Showing papers by "Ralf Srama published in 2015"
TL;DR: Analysis of silicon-rich, nanometre-sized dust particles (so-called stream particles) that stand out from the water-ice-dominated objects characteristic of Saturn indicate ongoing high-temperature (>90 °C) hydrothermal reactions associated with global-scale geothermal activity that quickly transports hydroThermal products from the ocean floor at a depth of at least 40 kilometres up to the plume of Enceladus.
Abstract: Analysis of silicon-rich, nanometre-sized dust particles near Saturn shows them to consist of silica, which was initially embedded in icy grains emitted from Enceladus’ subsurface waters and released by sputter erosion in Saturn’s E ring; their properties indicate their ongoing formation and transport by high-temperature hydrothermal reactions from the ocean floor and up into the plume of Enceladus. Hsiang-Wen Hsu et al. have analysed the silicon-rich, nanometre-sized dust stream particles in the Saturnian system using the Cosmic Dust Analyser (CDA) onboard the Cassini spacecraft. With the help of experiments and modelling, the particles are interpreted as silica grains that were initially embedded in the icy plume emitted from subsurface waters on Enceladus and released by sputter erosion in Saturn's E ring. Their properties indicate their formation and transport by high-temperature hydrothermal reactions from the ocean floor and up into the plume of Enceladus. Detection of sodium-salt-rich ice grains emitted from the plume of the Saturnian moon Enceladus suggests that the grains formed as frozen droplets from a liquid water reservoir that is, or has been, in contact with rock1,2. Gravitational field measurements suggest a regional south polar subsurface ocean of about 10 kilometres thickness located beneath an ice crust 30 to 40 kilometres thick3. These findings imply rock–water interactions in regions surrounding the core of Enceladus. The resulting chemical ‘footprints’ are expected to be preserved in the liquid and subsequently transported upwards to the near-surface plume sources, where they eventually would be ejected and could be measured by a spacecraft4. Here we report an analysis of silicon-rich, nanometre-sized dust particles5,6,7,8 (so-called stream particles) that stand out from the water-ice-dominated objects characteristic of Saturn. We interpret these grains as nanometre-sized SiO2 (silica) particles, initially embedded in icy grains emitted from Enceladus’ subsurface waters and released by sputter erosion in Saturn’s E ring. The composition and the limited size range (2 to 8 nanometres in radius) of stream particles indicate ongoing high-temperature (>90 °C) hydrothermal reactions associated with global-scale geothermal activity that quickly transports hydrothermal products from the ocean floor at a depth of at least 40 kilometres up to the plume of Enceladus.
394 citations
TL;DR: Observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds are reported.
Abstract: Observations are reported of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. Before its planned demise on lunar impact in April 2014, NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) spent some seven months orbiting the Moon's equator, collecting dust particles for spectroscopic analysis. Sketches made by the Apollo 17 astronauts famously showed a lunar horizon glow, triggering suggestions that electrostatic lofting might be generating dense clouds of small dust particles high above the lunar surface. In this first report on the observations made by the Lunar Dust Experiment (LDEX) onboard LADEE, Mihaly Horanyi et al. find no evidence for such clouds. However, they have detected a permanent asymmetric dust cloud around the Moon, supplied by secondary ejecta dust particles produced by the continual surface impacts of high-speed cometary dust particles in eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths and striking the Moon at lower speeds. The lunar surface is exposed to the same stream of interplanetary dust particles as the Earth, and the LDEX data show that the density of the lunar ejecta cloud increases during meteor showers such as the Geminids. Interplanetary dust particles hit the surfaces of airless bodies in the Solar System, generating charged1 and neutral2 gas clouds, as well as secondary ejecta dust particles3. Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter4 and Saturn5, but have hitherto not been observed near bodies with refractory regolith surfaces. High-altitude Apollo 15 and 17 observations of a ‘horizon glow’ indicated a putative population of high-density small dust particles near the lunar terminators6,7, although later orbital observations8,9 yielded upper limits on the abundance of such particles that were a factor of about 104 lower than that necessary to produce the Apollo results. Here we report observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. The density of the lunar ejecta cloud increases during the annual meteor showers, especially the Geminids, because the lunar surface is exposed to the same stream of interplanetary dust particles. We expect all airless planetary objects to be immersed in similar tenuous clouds of dust.
