Km In Radius Research Articles

A Comet Active Beyond the Crystallization Zone

Abstract We present observations showing inbound long-period comet C/2017 K2 (PANSTARRS) to be active at a record heliocentric distance. Nucleus temperatures are too low (60–70 K) either for water ice to sublimate or for amorphous ice to crystallize, requiring another source for the observed activity. Using the Hubble Space Telescope we find a sharply bounded, circularly symmetric dust coma 105 km in radius, with a total scattering cross-section of ∼105 km2. The coma has a logarithmic surface brightness gradient −1 over much of its surface, indicating sustained, steady-state dust production. A lack of clear evidence for the action of solar radiation pressure suggests that the dust particles are large, with a mean size ≳0.1 mm. Using a coma convolution model, we find a limit to the apparent magnitude of the nucleus (absolute magnitude ). With assumed geometric albedo p V = 0.04, the limit to the nucleus circular equivalent radius is <9 km. Prediscovery observations from 2013 show that the comet was also active at 23.7 au heliocentric distance. While neither water ice sublimation nor exothermic crystallization can account for the observed distant activity, the measured properties are consistent with activity driven by sublimating supervolatile ices such as CO2, CO, O2, and N2. Survival of supervolatiles at the nucleus surface is likely a result of the comet’s recent arrival from the frigid Oort Cloud.

From Neutron Star Observables to the Equation of State. II. Bayesian Inference of Equation of State Pressures

Abstract One of the key goals of observing neutron stars is to infer the equation of state (EoS) of the cold, ultradense matter in their interiors. Here, we present a Bayesian statistical method of inferring the pressures at five fixed densities, from a sample of mock neutron star masses and radii. We show that while five polytropic segments are needed for maximum flexibility in the absence of any prior knowledge of the EoS, regularizers are also necessary to ensure that simple underlying EoS are not over-parameterized. For ideal data with small measurement uncertainties, we show that the pressure at roughly twice the nuclear saturation density, , can be inferred to within 0.3 dex for many realizations of potential sources of uncertainties. The pressures of more complicated EoS with significant phase transitions can also be inferred to within ∼30%. We also find that marginalizing the multi-dimensional parameter space of pressure to infer a mass–radius relation can lead to biases of nearly 1 km in radius, toward larger radii. Using the full, five-dimensional posterior likelihoods avoids this bias.

Antisymmetry of oceanic eddies across the Kuroshio over a shelfbreak

From the analysis of oceanic eddies detected in the drifter trajectories of the Global Drifter Program (GDP) data set, it was found that oceanic eddies are asymmetrically distributed across the Kuroshio in the East China Sea: predominant cyclonic (anticyclonic) eddies are on the western (eastern) sides of Kuroshio. This distribution is confirmed by high-resolution numerical modeling output as well. Most of these eddies are 5~20 km in radius, less than the local first baroclinic deformation radius, thus categorized as submesoscale. The generation mechanism of these submesoscale eddies is speculated to be related to the horizontal velocity shear of the Kuroshio when it flows northeastward along the shelf break in the East China Sea. The budget analysis of eddy kinetic energy shows that both the horizontal shear and vertical buoyancy flux are important energy sources for eddy generation on the two sides of Kuroshio axis. The finding highlights the unique feature of oceanic eddies along the western boundary currents.

Formation of a solid inner core during the accretion of Earth

AbstractThe formation of an inner core during the accretion of Earth is investigated by using self‐gravitating and compressible Earth models formed by accreting a total of 25 or 50 Moon to Mars‐sized planetary embryos. The impact of an embryo heats the proto‐Earth's interior differentially, more below the impact site than elsewhere. The rotating core dynamically overturns and stratifies shortly after each impact, creating a spherically symmetric and radially increasing temperature distribution relative to an adiabatic profile. Merging of an embryo to the proto‐Earth increases the lithostatic pressure that results in compressional temperature increase while further enhances the melting temperature of the core causing solidification. A total of 36 thermal evolution models of the growing proto‐Earth's core are calculated to investigate effects of major physical parameters. No solidification is considered in the first 21 models where modified two‐body escape velocities are used as the impact velocities of the embryos. At the end of accretion, temperatures in the upper part of the core are significantly different among these models, whereas temperatures in the deeper parts are similar. The core solidification considered in the remaining 15 models, where impact velocities higher than the modified two‐body escape velocities are adopted, drastically changes the temperature distribution in the deeper parts of the core. All of the models produce partially solidified stiff inner cores, 1000–2100 km in radius, at the end of accretion, where the solid fraction is larger than 50%. The innermost of the stiff inner cores is completely solidified to radii 250–1500 km.

