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Surface Properties of Asteroids from Mid-Infrared Observations and Thermophysical Modeling

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Surface Properties of Asteroids from Mid-Infrared Observations and Thermophysical Modeling Dissertation zur Erlangung des akademischen Grades Doktor der Naturwissenschaften (doctor rerum naturalium) Surface Properties of Asteroids from Mid-Infrared Observations and Thermophysical Modeling Dipl.-Phys. Michael M¨uller 2007 Eingereicht und verteidigt am Fachbereich Geowissenschaften der Freien Universit¨atBerlin. Angefertigt am Institut f¨ur Planetenforschung des Deutschen Zentrums f¨urLuft- und Raumfahrt e.V. (DLR) in Berlin-Adlershof. arXiv:1208.3993v1 [astro-ph.EP] 20 Aug 2012 Gutachter Prof. Dr. Ralf Jaumann (Freie Universit¨atBerlin, DLR Berlin) Prof. Dr. Tilman Spohn (Westf¨alische Wilhems-Universit¨atM¨unster,DLR Berlin) Tag der Disputation 6. Juli 2007 In loving memory of Felix M¨uller(1948{2005). Wish you were here. Abstract The subject of this work is the physical characterization of asteroids, with an emphasis on the thermal inertia of near-Earth asteroids (NEAs). Thermal inertia governs the Yarkovsky effect, a non-gravitational force which significantly alters the orbits of asteroids up to ∼ 20 km in diameter. Yarkovsky-induced drift is important in the assessment of the impact hazard which NEAs pose to Earth. Yet, very little has previously been known about the thermal inertia of small asteroids including NEAs. Observational and theoretical work is reported. The thermal emission of aster- oids has been observed in the mid-infrared (5{35 µm) wavelength range using the Spitzer Space Telescope and the 3.0 m NASA Infrared Telescope Facility, IRTF; techniques have been established to perform IRTF observations remotely from Berlin. A detailed thermophysical model (TPM) has been developed and exten- sively tested; this is the first detailed TPM shown to be applicable to NEA data. Our main result is the determination of the thermal inertia of 5 NEAs, increasing the total number of NEAs with measured thermal inertia to 6. For two of our targets, previously available estimates are refined, no reliable estimates have been available for the remaining three. The diameter range spanned by our targets is 0.1{17 km. Our results allow the first determination of the typical thermal inertia of NEAs, which is around 300 J s-1/2K-1m-2 (corresponding to a thermal conductivity of ∼ 0.08 W K-1m-1), larger than the typical thermal inertia of large main-belt asteroids (MBAs) by more than an order of magnitude. In particular, thermal inertia appears to increase with decreasing asteroid diameter. Our results have been used by colleagues to estimate the size dependence of the Yarkovsky effect, thus explaining the apparent difference in the size-frequency distribution of NEAs and similarly sized MBAs. Thermal inertia is a very sensitive indicator for the presence or absence of par- ticulate material on the surface, a fact that is widely used in, e.g., Martian geology. Our estimate of the typical thermal inertia of NEAs is intermediate between val- v Abstract ues for lunar regolith and bare rock, indicating that even sub-kilometer asteroids are covered with coarse regolith. This is consistent with spacecraft observations of the 0.32 km wide NEA (25143) Itokawa obtained in 2005. The correlation of thermal inertia with size indicates a trend of smaller objects having coarser and/or thinner regolith than larger objects. This may allow an improved understanding of regolith formation through impact processes. The first thermal-infrared observations of an eclipsing binary asteroid system are reported. To this end, the Trojan system (617) Patroclus has been observed using Spitzer. It is demonstrated that such observations enable uniquely direct thermal-inertia measurements. In particular, we report the first reliable estimate of the thermal inertia of a Trojan. Additionally, two targets of future spacecraft encounters, (21) Lutetia and (10302) 1989 ML, have been observed. Their size and albedo has been deter- mined and their surface mineralogy constrained. Our results for 1989 ML, in particular, are relevant in the current planning of the ESA mission Don Quijote. vi Contents Abstract v 1 Introduction 1 1.1 Space missions to asteroids . 