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Observations of hydroxyl in the Fra Mauro regionтезисы доклада
- Авторы: Wöhler C., Berezhnoy A.A., Feoktistova E.A., Grumpe A., Sinitsyn M.P., Shevchenko V.V.
- Сборник: 40th COSPAR Scientific Assembly
- Тезисы
- Год издания: 2014
- Аннотация: Lunar pyroclastic deposits (LPDs) are known to consist of volcanic material (basalt and glass) ejected by eruptions [1]. In the southern part of the crater Fra Mauro, a localised LPD is associated with Rima Parry V [2]. In [3], a suppressed neutron flux is described for the Fra Mauro region based on measurements of the Lunar Exploration Neutron Detector (LEND) [4], interpreted as an indicator of hydroxyl (OH). In this study we compare these measurements with NIR hyperspectral data acquired by the Moon Mineralogy Mapper (M3) [5] instrument. The suppression factor of the neutron flux is defined according to Delta = (N_ref - N_ex)/ N_ref [6,7] with N_ex as the average count rate of the omnidirectional sensor (SETN) [8] of LEND for the region under study (here: the Fra Mauro region) and N_ref as the average count rate for a reference area (here: immediately west of Fra Mauro). For Fra Mauro crater, a suppression factor of 2.4% with a standard error of 0.41% was found. The epithermal neutron flux can be assumed to be inversely proportional to the hydrogen content. Hence, the measured positive suppression factor indicates a positive anomaly of the hydrogen content at up to 1 m depth. Under the approximative assumption of a proportional relation between the suppression factor and the hydrogen content, the observation in [7] of a suppression factor of 18% in the crater Cabeus associated with a homogeneous hydrogen content of about 500 ppm implies an enrichment in hydrogen by about 70 ppm for the Fra Mauro region. However, these values do not specifically refer to the small Fra Mauro LPD, but to a larger area of about 150 km diameter. To identify the LPD-specific suppression factor, it would be necessary to acquire collimated neutron flux measurements. We have complemented the LEND-based measurements by the analysis of spectral reflectance data acquired by the M3 instrument. The presence of OH in the surface material leads to an absorption band beyond 2700 nm wavelength [9]. Based on the M3 version V03 radiance data [10] and GLD100 topographic data [11], a topographically and photometrically corrected spectral reflectance data set normalised to 30 incidence angle, 0 emission angle and 30 phase angle [12] has been generated along with a refined DEM of high lateral resolution (cf. [13] for a detailed description of the applied processing steps). The depth of the OH absorption band is indicated by the reflectance ratio between the 2657 nm and 2817 nm bands (M3 channels 77 and 81). In our map of the 2657/2817 re ectance ratio, the Fra Mauro LPD as well as a similar deposit in the northern part of the crater Bonpland stand out as positive anomalies exhibiting an increase of 2 - 2.5% with respect to the surrounding crater oor material. This observation indicates the presence of OH in the LPD surface material. Similar absorption depths can be observed for the inner walls and ejecta of a crater of 3 km diameter located north-northwest of the Fra Mauro LPD and of several craters with diameters less than 2 km in the northern floor of Bonpland. This behaviour may indicate the presence of OH also at larger depth below the surface. [1] Gaddis et al. (2003) Icarus 161(2), 262-280 [2] Head, J. W., Wilson, L. (1993) PSS 41(10), 719-727 [3] Sinitsyn et al. (2013) 4th Moscow Solar System Symp., 4MS3-PS14 [4] Mitrofanov et al. (2010a) Space Sci. Rev. 150(1-4), 183-207 [5] Pieters et al. (2009) Current Science 96(4), 500-505 [6] Boynton et al. (2012) JGR 117, E00H33 [7] Mitrofanov et al. (2010b) Science 330, 483-485 [8] Litvak et al. (2012) JGR 117, E00H22 [9] Clark et al. (2010) LPSC XXXXI, abstract 1533 [10] http://pds-maging.jpl.nasa.gov/volumes/m3.html [11] Scholten et al. (2012) JGR 117, E00H17 [12] Pieters (1999) Workshop on New Views of the Moon II, abstract #8025 [13] Grumpe et al. (2013) Adv. Space Res., in press.
- Добавил в систему: Бережной Алексей Андреевич