Seismo-ionospheric anomalies in ionospheric TEC and plasma density before the 17 July 2006 <i>M</i>7.7 south of Java earthquake

Abstract. In this paper, we report significant evidence for preseismic ionospheric anomalies in total electron content (TEC) of the global ionosphere map (GIM) and plasma density appearing on day 2 before the 17 July 2006 M7.7 south of Java earthquake. After distinguishing other anomalies related to the geomagnetic activities, we found a temporal precursor around the epicenter on day 2 before the earthquake (15 July 2006), which agrees well with the spatial variations in latitude–longitude–time (LLT) maps. Meanwhile, the sequences of latitude–time–TEC (LTT) plots reveal that the TECs on epicenter side anomalously decrease and lead to an anomalous asymmetric structure with respect to the magnetic equator in the daytime from day 2 before the earthquake. This anomalous asymmetric structure disappears after the earthquake. To further confirm these anomalies, we studied the plasma data from DEMETER satellite in the earthquake preparation zone (2046.4 km in radius) during the period from day 45 before to day 10 after the earthquake, and also found that the densities of both electron and total ion in the daytime significantly increase on day 2 before the earthquake. Very interestingly, O+ density increases significantly and H+ density decreases, while He+ remains relatively stable. These results indicate that there exists a distinct preseismic signal (preseismic ionospheric anomaly) over the epicenter.

Guadalajara, Mexico, Earthquake Sequence of December 2015 and May 2016: Source, Q, and Ground Motions

We analyze an earthquake sequence which occurred 24 km NNW from Guadalajara Cathedral. The activity began on 15 December, 2015 with an Mw4.1 earthquake and resumed, again, on 11 May, 2016 when the largest earthquake of the sequence, Mw4.8, occurred. The earthquakes were recorded in the city by two accelerographs and at larger distances by broadband seismographs. The source region of the sequence is located in the north of the broad junction formed by NW-SE trending Tepic-Zacoalco rift, N-S trending Colima rift, and E-W trending Chapala rift. The focal mechanism of the largest event is characterized by: strike Ï• = 21º, dip δ= 49º, and rake λ= -86º, with horizontal tensional axis trending 110º. This mechanism is not consistent with NE-SW direction of motion measured on fault strike by Barrier et al. (1990) ~ 35 km WNW of Guadalajara. It also differs from the mechanism reported in a previous study for a seismic sequence in the Zacoalco rift about 60 km south of Guadalajara. Similarities among recorded waveforms at the accelerographic stations suggest that the mechanism of other events in the sequence were similar and the events were located in a small volume, probably 1 to 2 km in radius. From the analysis of the recordings of the 11 May shock along the Trans-Mexican Volcanic Belt, from Ceboruco volcano to the west and Popocatepetl volcano to the east, the attenuation parameter Q as 141f 0.63 was estimated. The earthquake on 11 May was felt very strongly in the city and produced peak ground acceleration (PGA) reaching 80 cm/s2. Relatively large earthquakes have been reported in the region; e.g., on 11 February, 1875 near the town of San Cristobal de la Barranca, about 40 km NNW of Guadalajara, therefore, the interest on using recordings from accelerographic stations to estimate peak ground motion parameters for scenario earthquakes of Mw≤ 6.0 using random vibration theory (RVT) and empirical Green’s function (EGF) techniques. From RVT simulations, the expected PGA, peak ground velocity (PGV), and peak ground displacement (PGD) at the accelerographic stations from an Mw6.0 earthquake in the source region of the sequence are 195 cm/s2, 10 cm/s, and 3 cm, respectively. The predictions from the EGF technique are somewhat higher but still within a factor of two of the RVT predictions.

Relationships between airborne pollen grains, wind direction and land cover using GIS and circular statistics.