2 1.2 Asteroid populations and their origins . 4 1.3 The Yarkovsky and YORP effects . 8 1.4 Asteroids impacting Earth: Hazard and link to meteorites . 11 1.5 Physical properties of asteroids . 12 1.5.1 Diameter and albedo . 13 1.5.2 Taxonomy . 14 1.5.3 Spin rate . 15 1.5.4 Shape and spin axis . 16 1.5.5 Internal structure|are asteroids piles of rubble? . 16 1.5.6 Binarity . 18 1.5.7 Regolith . 18 1.5.8 Thermal inertia . 19 1.6 Scope of this work . 21 2 Thermal emission of asteroids 23 2.1 Overview . 23 2.2 Relevant physical properties . 26 2.2.1 Size and albedo . 28 2.2.2 Thermal inertia . 29 2.2.3 Beaming due to surface roughness . 31 2.2.4 Shape and spin state . 32 2.2.5 Observation geometry . 33 2.2.6 Thermal emission model . 34 2.3 Observability . 35 2.4 Simple models: STM and FRM . 36 2.4.1 Standard Thermal Model (STM) . 36 2.4.2 Fast Rotating Model (FRM) . 38 2.5 NEATM . 39 2.5.1 Model description . 40 2.5.2 Physical significance of η ................... 41 2.5.3 Accuracy of NEATM-derived diameters and albedos . 43 2.5.4 NEATM fitting routine . 44 vii Contents 3 Detailed thermophysical modeling 47 3.1 Overview . 47 3.1.1 Model description . 49 3.1.2 Model parameters . 49 3.1.3 Implementation overview . 50 3.2 Thermal physics . 50 3.2.1 Global shape and spin state . 50 3.2.2 Thermal conduction . 52 3.2.3 Thermal-infrared beaming . 60 3.3 Implementation . 73 3.3.1 Class structure . 73 3.3.2 Thermal conduction . 75 3.3.3 Beaming . 76 3.4 Validation . 77 3.4.1 Thermal conduction . 77 3.4.2 Beaming without thermal conduction . 78 3.4.3 Beaming with thermal conduction . 81 3.5 TPM fits to asteroid data . 82 4 IRTF observations 85 4.1 IRTF setup and instrumentation . 85 4.1.1 MIRSI . 87 4.1.2 Apogee . 89 4.2 Remote control of observations . 89 4.3 IRTF observing strategy . 91 4.4 Data reduction . 94 4.4.1 MIRSI . 94 4.4.2 Apogee . 97 5 Spitzer observations 99 5.1 Spitzer overview . 100 5.1.1 Spacecraft design and orbit . 101 5.1.2 Optical design . 103 5.1.3 Planning and proposing observations . 104 5.1.4 Asteroid-specific aspects to observation planning . 105 5.1.5 Spitzer data reduction . 107 5.2 InfraRed Array Camera (IRAC) . 107 5.2.1 Instrument layout . 108 5.2.2 Observing strategy for asteroid photometry . 111 5.2.3 IRAC BCD pipeline . 114 5.2.4 Data reduction steps taken by us . 116 5.2.5 Validation of our data reduction pipeline . 123 5.3 InfraRed Spectrograph (IRS) . 125 5.3.1 Instrument layout . 125 viii Contents 5.3.2 Peak-Up Imaging (PUI) . 127 5.3.3 Low-resolution spectroscopy . 133 6 Results 137 6.1 (433) Eros . 137 6.1.1 Introduction . 138 6.1.2 Modeling . 139 6.1.3 Results . 140 6.1.4 Discussion . 141 6.1.5 Summary . 142 6.2 (25143) Itokawa . 143 6.2.1 Introduction . 144 6.2.2 Thermal-infrared observations . 147 6.2.3 Results . 148 6.2.4 Discussion . 151 6.2.5 Summary . 155 6.3 (1580) Betulia . 155 6.3.1 Introduction . 156 6.3.2 Observations and data reduction . 157 6.3.3 Results . 159 6.3.4 Discussion . 161 6.3.5 Summary . 163 6.4 (54509) YORP . 164 6.4.1 Introduction . 165 6.4.2 Spitzer observations . 166 6.4.3 Results . 169 6.4.4 Discussion . 171 6.4.5 Summary . 172 6.5 (33342) 1998 WT24 . 173 6.5.1 Introduction . 173 6.5.2 Data . 175 6.5.3 Results . 177 6.5.4 Discussion . 180 6.5.5 Summary . 182 6.6 (21) Lutetia, Rosetta flyby target . 182 6.6.1 Introduction . 183 6.6.2 Observations . 184 6.6.3 Data analysis . 185 6.6.4 Discussion . 190 6.6.5 Summary . ..
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