Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed seventeen Ryugu samples measuring 1-8 mm. CO 2 -bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and Ca, Al-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed by aqueous alteration reactions at low temperature, high pH, and water/rock ratios < 1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate Ryugu’s parent body formed ~ 2 million years after the beginning of Solar System formation.
Multiple space exploration missions have recently revealed that asteroids have complex and variable surfaces. Geomorphological features are clues for mass transport processes occurring on small bodies. For example, the NEAR Shoemaker mission discovered a loose regolith layer on asteroid Eros. There are direct evidences of downslope motions, such as flat-floored sediments ponds in craters, a deficit of small craters, and large blocks surrounded by debris aprons (e.g., Thomas et al., 2002; Veverka et al., 2001). Miyamoto et al. (2007) report from images taken by Hayabusa that the surface regolith on Itokawa is segregated. However, transporting processes of asteroid regolith in both lateral and vertical directions remain ambiguous.
<p><strong>Introduction</strong></p><p>Since 2015, the Japanese meteorological satellite Himawari-8 has been observing weather in the Asia-Pacific region every 10 minutes [1]. Scanning the full-disk Earth also covers space adjacent to the Earth, where celestial objects, such as the Moon, are occasionally included (Figure 1). Onboard Himawari-8 is an Advanced Himawari Imager (AHI), whose high spatio-temporal resolution can provide over 900 lunar images with 23 km/pix or better.</p><p>Furthermore, various AHI bands from visible to infrared wavelengths possibly provide multispectral information about planets. In particular, some of the nine infrared wavelength bands on AHI have not been used for observing the Moon by any other spaceborne telescopes. For example, within the AHI infrared coverage of 6&#8211;14 &#956;m, the Diviner radiometer onboard Lunar Reconnaissance Orbiter (LRO) has only three channels around 8 &#956;m [2].</p><p>Despite such potential, the AHI images have never been recognized as a useful dataset in planetary science. In order to demonstrate the possibility of utilization of AHI data, we investigate the lunar thermal environment with the AHI data. We develop the procedure to extract lunar multiband brightness temperatures. Then, we compare them with the Diviner measurements and numerical simulations, posing some constraints on lunar thermophysical conditions.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.39d45d49cc7265615432561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=18a9d275bcae0b4da135d33df42b0d3d&ct=x&pn=gnp.elif&d=1" alt="" width="494" height="494"></p><p>Figure 1. Synthesized images of the Moon captured within AHI.</p><p>&#160;</p><p><strong>Method</strong></p><p>To derive lunar brightness temperatures, we use the Himawari Standard Data (HSD) published by the Japanese Meteorological Agency. First, we extract lunar images from all HSDs taken by the end of November 2021. Next, we manually choose 248 lunar images in which more than a half part of the Moon is captured (Figure 2). Then, we calculate the lunar orientation with SPICE to obtain the lunar longitude and latitude of each HSD pixel. Finally, after estimating background noise, we convert the radiance value to brightness temperatures at all nine bands.</p><p>To interpret the derived brightness temperatures, we employ a 1-D conductive model to simulate the temperature on the Moon. In this simulation, the material differences must be considered because the thermal inertia of the media influences the temperature prediction, particularly on the nightside. Thus, the calculation is conducted for both rock and regolith, using the Apollo data [3]. Moreover, surficial roughness is critical because of its effects on shadows, notably in the morning and evening. Thus, we incorporate the statistical model among incidence angle, emission angle, shadow ratio, and roughness parameterized by the RMS mean slope [4, 5].</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.ec8a88b9cc7262725432561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=306d97d3fdc8a8fb71f9c3a80ee20937&ct=x&pn=gnp.elif&d=1" alt="" width="921" height="396"></p><p><span>Figure 2. (a, b) Images of the half-Moon and almost-new Moon taken on 2018/12/28 08:20 and 2015/11/09 13:10 in UTC, respectively.</span></p><p>&#160;</p><p><strong>Brightness temperatures</strong></p><p>Lunar brightness temperatures derived from HSD are consistent with those measured by Diviner. Due to the similarity between band 11 on AHI (8.40&#8211;8.78 &#956;m) and channel 5 on Diviner (8.38&#8211;8.68 &#956;m), we compare HSD with the Diviner Global Cumulative Product (GCP) [6]. Although HSD is saturated at local times from 8 to 16 hours due to its dynamic range, the brightness temperatures from HDS match those from Diviner GCP in the morning and evening. In addition, HSD at Tycho crater also agrees with Diviner GCP even in the nightside, indicating the reliability of AHI for planetary science.