Heated Synthetic Murchison Reflectance Spectra

1

S. Sidhu1, P. Mann2 and E. Cloutis1
Centre for Terrestrial and Planetary Exploration (C-TAPE), The University of Winnipeg
2Department of Geography, The University of Winnipeg

Introduction: Several carbonaceous chondrites (CCs) display evidence of thermal alteration [1]. Remote sensing data from theJAXA Hayabusa2 asteroid sample return mission suggests that asteroid (162173) Ryugu’s parent body may have undergoneinternal or impact-generated heating prior to breakup [2]. Furthermore, models of orbital and thermal evolution of Ryugu suggestthat the asteroid was likely exposed to surficial heating during close passes to the Sun. [3]. Recently, geomorphological features
observed on Bennu’s surface indicate alteration of rocks due to thermal fatigue [4]. Constraining the spectral characteristics ofheating will aid in identifying the timing and intensity of heating experienced by Ryugu and its parent body, as well as otherextraterrestrial bodies which may have also undergone a period of thermal alteration.
Methods: The Centre for Terrestrial and Planetary Exploration (C-TAPE) at the University of Winnipeg conducted aheating experiment on a synthetic Murchison simulant. The synthetic mixture, named WMM, consisted of various major
representative phases in CM carbonaceous chondrites as following: 85 wt. % ASB267, a dark serpentinite representative of CM
phyllosilicates. 5 wt. % SHU102, shungite, terrestrial organic representative of organic phases. 5 wt. % TRO203, synthetic
troilite representative of sulfides, and 5 wt. % MAG200, magnetite. The simulant consisted of <63 µm powders of serpentinite,shungite, and troilite, and ~20 nm sized magnetite that appeared to be maghemite from X-ray diffractometry (likely due to surface
oxidation of the nanoparticles). The samples were heated up to 1200C at 100C increments. XRD and reflectance spectra werecollected prior to heating, and after each temperature increment. A new sample powder was used for each increment, and samples
were heated under continuous nitrogen flow. Samples were heated for one week at intervals of 100C to 1000C. However, due
to the oven’s constraints, samples were only heated for 2 hours at 1100C and half an hour at 1200C. The reflectance spectrumfor 300C is unavailable due to an accidental sample spill while removing the powder from the oven. The spectra were collectedwith an Analytical Spectral Devices (Boulder, CO) LabSpec 4 Hi-Res® spectrophotometer between 350 and 2500 nm. Spectra
were measured at a viewing geometry of i = 30º and e = 0º with incident light being provided by an in-house 150 W quartztungsten-halogen collimated light source. Sample spectra were measured relative to a Labsphere Spectralon ® 100% diffuse
reflectance standard and corrected for minor (less than ~2%) irregularities in its absolute reflectance.
Results: Heating the Murchison simulant samples resulted in spectral variation across the temperature increments. Thespectra show little change up to ~500C (Figure 1). Beyond 500C, the spectra display features associated with Fe3+
oxyhydroxides, such as low reflectance below ~500nm, a steep red slope in the visible region, and absorption features near 870
nm. Absorption bands associated with serpentine, at 1400 and 2320 nm, are evident up to ~600-700C. The spectra generally
become brighter up to ~800C, and then darken substantially beyond this temperature. At and above 1100C, the spectra are
generally dark and featureless, with a broad absorption region centered near 1600 nm.
1Absolute Reflectance

0.8
0.6
0.4
0.2
0300

800
Before Heating
500°C
900°C
Figure 1.

100°C
600°C
1000°C

1300
1800
Wavelength (nm)
200°C
700°C
1100°C

Reflectance Spectra of Simulated Murchison Mixture WMM.

2300
400°C
800°C
1200°C

Additionally, we heated end members used to produce the Murchison simulant to better understand the causes of thespectral changes observed. These samples (serpentine, shungite, troilite, and magnetite) were heated under the same conditions
as the simulant. Serpentine spectra remained unchanged up to ~200C, after which the spectra display an increasing red slope
below ~700 nm and an absorption feature near 860 nm, both of which are ascribed to Fe3+ oxyhydroxides. Furthermore, theOH absorption bands near 1400 and 2320 nm associated with serpentine were evident up to ~700C. Reflectance generally
increased up to ~600C, and decreased at higher temperatures. Shungite was stable up to ~500C. However, at 600C, theorganic component of the sample was volatilized. Troilite remained stable up to ~300C, after which Fe3+ oxyhydroxides wereevident in its spectra. Fine-grained magnetite remained stable up to ~100C, after which it also transformed to Fe3+oxyhydroxides.
Discussion: Temperature-induced spectral changes are evident in the Murchison simulant used in this study. XRD
measurements confirm that the changes seen in the spectra can be related to the changes in the mineralogy of the samples. The
spectral changes occur in multiple wavelength regions, suggesting that various spectral metrics could be robust indicators oftemperatures experienced by a serpentine-rich carbonaceous chondrite.
Spectral changes observed in this study are associated with short heating periods (hours to days), and are thus,
applicable to short-term heating events such as impact heating. The applicability of these results to long-term heating events,
such as internal heating within larger asteroids, is unknown. Furthermore, the dry nitrogen environment under which thisheating experiment was conducted only represents one scenario of the conditions which may have prevailed on carbonaceouschondrite parent bodies during any heating events. Nevertheless, the results of our experiments suggest that spectroscopic datain the 350-2500 nm can provide insights into the intensity and duration of heating events affecting carbonaceous chondrite
asteroids. It is also worth noting that naturally-heated carbonaceous chondrites show spectral differences associated withchanges in mineralogy [1]. We are in the process of relating the results of these experiments to other carbonaceous chondrite
heating experiments [5, 6, 7, 8] and naturally-heated carbonaceous chondrites [9] to better understand the full applicability ofour results to observational data.
References[1] Cloutis E. et al. 2012. Icarus 220:586-617. [2] Sugita S. et al. 2019. Science 6437:252. [3] Michel P. & Delbo M. 2010.
Icarus 209:520-534. [4] Molaro J.L. et al. 2020. Nature Communications 11:2913. [5] Hiroi T. et al. 1993. Science 261:10161018. [6] Hiroi T. et al. 1993. Symposium on Antarctic Meteorites 18:93-96. [7] Hiroi T. et al. 1994. Proceedings of the NIPR
Symposium on Antarctic Meteorites 7:230-243. [8] Hiroi T. et al. 1996. Meteoritics and Planetary Science 31:321-327. [9]Hiroi T. et al. 1997. Lunar and Planetary Science Conference 28:577-578.