179 citations
01 Jan 2015
TL;DR: The Lunar Dust Experiment (LDEX) is an in situ dust detector onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission as mentioned in this paper, which is designed to characterize the variability of the dust in the lunar exosphere by mapping the size and spatial distributions of dust grains in a lunar environment as a function of local time and the position of the Moon with respect to the magnetosphere of the Earth.
Abstract: The Lunar Dust Experiment (LDEX) is an in situ dust detector onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission. It is designed to characterize the variability of the dust in the lunar exosphere by mapping the size and spatial distributions of dust grains in the lunar environment as a function of local time and the position of the Moon with respect to the magnetosphere of the Earth. LDEX gauged the relative contributions of the two competing dust sources: (a) ejecta production due to the continual bombardment of the Moon by interplanetary micrometeoroids, and (b) lofting of small grains from the lunar surface due to plasma-induced near-surface electric fields.
32 citations
TL;DR: It is shown that charge separation naturally occurs if water droplets/clusters or ice particles with embedded charge carriers encounter a high energy impact with subsequent dispersion - even if the involved kinetic energy is significantly below the molecular ionization energy.
Abstract: Charge separation in condensed matter after strong impacts is a general and intriguing phenomenon in nature, which is often identified and described but not necessarily well understood in terms of a quantitative mechanistic picture. Here we show that charge separation naturally occurs if water droplets/clusters or ice particles with embedded charge carriers, e.g., ions, encounter a high energy impact with subsequent dispersion – even if the involved kinetic energy is significantly below the molecular ionization energy. We find that for low charge carrier concentrations (c < 0.01 mol L−1) a simple statistical Poisson model describes the charge distribution in the resulting molecular “fragments” or aggregates. At higher concentrations Coulomb interactions between the charge carriers become relevant, which we describe by a Monte Carlo approach. Our models are compared to experimental data for strong (laser) impacts on liquid micro beams and discussed for the charge generation in cluster-impact mass spectrometry on cosmic dust detectors where particle kinetic energies are below the plasma threshold. Taken together, a simple and intuitive but quantitative microscopic model is obtained, which may contribute to the understanding of a larger range of phenomena related to charge generation and separation in nature.
27 citations
TL;DR: In this article, a new model of the dust streams of comet 67P/Churyumov-Gerasimenko was developed using the Interplanetary Meteoroid Environment for Exploration (IMEX).
Abstract: Context. Here we describe a new model of the dust streams of comet 67P/Churyumov-Gerasimenko that has been developed using the Interplanetary Meteoroid Environment for Exploration (IMEX). This is a new universal model for recently created cometary meteoroid streams in the inner solar system.Aims. The model can be used to investigate characteristics of cometary trails: here we describe the model and apply it to the trail of comet 67P/Churyumov-Gerasimenko to develop our understanding of the trail and assess the reliability of the model.Methods. Our IMEX model provides trajectories for a large number of dust particles released from ~400 short-period comets. We use this to generate optical depth profiles of the dust trail of comet 67P/Churyumov-Gerasimenko and compare these to Spitzer observations of the trail of this comet from 2004 and 2006.Results. We find that our model can match the observed trails if we use very low ejection velocities, a differential size distribution index of α ≈ −3.7, and a dust production rate of 300–500 kg s-1 at perihelion. The trail is dominated by mm-sized particles and can contain a large proportion of dust produced before the most recent apparition. We demonstrate the strength of IMEX in providing time-resolved histories of meteoroid streams. We find that the passage of Mars through the stream in 2062 creates visible gaps. This indicates the utility of this model in providing insight into the dynamical evolution of streams and trails, as well as impact hazard assessment for spacecraft on interplanetary missions.
22 citations
TL;DR: In this paper, the authors used AUTODYN15.0/2D software to simulate the velocity and angular distributions of ejecta created by the primary impacts of interplanetary dust particles.
6 citations
TL;DR: In this paper, a new design of Lunar Dust eXplorer (LDX) is presented, which is well suited for lunar or asteroid landers with a broad range of particle charges and sizes.