</p><p>Multiband lunar images by AHI reveal differences in brightness temperature among wavelengths, so-called anisothermality. In the morning and evening, the brightness temperature differences among bands increase with the incident angle. Nightside temperature differences also exist in Tycho crater. These anisothermal features are consistent with the Diviner measurements and indicate the mixture of various temperatures even within a pixel of HSD.</p><p>&#160;</p><p><strong>Lunar thermophysical conditions</strong></p><p>The anisothermality in the morning and evening is caused by the surface roughness of the Moon. Due to various slopes on millimetric to centimetric scales, the surface temperature on the Moon shows a wide-ranging distribution, increasing anisothermality with the incident angles. In addition, our radiance simulation with the RMS mean slope of 16&#8211;20 degrees is consistent with the observed values at incident angles lower than 70 degrees. These slope angles also match those at Apollo landing sites [7].</p><p>On the other hand, the nightside anisothermality reflects rock abundance. Because of the high thermal inertia of rocks, the midnight temperature of rocks is expected to be higher than that of regolith by over 100 K. As a result, only a slight mixture of rock in regolith can make brightness temperatures higher than regolith (Figure 3). Our mixture model of rock and regolith estimates the rock abundances at the equator and Tycho crater are 0.18&#8211;0.48 and 6.1&#8211;10.3%, respectively. These estimates are also consistent with the Diviner constraints [8].</p><p><span><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.ad922dd9cc7269035432561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=63a652557cb5b0e11aa02b190435a853&ct=x&pn=gnp.elif&d=1" alt="" width="609" height="410"></span></p><p><span>Figure 3 Temporal temperature variations at the lunar equator after sunset. The black points are the HSD brightness temperatures at band 14. The black dashed and dotted lines show the rock and regolith temperatures in our simulation. </span><span>The solid lines show rock-regolith mixture models with rock ratios of 0.2, 0.5, 2, 10, 25, and 50 %.</span></p><p>&#160;</p><p><strong>Conclusion</strong></p><p>Consistencies between AHI and Diviner indicate that the quality of HSD is sufficiently high for planetary science. Despite the spatial resolution lower than Diviner, the unique wavelength coverages of AHI can provide new spaceborne infrared datasets. In addition, AHI sometimes captures other planets like Mercury, Venus, Mars, and Jupiter. Thus, HSD can be a valuable data source for calibrations and future planetary sciences.</p><p>&#160;</p><p><strong>Note</strong></p><p>The contents of this presentation have been submitted by Nishiyama et al. to Earth, Planets, and Space.</p><p>&#160;</p><p><strong>Reference</strong></p><p>[1] Okuyama, A. et al., 2018, Journal of the Meteorological Society of Japan 96B, 91.</p><p>[2] Paige, D.A. et al., 2010, Space Science Reviews 150, 125&#8211;160.</p><p>[3] Horai, K. & Simmons, G., 1972, Thermal Characteristics of the Moon. pp. 243&#8211;267.</p><p>[4] Bandfield, J.L. et al., 2015, Icarus 248, 357&#8211;372.</p><p>[5] Davidsson, B.J.R. et al., 2015, Icarus 252, 1&#8211;21.</p><p>[6] Williams, J.P. et al., 2017, Icarus 283, 300&#8211;325.</p><p>[7] Helfenstein, P. & Shepard, M.K., 1999, Icarus 141, 107&#8211;131.</p><p>[8] Bandfield, J.L. et al., 2011, JGR: Planets 116.</p>
<p><strong>Introduction</strong></p> <p>The ESA/JAXA joint mission BepiColombo comprises two spacecraft, one of which is the Mercury Planetary Orbiter (MPO). It will arrive at the Mercury orbit in 2025, and the BepiColombo Laser Altimeter (BELA) onboard MPO will start to characterize the Mercury&#8217;s surface, such as topographic reliefs. During the nominal and extended operations, the whole surface of Mercury will be covered by the BELA footprints, and separations between neighboring tracks were expected to vary from 3 km at the equator down to less than 250 m in polar regions [1].