Publication archive

Publication archive

Understanding the origin (volcanic or sedimentary) and timing of intercrater plains is crucial for deciphering the geological evolution of Mars. We have produced a detailed geological map of the intercrater plains north of the Hellas basin, based on images from the Mars Express High-Resolution Stereo Camera, the Mars Reconnaissance High-Resolution Imaging Science Experiment, and Context. Erosional windows and fresh impact craters provide a way of studying the lithology of intercrater plain units. They are composed predominantly of light-toned sedimentary rocks with subhorizontal bedding over a broad extent (greater than tens of kilometers), showing cross-bedding stratifications locally. The broad extent, geometry, and flat topography of these sediments favor a formation by aqueous processes (alluvial and lacustrine) rather than airfall (eolian and volcaniclastic). The Late Noachian (~3.7 Ga) sedimentary plains are locally covered by dark-toned, rough-textured lava flows of Late Hesperian age (~3.3 Ga). Fe/Mg phyllosilicates were detected within sedimentary rocks, whereas volcanic rocks contain pyroxene and lack signatures of alteration, in agreement with interpretations made from texture and morphology. In erosional windows, the superimposition of sedimentary rocks by younger volcanic flows enables the estimation of an erosion rate of ~1000 nm yr-1 during the Hesperian period (3.3–3.7 Ga). Thus, our study shows that an intense sedimentary cycle occurred on the northern rim of the Hellas basin before and during the Late Noachian, leading to the formation of widespread sedimentary plains, which were then eroded, in agreement with a gradual change in the climatic conditions in this period, and later covered by volcanic flows.
Published: 14 November 2016

This issue of the ISSI-BJ Magazine 'TAIKONG' provides an overview of the scientific objectives and the overall design of the SMILE project, including spacecraft and instrumentation, as discussed during the two-day forum "The Link between Solar Wind, Magnetosphere, Ionosphere", which was held on 6-7 July 2016 in Beijing, China.

Contents:

  • Foreword
  • Introduction
  • Global measurements and the solar wind-magnetosphere interaction
  • A novel method to image the magnetosphere
  • Aurora and substorm
  • Cusp dynamics
  • Modeling solar wind-magnetosphere interaction and field of view
  • SMILE scientific objectives and mission overview
  • Summary and recommendations

Published: 02 September 2016
Comet 67P/Churyumov-Gerasimenko currently spins with a period of about 12.4 hours, but its spin period used to be about 12.8 hours in 2002. It has been suggested that the difference is due to a period decrease caused by the comet's activity during its last close approach to the Sun in 2009. However, rather than the expected decrease in the spin period, recent navigation data from the Rosetta spacecraft has shown that the period has increased by 80.5 s as of Apr 13, 2015, as the comet approaches perihelion. Keller et al. estimate the water sublimation rate on each facet of a 3D model of the comet to calculate the global torque acting on the comet and infer changes in rotation rate. They show that the model qualitatively explains the modest increase in period observed in the first half of 2015 and predict that the rotation period should then decrease rapidly by about 13 minutes after perihelion. This will be directly tested by measurements in the next months.
Published: 30 September 2016

This press kit contains background information about the Rosetta mission. It has been prepared to accompany Rosetta's grand finale: the end of mission on 30 September 2016.

Contents:
Rosetta at a glance
Fast Facts
Landing Rosetta on the comet
Collecting science until the very end
Rosetta's final resting place
Highlights from the Rosetta mission thus far
No ordinary spacecraft: the challenges of flying Rosetta
Meet Comet 67P/Churyumov-Gerasimenko
Comets - an introduction
Missions to comets - Rosetta in context
Appendices: Mission milestones; Distance, dates, and times for mission milestones; Selected images and videos; Online resources; Media contacts

To download the pdf file click on the image or on the link to publication below.

Published: 26 September 2016
During its two years mission around comet 67P/Churyumov-Gerasimenko, ESA's Rosetta spacecraft had the unique opportunity to follow closely a comet in the most active part of its orbit. Many studies have presented the typical features associated to the activity of the nucleus, such as localized dust and gas jets. Here we report on series of more energetic transient events observed during the three months surrounding the comet's perihelion passage in August 2015. We detected and characterized 34 outbursts with the Rosetta cameras, one every 2.4 nucleus rotation. We identified 3 main dust plume morphologies associated to these events: a narrow jet, a broad fan, and more complex plumes featuring both previous types together. These plumes are comparable in scale and temporal variation to what has been observed on other comets. We present a map of the outbursts source locations, and discuss the associated topography. We find that the spatial distribution sources on the nucleus correlates well with morphological region boundaries, especially in areas marked by steep scarps or cliffs. Outbursts occur either in the early morning or shortly after the local noon, indicating two potential processes: Morning outbursts may be triggered by thermal stresses linked to the rapid change of temperature; afternoon events are most likely related to the diurnal or seasonal heat wave reaching volatiles buried under the first surface layer. In addition, we propose that some events can be the result of a completely different mechanism, in which most of the dust is released upon the collapse of a cliff.
Published: 23 September 2016

This media kit contains background information about the Gaia mission. It has been prepared to accompany Gaia Data Release 1.

