ESA Science & Technology - Publication Archive

In Press Solar wind controls non-thermal escape of planetary atmospheric volatiles, regardless of the strength of planetary magnetic fields. For both Earth with a strong dipole and Mars with weak remnant fields, the oxygen ion (O⁺) outflow has been separately found to be enhanced during corotating interaction region (CIR) passage. Here we compared the enhancements of O⁺ outflow on Earth and Mars driven by a CIR in January, 2008 when Sun, Earth and Mars were approximately aligned. The CIR propagation was recorded by STEREO, ACE, Cluster and Mars Express (MEX). During the CIR passage, Cluster observed enhanced flux of upwelling oxygen ions above the Earth's polar region, while MEX detected an increased escape flux of oxygen ions in the Martian magnetosphere. We found that, (1) under a solar wind dynamic pressure increase by 2-3 nPa, the rate of increase in Martian O⁺ outflow flux was one order higher than those on Earth; (2) as response to the same part of the CIR body, the rate of increase in Martian O⁺ outflow flux was on the same order as for Earth. The comparison results imply that the dipole effectively prevents coupling of solar wind kinetic energy to planetary ions, and the distance to the Sun is also crucially important for planetary volatile loss in our inner solar system.

We report the first detection in the atmosphere of Mars of the nightside O₂(a¹Delta_g) emission at 1.27 µm from limb observations of the OMEGA imaging spectrometer on board Mars Express (MEX). The emission, detected in three cases out of 40 observations, is due to recombination in a downwelling air parcel of O atoms produced by photodissociation of CO₂ on the dayside in the upper atmosphere (O + O + M -> O₂* + M), and not from ozone UV photodissociation, as is often seen on the dayside. Observed vertical profiles and total retrieved vertical intensities are compared with models. When detected, the emission is 10 times larger than previous predictions, at ~240 kR. This can be explained in the frame of a general circulation model (GCM) of Mars. As predicted by the GCM, all positive observations were obtained at high latitudes, during the winter night. The model is validated, which simulates the large Hadley cell characterizing the meridional circulation, ascending from the summer pole and descending to the winter pole. This new emission is tracing uniquely a downward advection transport mechanism, and therefore its detailed study will provide important constraints on the overall aeronomy and dynamics of Mars. The impact on long-term stability of methane is examined. It is found that recycling through the mesosphere will not decrease significantly the overall lifetime of CH₄ (~300 years), because the descent of air is confined to high latitudes and winter seasons. These observations are demonstrating a new diagnosis of the aeronomy and atmospheric dynamics of Mars.

Magnetospheric substorms are elemental processes of solar wind energy storage and explosive release in Earth's magnetosphere. They encompass fundamental plasma physics questions, are ubiquitous during all types of geomagnetic conditions, contribute significantly to magnetic storms, and are a key element of Space Weather applications. This paper reviews recent major advances enabled by modern multi-point space-based and ground-based platforms. These datasets have also empowered a system-wide perspective and advanced modeling. We particularly highlight progress in two areas: (1) substorm onset timing and evidence for current sheet preconditioning and destabilization and (2) fast flows and dipolarizations, including the role of entropy in magnetotail plasma propagation.

Recent X-ray observations have enabled the study of reverberation delays in active galactic nuclei (AGN) for the first time. All the detections so far are in sources with a strong soft excess, and the measured delay is between the hard (1-3 keV) direct continuum and the soft excess (0.5-1 keV), interpreted as the reflection continuum smeared by relativistic effects. There is however an inherent ambiguity in identifying and studying the details of the lines in the soft excess. Here we report the first detection of reverberation in the iron K band in any AGN. Using XMM-Newton observations of NGC 4151, we find delays of the order of 2000 s on time-scales of 10⁵ s between the 5-6 keV band and the 2-3 and 7-8 keV bands, with a broad lag profile resembling a relativistically broadened iron line. The peak of the lag spectra shifts to lower energies at higher frequencies, consistent with the red wing of the line being emitted at smaller radii, as expected from reflection off the inner accretion disc. This is a first detection of a broad iron line using timing studies.

Reference: LISA-EST-RP-1018

L1 Mission Reformulation, NGO

This report summarises the findings of the ESA review on the reformulation of LISA, the Laser Interferometer Space Antenna (L class mission candidate of the Cosmic Vision 2015-2025 programme), into the new mission concept named NGO (New Gravitational wave Observer).

The review, completed at the end of the reformulation exercise, establishes the overall feasibility and credibility of the L1 mission candidate reformulated concept - for both platform and payload - for a launch in 2022, and an ESA cost at completion of 850 MEuro (e.c. 2010).

