Showing papers by "Jian-Yang Li published in 2020"
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We will present the results of coordinated observations of 2I/Borisov with the Neil Gehrels-Swift observatory (Swift) and Hubble Space Telescope (HST), which allowed us to provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star. Comets are condensed samples of the gas, ice and dust that were in a star’s protoplanetary disk during the formation of its planets, and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration distributes volatiles [1], organic material and prebiotic chemicals around their host system [2]. In our Solar System, hundreds of comets have been observed remotely, and a few have been studied up close by space missions [3]. Similarly, interstellar comets offer a glimpse into the building blocks, formation, and evolution of other planetary systems. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars. 2I/Borisov, discovered in Aug. 2019, is the first notably active interstellar comet discovered in our Solar System [4].
We used the UltraViolet Optical Telescope (UVOT) of Swift to determine 2I/Borisov’s water production rates and dust content surrounding the nucleus at six epochs spaced before and after perihelion on Dec. 8.55, 2019 UTC (-2.56AU to 2.54AU) [5]. Water production rates increased steadily before perihelion at a rate of increase quicker than that of most dynamically new comets but slower than most Jupiter-family comets. After perihelion, the water production rate decreased much more rapidly than that of all previously observed comets. We used a sublimation model to constrain the active area and minimum radius of the nucleus, and found that a significant fraction of the surface of Borisov is active.
We also used Cosmic Origins Spectrograph (COS) on the HST during four epochs around the perihelion and clearly detected the emissions of several bands of the CO Fourth Positive system, which we used to derive CO production rates [6]. Comparing these with the water production rates determined by Swift, we found that after perihelion, the coma of 2I/Borisov contains substantially more CO than H2O gas. Our abundances were more than three times higher than previously measured for any comet in the inner (<2.5 au) Solar System [3]. The derived high abundance ratio of CO/H2O and high elemental abundance of carbon relative to oxygen firmly sets 2I/Borisov apart from solar system comets, and suggest that the physical and chemical environment were Borisov was formed are substantially different from those in our solar system [6, 7] .
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1. Introduction
Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) is a NASA New Frontiers mission to return a sample of near-Earth asteroid (101955) Bennu. The OSIRIS-REx spacecraft is equipped with a suite of scientific instruments [1], including the OSIRIS-REx Camera Suite (OCAMS) and the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS), among others. OCAMS’s high-spatial-resolution images of Bennu’s surface facilitate the identification of regions of interest, characterization of surface morphology, and mapping of relative surface albedo [2]. OVIRS is a point spectrometer that measures surface composition [3]. The OVIRS footprint during the Reconnaissance phase of the mission [1] had an observational field of view with diameters between 5 to 9 m.
We search for and distinguish organics and carbonates on the surface of Bennu by studying the shape of the 3.4-micron feature, as observed by the OVIRS spectrometer during reconnaissance of candidate sampling sites [4]. Aliphatic organics have vibrational stretching bands at 3.4 microns, and the CO32- anion in carbonate minerals has an overtone of a fundamental asymmetric vibrational stretch, also at 3.4 microns. Organics on asteroids are hypothesized to be related to the organic materials delivered to Earth during the early bombardment phase of Earth’s development [5]. Carbonates record evidence of aqueous processes on Bennu [6], and together the organics and carbonates provide evidence for Bennu’s alteration history.
We present the results of a search for Bennu spectral matches to specific laboratory carbonate spectra and meteoritic aliphatic organic spectra, in the wavelength range from 3.2 to 3.6 microns. The carbonate spectra were obtained from the RELAB facility at Brown University [7], and the insoluble organic material (IOM) spectra were obtained from [8]. For the carbonates, we test ~10 representative spectra each of calcite, dolomite, and magnesite. For the organic IOM, we test spectra such as Tagish Lake, Cold Bokkeveld, Mighei, Murchison, and Orgueil.
2 Data collection and preparation
2.1 Spectrum preparation
To prepare laboratory and OVIRS spectra for band-match testing, we first divide every spectrum by a continuum, defined as a second order polynomial. The parabola is constrained using reflectance values at the wavelengths 2.95, 3.24, 3.6 microns. We then “stretch” the band by normalizing each spectrum such that the band minimum occurs at 0.0 and the maximum occurs at 1.0. Thus, we are only comparing the band shapes, as all other quantitative information has been removed by the continuum normalization and the band stretching.
2.2 K-S parameter test
We apply a Kolmogorov-Smirnov (K-S) parameter test to find which laboratory spectra best fit the OVIRS spectra. We compare each laboratory spectrum to the entire OVIRS data set. The K-S parameter is an evaluation of the maximum discrepancy between two cumulative distribution functions (Figure 1). That is, we calculate the cumulative sum of the laboratory and OVIRS spectra and find the points of maximum discrepancy. The smaller the discrepancy, the better the fit.
3 Results
Figure 1 shows an example match between an OVIRS spectrum from Bennu and a laboratory spectrum of calcite. Because the K-S parameter has a very low value (<0.013), we consider this a strong calcite detection. We perform the same test for the different types of carbonates and detect calcite more frequently than dolomite or magnesite. We perform the K-S test for all different types of IOM in our spectral library, and we find good matches on Bennu for the IOM in the Tagish Lake, Cold Bokkeveld, Mighei, Murchison, and Orgueil meteorites.
We will present maps of where carbonate and organic material is detected on Bennu and discuss our search for associations or correlations with boulders or other surface properties.
Acknowledgements
This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. INAF participation was supported by Italian Space Agency grant agreement n. 2017-37-H.0. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible.
References
[1] Lauretta, D.S. et al. (2017). OSIRIS-REx: Sample Return from Asteroid (101955) Bennu. Space Sci. Rev. 212, 925–984.
[2] Rizk, B. et al. OCAMS: the OSIRIS-REx camera suite. Space Sci Rev (2018) 214:26 https://doi.org/10.1007/s11214-017-0460-7
[3] Reuter, D.C. et al. (2018). The OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS): Spectral Maps of the Asteroid Bennu. Space Sci. Rev. 214, 54.
[4] V. E. Hamilton. VNIR-TIR spectroscopy of (101955) Bennu. This conference.
[5] Anders, Edward. Pre-biotic organic matter from comets and asteroids. Nature 342.6247 (1989): 255-257.
[6] Kaplan, H. H., et al. Evidence of Organics and Carbonates on (101955) Bennu. LPI 2326 (2020): 1050.
[7] Pieters, C.M., et al. Reflectance Experiment Laboratory (RELAB) Description and User's Manual. NASA Technical Reports Server. (2004). Document ID, 20040129713
[8] Kaplan, H.H., et al. Reflectance spectroscopy of insoluble organic matter (IOM) and carbonaceous meteorites. Meteorit Planet Sci, 54 (2019): 1051-1068. doi:10.1111/maps.13264