Contents:
Gaia - the Billion Star Surveyor
Fast Facts
Mapping the Galaxy with Gaia
Gaia's first data release - the Galactic Census begins
Towards the final Gaia catalogue
Making sense of it all - the role of the Gaia Data Processing and Analysis Consortium
Where is Gaia and why do we need to know?
From ancient star maps to precision astrometry
Appendices: Resources, Speakers at the press event, Media contacts

To download the pdf file click on the image or on the link to publication below.

Published: 09 September 2016
Published online 7 September 2016

The presence of solid carbonaceous matter in cometary dust was established by the detection of elements such as carbon, hydrogen, oxygen and nitrogen in particles from comet 1P/Halley. Such matter is generally thought to have originated in the interstellar medium, but it might have formed in the solar nebula – the cloud of gas and dust that was left over after the Sun formed. This solid carbonaceous material cannot be observed from Earth, so it has eluded unambiguous characterization. Many gaseous organic molecules, however, have been observed; they come mostly from the sublimation of ices at the surface or in the subsurface of cometary nuclei. These ices could have been formed from material inherited from the interstellar medium that suffered little processing in the solar nebula. Here we report the in situ detection of solid organic matter in the dust particles emitted by comet 67P/Churyumov–Gerasimenko; the carbon in this organic material is bound in very large macromolecular compounds, analogous to the insoluble organic matter found in the carbonaceous chondrite meteorites. The organic matter in meteorites might have formed in the interstellar medium and/or the solar nebula, but was almost certainly modified in the meteorites' parent bodies. We conclude that the observed cometary carbonaceous solid matter could have the same origin as the meteoritic insoluble organic matter, but suffered less modification before and/or after being incorporated into the comet.

Published: 08 September 2016
Comets are thought to preserve almost pristine dust particles, thus providing a unique sample of the properties of the early solar nebula. The microscopic properties of this dust played a key part in particle aggregation during the formation of the Solar System. Cometary dust was previously considered to comprise irregular, fluffy agglomerates on the basis of interpretations of remote observations in the visible and infrared and the study of chondritic porous interplanetary dust particles that were thought, but not proved, to originate in comets. Although the dust returned by an earlier mission has provided detailed mineralogy of particles from comet 81P/Wild, the fine-grained aggregate component was strongly modified during collection. Here we report in situ measurements of dust particles at comet 67P/Churyumov–Gerasimenko. The particles are aggregates of smaller, elongated grains, with structures at distinct sizes indicating hierarchical aggregation. Topographic images of selected dust particles with sizes of one micrometre to a few tens of micrometres show a variety of morphologies, including compact single grains and large porous aggregate particles, similar to chondritic porous interplanetary dust particles. The measured grain elongations are similar to the value inferred for interstellar dust and support the idea that such grains could represent a fraction of the building blocks of comets. In the subsequent growth phase, hierarchical agglomeration could be a dominant process and would produce aggregates that stick more easily at higher masses and velocities than homogeneous dust particles. The presence of hierarchical dust aggregates in the near-surface of the nucleus of comet 67P also provides a mechanism for lowering the tensile strength of the dust layer and aiding dust release.
Published: 02 August 2016
Published online 19 April 2016

SPICAV VIS-IR spectrometer on-board the Venus Express mission measured the H2O abundance above Venus' clouds in the 1.38 µm band, and provided an estimation of the cloud top altitude based on CO2 bands in the range of 1.4–1.6 µm. The H2O content and the cloud top altitude have been retrieved for the complete Venus Express dataset from 2006 to 2014 taking into account multiple scattering in the cloudy atmosphere. The cloud top altitude, corresponding to unit nadir aerosol optical depth at 1.48 µm, varies from 68 to 73 km at latitudes from 40°S to 40°N with an average of 70.2 ± 0.8 km assuming the aerosol scale height of 4 km. In high northern latitudes, the cloud top decreases to 62–68 km. The altitude of formation of water lines ranges from 59 to 66 km. The H2O mixing ratio at low latitudes (20°S-20°N) is equal to 6.1 ± 1.2 ppm with variations from 4 to 11 ppm and the effective altitude of 61.9 ± 0.5 km. Between 30° and 50° of latitude in both hemispheres, a local minimum was observed with a value of 5.4 ± 1 ppm corresponding to the effective altitude of 62.1 ± 0.6 km and variations from 3 to 8 ppm. At high latitudes in both hemispheres, the water content varies from 4 to 12 ppm with an average of 7.2 ± 1.4 ppm which corresponds to 60.6 ± 0.5 km. Observed variations of water vapor within a factor of 2-3 on the short timescale appreciably exceed individual measurement errors and could be explained as a real variation of the mixing ratio or/and possible variations of the cloud opacity within the clouds. The maximum of water at lower latitudes supports a possible convection and injection of water from lower atmospheric layers.
[Remainder of abstract truncated due to character limitations]