Reference: SRE-PA/2011/117

L1 Mission Reformulation, ATHENA

This report summarises the findings of the ESA review on the reformulation of IXO, the International X-ray Observatory (L class mission candidate of the Cosmic Vision 2015-2025 programme), into the new mission concept named ATHENA (Advanced Telescope for High Energy Astrophysics).

The review, completed at the end of the reformulation exercise, establishes the overall feasibility and credibility of the L1 mission candidate reformulated concept - for both platform and payload - for a launch in 2022, and an ESA cost at completion of 850 MEuro (e.c. 2010).

In Press

Using the results of a previous X-ray photo-ionization modelling of blue-shifted Fe K absorption lines on a sample of 42 local radio-quiet AGNs observed with XMM-Newton, in this letter we estimate the location and energetics of the associated ultra-fast outflows (UFOs). Due to significant uncertainties, we are essentially able to place only lower/upper limits. On average, their location is in the interval ~0.0003-0.03pc (~10²-10⁴ r_s) from the central black hole, consistent with what is expected for accretion disk winds/outflows. The mass outflow rates are constrained between ~0.01-1 M_Sun/yr, corresponding to >5-10% of the accretion rates. The average lower-upper limits on the mechanical power are log(E_K_dot)~42.6-44.6 erg/s. However, the minimum possible value of the ratio between the mechanical power and bolometric luminosity is constrained to be comparable or higher than the minimum required by simulations of feedback induced by winds/outflows. Therefore, this work demonstrates that UFOs are indeed capable to provide a significant contribution to the AGN cosmological feedback, in agreement with theoretical expectations and the recent observation of interactions between AGN outflows and the interstellar medium in several Seyferts galaxies.

We present Herschel/SPIRE observations for the 2MASS 1207334-393254 (2M1207) system. Based on radiative transfer modelling of near-infrared to submillimetre data, we estimate a disc mass of 3 ± 2 M_Jup and an outer disc radius of 50-100 AU for the 2M1207A disc. The relative disc mass for 2M1207A is similar to the T Tauri star TW Hya, which indicates that massive discs are not underabundant around substellar objects. In probing the various formation mechanisms for this system, we find that core accretion is highly uncertain mainly due to the large separation between the primary and the companion. Disc fragmentation could be a likely scenario based on analytical models, and if the disc initially was more massive than its current estimate. Considering that the TW Hydrae Association (TWA) is sparsely populated, this system could have formed via one of the known binary formation mechanisms (e.g. turbulent fragmentation of a core) and survived disruption at an early stage.

The prospects for continued exploration and discovery in the Outer Planets of the Solar System have never been better. Among others, the Rosetta, Dawn, and New Horizons missions are enroute, the Cassini mission continues its comprehensive exploration of the saturnian system, and the Juno mission is preparing for a 2011 launch to Jupiter. Elsewhere, NASA and ESA are focusing on a flagship-class mission opportunity to Europa and the rest of the Jovian system, followed by a mission back to Titan, and a New Frontiers announcement of opportunity has been released by NASA to solicit missions across a wide spectrum, including missions to the South Pole of the Earth's Moon, Venus, the surface of a comet (and back again), Mars, Mercury, the Trojan or Centaur asteroids in Jupiter's orbit, other asteroids (and back again), Io, or Ganymede. New missions are also under way at ESA.

This issue of Icarus presents papers on the planet Venus based principally on presentations at two international conferences during the summer of 2010. Under the sponsorship of the European Space Agency, the International Venus Conference (Aussois, France, 20-26 June 2010) focused on the results from the Venus Express Mission. Venus Express is expected to continue operations through December 2014 and beyond. The second conference, "Venus Our Closest Earth-like Planet: From Surface to Thermosphere - How does it work?", was sponsored by the Venus Exploration Analysis Group (VEXAG) chartered by NASA in Madison, Wisconsin (29 August-1 September, 2010). The work presented at these conferences illustrates the resurgence in Venus research since the arrival at Venus of the European Space Agency's Venus Express orbiter in April 2006. The issue also includes papers that were inspired by JAXA's launch of Venus Climate Orbiter (also known as Akatsuki) in May 2010.

The papers reflect the international interest in Venus and cover many different aspects of the planet, ranging from interior and surface to the upper atmosphere, with many results focusing on the coupling between different layers.

- The remainder of the abstract is truncated -

We present in situ Cassini Radio Plasma Wave Science observations in the vicinity of Enceladus and in the E ring of Saturn that indicate the presence of dusty plasma. The four flybys of Enceladus in 2008 revealed the following cold plasma characteristics: (1) there is a large plasma density (both ions and electrons) within the Enceladus plume region, (2) no plasma wake effect behind Enceladus was detected, (3) electron densities are generally much lower than the ion densities in the E ring (n_e/n_i < 0.5) as well as in the plume (n_e/n_i < 0.01), and (4) the average bulk ion drift speed is significantly less than the corotation speed and is instead close to the Keplerian speed. These signatures result from half or more of the electrons being attached to dust grains and by the interaction between the surrounding cold plasma and the predominantly negatively charged submicrometer-sized dust grains. The dust and plasma properties estimated from the observations clearly show that the dust-plasma interaction is collective. This strong dust-plasma coupling appears not only in the Enceladus plume but also in the Enceladus torus, typically from about 20 R_E (~5000 km) north and about 60 R_E (~15,000 km) south of Enceladus. We also suggest that the dust-plasma interaction in the E ring is the cause of the planetary spin-modulated dynamics of Saturn's magnetosphere at large.

