(2020) Constraints on the Surface Materials of Ryugu from Spectral and Water Analysis of Experimentally-Heated CI Carbonaceous Chondrite
Amano K, Nakamura T, Usui F, Okumura S & Zolensky M
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01c: Room 1, Tuesday 23rd June 00:57 - 01:00
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The 2.7-um band depth clearly changes between 400 and 500°C. Does decomposition of serpentine change the band depth? If so, does it occur at 400°C or lower temperatures for longer duration?
Thank you for your important question. Just as you say, synchrotron-radiation XRD analysis indicates that 400-C heated sample still contains serpentine while 500-C heated sample does not contain serpentine but only saponite. The change in the band depth should be due to decomposition of serpentine. In the case of longer heating, phyllosilicates can decompose at lower temperature about by ~100 °C as far as Akai(1992) examined. Therefore when we think that Ryugu experienced longer heating such as internal heating in the parent body o Ryugu or repeating solar heating, we can suggest that Ryugu experienced heating at around 400 °C or lower, in other words, 500 °C is just maximum.
What process do you think heated Ryugu?
I think the heat source is solar heating or internal heating. Shock heating should result in a great variation of 2.7-µm band properties however we can see only a small variation of that on Ryugu. To identify the process, we have to combine the results of other observations and return sample analysis.
Nice talk and interesting results. Slide 3 shows that previous measurements of heated CMs/CIs are brighter than Ryugu, but your expt. shows that heating a CI to 500C gets to the 2% reflectance. Does this mean the previous comparisons were to meteorites that weren't heated to as high a temperature? Also, you compare the reflectance at 550 nm and the 2.7 micron band parameters, but didn't show the actual spectrum of the 500C Orgueil sample. Does it match overall what is seen at Ryugu? thanks.
Thank you for your questions. (1) the right figure in slide 3 shows naturally-heated 2 CM heated below ~500C and a CI heated above ~600C. In general, CM chondrites display brighter spectra than CI and samples heated at high temperature display brighter spectra than moderately heated samples. Orgueil CI chondrite which we selected to use originally has dark reflectance (~3% in the VIS) and heated samples are darker than unheated. (2) Our sample heated at 500C shows a good spectral match with Ryugu in terms of reflectance at 550 nm and 2.7-µm band properties. However, as you suppose, spectral slope from VIS to NIR of our fine-grained sample is redder than that of Ryugu possibly due to grain size effects.
Is Ryugu expected to have migrated inwards since its formation?
Yes, exactly. Ryugu is expected to be derived from the Main Asteroids Belt and some studies suggest that some near-Earth asteroids such as Ryugu might have experienced in the past frequent close approaches to the Sun.
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