arXiv subjects

Peter Wurz

Publications and source records attributed to Peter Wurz.

At least 37 records · Page 2Linked to original sources

arXiv2019

Velocity distribution function of Na released by photons from planetary surfaces

In most surface-bound exospheres Na has been observed at altitudes above what is possible by thermal release. Photon stimulated desorption of adsorbed Na on solid surfaces has been commonly used to explain observations at high altitudes. We investigate three model velocity distribution functions (VDF) that have been previously used in several studies to describe the desorption of atoms from a solid surface either by electron or by photon bombardment, namely: the Maxwell-Boltzmann (M-B) distribution, the empirical distribution proposed by [Wurz et al., 2010] for PSD, and the Weibull distribution. We use all available measurements reported by [Yakshinskiy and Madey, 2000, 2004] to test these distributions and determine which one fits best (statistically) and we discuss their physical validity. Our results show that the measured VDF of released Na atoms are too narrow compared to Maxwell-Boltzmann fits with supra-temperatures as suggested by [3]. We found that a good fit with M-B is only achieved with a speed offset of the whole distribution to higher speeds and a lower temperature, with the offset and the fit temperature not showing any correlation with the surface temperature. From the three distributions we studied, we find that the Weibull distribution provides the best fits using the temperature of the surface, though an offset towards higher speeds is required. This work confirms that Electron-Stimulated Desorption (ESD) and Photon-Stimulated Desorption (PSD) should produce non-thermal velocity (or energy) distributions of the atoms released via these processes, which is expected from surface physics. We recommend to use the Weibull distribution with the shape parameter kappa=1.7, the speed offset v0=575 m/s, and the surface temperature to model PSD distributions at planetary bodies.

astro-ph.EP↗

arXiv2019

Three-Dimensional Modeling of Callisto's Surface Sputtered Exosphere Environment

We study the release of various elements from Callisto's surface into its exosphere by plasma sputtering. The cold Jovian plasma is simulated with a 3D plasma-planetary interaction hybrid model, which produces 2D surface precipitation maps for magnetospheric H+ , O+ , O++ , and S++ . For the hot Jovian plasma, we assume isotropic precipitation onto the complete spherical surface. Two scenarios are investigated: One where no ionospheric shielding takes place and accordingly full plasma penetration is implemented ('no ionosphere' scenario), and one where an ionosphere lets virtually none of the cold plasma but all of the hot plasma reach Callisto's surface ('ionosphere' scenario). In the 3D exosphere model, neutral particles are sputtered from the surface and followed on their individual trajectories. The 3D density profiles show that whereas in the 'no ionosphere' scenario the ram direction is favored, the 'ionosphere' scenario produces almost uniform density profiles. In addition, the density profiles in the 'ionosphere' scenario are reduced by a factor of ~2.5 with respect to the 'no ionosphere' scenario. We find that the Neutral gas and Ion Mass spectrometer, which is part of the Particle Environment Package on board the JUICE mission, will be able to detect the different sputter populations from Callisto's icy surface and the major sputter populations from Callisto's non-icy surface. The chemical composition of Callisto's exosphere can be directly linked to the chemical composition of its surface, and will offer us information not only on Callisto's formation scenario but also on the building blocks of the Jupiter system.

astro-ph.EP↗

arXiv2019

Elemental and molecular abundances in comet 67P/Churyumov-Gerasimenko

Comets are considered to be some of the most pristine and unprocessed solar system objects accessible to in-situ exploration. Investigating their molecular and elemental composition takes us on a journey back to the early period of our solar system and possibly even further. In this work, we deduce the bulk abundances of the major volatile species in comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency's Rosetta mission. The basis are measurements obtained with the ROSINA instrument suite on board the Rosetta orbiter during a suitable period of high outgassing near perihelion. The results are combined with both gas and dust composition measurements published in the literature. This provides an integrated inventory of the major elements present in the nucleus of 67P/Churyumov-Gerasimenko. Similar to comet 1P/Halley, which was visited by ESA's Giotto spacecraft in 1986, comet 67P/Churyumov-Gerasimenko also shows near-solar abundances of oxygen and carbon, whereas hydrogen and nitrogen are depleted compared to solar. Still, the degree of devolatilization is lower than that of inner solar system objects, including meteorites and the Earth. This supports the idea that comets are among the most pristine objects in our solar system.

