Self-consistent modelling of Mercury’s surface composition and exosphere by solar wind sputtering

A Monte-Carlo model of exospheres was extended by treating the solar wind ion induced sputtering process, quantitatively in a self-consistent way starting with the actual release of particles from the mineral surface of Mercury. Mercury is a body without a significant atmosphere, thus, the surface is effected by different processes that are mainly related to the radiation and plasma environment of the Sun and to micrometeorites, which are delivered to Mercury’s surface. In such a case it can be assumed that the composition of Mercury’s thin collisionless atmosphere, the exosphere, is related to the composition of the planetary crustal materials. If so, then inferences regarding the bulk chemistry of the planet can be made from a study of atoms and molecules in the exosphere after they are released from the mineral surface by a variety of release processes. One difficult challenge is the identification of the main source of some elements like H, He, Na or K. Generally it is believed that H and He come primarily from the solar wind, while Na and K originate from volatilized materials partitioned between Mercury’s crust and impacts from meteorites. Besides the before mentioned elements corresponding to spectroscopic observations and experiments with soil analogues, other elements such as O, Na, Mg, Al, Si, P, S, K, Ca, Ti, Cr, Fe, Ni, Zn, OH should also be related with Mercury’s surface soils (Wurz et al., 2010, and references therein). Based on available observational data and literature data we established a global model for the surface mineralogy of Mercury and from that derived the average elemental composition of the surface. Compositional data analysis has been employed for Mercury’s surface minerals recently by (Sprague et al., 2009). In these cases the applied method was based on simple correlation methods, which do not exploit the full potential of the available data. In addition, the closed nature of compositional data, i.e., the assumption that component concentrations have to sum up to 100% in an analysis, bears important implications for the statistical analysis of compositional data, which do not seem to have been sufficiently appreciated until now. To investigate the default of the classical additive analysis method our research group applied recently a more realistic multiplicative method (Aitchison, 1986) based on the Euclidean space geometry of the simplex (see the chapter Elements of simplicial linear algebra and geometry). Our recent results presented in detail in Wurz et al., (2010) for Mercury will be discussed. This model serves as a tool to estimate densities of species in the exosphere depending on the release mechanism and the associated physical parameters quantitatively describing the particle release from the surface

The twin paradox and Mach's principle

The problem of absolute motion in the context of the twin paradox is discussed. It is shown that the various versions of the clock paradox feature some aspects which Mach might have been appreciated. However, the ultimate cause of the behavior of the clocks must be attributed to the autonomous status of spacetime, thereby proving the relational program advocated by Mach as impracticable.Comment: Latex2e, 11 pages, 6 figures, 33 references, no tables. Accepted for publication in The European Physical Journal PLUS (EPJ PLUS

Phenomenology of the Lense-Thirring effect in the Solar System

Recent years have seen increasing efforts to directly measure some aspects of the general relativistic gravitomagnetic interaction in several astronomical scenarios in the solar system. After briefly overviewing the concept of gravitomagnetism from a theoretical point of view, we review the performed or proposed attempts to detect the Lense-Thirring effect affecting the orbital motions of natural and artificial bodies in the gravitational fields of the Sun, Earth, Mars and Jupiter. In particular, we will focus on the evaluation of the impact of several sources of systematic uncertainties of dynamical origin to realistically elucidate the present and future perspectives in directly measuring such an elusive relativistic effect.Comment: LaTex, 51 pages, 14 figures, 22 tables. Invited review, to appear in Astrophysics and Space Science (ApSS). Some uncited references in the text now correctly quoted. One reference added. A footnote adde

Escape and fractionation of volatiles and noble gases from Mars-sized planetary embryos and growing protoplanets.