Published: 02 August 2016
A&A, Forthcoming article

This paper describes the identification, modelling, and removal of previously unexplained systematic effects in the polarization data of the Planck High Frequency Instrument (HFI) on large angular scales, including new mapmaking and calibration procedures, new and more complete end-to-end simulations, and a set of robust internal consistency checks on the resulting maps. These maps, at 100, 143, 217, and 353 GHz, are early versions of those that will be released in final form later in 2016. The improvements allow us to determine the cosmic reionization optical depth τ using, for the first time, the low-multipole EE data from HFI, reducing significantly the central value and uncertainty, and hence the upper limit. Two different likelihood procedures are used to constrain τ from two estimators of the CMB E- and B-mode angular power spectra at 100 and 143 GHz, after debiasing the spectra from a small remaining systematic contamination. These all give fully consistent results. A further consistency test is performed using cross-correlations derived from the Low Frequency Instrument maps of the Planck 2015 data release and the new HFI data. For this purpose, end-to-end analyses of systematic effects from the two instruments are used to demonstrate the near independence of their dominant systematic error residuals. The tightest result comes from the HFI-based τ posterior distribution using the maximum likelihood power spectrum estimator from EE data only, giving a value 0.055 ± 0.009. In a companion paper these results are discussed in the context of the best-fit Planck ΛCDM cosmological model and recent models of reionization.

Published: 01 September 2016
A&A, Forthcoming article.

We investigate constraints on cosmic reionization extracted from the Planck cosmic microwave background (CMB) data. We combine the Planck CMB anisotropy data in temperature with the low-multipole polarization data to fit ΛCDM models with various parameterizations of the reionization history. We obtain a Thomson optical depth τ = 0.058 ± 0.012 for the commonly adopted instantaneous reionization model. This confirms, with data solely from CMB anisotropies, the low value suggested by combining Planck 2015 results with other data sets, and also reduces the uncertainties. We reconstruct the history of the ionization fraction using either a symmetric or an asymmetric model for the transition between the neutral and ionized phases. To determine better constraints on the duration of the reionization process, we also make use of measurements of the amplitude of the kinetic Sunyaev-Zeldovich (kSZ) effect using additional information from the high-resolution Atacama Cosmology Telescope and South Pole Telescope experiments. The average redshift at which reionization occurs is found to lie between z = 7.8 and 8.8, depending on the model of reionization adopted. Using kSZ constraints and a redshift-symmetric reionization model, we find an upper limit to the width of the reionization period of ∆z < 2.8. In all cases, we find that the Universe is ionized at less than the 10 % level at redshifts above z ~ 10. This suggests that an early onset of reionization is strongly disfavoured by the Planck data. We show that this result also reduces the tension between CMB-based analyses and constraints from other astrophysical sources.

Published: 01 September 2016
Published online 25 August 2016

On 19 Feb. 2016 nine Rosetta instruments serendipitously observed an outburst of gas and dust from the nucleus of comet 67P/Churyumov-Gerasimenko. Among these instruments were cameras and spectrometers ranging from UV over visible to microwave wavelengths, in-situ gas, dust and plasma instruments, and one dust collector. At 9:40 a dust cloud developed at the edge of an image in the shadowed region of the nucleus. Over the next two hours the instruments recorded a signature of the outburst that significantly exceeded the background. The enhancement ranged from 50% of the neutral gas density at Rosetta to factors >100 of the brightness of the coma near the nucleus. Dust related phenomena (dust counts or brightness due to illuminated dust) showed the strongest enhancements (factors >10). However, even the electron density at Rosetta increased by a factor 3 and consequently the spacecraft potential changed from ~-16 V to -20 V during the outburst. A clear sequence of events was observed at the distance of Rosetta (34 km from the nucleus): within 15 minutes the Star Tracker camera detected fast particles (~25 m s−1) while 100 μm radius particles were detected by the GIADA dust instrument ∼1 hour later at a speed of ~6 m s−1. The slowest were individual mm to cm sized grains observed by the OSIRIS cameras. Although the outburst originated just outside the FOV of the instruments, the source region and the magnitude of the outburst could be determined.

