Lunar X ray and Gamma Ray spectroscopy with miniaturized detector on cubesat platform

<p>Assessing the chemical composition of the Moon is a key part of its investigation. Elements can be grouped according to their condensation and geochemical behavior, and thus chemical abundances of key elements can be used to asses both the origin and the evolution of the body.</p> <p>The relative abundance of the surface (down to a few µm) major elements with atomic number £20 can be estimated using X-ray fluorescence, using solar X-rays to excite element’s atoms (both quiescent Sun X-ray emission, in particular during solar maxima, and solar flares).</p> <p>Only an handle of successful X-ray fluorescence experiments have been flown so far, including Apollo15 XRFS, Chandrayaan-1 C1XS, SMART-1 D-C1XS at the Moon, Messanger-XRS, BepiColombo-MIXS at Mercury, and NEAR-XRS at EROS, Hayabusa-XRS at Itokawa, OSIRIS-REX/REXIS at Bennu (Allen et al. 2013, arXiv:1309.6665 and refs. therein).</p> <p>The main goal of fluorescent X-ray spectroscopy of asteroids is to determine the intensity of Fe-L, Fe-K, Al-K, Mg-K, Si-K complexes and S-Ka, S-Kb fluorescent lines. From these lines mass abundance ratios can be determined [Mg/Si], [Fe/Si]. The comparison of these abundance ratio to those of meteorites can understand whether the surface of the Moon resembles a particular chondritic meteorite.</p> <p>Gamma-ray spectroscopy of nuclear lines can also be used to assess abundances of elements in the planetary surface (down to 10-20cm, e.g. Reedy et al. 1978, Proc. Lunar Planet Sci. Conf. 9th 1978). The main source of gamma-ray lines are: decay of natural radionuclides, reaction induced by energetic cosmic rays, capture of low-energy neutrons, and solar-proton induced radioactivity. Gamma-ray spectroscopy can provide information on many elements, including those with high atomic number, hardly accessible to X-ray fluorescence spectroscopy. Several successful gamma-ray experiment have been performed so far, including early spectroscopy of the Moon (Luna10, Apollo15, 16), Mars (Mars-5). More recently, Kayuga gamma-ray spectrometer obtained a detailed mapping of radioactive elements on the Moon (K, Th, U), as well major elements, O, Al, Mg, Si, Ca, Ti, Fe (Yamashita et al. EPSC Abstracts Vol. 5, EPSC210-580, 2010, Hasebe et al. Proc. Int. Workshop Advances in Cosmic Ray Science J. Phys. Soc. Jpn. 78, 2009). Other lunar experiments include Lunar prospector and Chang’E. Other experiments include more Mars spectroscopy (MARS Odyssey) Mercury (Messenger) and the asteroids EROS (NEAR) and 4 Vesta (Dawn). The K/Th ratio is a diagnostic of the body bulk composition and provide information about where and when the body was formed. The collecting area of most X-ray and gamma-ray spectrometers used for planetary observations is quite small, a few cm2. The REXIS instrument working today at Bennu as a collecting area of 25cm2. The largest instrument flown so far is the Kaguya-XRS with a collecting are of 100cm2. A new miniaturize payload is here proposed, currently under development for astrophysics goals and to be installed on a 3U cubesat platform; the sensors uses GAGG scintillator crystals and Silicon Drift Detectors to both detect direct X-ray photons and optical photons produced in the scintillators by interaction with gamma-rays, and can cover a unique wide band, from a few keV to several MeV, providing at the same time both X-ray fluorescent spectroscopy and gamma-ray spectroscopy. The collecting area of a compact instrument (~3-4kg) can be conservatively ~100cm2. A passive collimator can shield the SDD from most Cosmic X-ray Background, reducing the background at energies <20keV, where it is largely dominated by the CXB. Laboratory calibration in the next few months will provide more precise numbers on the resolution achievable. The gamma-ray line sensitivity of this instruments with collecting area ~ 100 cm2 and active anticoincidences (efficiency  » twice that of Kayuga spectrometer) should therefore be a factor several better than that of other planetary gamma-ray spectrometers. The paper shows the proposed scientific mission goals around which the miniaturized payload is designed, and its possible embarking on a cubesat like space segments, 6U large; the whole feasibility study for a smallsat scientific mission is presented, showing the generality of the approach and its applicability also for small bodies exploration as well.</p>

