820 research outputs found
Space Weathering on Near-Earth Objects investigated by neutral-particle detection
The ion-sputtering (IS) process is active in many planetary environments in
the Solar System where plasma precipitates directly on the surface (for
instance, Mercury, Moon, Europa). In particular, solar-wind sputtering is one
of the most important agents for the surface erosion of a Near-Earth Object
(NEO), acting together with other surface release processes, such as Photon
Stimulated Desorption (PSD), Thermal Desorption (TD) and Micrometeoroid Impact
Vaporization (MIV). The energy distribution of the IS-released neutrals peaks
at a few eVs and extends up to hundreds of eVs. Since all other release
processes produce particles of lower energies, the presence of neutral atoms in
the energy range above 10 eV and below a few keVs (Sputtered High-Energy Atoms
- SHEA) identifies the IS process. SHEA easily escape from the NEO, due to
NEO's extremely weak gravity. Detection and analysis of SHEA will give
important information on surface-loss processes as well as on surface elemental
composition. The investigation of the active release processes, as a function
of the external conditions and the NEO surface properties, is crucial for
obtaining a clear view of the body's present loss rate as well as for getting
clues on its evolution, which depends significantly on space weather. In this
work, an attempt to analyze the processes that take place on the surface of
these small airless bodies, as a result of their exposure to the space
environment, has been realized. For this reason a new space weathering model
(Space Weathering on NEO - SPAWN), is presented. Moreover, an instrument
concept of a neutral-particle analyzer specifically designed for the
measurement of neutral density and the detection of SHEA from a NEO is proposedComment: 36 page
Residual Stresses from Incremental Hole Drilling Using Directly Deposited Thin Film Strain Gauges
Background: Commonly, polymer foil-based strain gauges are used for the incremental hole drilling method to obtain residual stress depth profiles. These polymer foil-based strain gauges are prone to errors due to application by glue. For example zero depth setting is thus often erroneous due to necessary removal of polymer foil and glue. This is resulting in wrong use of the calibration coefficients and depth resolution and thus leading to wrong calculations of the obtained residual stress depth profiles. Additionally common polymer foil-based sensors are limited in their application regarding e.g. exposure to high temperatures. Objective: This paper aims at a first step into the qualification of directly deposited thin film strain gauges for use with the incremental hole drilling method. With the directly deposited sensors, uncertainties regarding the determination of calibration coefficients and zero depth setting due to the absence of glue can be reduced to a minimum. Additionally, new areas of interest such as the investigation of thermally sprayed metallic layers can be addressed by the sensors due to their higher temperature resilience and their component inherent minimal thickness. Methods: For the first time, different layouts of directly deposited thin film strain gauges for residual stress measurements were manufactured on a stainless steel specimen. Strain measurements during incremental hole drilling using a bespoke hole drilling device were conducted. Residual stress depth profiles were calculated using the Integral method of the ASTM E837 standard. Afterwards, strain measurements with conventional polymer foil-based strain gauges during incremental hole drilling were conducted and residual stress depth profiles were calculated accordingly. Finally the obtained profiles were compared regarding characteristic values. Results: The residual stress depth profiles obtained from directly deposited strain gauges generally match the ones obtained from conventional polymer foil based strain gauges. With the novel strain gauges, zero depth setting is simplified due to the absence of glue and polymer foil. With the direct deposition, a wide variety of rosette designs is possible, enabling a more detailed evaluation of the strain field around the drilled hole. Conclusions: The comparative analysis of the obtained residual stress depth profiles shows the general feasibility of directly deposited strain gauges for residual stress measurements. Detailed investigations on uncertainty sources are still necessary. © 2022, The Author(s)
Warm Breeze from the starboard bow: a new population of neutral helium in the heliosphere
We investigate the signals from neutral He atoms observed from Earth orbit in
2010 by IBEX. The full He signal observed during the 2010 observation season
can be explained as a superposition of pristine neutral interstellar He gas and
an additional population of neutral He that we call the Warm Breeze. The Warm
Breeze is approximately two-fold slower and 2.5 times warmer than the primary
interstellar He population, and its density in front of the heliosphere is ~7%
that of the neutral interstellar helium. The inflow direction of the Warm
Breeze differs by ~19deg from the inflow direction of interstellar gas. The
Warm Breeze seems a long-term feature of the heliospheric environment. It has