Planetary embryos form protoplanets via mutual collisions, which can lead to the development of magma oceans. During their solidification, significant amounts of the mantles’ volatile contents may be outgassed. The resulting H2O/CO2 dominated steam atmospheres may be lost efficiently via hydrodynamic escape due to the low gravity of these Moon- to Mars-sized objects and the high stellar EUV luminosities of the young host stars. Protoplanets forming from such degassed building blocks after nebula dissipation could therefore be drier than previously expected. We model the outgassing and subsequent hydrodynamic escape of steam atmospheres from such embryos. The efficient outflow of H drags along heavier species like O, CO2, and noble gases. The full range of possible EUV evolution tracks of a young solar-mass star is taken into account to investigate the atmospheric escape from Mars-sized planetary embryos at different orbital distances. The estimated envelopes are typically lost within a few to a few tens of Myr. Furthermore, we study the influence on protoplanetary evolution, exemplified by Venus. In particular, we investigate different early evolution scenarios and constrain realistic cases by comparing modeled noble gas isotope ratios with present observations. Isotope ratios of Ne and Ar can be reproduced, starting from solar values, under hydrodynamic escape conditions. Solutions can be found for different solar EUV histories, as well as assumptions about the initial atmosphere, assuming either a pure steam atmosphere or a mixture with accreted hydrogen from the protoplanetary nebula. Our results generally favor an early accretion scenario with a small amount of residual hydrogen from the protoplanetary nebula and a low-activity Sun, because in other cases too much CO2 is lost during evolution, which is inconsistent with Venus’ present atmosphere. Important issues are likely the time at which the initial steam atmosphere is outgassed and/or the amount of CO2 which may still be delivered at later evolutionary stages. A late accretion scenario can only reproduce present isotope ratios for a highly active young Sun, but then unrealistically massive steam atmospheres (few kbar) would be required

The sodium exosphere of Mercury: Comparison between observations during Mercury's transit and model results

International audienc

Loss of hydrogen and oxygen from the upper atmosphere of Venus

Atmospheric escape from the upper atmosphere of Venus is mainly influenced by the loss of hydrogen and oxygen caused by the interaction of solar radiation and particle flux with the unprotected planetary environment. Because one main aim of the ASPERA-4 particle/plasma and VEX-MAG magnetic field experiments on board of ESA's forthcoming Venus Express mission is the investigation of atmospheric erosion processes from the planet's ionosphere–exosphere environment, we study the total loss of hydrogen and oxygen and identified the efficiency of several escape mechanisms involved. For the estimation of pick up loss rates we use a gas dynamic test particle model and obtained average loss rates for , and pick up ions of about and about , respectively. Further, we estimate ion loss rates due to detached plasma clouds, which were observed by the pioneer Venus orbiter and may be triggered by the Kelvin–Helmholtz instability of about . Thermal atmospheric escape processes and atmospheric loss by photo-chemically produced oxygen atoms yield negligible loss rates. Sputtering by incident pick up ions give O atom loss rates in the order of about . On the other hand, photo-chemically produced hot hydrogen atoms are a very efficient loss mechanism for hydrogen on Venus with a global average total loss rate of about , which is in agreement with Donahue and Hartle [1992. Solar cycle variations in and densities in the Venus ionosphere: implications for escape. Geophys. Res. Lett. 12, 2449–2452] and of the same order but less than the estimated ion outflow on the Venus nightside of about due to acceleration by an outward electric polarization force related to ionospheric holes by Hartle and Grebowsky [1993. Light ion flow in the nightside ionosphere of Venus. J. Geophys. Res. 98, 7437–7445]. Our study indicates that on Venus, due to its larger mass and size compared to Mars, the most relevant atmospheric escape processes of oxygen involve ions and are caused by the interaction with the solar wind. The obtained results indicate that the ratio between H/O escape to space from the Venusian upper atmosphere is about 4, and is in a much better agreement with the stoichiometrically H/O escape ratio of 2:1, which is not the case on Mars. However, a detailed analysis of the outflow of ions from the Venus upper atmosphere by the ASPERA-4 and VEX-MAG instruments aboard Venus Express will lead to more accurate atmospheric loss estimations and a better understanding of the planet's water inventory