astro-ph.EP↗

arXiv2019

In situ Exploration of the Giant Planets

Remote sensing observations suffer significant limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. This impacts our knowledge of the formation of these planets and the physics of their atmospheres. A remarkable example of the superiority of in situ probe measurements was illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases' abundances and the precise measurement of the helium mixing ratio were only made available through in situ measurements by the Galileo probe. Here we describe the main scientific goals to be addressed by the future in situ exploration of Saturn, Uranus, and Neptune, placing the Galileo probe exploration of Jupiter in a broader context. An atmospheric entry probe targeting the 10-bar level would yield insight into two broad themes: i) the formation history of the giant planets and that of the Solar System, and ii) the processes at play in planetary atmospheres. The probe would descend under parachute to measure composition, structure, and dynamics, with data returned to Earth using a Carrier Relay Spacecraft as a relay station. An atmospheric probe could represent a significant ESA contribution to a future NASA New Frontiers or flagship mission to be launched toward Saturn, Uranus, and/or Neptune.

astro-ph.EP↗

arXiv2019

A comparison between the two lobes of comet 67P/Churyumov-Gerasimenko based on D/H ratios in H2O measured with the Rosetta/ROSINA DFMS

The nucleus of the Jupiter-family comet 67P/Churyumov-Gerasimenko was discovered to be bi-lobate in shape when the European Space Agency spacecraft Rosetta first approached it in July 2014. The bi-lobate structure of the cometary nucleus has led to much discussion regarding the possible manner of its formation and on how the composition of each lobe might compare with that of the other. During its two-year-long mission from 2014 to 2016, Rosetta remained in close proximity to 67P/Churyumov-Gerasimenko, studying its coma and nucleus in situ. Based on lobe-specific measurements of HDO and H2O performed with the ROSINA DFMS mass spectrometer on board Rosetta, the Deuterium-to-Hydrogen ratios in water from the two lobes could be compared. No appreciable difference was observed, suggesting that both lobes formed in the same region and are homogeneous in their Deuterium-to-Hydrogen ratios.

astro-ph.EP↗

arXiv2019

The $^{16}$O/$^{18}$O ratio in Water in the Coma of Comet 67P / Churyumov-Gerasimenko measured with the Rosetta / ROSINA Double-Focusing Mass Spectrometer

The European Space Agency (ESA) spacecraft Rosetta accompanied the Jupiter-family comet (JFC) 67P / Churyumov-Gerasimenko for over two years along its trajectory through the inner solar system. Between 2014 and 2016, it performed almost continuous in-situ measurements of the comet's gaseous atmosphere in close proximity to its nucleus. In this study, the $^{16}$O/$^{18}$O ratio of H$_2$O in the coma of 67P / Churyumov-Gerasimenko, as measured by the ROSINA DFMS mass spectrometer on board Rosetta, was determined from the ratio of H${_2}^{16}$O / H${_2}^{18}$O and $^{16}$OH / $^{18}$OH. The value of 445 $\pm$ 35 represents an $\sim$ 11% enrichment of $^{18}$O compared with the terrestrial ratio of 498.7 $\pm$ 0.1. This cometary value is consistent with the comet containing primordial water, in accordance with leading self-shielding models. These models predict primordial water to be between 5% to 20% enriched in heavier oxygen isotopes compared to terrestrial water. Addendum: The $^{16}$O/$^{17}$O ratio of H$_2$O in the coma of 67P / Churyumov-Gerasimenko.

astro-ph.EP↗

arXiv2018

Krypton isotopes and noble gas abundances in the coma of comet 67P/Churyumov-Gerasimenko