This double-sided poster summarises ATHENA, a mission devised to reveal the structure of the extreme Universe, from black holes to large-scale structure.

The 1.02 micron wavelength thermal emission of the nightside of Venus is strongly anti-correlated to the elevation of the surface. The VIRTIS instrument on Venus Express has mapped this emission and therefore gives evidence for the orientation of Venus between 2006 and 2008. The Magellan mission provided a global altimetry data set recorded between 1990 and 1992. Comparison of these two data sets reveals a deviation in longitude indicating that the rotation of the planet is not fully described by the orientation model recommended by the IAU. This deviation is sufficiently large to affect estimates of surface emissivity from infrared imaging. A revised period of rotation of Venus of 243.023 ± 0.002 d aligns the two data sets. This period of rotation agrees with pre-Magellan estimates but is significantly different from the commonly accepted value of 243.0185 ± 0.0001 d estimated from Magellan radar images. It is possible that this discrepancy stems from a length of day variation with the value of 243.023 ± 0.002 d representing the average of the rotation period over 16 years.

The plasma density above the Earth's polar caps provide crucial information about the state of the magnetosphere. This region of space is known for its tenuous plasma and extremely low plasma densities, thus making traditional measurements with particle and plasma instruments extremely difficult. A new method based on spacecraft potential measurements from the electric field instrument onboard the Cluster satellites has shown that more reliable density measurements can be obtained. In this paper, we utilize this method and present a survey of the polar cap densities and the response to changes in the solar irradiation, solar wind parameters as well as processes internal to the magnetosphere. Our observations spans a time interval of almost 10 years, thus covering almost a full solar cycle. The observations seem to confirm that solar irradiance, and thus ionization through UV absorption in the atmosphere is the most important mechanism controlling the polar cap cold plasma density. We also find positive correlations between polar cap density and solar wind density and solar wind dynamic pressure, as well as geomagnetic activity levels.

Aims. One of the closest and brightest massive stars, zeta Puppis, was the first early-type object observed by the current generation of X-ray observatories. These observations provided some surprising results, partly confirming the theoretical predictions while simultaneously unveiling some problematic mismatches with expectations. In this series of papers, we perform a thorough study of zeta Puppis in X-rays, using a decade of XMM-Newton observations.

Methods. The star zeta Puppis was observed 18 times by XMM-Newton, totaling 1 Ms in exposure. This provides the highest quality high-resolution X-ray spectrum of a massive star to date, as well as a perfect dataset for studying X-ray variability in an "archetype" object.

Results. This first paper reports on the data reduction of this unique dataset and provides a few preliminary results. On the one hand, analysis of EPIC low-resolution spectra shows the star to have a remarkably stable X-ray emission from one observation to the next. On the other hand, fitting by a wind model of individual line profiles recorded by RGS confirms the wavelength dependence of the line morphology.

Theoretical and numerical modeling of the assembly of dark-matter halos predicts that the most massive and luminous galaxies at high redshift are surrounded by overdensities of fainter companions. We test this prediction with HST observations acquired by our Brightest-of-Reionizing Galaxies (BoRG) survey, which identified four very bright z ~ 8 candidates as Y₀₉₈-dropout sources in four of the 23 non-contiguous Wide Field Camera 3 fields observed. We extend here the search for Y₀₉₈-dropouts to fainter luminosities (M* galaxies with M_AB ~ -20), with detections at e5 sigma confidence (compared to the 8 sigma confidence threshold adopted earlier) identifying 17 new candidates. We demonstrate that there is a correlation between number counts of faint and bright Y₀₉₈-dropouts at > 99.84% confidence. Field BoRG58, which contains the best bright z ~ 8 candidate (M_AB = -21.3), has the most significant overdensity of faint Y₀₉₈-dropouts. Four new sources are located within 70'' (corresponding to 3.1 comoving Mpc at z = 8) from the previously known brighter z ~ 8 candidate. The overdensity of Y₀₉₈-dropouts in this field has a physical origin to very high confidence (p > 99.975%), independent of completeness and contamination rate of the Y₀₉₈-dropout selection. We modeled the overdensity by means of cosmological simulations and estimate that the principal dark-matter halo has mass M_h ~ (4-7)×10¹¹ M_sun (~5 sigma density peak) and is surrounded by several M_h ~ 10¹¹ M_sun halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M_h > 2×10¹⁴ M_sun galaxy cluster by z = 0.