</p> <p>One of the scientific objectives of BELA is to measure surface roughness, local slope, and albedo variations of Mercury [2]. In general, laser altimetry is a useful remote sensing tool to reveal the surficial features because the shape of the received pulse contains various information about footprints. For example, vegetation structure within footprints was observed in the waveforms measured by the Ice, Cloud, and land Elevation Satellite (ICESat) [3], and the martian roughness was estimated from the pulse width received by the Mars Orbiter Laser Altimeter [4]. Thus, BELA is expected to help characterize geologic features on Mercury, such as hollows [5]</p> <p>In contrast, the effects of surficial features on the BELA pulse shape have not been investigated numerically. Until recently, semi-analytical pulse shape models have been employed to demonstrate the BELA measurements after the operation starts [1, 6]. Therefore, we develop the procedure to simulate realistic pulse shapes, incorporating digital terrain models (DTMs) and noise data from previous in-cruse checkouts of BELA. Then, we discuss the detectability of the surficial properties of Mercury.</p> <p>&#160;</p> <p><strong>Method</strong></p> <p>To simulate the realistic pulse shape returning from the Mercury surface, we first numerically model pulses returning from DTMs. Assuming a Gaussian pulse transmission and a Lambertian surface, we integrate photons from all the facets. Then, after the pulse shape is widened by the receiver optics, the number of photons is converted to signals in the avalanche photodiode. With 16 gain channels ranging from 4 to 44 dB, the Analog Electronics Unit amplifies the signal.</p> <p>The simulated signal is finally compared with the sum of dark, solar, and shot noises. Data from in-cruise checkouts by June 2021 is used as the dark noise for respective gain channels. The solar and shot noise levels are analytically estimated [1]. By incorporating these noises, the signal-to-noise ratio (SNR) of the received pulse is calculated for various surfaces.</p> <p>&#160;</p> <p><strong>Results and discussions </strong></p> <p>Figure 1 shows pulse shape examples simulated with various ideal topographies. For the flat surface, time of flight is almost uniform for all the photons from the footprint. However, slopes change distances between facets and BELA, widening pulse shapes. If bimodal height distribution, such as depression, cliff, and rocks, is contained within the footprint, a split pulse can be detected, depending on the height gaps.</p> <p><img src="" alt="" width="825" height="440" /></p> <p>Figure 1. Pulse shapes returning from a flat surface, slope (20 degrees), and round depression. The altitude is set to be 1000 km above the Mercury surface.</p> <p>&#160;</p> <p>Detectability of rocks within a footprint is also examined in our analysis. Locating a hemispherical rock on a flat plain and changing its radius, we calculated pulse shapes with two peaks from the rock and its surroundings (Figure 2). In order to receive the rock&#8217;s signal with an SNR of 1 or higher, our estimate suggests that the radius of the rock needs to be larger than a few meters at an altitude higher than 400 km.</p> <p><img src="" alt="" width="873" height="354" /></p> <p>Figure 2. Pulse shapes without noises simulated for hemispherical rocks with various radii. Black shades show a standard deviation of dark noise. Dashed lines show the sum of dark, shot, and solar noises. Left peaks correspond to laser pulses reflected by the rock surface.</p> <p>&#160;</p> <p>Furthermore, roughness estimation within the footprint requires accurate slope values because they similarly widen the pulse shape. Although only one-dimensional slopes can be constrained with BELA, possible constraints on roughness will be estimated using these pulse shape simulations. This presentation will report on such further implications for Mercury&#8217;s surfaces in future BELA observations.</p> <p>&#160;</p> <p><strong>Acknowledgment</strong></p> <p>This research is granted by JSPS Overseas Challenge Program for Young Researchers.</p> <p>&#160;</p> <p><strong>Reference</strong></p> <p>[1] G. Steinbr&#252;gge, A. Stark, H. Hussmann, K. Wickhusen, J. Oberst, The performance of the BepiColombo Laser Altimeter (BELA) prior launch and prospects for Mercury orbit operations. Planetary and Space Science. 159, 84-92 (2018).</p> <p>[2] N. Thomas et al., The BepiColombo Laser Altimeter. Space Sci Rev. 217(2021).</p> <p>[3] D. J. Harding, C. C. Carabajal, ICESat waveform measurements of within-footprint topographic relief and vegetation vertical structure. Geophysical Research Letters. 32, L21S10-n/a (2005).</p> <p>[4] G. A. Neumann et al., Mars Orbiter Laser Altimeter pulse width measurements and footprint-scale roughness. Geophysical Research Letters. 30, 1561-n/a (2003).</p> <p>[5] T. Blewett David et al., Hollows on Mercury: MESSENGER Evidence for Geologically Recent Volatile-Related Activity. Science. 333, 1856-1859 (2011).</p> <p>[6] A. HosseiniArani et al., Comprehensive in-orbit performance evaluation of the BepiColombo Laser Altimeter (BELA). Planetary and Space Science. 195, 105088 (2021).</p>
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