Airborne bio-aerosol content (mainly pollen and spores) depends on the surrounding vegetation and weather conditions, particularly wind direction. In order to understand this issue, maps of the main land cover in influence areas of 10km in radius surrounding pollen traps were created. Atmospheric content of the most abundant 14 pollen types was analysed in relation to the predominant wind directions measured in three localities of SW of Iberian Peninsula, from March 2011 to March 2014. Three Hirst type traps were used for aerobiological monitoring. The surface area for each land cover category was calculated and wind direction analysis was approached by using circular statistics. This method could be helpful for estimating the potential risk of exposure to various pollen types. Thus, the main land cover was different for each monitoring location, being irrigated crops, pastures and hardwood forests the main categories among 11 types described. Comparison of the pollen content with the predominant winds and land cover shows that the atmospheric pollen concentration is related to some source areas identified in the inventory. The study found that some pollen types (e.g. Plantago, Fraxinus-Phillyrea, Alnus) come from local sources but other pollen types (e.g. Quercus) are mostly coming from longer distances. As main conclusions, airborne particle concentrations can be effectively split by addressing wind with circular statistics. By combining circular statistics and GIS method with aerobiological data, we have created a useful tool for understanding pollen origin. Some pollen loads can be explained by immediate surrounding landscape and observed wind patterns for most of the time. However, other factors like medium or long-distance transport or even pollen trap location within a city, may occasionally affect the pollen load recorded using an air sampler.

CONSTRAINTS ON COMET 332P/IKEYA-MURAKAMI

ABSTRACT Encke-type comet 332P/Ikeya-Murakami is experiencing cascading fragmentation events during its 2016 apparition. It is likely the first splitting Encke-type comet ever observed. A nongravitational solution to the astrometry reveals a statistical detection of the radial and transverse nongravitational parameters, au day−2 and au day−2, respectively, which implies a nucleus erosion rate of ‰ per orbital revolution. The mass-loss rate likely has to be supported by a much larger fraction of an active surface area than known cases of short-period comets; it may be relevant to the ongoing fragmentation. We failed to detect any serendipitous pre-discovery observations of the comet in archival data from major sky surveys, whereby we infer that 332P used to be largely inactive, and is perhaps among the few short-period comets that have been reactivated from weakly active or dormant states. We therefore constrain an upper limit to the nucleus size as 2.0 ± 0.2 km in radius. A search for small bodies in similar orbits to that of 332P reveals comet P/2010 B2 (WISE) to be the best candidate. From an empirical generalized Jupiter-family (Encke-type included) comet population model, we estimate the likelihood of a chance alignment of the 332P–P/2010 B2 pair to be 1 in 33, a small number indicative of a genetic linkage between the two comets on a statistical basis. The pair possibly originated from a common progenitor, which underwent a disintegration event well before the twentieth century.

ARE THERE MOONLETS NEAR THE URANIAN α AND β RINGS?

ABSTRACT The Voyager 2 Radio Science Subsystem occultations of the Uranian α and β rings exhibit quasi-periodic optical depth variations with radial wavelengths that vary with longitude. These patterns may be wakes from small moonlets orbiting exterior to these rings. Based on the observed structures in the rings, we estimate that the moonlets would need to be located ∼100 km exterior to the rings’ semimajor axes ( km for α and km for β) and be 2–7 km in radius. Such moonlets could help keep the rings confined. Due to their small radii and presumed low albedo, the expected brightness of these moonlets is on the order of the noise in Voyager 2 images.

Onset of oligarchic growth and implication for accretion histories of dwarf planets

We investigate planetary accretion that starts from equal-mass planetesimals using an analytic theory and numerical simulations. We particularly focus on how the planetary mass Moli at the onset of oligarchic growth depends on the initial mass m0 of a planetesimal. Oligarchic growth commences when the velocity dispersion relative to the Hill velocity of the protoplanet takes its minimum. We find that if m0 is small enough, this normalized velocity dispersion becomes as low as unity during the intermediate stage between the runaway and oligarchic growth stages. In this case, Moli is independent of m0. If m0 is large, on the other hand, oligarchic growth commences directly after runaway growth, and Moli∝m03/7. The planetary mass Moli for the solid surface density of the Minimum Mass Solar Nebula is close to the masses of the dwarf planets in a reasonable range of m0. This indicates that they are likely to be the largest remnant planetesimals that failed to become planets. The power-law exponent q of the differential mass distribution of remnant planetesimals is typically −2.0 and −2.7 to −2.5 for small and large m0. The slope, q≃−2.7, and the bump at 1021 g (or 50 km in radius) for the mass distribution of hot Kuiper belt objects are reproduced if m0 is the bump mass. On the other hand, small initial planetesimals with m0 ∼ 1013 g or less are favored to explain the slope of large asteroids, q≃−2.0, while the bump at 1021 g can be reproduced by introducing a small number of asteroid seeds each with mass of 1019 g.