Published: 26 August 2016

Aims. We provide a detailed morphological analysis of the Aswan site on comet 67P/Churyumov-Gerasimenko (67P). We derive the size-frequency distribution of boulders ≥2 m and correlate this distribution with the gravitational slopes for the first time on a comet. We perform the spectral analysis of this region to understand if possible surface variegation is related to the different surface textures observable on the different units.

Methods. We used two OSIRIS Narrow Angle Camera (NAC) image data sets acquired on September 19 and 22, 2014, with a scale of 0.5 m/px. Gravitational slopes derived from the 3D shape model of 67P were used to identify and interpret the different units of the site. By means of the high-resolution NAC data sets, boulders ≥2.0 m can be unambiguously identified and extracted using the software ArcGIS. Coregistered and photometrically corrected color cubes were used to perform the spectral analyses, and we retrieved the spectral properties of the Aswan units.

Results. The high-resolution morphological map of the Aswan site (0.68 km²) shows that this site is characterized by four different units: fine-particle deposits located on layered terrains, gravitational accumulation deposits, taluses, and the outcropping layered terrain. Multiple lineaments are identified on the Aswan cliff, such as fractures, exposed layered outcrops, niches, and terraces. Close to the terrace margin, several arched features observed in plan view suggest that the margin progressively retreats as a result of erosion. The size-frequency of boulders ≥2 m in the entire study area has a power-law index of -3.9 +0.2/-0.3 (1499 boulders ≥2 m/km²), suggesting that the Aswan site is mainly dominated by gravitational events triggered by sublimation and/or thermal insolation weathering causing regressive erosion.

[Remainder of abstract truncated due to character limitations]

Published: 29 July 2016
Published online 28 July 2016

Context. We investigate the formation and evolution of comet nuclei and other trans-Neptunian objects (TNOs) in the solar nebula and primordial disk prior to the giant planet orbit instability foreseen by the Nice model.

Aims. Our goal is to determine whether most observed comet nuclei are primordial rubble-pile survivors that formed in the solar nebula and young primordial disk or collisional rubble piles formed later in the aftermath of catastrophic disruptions of larger parent bodies. We also propose a concurrent comet and TNO formation scenario that is consistent with observations.

Methods. We used observations of comet 67P/Churyumov-Gerasimenko by the ESA Rosetta spacecraft, particularly by the OSIRIS camera system, combined with data from the NASA Stardust sample-return mission to comet 81P/Wild 2 and from meteoritics; we also used existing observations from ground or from spacecraft of irregular satellites of the giant planets, Centaurs, and TNOs. We performed modeling of thermophysics, hydrostatics, orbit evolution, and collision physics.

Results. We find that thermal processing due to short-lived radionuclides, combined with collisional processing during accretion in the primordial disk, creates a population of medium-sized bodies that are comparably dense, compacted, strong, heavily depleted in supervolatiles like CO and CO2; they contain little to no amorphous water ice, and have experienced extensive metasomatism and aqueous alteration due to liquid water. Irregular satellites Phoebe and Himalia are potential representatives of this population. Collisional rubble piles inherit these properties from their parents. Contrarily, comet nuclei have low density, high porosity, weak strength, are rich in supervolatiles, may contain amorphous water ice, and do not display convincing evidence of in situ metasomatism or aqueous alteration.
[Remainder of abstract truncated]