Simulating the Reiner Gamma Swirl and Magnetic Anomaly: The Impact of the Solar Wind Alpha Population

<p>The Reiner Gamma swirl is one of the most prominent albedo features on the lunar surface. Its modest spatial scales and structure allows fully kinetic modelling. The region therefore presents a prime location to investigate the lunar albedo patterns and their co-location with magnetic anomalies. The precise relationship between the impinging plasma and the swirl, and in particular, how these interactions vary over the course of a lunar day, remains an open issue.</p><p>Here we use the fully kinetic particle-in-cell code,  iPIC3D, coupled with a magnetic field model based on Kaguya and Lunar Prospector observations, and simulate the interaction with the Reiner Gamma anomaly for all plasma regimes the region is exposed to along a typical orbit, including different solar wind incidence angles and the Moon's crossing through the terrestrial magnetosphere. We focus on the impact of the solar wind alpha population and construct energy and velocity distributions in key locations surrounding the interaction region of the anomaly.</p><p>The energy flux profile provides a better match to the albedo pattern only when integrating over the full lunar orbit. Including He<sup>2+</sup> as a self-consistent plasma species improves the brightness ratios between the inner and outer bright lobes, the dark lanes, and the mare background. However, substantial differences between the observed albedo pattern and the predicted flux remain.  For example, the bright outer lobes are substantially brighter than predicted and the central portion of the anomaly is darker than predicted. This is likely due to an incomplete model of the near-surface field structure.</p><p>Solar wind standoff can explain the large-scale correlation between the Reiner Gamma swirl and the co-located magnetic anomaly. In particular, the outer bright lobes emerge in the simulated weathering pattern only when integrating over the entire lunar orbit, although they are much weaker than observed. Both the proton and helium energy flux to the surface need to be taken into account to best reproduce the swirl pattern. A complete understanding of the solar wind interaction with lunar magnetic anomalies and swirl formation could be vastly improved by low altitude measurements of the magnetic field and solar wind.</p>

Retrieving paleopoles using newly mapped lunar magnetic anomalies within basins

<p>Orbital spacecraft magnetic field observations show that several isolated magnetic anomalies are found to be heterogeneously distributed over the lunar surface. The magnetic anomalies origin is still debated; however, it is largely accepted that an ambient core magnetic field was present during their formation. Contrary to previous studies, here we focus only on anomalies that are related to basins/craters, which correspond to the best possibility to hold ancient core field information. In particular, the basin rocks become thermoremanently magnetized as the melt sheet cools down slowly recording the ambient magnetic field that was present when the crater was formed.</p><p>We build regional magnetic field maps using data from quiet orbits of Lunar Prospector and Kaguya spacecraft. When comparing these regional maps to existing global models, several differences and details are discovered. Further investigation is required to understand why small scales are missing from global models. For each mapped crater, we perform inversions for the magnetization direction to estimate the corresponding paleopole position (defined as the north magnetic pole when the anomaly formed). In detail, a grid of dipoles is placed over the basin inner depression, where the melt sheet is believed to be. All dipoles have the same common direction, nonetheless different dipole moments.</p><p>Preliminary results show that paleopole positions of regionally mapped anomalies associated with craters are not in absolute agreement with previous paleopole studies. Also of significance is the distribution of dipoles obtained, which seem to be consistent with inferred impactor trajectories. We conclude that paleopole position results are highly dependent on the technique and choices we make to construct the magnetic field maps. Further studies of several other craters will be performed, but we expect large differences when using regionally mapped anomalies. Our results will help to better constrain the lunar ancient core field morphology.</p>

Global mapping of lunar refractory elements: multivariate regression vs. machine learning