THE COLLISIONAL EVOLUTION OF UNDIFFERENTIATED ASTEROIDS AND THE FORMATION OF CHONDRITIC METEOROIDS

ABSTRACT Most meteorites are fragments from recent collisions experienced in the asteroid belt. In such a hypervelocity collision, the smaller collision partner is destroyed, whereas a crater on the asteroid is formed or it is entirely disrupted, too. The present size distribution of the asteroid belt suggests that an asteroid with a 100 km radius is encountered 1014 times during the lifetime of the solar system by objects larger than 10 cm in radius; the formed craters cover the surface of the asteroid about 100 times. We present a Monte Carlo code that takes into account the statistical bombardment of individual infinitesimally small surface elements, the subsequent compaction of the underlying material, and the formation of a crater and a regolith layer. For the entire asteroid, 10,000 individual surface elements are calculated. We compare the ejected material from the calculated craters with the shock stage of meteorites with low petrologic type and find that these most likely stem from smaller parent bodies that do not possess a significant regolith layer. For larger objects, which accrete a regolith layer, a prediction of the thickness depending on the largest visible crater can be made. Additionally, we compare the crater distribution of an object initially 100 km in radius with the shape model of the asteroid (21) Lutetia, assuming it to be initially formed spherical with a radius that is equal to its longest present ellipsoid length. Here we find the shapes of both objects to show resemblance to each other.

Stroke multiplicity and horizontal scale of negative charge regions in thunderclouds

AbstractAn X‐band polarimetric radar and multiple lightning detection systems are used to document the initial cloud‐to‐ground lightning flash in a large number (46 cases) of incipient thunderstorms, as part of the CHUVA‐Vale field campaign during the 2011/2012 spring‐summer in southeast Brazil. The results show an exceptionally low stroke multiplicity (87% of flashes with single stroke) in the initial ground flashes, a finding consistent with the limited space available for the positive leader extension into new regions of negative space charge in compact cells. The results here are contrasted with the behavior of ground flashes in mesoscale thunderstorms in previous studies. Additionally, we found evidence for a minimum scale (radar echo >20 dBZ) for lightning initiation (>3 km in radius) and that the peak currents of initial cloud‐to‐ground flashes in these compact thunderstorms are only half as large as return stroke peak currents in general.

Origin of the eclogitic clasts with graphite-bearing and graphite-free lithologies in the Northwest Africa 801 (CR2) chondrite: Possible origin from a Moon-sized planetary body inferred from chemistry, oxygen isotopes and REE abundances

In order to clarify the origin of the eclogitic clasts found in the NWA801 (CR2) chondrite (Kimura et al., 2013), especially, that of the high pressure and temperature (P–T) condition (∼3GPa and ∼1000°C), we conducted ion microprobe analyses of oxygen isotopes and rare earth element (REE) abundances in the clasts. Oxygen isotopic compositions of the graphite-bearing lithology (GBL) and graphite-free lithology (GFL) show a slope ∼0.6 correlation slightly below the CR-CH-CB chondrites field in the O three-isotope-diagram, with a large variation for the former and almost homogeneous composition for the latter. The average REE abundances of the two lithologies show almost unfractionated patterns. Based on these newly obtained data, as well as mineralogical observations, bulk chemistry, and considerations about diffusion timescales for various elements, we discuss in detail the formation history of the clasts. Consistency of the geothermobarometers used by Kimura et al. (2013), suggesting equilibration of various elements among different mineral pairs, provides a strong constraint for the duration of the high P–T condition. We suggest that the high P–T condition lasted 102–103years. This clearly precludes a shock high pressure (HP) model, and hence, strongly supports a static HP model. A static HP model requires a Moon-sized planetary body of ∼1500km in radius. Furthermore, it implies two successive violent collisions, first at the formation of the large planetary body, when the clasts were placed its deep interior, and second, at the disruption of the large planetary body, when the clasts were expelled out of the parent body and later on transported to the accretion region of the CR chondrites. We also discuss possible origin of O isotopic variations in GBL, and presence/absence of graphite in GBL/GFL, respectively, in relation to smelting possibly occurred during the igneous process(es) which formed the two lithologies. Finally we present a possible formation scenario of the eclogitic clasts.