Published: 29 July 2016
White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 107-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they mainly originate from the cool star. AR Sco's broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.
Published: 28 July 2016
The tadpole galaxy Kiso 5639 has a slowly rotating disk with a drop in metallicity at its star-forming head, suggesting that star formation was triggered by the accretion of metal-poor gas. We present multi-wavelength Hubble Space Telescope Wide Field Camera 3 images of UV through I band plus Hα to search for peripheral emission and determine the properties of various regions. The head has a mass in young stars of ~106 M and an ionization rate of 6.4 × 1051 s−1, equivalent to ~2100 O9-type stars. There are four older star-forming regions in the tail, and an underlying disk with a photometric age of ~1 Gyr. The mass distribution function of 61 star clusters is a power law with a slope of −1.73 ± 0.51. Fourteen young clusters in the head are more massive than 104 M, suggesting a clustering fraction of 30%–45%. Wispy filaments of Hα emission and young stars extend away from the galaxy. Shells and holes in the head H II region could be from winds and supernovae. Gravity from the disk should limit the expansion of the H II region, although hot gas might escape through the holes. The star formation surface density determined from Hα in the head is compared to that expected from likely pre-existing and accreted gas. Unless the surface density of the accreted gas is a factor of ~3 or more larger than what was in the galaxy before, the star formation rate has to exceed the usual Kennicutt–Schmidt rate by a factor of ≥5.
Published: 14 July 2016
Accreting stellar-mass black holes often show a 'Type-C' quasi-periodic oscillation (QPO) in their X-ray flux and an iron emission line in their X-ray spectrum. The iron line is generated through continuum photons reflecting off the accretion disc, and its shape is distorted by relativistic motion of the orbiting plasma and the gravitational pull of the black hole. The physical origin of the QPO has long been debated, but is often attributed to Lense–Thirring precession, a General Relativistic effect causing the inner flow to precess as the spinning black hole twists up the surrounding space–time. This predicts a characteristic rocking of the iron line between red- and blueshift as the receding and approaching sides of the disc are respectively illuminated. Here we report on XMM–Newton and NuSTAR observations of the black hole binary H1743−322 in which the line energy varies systematically over the ~4 s QPO cycle (3.70σ significance), as predicted. This provides strong evidence that the QPO is produced by Lense–Thirring precession, constituting the first detection of this effect in the strong gravitation regime. There are however elements of our results harder to explain, with one section of data behaving differently than all the others. Our result enables the future application of tomographic techniques to map the inner regions of black hole accretion discs.
Published: 13 July 2016
Proc. SPIE 9905, Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray, 990502 (11 July 2016); doi: 10.1117/12.2231984
Published: 12 July 2017
Clusters of galaxies are the most massive gravitationally bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and many astrophysical processes. However, knowledge of the dynamics of the pervasive hot gas, the mass of which is much larger than the combined mass of all the stars in the cluster, is lacking. Such knowledge would enable insights into the injection of mechanical energy by the central supermassive black hole and the use of hydrostatic equilibrium for determining cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50-million-kelvin diffuse hot plasma filling its gravitational potential well. The active galactic nucleus of the central galaxy NGC 1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These bubbles probably induce motions in the intracluster medium and heat the inner gas, preventing runaway radiative cooling – a process known as active galactic nucleus feedback. Here we report X-ray observations of the core of the Perseus cluster, which reveal a remarkably quiescent atmosphere in which the gas has a line-of-sight velocity dispersion of 164 ± 10 kilometres per second in the region 30–60 kiloparsecs from the central nucleus. A gradient in the line-of-sight velocity of 150 ± 70 kilometres per second is found across the 60-kiloparsec image of the cluster core. Turbulent pressure support in the gas is four per cent of the thermodynamic pressure, with large-scale shear at most doubling this estimate. We infer that a total cluster mass determined from hydrostatic equilibrium in a central region would require little correction for turbulent pressure.
Published: 08 July 2016
Based on analysis of UV images (at 365 nm) of Venus cloud top (altitude 67±2 km) collected with Venus Monitoring Camera on board Venus Express (VEX), it is found that the zonal wind speed south of the equator (from 5°S to 15°S) shows a conspicuous variation (from −101 to −83 m/s) with geographic longitude of Venus, correlated with the underlying relief of Aphrodite Terra. We interpret this pattern as the result of stationary gravity waves produced at ground level by the uplift of air when the horizontal wind encounters a mountain slope. These waves can propagate up to the cloud top level, break there, and transfer their momentum to the zonal flow. Such upward propagation of gravity waves and influence on the wind speed vertical profile was shown to play an important role in the middle atmosphere of the Earth by Lindzen (1981) but is not reproduced in the current GCM of Venus atmosphere from LMD. (Laboratoire de Météorologie Dynamique) In the equatorial regions, the UV albedo at 365 nm varies also with longitude. We argue that this variation may be simply explained by the divergence of the horizontal wind field. In the longitude region (from 60° to −10°) where the horizontal wind speed is increasing in magnitude (stretch), it triggers air upwelling which brings the UV absorber at cloud top level and decreases the albedo and vice versa when the wind is decreasing in magnitude (compression). This picture is fully consistent with the classical view of Venus meridional circulation, with upwelling at equator revealed by horizontal air motions away from equator: the longitude effect is only an additional but important modulation of this effect. This interpretation is comforted by a recent map of cloud top H2O, showing that near the equator the lower UV albedo longitude region is correlated with increased H2O. We argue that H2O enhancement is the sign of upwelling, suggesting that the UV absorber is also brought to cloud top by upwelling.
Published: 01 July 2016
22-Nov-2019 07:42 UT

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