The ROSINA mass spectrometer DFMS on board ESA's Rosetta spacecraft detected the major isotopes of the noble gases argon, krypton, and xenon in the coma of comet 67P/Churyumov-Gerasimenko. Earlier, it has been shown that xenon exhibits an isotopic composition distinct from anywhere else in the solar system. However, argon isotopes, within error, were shown to be consistent with solar isotope abundances. This discrepancy suggested an additional exotic component of xenon in comet 67P/Churyumov-Gerasimenko. Here we show that also krypton exhibits an isotopic composition close to solar. Furthermore, we found a depletion compared to solar of argon with respect to krypton and of krypton with respect to xenon, which is a necessity to postulate an addition of exotic xenon in the comet.

astro-ph.EP↗

arXiv2018

First experimental data of sulphur ions sputtering water ice

This paper presents the first experimental sputtering yields for sulphur ions with energies between 10 keV and 140 keV irradiating water ice films on a microbalance. The measured sputtering yields exceed theoretical predictions based on other ion species by a factor of 2 to 3 for most energies. Moreover, the sputtering yield of SF+ molecules is compared to the yield of atomic species S+ and F+. As found for atomic versus molecular oxygen, the sputtering yield caused by molecules is two times higher than expected. Finally, the implications of the enhanced sulphur sputtering yield for Europa's atmosphere are discussed.

astro-ph.IM↗

arXiv2018

Interstellar neutral helium in the heliosphere from IBEX observations. V. Observations in IBEX-Lo ESA steps 1, 2, & 3

Direct-sampling observations of interstellar neutral (ISN) He by Interstellar Boundary Explorer (IBEX) provide valuable insight into the physical state of and processes operating in the interstellar medium ahead of the heliosphere. The ISN He atom signals are observed at the four lowest ESA steps of the IBEX-Lo sensor. The observed signal is a mixture of the primary and secondary components of ISN He and H. Previously, only data from one of the ESA steps have been used. Here, we extended the analysis to data collected in the three lowest ESA steps with the strongest ISN He signal, for the observation seasons 2009-2015. The instrument sensitivity is modeled as a linear function of the atom impact speed onto the sensor's conversion surface separately for each ESA step of the instrument. We found that the sensitivity increases from lower to higher ESA steps, but within each of the ESA steps it is a decreasing function of the atom impact speed. This result may be influenced by the hydrogen contribution, which was not included in the adopted model, but seems to exist in the signal. We conclude that the currently accepted temperature of ISN He and velocity of the Sun through the interstellar medium do not need a revision, and we sketch a plan of further data analysis aiming at investigating ISN H and a better understanding of the population of ISN He originating in the outer heliosheath.

astro-ph.SR↗

arXiv2017

On the in-situ detectability of Europa's water vapour plumes from a flyby mission

We investigate the feasibility of detecting water molecules (H2O) and water ions (H20+) from the Europa plumes from a flyby mission. A Monte Carlo particle tracing method is used to simulate the trajectories of neutral particles under the influence of Europa's gravity field and ionized particles under the influence of Jupiter's magnetic field and the convectional electric field. As an example mission case we investigate the detection of neutral and ionized molecules using the Particle Environment Package (PEP), which is part of the scientific payload of the future JUpiter ICy moon Explorer mission (JUICE). We consider plumes that have a mass flux that is three orders of magnitude lower than what has been inferred from recent Hubble observations (Roth et al., 2014a). We demonstrate that the in-situ detection of H2O and H2O+ from these low mass flux plumes is possible by the instruments with large margins with respect to background and instrument noise. The signal to noise ratio for neutrals is up to ~5700 and ~33 for ions. We also show that the geometry of the plume source, either a point source or 1000 km-long crack, does not influence the density distributions, and thus, their detectability. Furthermore, we discuss how to separate the plume-originating H2O and H2O+ from exospheric H2O and H2O+ . The separation depends strongly on knowledge of the density distribution of Europa's exosphere.

astro-ph.EP↗

arXiv2016

Characteristics of proton velocity distribution functions in the near-lunar wake from Chandrayaan-1/SWIM observations

