coma above the limbs of the nucleus of the comet from cometocentric distances varying between 10 and 80 km. Depending on the position and orientation of the slit relative to the nucleus, emissions of atomic hydrogen and oxygen were initially detected. These emissions are spatially localized close to the nucleus and spatially variable with a strong enhancement above the comet’s neck at northern latitudes. Weaker emission from atomic carbon and CO were subsequently detected. Results. Analysis of the relative line intensities suggests photoelectron impact dissociation of H2O vapor as the source of the observed H i and O i emissions. The electrons are produced by photoionization of H2O. The observed C i emissions are also attributed to electron impact dissociation, of CO2, and their relative brightness to H i reflects the variation of CO2 to H2O column abundance in the coma.
NASA’s New Horizons (NH) Pluto–Kuiper Belt (PKB) mission was selected for development on 29 November 2001 following a competitive selection resulting from a NASA mission Announcement of Opportunity. ...New Horizons is the first mission to the Pluto system and the Kuiper belt, and will complete the reconnaissance of the classical planets. New Horizons was launched on 19 January 2006 on a Jupiter Gravity Assist (JGA) trajectory toward the Pluto system, for a 14 July 2015 closest approach to Pluto; Jupiter closest approach occurred on 28 February 2007. The ∼400 kg spacecraft carries seven scientific instruments, including imagers, spectrometers, radio science, a plasma and particles suite, and a dust counter built by university students. NH will study the Pluto system over an 8-month period beginning in early 2015. Following its exploration of the Pluto system, NH will go on to reconnoiter one or two 30–50 kilometer diameter Kuiper Belt Objects (KBOs) if the spacecraft is in good health and NASA approves an extended mission. New Horizons has already demonstrated the ability of Principal Investigator (PI) led missions to use nuclear power sources and to be launched to the outer solar system. As well, the mission has demonstrated the ability of non-traditional entities, like the Johns Hopkins Applied Physics Laboratory (JHU/APL) and the Southwest Research Institute (SwRI) to explore the outer solar system, giving NASA new programmatic flexibility and enhancing the competitive options when selecting outer planet missions. If successful, NH will represent a watershed development in the scientific exploration of a new class of bodies in the solar system—dwarf planets, of worlds with exotic volatiles on their surfaces, of rapidly (possibly hydrodynamically) escaping atmospheres, and of giant impact derived satellite systems. It will also provide other valuable contributions to planetary science, including: the first dust density measurements beyond 18 AU, cratering records that shed light on both the ancient and present-day KBO impactor population down to tens of meters, and a key comparator to the puzzlingly active, former dwarf planet (now satellite of Neptune) called Triton which is in the same size class as the small planets Eris and Pluto.
A bacterial flagellar filament is a cylindrical crystal of a protein known as flagellin. Flagellin subunits travel from the cytoplasm through a 2 nm axial pore and polymerize at the filament’s distal ...end. They are supplied by a pump in the cell membrane powered by a proton-motive force. In a recent experiment, it was observed that growth proceeded at a rate of approximately one subunit every 2 s. Here, we asked whether transport of subunits through the pore at this rate could be effected by single-file diffusion, which we simulated by a random walk on a one-dimensional lattice. Assuming that the subunits are α-helical, the answer is yes, by a comfortable margin.
Abstract
The NASA New Horizons Venetia Burney Student Dust Counter (SDC) measures dust particle impacts along the spacecraft’s flight path for grains with mass ≥10
−12
g, mapping out their spatial ...density distribution. We present the latest SDC dust density, size distribution, and flux measurements through 55 au and compare them to numerical model predictions. Kuiper Belt objects (KBOs) are thought to be the dominant source of interplanetary dust particles in the outer solar system due to both collisions between KBOs and their continual bombardment by interstellar dust particles. Continued measurements through 55 au show higher than model-predicted dust fluxes as New Horizons approaches the putative outer edge of the Kuiper Belt (KB). We discuss potential explanations for the growing deviation: radiation pressure stretches the dust distribution to further heliocentric distances than its parent body distribution; icy dust grains undergo photosputtering that rapidly increases their response to radiation pressure forces and pushes them further away from the Sun; and the distribution of KBOs may extend much further than existing observations suggest. Ongoing SDC measurements at even larger heliocentric distances will continue to constrain the contributions of dust production in the KB. Continued SDC measurements remain crucial for understanding the Kuiper Belt and the interpretation of dust disks around other stars.
Comets are remnants from the time when the outer planets formed, approximately 4-4.5 billion years ago. They have been in storage since then in the Oort cloud and Kuiper belt-distant regions that are ...so cold and sparsely populated that it was long thought that comets approaching the Sun were pristine samples from the time of Solar System formation. It is now recognized, however, that a variety of subtle but important evolutionary mechanisms operate on comets during their long storage, so they can no longer be regarded as wholly pristine.
•First spectral analysis of Nix, Hydra and Kerberos.•Crystalline water ice found on all three.•2.21 µm band seen on Nix and Hydra indicating an ammoniated species.•Disk resolved spectroscopy of ...Nix.•Temperature and crystalline H2O-ice fraction estimated for Nix and Hydra.
On July 14, 2015, NASA’s New Horizons spacecraft encountered the Pluto-system. Using the near-infrared spectral imager, New Horizons obtained the first spectra of Nix, Hydra, and Kerberos and detected the 1.5 and 2.0 µm bands of H2O-ice on all three satellites. On Nix and Hydra, New Horizons also detected bands at 1.65 and 2.21 µm that indicate crystalline H2O-ice and an ammoniated species, respectively. A similar band linked to NH3-hydrate has been detected on Charon previously. However, we do not detect the 1.99 µm band of NH3-hydrate. We consider NH4Cl (ammonium chloride), NH4NO3 (ammonium nitrate) and (NH4)2CO3 (ammonium carbonate) as potential candidates, but lack sufficient laboratory measurements of these and other ammoniated species to make a definitive conclusion. We use the observations of Nix and Hydra to estimate the surface temperature and crystalline H2O-ice fraction. We find surface temperatures < 20 K ( <70 K with 1-σ error) and 23 K ( < 150 K with 1-σ error) for Nix and Hydra, respectively. We find crystalline H2O-ice fractions of 78−22+12% and > 30% for Nix an Hydra, respectively. New Horizons observed Nix and Hydra twice, about 2–3 hours apart, or 5 and 25% of their respective rotation periods. We find no evidence for rotational differences in the disk-averaged spectra between the two observations of Nix or Hydra. We perform a pixel-by-pixel analysis of Nix’s disk-resolved spectra and find that the surface is consistent with a uniform crystalline H2O-ice fraction, and a ∼ 50% variation in the normalized band area of the 2.21 µm band with a minimum associated with the red blotch seen in color images of Nix. Finally, we find evidence for bands on Nix and Hydra at 2.42 and possibly 2.45 µm, which we cannot identify, and, if real, do not appear to be associated with the ammoniated species. We do not detect other ices, such as CO2, CH3OH and HCN.
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