Large expanses of linear dunes cover Titan's equatorial regions. As the Cassini mission continues, more dune fields are becoming unveiled and examined by the microwave radar in all its modes of operation (SAR, radiometry, scatterometry, altimetry) and with an increasing variety of observational geometries. In this paper, we report on Cassini's radar instrument observations of the dune fields mapped through May 2009 and present our key findings in terms of Titan's geology and climate. We estimate that dune fields cover ~12.5% of Titan's surface, which corresponds to an area of ~10 million km², roughly the area of the United States. If dune sand-sized particles are mainly composed of solid organics as suggested by VIMS observations (Cassini Visual and Infrared Mapping Spectrometer) and atmospheric modeling and supported by radiometry data, dune fields are the largest known organic reservoir on Titan. Dune regions are, with the exception of the polar lakes and seas, the least reflective and most emissive features on this moon. Interestingly, we also find a latitudinal dependence in the dune field microwave properties: up to a latitude of ~11°, dune fields tend to become less emissive and brighter as one moves northward. Above ~11° this trend is reversed. The microwave signatures of the dune regions are thought to be primarily controlled by the interdune proportion (relative to that of the dune), roughness and degree of sand cover. In agreement with radiometry and scatterometry observations, SAR images suggest that the fraction of interdunes increases northward up to a latitude of ~14°.

Dune fields dominate ~13% of Titan's surface and represent an important sink of carbon in the methane cycle. Herein, we discuss correlations in dune morphometry with altitude and latitude. These correlations, which have important implications in terms of geological processes and climate on Titan, are investigated through the microwave electromagnetic signatures of dune fields using Cassini radar and radiometry observations. The backscatter and emissivity from Titan's dune terrains are primarily controlled by the amount of interdune area within the radar footprint and are also expected to vary with the degree of the interdunal sand cover. Using SAR-derived topography, we find that Titan's main dune fields (Shangri-La, Fensal, Belet and Aztlan) tend to occupy the lowest elevation areas in Equatorial regions occurring at mean elevations between ~-400 and ~0 m (relative to the geoid). In elevated dune terrains, we show a definite trend towards a smaller dune to interdune ratio and possibly a thinner sand cover in the interdune areas. A similar correlation is observed with latitude, suggesting that the quantity of windblown sand in the dune fields tends to decrease as one moves farther north. The altitudinal trend among Titan's sand seas is consistent with the idea that sediment source zones most probably occur in lowlands, which would reduce the sand supply toward elevated regions. The latitudinal preference could result from a gradual increase in dampness with latitude due to the asymmetric seasonal forcing associated with Titan's current orbital configuration unless it is indicative of a latitudinal preference in the sand source distribution or wind transport capacity.

A number of observations suggest that an extended ocean once covered a significant part of the Martian northern hemisphere. By probing the physical properties of the subsurface to unprecedented depth, the MARSIS/Mars Express provides new geophysical evidences for the former existence of a Late Hesperian ocean. The Vastitas Borealis formation, located inside a putative shoreline of the ancient ocean, has a low dielectric constant compared with that of typical volcanic materials. We show that the measured value is only consistent with low-density sedimentary deposits, massive deposits of ground-ice, or a combination of the two. In contrast, radar observations indicate a distribution of shallow ground ice in equilibrium with the atmosphere in the south polar region. We conclude that the northern plains are filled with remnants of a late Hesperian ocean, fed by water and sediments from the outflow channels about 3 Gy ago.

Made available online 29 November 2011, before print publication

We report on a new Be/X-ray pulsar binary located in the Wing of the Small Magellanic Cloud (SMC). The strong pulsed X-ray source was discovered with the Chandra and XMM-Newton X-ray observatories. The X-ray pulse period of 1062 s is consistently determined from both Chandra and XMM-Newton observations, revealing one of the slowest rotating X-ray pulsars known in the SMC. The optical counterpart of the X-ray source is the emission-line star 2dFS 3831. Its B0-0.5(III)e+ spectral type is determined from VLT-FLAMES and 2dF optical spectroscopy, establishing the system as a Be/X-ray binary (Be-XRB). The hard X-ray spectrum is well fitted by a power law with additional thermal and blackbody components, the latter reminiscent of persistent Be-XRBs. This system is the first evidence of a recent supernova in the low-density surroundings of NGC 602. We detect a shell nebula around 2dFS 3831 in Ha and [O III] images and conclude that it is most likely a supernova remnant. If it is linked to the supernova explosion that created this new X-ray pulsar, its kinematic age of (2-4) × 10⁴ yr provides a constraint on the age of the pulsar.