Context.The quantitative estimation of elemental concentrations at the spatial resolution of hyperspectral near-infrared (NIR) images of the lunar surface is an important tool for understanding the processes relevant for the origin and evolution of the Moon.Aims.We aim to map the abundances of the elements Fe, Ca, and Mg at a typical accuracy of about 1 wt.% at the spatial resolution of the Moon Mineralogy Mapper (M3) instrument on-board Chandrayaan-1 lunar mission.Methods.The NIR reflectance of the lunar regolith is an integrated response to the presence of refractory elements and soil alteration processes. Our approach was to define a combination of spectral parameters that are robust with respect to the effects of soil maturity. We calibrated the spectral parameters with respect to elemental abundances measured by the Lunar Prospector Gamma Ray Spectrometer (LP GRS) and the Kaguya GRS (KGRS). For this purpose, we compared a classical multivariate linear regression (MLR) approach and the machine learning based support vector regression (SVR) technique applied to M3global observations.Results.The M3-based global elemental maps are consistent in distribution and range with the LP GRS and KGRS elemental maps and do not show artifacts in immature areas such as small fresh craters. The results derived using MLR and SVR are compared to sample-based ground truth data of the Apollo and Luna sample-return sites, where the root-mean-square deviations obtained by the two regression models are similar.Conclusions.The main advantage of the proposed new algorithm is its ability to minimize artifacts due to space-weathering effects. The elemental maps of Mg and Ca provide additional information and reveal structures not always visible in the Fe map. The global elemental abundance maps derived for the fully calibrated M3observations might thus serve as important tools to investigate the lunar geology and evolution.

LATERAL VARIATIONS IN BULK DENSITY AND POROSITY OF THE UPPER LUNAR CRUST FROM HIGH-RESOLUTION GRAVITY AND TOPOGRAPHY DATA: COMPARISON OF DIFFERENT ANALYSIS TECHNIQUES

<p><strong>Abstract.</strong> We map lateral variations in bulk density of the upper lunar highland crust using the most recent GRAIL gravity field solution of degree and order 1500 in combination with LOLA topography data, both truncated to an upper limit of degree and order 700. Our maps have a spatial resolution of 0.75&amp;deg;, where each grid point was calculated using circular analysis regions of 3&amp;deg; radius. We apply two methods, which yield similar results for most parts of the study area. The first method minimizes the correlation between topography and Bouguer anomalies, the second maximizes the smoothness of the Bouguer anomalies. Both approaches suffer in the case that terrain is flat and lacks topographic features; consequently, this is where results from the two methods differ. We also mapped porosity of the crust using grain densities derived from Lunar Prospector spectrometry and sample analysis. It appears that variations in bulk density are mostly related to differences in crustal porosity. We find that high porosity is often associated with areas of impact basins. This confirms earlier studies, that impacts changed the geophysical characteristics of the lithosphere sustainably and that the high porosity of the upper lunar crust is most likely impact induced.</p>

Study of Chang’E-2 Microwave Radiometer Data in the Lunar Polar Region

The Chang’E-2 (CE-2) four-channel microwave radiometer (MRM) data with frequencies of 3 GHz, 7.8 GHz, 19.35 GHz, and 37 GHz have been used to investigate the properties of lunar surface such as regolith thickness, dielectric constant, and titanium abundance within a depth of several meters in middle and low latitudes. The purpose of this work is to take a close look at MRM data in the polar regions of the Moon and analyze the characteristics of the brightness temperature (TB) in permanently shadowed regions (PSRs), especially where evidence of water ice has been found. First, the comparisons of brightness temperature values in the polar region and in low latitudes show that (1) the periodic diurnal (day/night) variation of TB becomes weak in high latitudes since topography plays a dominant role in determining TB in polar region and (2) seasonal effects are more recognizable in polar region than in low latitudes due to the weak illumination condition. Second, even without direct sun illumination, significant seasonal variations of TBs are observed in PSRs, probably caused by the scattering flux from neighboring topography. TB Ratio (TBR) between channel 1 and channel 4, which indicates the differences of TB at different depths of lunar regolith, is higher and shows stronger seasonal variation in PSR than regions with direct illumination. Third, overall the distribution of high TBR values is in consistence with the water ice distributions obtained by the Moon Mineralogy Mapper instrument, the LAMP UV spectra, and the Lunar Prospector Neutron Spectrometer. The proportion of the summation over area with water ice proof in the regions of interest is 0.89 and 0.56 in south pole and north pole, respectively. The causes of the correlation of high TBR between different microwave frequencies and stability of water ice deposits still require further investigation, but MRM data shows unique characteristic in PSRs and could provide important information about the upper few meters of lunar regolith.