not been detected earlier because it is strongly ionized inside the
heliosphere, which brings it below the threshold of detection via pickup ion
and heliospheric backscatter glow observations, as well as by the direct
sampling of GAS/Ulysses. Possible sources for the Warm Breeze include (1) the
secondary population of interstellar helium, created via charge exchange and
perhaps elastic scattering of neutral interstellar He atoms on interstellar He+
ions in the outer heliosheath, or (2) a gust of interstellar He originating
from a hypothetic wave train in the Local Interstellar Cloud. A secondary
population is expected from models, but the characteristics of the Warm Breeze
do not fully conform to modeling results. If, nevertheless, this is the
explanation, IBEX-Lo observations of the Warm Breeze provide key insights into
the physical state of plasma in the outer heliosheath. If the second hypothesis
is true, the source is likely to be located within a few thousand of AU from
the Sun, which is the propagation range of possible gusts of interstellar
neutral helium with the Warm Breeze characteristics against dissipation via
elastic scattering in the Local Cloud.Comment: submitted to ApJ
Ochre use at Olieboomspoort, South Africa: insights into specular hematite use and collection during the Middle Stone Age
Recent excavations at Olieboomspoort (OBP) in the Waterberg Mountains of South Africa confirmed previous research at the site that highlighted an abundance of ochre in the Middle Stone Age (MSA) deposits. Here, we report on the results of an analysis of the ochre from the MSA deposits excavated in 2018-2019. Fossilised equid teeth from these deposits were recently dated to approximately 150 ka, an early date for such a sizeable ochre assemblage in southern Africa. Calcium carbonate concretions were removed from ochre pieces using hydrochloric acid. Macro- and microscopic analyses were undertaken to identify raw material types and to investigate utilisation strategies. There are 438 pieces in the assemblage and only 14 of them show definite use-traces. The predominant raw material is a micaceous, hard specular hematite, which is rare at MSA sites elsewhere in southern Africa. A preliminary investigation into the geological nature of the ochreous materials in the archaeological sample and those available in the area was performed using semi-quantitative portable X-ray fluorescence (pXRF), XRF, and inductively coupled plasma mass spectrometry (ICP-MS). Together with site formation processes, we suggest possible, primarily local sources of the ochre found in the deposits. The data do not support previous suggestions that OBP was used as an ochre caching site that may have formed part of an exchange network during the MSA. Instead, the local abundance of nodules of specular hematite within the Waterberg sandstone, the limited number of used pieces in the assemblage, and the stratigraphic context indicate a more natural, less anthropogenic explanation for the abundance of ochre at the site.European Social Fund (ESF) RYC2020-029506-Iinfo:eu-repo/semantics/publishedVersio
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
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
Interstellar neutral helium in the heliosphere from IBEX observations. III. Mach number of the flow, velocity vector, and temperature from the first six years of measurements
We analyzed observations of interstellar neutral helium (ISN~He) obtained
from the Interstellar Boundary Explorer (IBEX) satellite during its first six
years of operation. We used a refined version of the ISN~He simulation model,
presented in the companion paper by Sokol_et al. 2015, and a sophisticated data
correlation and uncertainty system and parameter fitting method, described in
the companion paper by Swaczyna et al 2015. We analyzed the entire data set
together and the yearly subsets, and found the temperature and velocity vector
of ISN~He in front of the heliosphere. As seen in the previous studies, the
allowable parameters are highly correlated and form a four-dimensional tube in
the parameter space. The inflow longitudes obtained from the yearly data
subsets show a spread of ~6 degree, with the other parameters varying
accordingly along the parameter tube, and the minimum chi-square value is
larger than expected. We found, however, that the Mach number of the ISN~He
flow shows very little scatter and is thus very tightly constrained. It is in
excellent agreement with the original analysis of ISN~He observations from IBEX
and recent reanalyses of observations from Ulysses. We identify a possible
inaccuracy in the Warm Breeze parameters as the likely cause of the scatter in
the ISN~He parameters obtained from the yearly subsets, and we suppose that
another component may exist in the signal, or a process that is not accounted
for in the current physical model of ISN~He in front of the heliosphere. From
our analysis, the inflow velocity vector, temperature, and Mach number of the
flow are equal to lambda_ISNHe = 255.8 +/- 0.5 degree, beta_ISNHe = 5.16 +/-
0.10 degree, T_ISNHe = 7440 +/- 260 K, v_ISNHe = 25.8 +/- 0.4$ km/s, and
M_ISNHe = 5.079 +/- 0.028, with uncertainties strongly correlated along the
parameter tube.Comment: Updated reference
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