Due to the high absorption of solar wind plasma on the lunar dayside, a large scale wake structure is formed downstream of the Moon. However, recent in-situ observations have revealed the presence of protons in the near-lunar wake (100 km to 200 km from the surface). The solar wind, either directly or after interaction with the lunar surface (including magnetic anomalies), is the source of these protons in the near-wake region. Using the entire data from the SWIM sensor of the SARA experiment onboard Chandrayaan-1, we analysed the velocity distribution of the protons observed in the near-lunar wake. The average velocity distribution functions, computed in the solar wind rest frame, were further separated based on the angle between the upstream solar wind velocity and the IMF. Several proton populations were identified from the velocity distribution and their possible entry mechanism were inferred based on the characteristics of the velocity distribution. These entry mechanisms include (i) diffusion of solar wind protons into the wake along IMF, (ii) the solar wind protons with finite gyro-radii that are aided by the wake boundary electric field, (iii) solar wind protons with gyro-radii larger than lunar radii from the tail of the solar wind velocity distribution, and (iv) scattering of solar wind protons from the dayside lunar surface or from magnetic anomalies. In order to gain more insight into the entry mechanisms associated with different populations, backtracing is carried out for each of these populations. For most of the populations, the source of the protons obtained from backtracing is found to be in agreement with that inferred from the velocity distribution. There are few populations that could not be explained by the known mechanisms and remain unknown.

physics.space-ph↗

arXiv2015

A new view on the solar wind interaction with the Moon

Characterised by a surface bound exosphere and localised crustal magnetic fields, the Moon was considered as a passive object when solar wind interacts with it. However, the neutral particle and plasma measurements around the Moon by recent dedicated lunar missions, such as Chandrayaan-1, Kaguya, Chang'E-1, LRO, and ARTEMIS, as well as IBEX have revealed a variety of phenomena around the Moon which results from the interaction with solar wind, such as backscattering of solar wind protons as energetic neutral atoms (ENA) from lunar surface, sputtering of atoms from the lunar surface, formation of a "mini-magnetosphere" around lunar magnetic anomaly regions, as well as several plasma populations around the Moon, including solar wind protons scattered from the lunar surface, from the magnetic anomalies, pick-up ions, protons in lunar wake and more. This paper provides a review of these recent findings and presents the interaction of solar wind with the Moon in a new perspective.

physics.space-ph↗

arXiv2013

Local Interstellar Hydrogen's Disappearance at 1 Au: Four Years of IBEX in the Rising Solar Cycle

NASA's Interstellar Boundary Explorer (IBEX) mission has recently opened a new window on the interstellar medium (ISM) by imaging neutral atoms. One "bright" feature in the sky is the interstellar wind flowing into the solar system. Composed of remnants of stellar explosions as well as primordial gas and plasma, the ISM is by no means uniform. The interaction of the local ISM with the solar wind shapes our heliospheric environment with hydrogen being the dominant component of the very local ISM. In this paper, we report on direct sampling of the neutral hydrogen of the local ISM over four years of IBEX observations. The hydrogen wind observed at 1 AU has decreased and nearly disappeared as the solar activity has increased over the last four years; the signal at 1 AU has dropped off in 2012 by a factor of ~8 to near background levels. The longitudinal offset has also increased with time presumably due to greater radiation pressure deflecting the interstellar wind. We present longitudinal and latitudinal arrival direction measurements of the bulk flow as measured over four years beginning at near solar minimum conditions. The H distribution we observe at 1 AU is expected to be different from that outside the heliopause due to ionization, photon pressure, gravity, and filtration by interactions with heliospheric plasma populations. These observations provide an important benchmark for modeling of the global heliospheric interaction. Based on these observations we suggest a further course of scientific action to observe neutral hydrogen over a full solar cycle with IBEX.

astro-ph.SR↗

arXiv2012

Local Interstellar Neutral Hydrogen sampled in-situ by IBEX

Hydrogen gas is the dominant component of the local interstellar medium. However, due to ionization and interaction with the heliosphere, direct sampling of neutral hydrogen in the inner heliosphere is more difficult than sampling the local interstellar neutral helium, which penetrates deep into the heliosphere. In this paper we report on the first detailed analysis of the direct sampling of neutral hydrogen from the local interstellar medium. We confirm that the arrival direction of hydrogen is offset from that of the local Helium component. We further report the discovery of a variation of the penetrating Hydrogen over the first two years of IBEX observations. Observations are consistent with hydrogen experiencing an effective ratio of outward solar radiation pressure to inward gravitational force greater than unity (μ>1); the temporal change observed in the local interstellar hydrogen flux can be explained with solar variability.