5 citations
07 Mar 2015
TL;DR: The HyperDust instrument as discussed by the authors is a combination of a Dust Trajectory Sensor (DTS) with an in-situ chemical analyzer, which can measure dust particles as small as 3 µm in radius and dust speeds up to 100 km/s.
Abstract: The goal of dust astronomy is to uncover the information contained in individual cosmic dust grains A series of previous dust instruments lead to novel in-situ instrumentation suitable for determining the origin and chemical and elemental composition of dust particles A new instrument was developed that combines large target area, high mass-resolution, and accurate trajectory determination The Hyperdust instrument is a combination of a Dust Trajectory Sensor (DTS) with an in-situ chemical analyzer Dust particles' trajectories are determined by the measurement of induced charge signals, when a charged grain flies through a position-sensitive electrode system A modern DTS can measure dust particles as small as 03 µm in radius and dust speeds up to 100 km/s The chemical analyzer with ∼01 m2 sensitive target area has a mass resolution > 200 The Hyperdust instrument is capable of distinguishing interstellar and interplanetary grains based on their trajectory information The Hyperdust instrument is currently being developed to Technical Readiness Level (TRL) 6 funded by NASA's MatISSE program to be a low-mass, high performance instrument for future in-situ exploration This paper describes its current state of development
4 citations
TL;DR: In this article, the impact rates of dust particles recorded by the Cosmic Dust Analyzer (CDA) aboard the Cassini spacecraft were analyzed and the authors found that the dust density peaks at two times, at least, in a void region between Titan and Rhea.
2 citations
01 Mar 2015
TL;DR: Montag et al. as discussed by the authors proposed an approach for space physics and applied it to the field of astronomy and space physics at the University of Stuttgart in Germany, Germany.
Abstract: IMPACT TESTS. C. Montag , R. Laufer, R. Srama, G. Herdrich, O. Przybilski, T. W. Hyde, Institute of Aerospace Engineering, Technical University of Dresden, Marschner Strasse 32, 01062 Dresden, Germany, Email: christoph_montag@yahoo.de, Institute of Space Systems (IRS), University of Stuttgart, Pfaffenwaldring 29, 70569 Stuttgart, Germany, Center for Astrophysics, Space Physics & Engineering Research (CASPER), Baylor University, One Bear Place 97310, Waco, TX-76798-7310, USA.
14 Dec 2015
01 Apr 2015
TL;DR: In this article, the Cosmic Dust Analyzer (CDA) onboard the Cassini spacecraft is sampling these icy particles and producing TOF mass spectra of cations of impinging particles.
Abstract: The major source of ice particles in Saturn’s E-ring is Enceladus – a geological active moon of Saturn. Enceladus is emanating ice particles from its fractured south polar terrain (SPT), the so-called “Tiger Stripes”. The source of Enceladus activity and many of the ice particles is a subsurface ocean. The Cosmic Dust Analyzer (CDA) onboard the Cassini spacecraft is sampling these icy particles and producing TOF mass spectra of cations of impinging particles [1]. Three compositional types of ice particles have been identified from CDA-mass spectra: (i) pure water ice (Type-1) (ii) organic rich (Type-2) (iii) salt rich (Type-3) [2][3]. These organic rich (Type-2) spectra are particularly abundant in the icy jets of Enceladus as we found out during the Cassini’s Enceladus flybys (E17 and E18) in 2012 [4].
01 Apr 2015
TL;DR: In this paper, the footprints of Rhea's surface in the composition of E ring using Cosmic Dust Analyzer (CDA) during the closest approach of Cassini's Rhea flyby were identified.
Abstract: The Cassini spacecraft was launched in 2004 towards the Saturnian system to address major scientific questions about the planet, its magnetosphere, rings and icy moons. We have performed compositional mapping of Saturn’s E-ring during the Cassini’s flyby (R4) of Rhea, the second largest moon of Saturn, on 9th March 2013. The icy or rocky dust particles from the surface of moons without atmosphere are ejected from their surfaces by meteoroid bombardment. The ejected particles from the moon’s surface can be detected during a spacecraft flyby. In our campaign we try to identify the footprints of Rhea’s surface in the composition of E ring using Cosmic Dust Analyzer (CDA) during the closest approach of Cassini’s Rhea flyby. The flyby speed was 9.3km/s and the closest approach was at 997km from Rhea’s surface.