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Surface analysis of Mercury with a mass-spectrometer
Introduction: The European Space Agency BepiColombo mission to Mercury will include a lander, the Mercury Surface Element (MSE). Although the final configuration of instruments is still to be decided, we are developing a mass spectrometer suitable for use on this lander, or in other missions where low mass and low power consumption are a priority. Advantages of a mass-spectrometer over other analytical instruments include sensitivity to almost all elements, high dynamic range, spatially resolved measurements (with an appropriate sampling technique) and the potential to determine isotopic compositions
A homologue of the breast cancer associated gene BARD1 is involved in DNA repair in plants
hBRCA1 and hBARD1 are tumor suppressor proteins that are involved as heterodimer via ubiquitinylation in many cellular processes, such as DNA repair. Loss of BRCA1 or BARD1 results in early embryonic lethality and chromosomal instability. The Arabidopsis genome carries a BRCA1 homologue, and we were able to identify a BARD1 homologue. AtBRCA1 and the putative AtBARD1 protein are able to interact with each other as indicated by in vitro and in planta experiments. We have identified T-DNA insertion mutants for both genes, which show no visible phenotype under standard growth conditions and are fully fertile. Thus, in contrast to animals, both genes have no indispensable role during development and meiosis in plants. The two single as well as the double mutant are to a similar extent sensitive to mitomycin C, indicating an epistatic interaction in DNA crosslink repair. We could further demonstrate that in Arabidopsis BARD1 plays a prominent role in the regulation of homologous DNA repair in somatic cells
The downwind hemisphere of the heliosphere: Eight years of IBEX-Lo observations
We present a comprehensive study of energetic neutral atoms (ENAs) of 10 eV
to 2.5 keV from the downwind hemisphere of the heliosphere. These ENAs are
believed to originate mostly from pickup protons and solar wind protons in the
inner heliosheath. This study includes all low-energy observations made with
the Interstellar Boundary Explorer over the first 8 years. Since the protons
around 0.1 keV dominate the plasma pressure in the inner heliosheath in
downwind direction, these ENA observations offer the unique opportunity to
constrain the plasma properties and dimensions of the heliosheath where no
in-situ observations are available.
We first derive energy spectra of ENA intensities averaged over time for 49
macropixels covering the entire downwind hemisphere. The results confirm
previous studies regarding integral intensities and the roll-over around 0.1
keV energy. With the expanded dataset we now find that ENA intensities at 0.2
and 0.1 keV seem to anti-correlate with solar activity. We then derive the
product of total plasma pressure and emission thickness of protons in the
heliosheath to estimate lower limits on the thickness of the inner heliosheath.
The temporally averaged ENA intensities support a rather spherical shape of the
termination shock and a heliosheath thickness between 150 and 210 au for most
regions of the downwind hemisphere. Around the nominal downwind direction of
76{\deg} ecliptic longitude, the heliosheath is at least 280 au thick. There,
the neutral hydrogen density seems to be depleted compared to upwind directions
by roughly a factor of 2.Comment: Preprint of article in The Astrophysical Journa
The Hydrogen Exospheric Density Profile Measured with ASPERA-3/NPD
We have evaluated the Lyman-α limb emission from the exospheric hydrogen of Mars measured by the neutral particle detector of the ASPERA-3 instrument on Mars Express in 2004 at low solar activity (solar activity index = 42, F10.7=100). We derive estimates for the hydrogen exobase density, n H = 1010 m−3, and for the apparent temperature, T > 600 K. We conclude that the limb emission measurement is dominated by a hydrogen component that is considerably hotter than the bulk temperature at the exobase. The derived values for the exosphere density and temperature are compared with similar measurements done by the Mariner space probes in the 1969. The values found with Mars Express and Mariner data are brought in a broader context of exosphere models including the possibility of having two hydrogen components in the Martian exosphere. The present observation of the Martian hydrogen exosphere is the first one at high altitudes during low solar activity, and shows that for low solar activity exospheric densities are not higher than for high solar activit
An Impacting Descent Probe for Europa and the other Galilean Moons of Jupiter
We present a study of an impacting descent probe that increases the science
return of spacecraft orbiting or passing an atmosphere-less planetary body of
the solar system, such as the Galilean moons of Jupiter. The descent probe is a
carry-on small spacecraft (< 100 kg), to be deployed by the mother spacecraft,
that brings itself onto a collisional trajectory with the targeted planetary
body in a simple manner. A possible science payload includes instruments for
surface imaging, characterisation of the neutral exosphere, and magnetic field
and plasma measurement near the target body down to very low-altitudes (~1 km),
during the probe's fast (~km/s) descent to the surface until impact. The
science goals and the concept of operation are discussed with particular
reference to Europa, including options for flying through water plumes and
after-impact retrieval of very-low altitude science data. All in all, it is
demonstrated how the descent probe has the potential to provide a high science
return to a mission at a low extra level of complexity, engineering effort, and
risk. This study builds upon earlier studies for a Callisto Descent Probe (CDP)
for the former Europa-Jupiter System Mission (EJSM) of ESA and NASA, and
extends them with a detailed assessment of a descent probe designed to be an
additional science payload for the NASA Europa Mission.Comment: 34 pages, 11 figure
A protosolar nebula origin for the ices agglomerated by Comet 67P/Churyumov-Gerasimenko
The nature of the icy material accreted by comets during their formation in
the outer regions of the protosolar nebula is a major open question in
planetary science. Some scenarios of comet formation predict that these bodies
agglomerated from crystalline ices condensed in the protosolar nebula.
Concurrently, alternative scenarios suggest that comets accreted amorphous ice
originating from the interstellar cloud or from the very distant regions of the
protosolar nebula. On the basis of existing laboratory and modeling data, we
find that the N/CO and Ar/CO ratios measured in the coma of the Jupiter
family comet 67P/Churyumov-Gerasimenko by the ROSINA instrument aboard the
European Space Agency's Rosetta spacecraft match those predicted for gases
trapped in clathrates. If these measurements are representative of the bulk
N/CO and Ar/CO ratios in 67P/Churyumov-Gerasimenko, it implies that the
ices accreted by the comet formed in the nebula and do not originate from the
interstellar medium, supporting the idea that the building blocks of outer
solar system bodies have been formed from clathrates and possibly from pure
crystalline ices. Moreover, because 67P/Churyumov-Gerasimenko is impoverished
in Ar and N, the volatile enrichments observed in Jupiter's atmosphere
cannot be explained solely via the accretion of building blocks with similar
compositions and require an additional delivery source. A potential source may
be the accretion of gas from the nebula that has been progressively enriched in
heavy elements due to photoevaporation.Comment: The Astrophysical Journal Letters, in pres
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
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