FORMING CHONDRITES IN A SOLAR NEBULA WITH MAGNETICALLY INDUCED TURBULENCE

ABSTRACT Chondritic meteorites provide valuable opportunities to investigate the origins of the solar system. We explore impact jetting as a mechanism of chondrule formation and subsequent pebble accretion as a mechanism of accreting chondrules onto parent bodies of chondrites, and investigate how these two processes can account for the currently available meteoritic data. We find that when the solar nebula is ≤5 times more massive than the minimum-mass solar nebula at a ≃ 2–3 au and parent bodies of chondrites are ≤1024 g (≤500 km in radius) in the solar nebula, impact jetting and subsequent pebble accretion can reproduce a number of properties of the meteoritic data. The properties include the present asteroid belt mass, the formation timescale of chondrules, and the magnetic field strength of the nebula derived from chondrules in Semarkona. Since this scenario requires a first generation of planetesimals that trigger impact jetting and serve as parent bodies to accrete chondrules, the upper limit of parent bodies’ masses leads to the following implications: primordial asteroids that were originally ≥1024 g in mass were unlikely to contain chondrules, while less massive primordial asteroids likely had a chondrule-rich surface layer. The scenario developed from impact jetting and pebble accretion can therefore provide new insights into the origins of the solar system.

KIC 8462852: TRANSIT OF A LARGE COMET FAMILY

ABSTRACT We investigate the plausibility of a cometary source of the unusual transits observed in the KIC 8462852 light curve. A single comet of similar size to those in our solar system produces a transit depth of the order of 10−3 lasting less than a day which is much smaller and shorter than the largest dip observed ( ∼ 20 % ?> for ∼3 days), but a large, closely traveling cluster of comets can fit the observed depths and durations. We find that a series of large comet swarms, with all except one on the same orbit, provides a good fit for the KIC 8462852 data during Quarters 16 and 17, but does not explain the large dip observed during Quarter 8. However, the transit dips only loosely constrain the orbits and can be fit by swarms with periastrons differing by a factor of 10. To reach a transit depth of ∼0.2, the comets need to be in a close group of ∼30, if they are ∼100 km in radius or in a group of ∼300 if they are ∼10 km in radius. The total number of comets required to fit all of the dips is ∼70 ∼ 100 km or ∼700 ∼ 10 km comets. A single comet family from a tidally disrupted Ceres-sized progenitor or the start of a Late Heavy Bombardment period explains the last ∼60 days of the unusual KIC 8462852 light curve.

Epidemiology of infectious pancreatic necrosis virus (IPNV) in Atlantic salmon marine farms in Chile, from 2010-2014

Epidemiology of infectious pancreatic necrosis virus (IPNV) in Atlantic salmon marine farms in Chile, from 2010-2014

Diversidad de mariposas y su relación con el paisaje en la cuenca del río Lagunillas, Tolima, Colombia

El presente estudio se desarrolló al norte del departamento del Tolima con el objetivo de caracterizar las mariposas asociadas a fragmentos de bosque, matorral o rastrojo y pastos. Se registraron las coordenadas geográficas de los bordes de los hábitats encontrados en una parcela de 0,6 km de radio y se establecieron las correlaciones entre los atributos del paisaje y la comunidad de mariposas, representada por seis familias y 266 especies. Las diferencias estructurales entre los hábitats de estudio pueden ayudar a explicar las diferencias en la riqueza y la diversidad de mariposas debido a que las comunidades responden diferencialmente al hábitat.<br /><br />