astro-ph.SR↗

arXiv2010

Extremely high reflection of solar wind protons as neutral hydrogen atoms from regolith in space

We report on measurements of extremely high reflection rates of solar wind particles from regolith-covered lunar surfaces. Measurements by the Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft in orbit around the Moon show that up to 20% of the impinging solar wind protons are reflected from the lunar surface back to space as neutral hydrogen atoms. This finding, generally applicable to regolith-covered atmosphereless bodies, invalidates the widely accepted assumption that regolith almost completely absorbs the impinging solar wind.

astro-ph.EP↗

arXiv2010

Studying the Lunar-Solar Wind Interaction with the SARA Experiment aboard the Indian Lunar Mission Chandrayaan-1

The first Indian lunar mission Chandrayaan-1 was launched on 22 October 2008. The Sub-keV Atom Reflecting Analyzer (SARA) instrument onboard Chandrayaan-1 consists of an energetic neutral atom (ENA) imaging mass analyzer called CENA (Chandrayaan-1 Energetic Neutrals Analyzer), and an ion-mass analyzer called SWIM (Solar wind Monitor). CENA performed the first ever experiment to study the solar wind-planetary surface interaction via detection of sputtered neutral atoms and neutralized backscattered solar wind protons in the energy range ~0.01-3.0 keV. SWIM measures solar wind ions, magnetosheath and magnetotail ions, as well as ions scattered from lunar surface in the ~0.01-15 keV energy range. The neutral atom sensor uses conversion of the incoming neutrals to positive ions, which are then analyzed via surface interaction technique. The ion mass analyzer is based on similar principle. This paper presents the SARA instrument and the first results obtained by the SWIM and CENA sensors. SARA observations suggest that about 20% of the incident solar wind protons are backscattered as neutral hydrogen and ~1% as protons from the lunar surface. These findings have important implications for other airless bodies in the solar system.

astro-ph.EP↗

arXiv2010

The Sub-keV Atom Reflecting Analyzer (SARA) Experiment Aboard Chandrayaan-1 Mission: Instrument and Observations

SARA experiment aboard the first Indian lunar mission Chandrayaan-1 had the objective to explore the solar wind-lunar interaction using energetic neutral atoms (ENA) from the lunar surface as diagnostic tool. SARA consisted of an ENA imaging mass analyzer CENA (Chandrayaan-1 Energetic Neutral Analyzer) and an ion mass analyser SWIM (Solar Wind Monitor), along with a digital processing unit (DPU) which commands and controls the sensors and provides the interface to the spacecraft. Both sensors have provided excellent observational data. CENA has observed ENAs from the lunar surface and found that ~20% of the incident solar wind ions get backscattered as ENAs from the lunar surface. This is contrary to the previous assumptions of almost complete absorption of solar wind by the lunar surface. The observation is relevant for other airless bodies in the solar system.

astro-ph.EP↗

arXiv2010

First observation of a mini-magnetosphere above a lunar magnetic anomaly using energetic neutral atoms

The Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft has produced for the first time an image of a lunar magnetic anomaly in backscattered hydrogen atoms. The image shows that a partial void of the solar wind, a mini-magnetosphere, is formed above the strong magnetic anomaly near the Crisium antipode. The mini-magnetosphere is 360 km across at the surface and is surrounded by a 300-km-thick region of enhanced plasma flux that results from the solar wind flowing around the mini-magnetosphere. The mini-magnetosphere is visible only in hydrogen atoms with energy exceeding 150 eV. Fluxes with energies below 100 eV do not show corresponding spatial variations. While the high-energy atoms result from the backscattering process, the origin of the low-energy component is puzzling. These observations reveal a new class of objects, mini-magnetospheres, and demonstrate a new observational technique to study airless bodies, imaging in backscattered neutral atoms.

astro-ph.EP↗