Southern Hemisphere strong polar mesoscale cyclones in high-resolution datasets

Mesoscale cyclones are small low-pressure systems (usually <500 km in radius) that often appear embedded in synoptic structures. These events can be weak and short lived or vigorous and destructive. Here we use an automatic tracking scheme to investigate two subsets of Southern Hemisphere mesoscale cyclones that are strong and have the potential to cause damage, namely “polar lows” (i.e., strong and short lived) and “explosive cyclones” (i.e., rapid intensification but not necessarily short lived). A short climatology (2009–2012) is obtained by using high resolution (0.5°) Antarctic Mesoscale Prediction System (AMPS) mean sea level pressure. The results show a significant improvement of spatial detail compared to the 0.75° resolution ERA-interim dataset, with a total count approximately 46 % higher in AMPS. The subset of mesoscale cyclones that are explosive is small, with a total genesis number of about 13 % that of polar lows. In addition, only about 1 % of the polar lows are explosive, suggesting that cyclones that undergo rapid intensification tend to become larger longer lived (and hence are no longer regarded as polar lows). Mesoscale cyclones are more frequent in winter, with a maximum concentration around the Antarctic but also occurring as far north as Tasmania and New Zealand. Analysis of sensible heat flux and sea ice extent anomalies during the genesis days shows that there is a large spread of genesis points over both positive and negative flux anomalies in winter, with a somewhat random pattern in the other seasons.

The interior configuration of planet Mercury constrained by moment of inertia and planetary contraction

This paper presents an analysis of present-day interior configuration models for Mercury considering cores of Fe-S or Fe-Si alloy, the latter possibly covered by a solid FeS layer, in light of the improved limit of planetary contraction of 7 km derived from MErcury Surface, Space ENvironment, GEochemistry, and Ranging observations of surface landforms. Density profiles, generated by a Monte Carlo approach, are constrained by Mercury's mass, polar moment of inertia (C), fraction of polar moment corresponding to its outer solid shell (Cm/C), and observed planetary contraction. Results show that the outer liquid core boundary is constrained to 1985–2090 km in radius, where large radii correspond to high Si and S core contents and high mantle densities or the presence of an FeS layer at the top of the outer core. The bulk core S and Si contents are within 2.8–8.9 wt % and above 8.5 wt %, respectively, where an increase of light element core content correlates positively with mantle density and core size. The size of the inner core is constrained by the observed planetary contraction to below 1454 or 1543 km in radius for bulk cores rich in S (near 8.9 wt %) or Si (near 25 wt %), respectively. For cores poor in light elements, inner cores up to 1690 km in radius remain consistent with the observed planetary contraction. Finally, we show that solid FeS at outer core conditions, previously argued to float on liquid Fe-S, may be denser than the residual liquid. This implies that a separate mechanism may be required to maintain an FeS layer at the suggested location.

GONE IN A BLAZE OF GLORY: THE DEMISE OF COMET C/2015 D1 (SOHO)

We present studies of C/2015 D1 (SOHO), the first sunskirting comet ever seen from ground stations over the past half century. The Solar and Heliospheric Observatory (SOHO) witnessed its peculiar light curve with a huge dip followed by a flareup around perihelion: the dip was likely caused by sublimation of olivines, directly evidenced by a coincident temporary disappearance of the tail. The flareup likely reflects a disintegration event, which we suggest was triggered by intense thermal stress established within the nucleus interior. Photometric data reveal an increasingly dusty coma, indicative of volatile depletion. A catastrophic mass loss rate of $\sim$10$^{5}$ kg s$^{-1}$ around perihelion was seen. Ground-based Xingming Observatory spotted the post-perihelion debris cloud. Our morphological simulations of post-perihelion images find newly released dust grains of size $a \gtrsim 10$ $\mu$m in radius, however, a temporal increase in $a_{\min}$ was also witnessed, possibly due to swift dispersions of smaller grains swept away by radiation forces without replenishment. Together with the fading profile of the light curve, a power law dust size distribution with index $\gamma = 3.2 \pm 0.1$ is derived. We detected no active remaining cometary nuclei over $\sim$0.1 km in radius in post-perihelion images acquired at Lowell Observatory. Applying radial non-gravitational parameter, $\mathcal{A}_{1} = \left(1.209 \pm 0.118 \right) \times 10^{-6}$ AU day$^{-2}$, from an isothermal water-ice sublimation model to the SOHO astrometry significantly reduces residuals and sinusoidal trends in the orbit determination. The nucleus mass $\sim$10$^{8}$--10$^{9}$ kg, and the radius $\sim$50--150 m (bulk density $\rho_{\mathrm{d}} = 0.4$ g cm$^{-3}$ assumed) before the disintegration are deduced from the photometric data; consistent results were determined from the non-gravitational effects.