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Spectroscopy and atmospheric transparency
The observation of celestial bodies using different types of ground-based telescopes is possible in the regions of electromagnetic spectrum for which the atmosphere is transparent. There are two spectral windows which allow the observation: the optical (V) up to the mid-infrared(the near-infrared 0.8 – 2.5 μm interval is denoted as NIR) and the radio one. The X-rays and ultraviolet wavelengths are blocked due to absorption by ozone and oxygen, while the far infrared radiation is blocked mainly due to absorption by water and carbon dioxide. While in the optical wavelength region the atmosphere is almost completely transparent, in the near-infrared there are absorption bands of water vapors making some regions like 1.4-1.5 μm and 1.8-2.0 μm poorly transparent (Fig. 2.2). Because of the effects of the atmosphere, observations with space telescopes, such as the Hubble and Spitzer telescopes, are very valuable. Another important difference between the V and NIR spectral intervals is the fact that the sky is brighter in the NIR region. For example in the J, H, K filters1 the estimated sky background has 15.7, 13.6, respectively 13 mag/arcsec2. Additional, important variations of the sky background could be observed in the intervals of tens of arc minutes of the sky.
Spectroscopy for asteroids
The knowledge of the surface mineralogy of individual asteroids and groups of asteroids can be inferred through the spectroscopy. The solar light reflected from the asteroids contains essential information regarding the optical properties of the materials found at the asteroids surface. The spectral interval 0.8 – 2.5 μm is very important to discriminate between different mineralogy of silicate-based compounds. Silicate minerals identification is based on the presence of broad bands of absorption around 1 and 2 μm. These bands are due essentially to the presence of olivine and pyroxene (or mixtures) on the surface of the asteroid.
Reflectance versus emission
The incident flux arriving from an asteroid surface is splitted in two contributions (Fig. 2.5): the solar radiation passively reflected by the surface material, and the solar radiation which has been absorbed, converted to heat, an re-emitted as thermal radiation [McCord & Adams, 1977].
IRTF Telescope and the SpeX instrument
Several large telescopes are equipped with a spectrograph. Some examples among those supporting research programs for planetary sciences are: the NASA InfraRed Telescope Facility (IRTF), the European Southern Observatory (ESO) Very Large Telescope (VLT), the ESO New Technology Telescope (NTT) and Telescopio Nazionale Galileo (TNG).
The NIR spectra presented in this thesis are obtained with NASA IRTF (Fig. 3.1a), a 3.0- meter telescope located on the top of Mauna Kea – Hawaii. It was built initially to support the Voyager missions, but today at least 50% of the observing time is devoted to planetary sciences. The IRTF hosts 6 facility instruments:, SpeX (Fig. 3.1b), NSFCAM2, CSHELL, MIRSI, Apogee,Moris. These instruments allow imaging, polarimetry, low and high resolution spectroscopy in the near to mid infrared (0.8 – 30) μm. SpeX – the most used instrument by planetologists from NASA IRTF telescope, is a low to medium resolution spectrograph and imager in the (0.8-5.5) μm. It provides spectral resolutions of R ≈ 1000 – 2000 across 0.8 – 2.4 μm, 2.0 – 4.1 μm, and 2.3 – 5.5 μm, using prism cross-disperser [Rayner et al., 2003]. Single order long slit modes are also available. A high throughput prism mode is provided for 0.8 – 2.5 μm spectroscopy at R ≈ 100. SpeX employs a 1024×1024 Aladdin3 InSBb CCD array for acquiring the spectra, while image acquisition could be made with a 512×512 Alladin2 CCD InSb array.
Spectral comparison – Comparative planetology
Spectroscopy of different samples performed in the laboratory provides the basis upon which compositional information about unexplored planetary surfaces can be understood from remotely obtained reflectance spectra. Thus, confronting the spectral data derived from telescopic observations with laboratory measurements is an important step in study of asteroid physical properties [Britt et al., 1992, Vernazza et al., 2007, Popescu et al., 2011]. Among the laboratory samples, meteorites can provide the most fruitful results for understanding asteroid composition. This is owed to the fact that, prior to their arrival meteorites are themselves small bodies of the solar system. Thus, spectral comparison represents a direct link for understanding of asteroid-meteorite relationships.
The traditional classification is based upon their appearance [de Pater & Lissauer, 2010]:
• metal meteorites are referred as iron meteorites. They are made primary of iron and nickel and smaller amounts of siderophile elements (elements which easily combine with molten iron);
• meteorites that contain comparable amounts of macroscopic metallic and rocky components are called stony irons.
• meteorites that do not contain large concentrations of metal are know as stones. A second classification of meteorites takes into account their mineralogic changes: achondrites are igneous bodies, the product of melting, changes in composition and recrystallization, while chondrites are the primitive meteorites composed of material that formed the solar nebula and surviving interstellar grains, little modified in some case by aqueous and/or thermal processes.
Space weathering effects
It is now widely accepted that the space environment alters the optical properties of airless body surfaces (Fig. 4.3). Space weathering is the term that describes the observed phenomena caused by these processes operating at or near the surface of an atmosphere-less solar system body, that modify the remotely sensed properties of this body surface away from those of the unmodified, intrinsic, subsurface bulk of the body [Chapman, 1996, 2004].
Table of contents :
Tables
Figures
I INTRODUCTION
1 Why asteroids?
1.1 The place of asteroids in the structure of the Solar System
1.2 The Discovery Of Asteroids
1.3 Distribution and diversity of asteroids
1.4 Asteroid brightness and albedo
1.5 My contribution to asteroids discovery
2 Why spectroscopy?
2.1 Diffraction gratings and prisms
2.2 Spectroscopy and atmospheric transparency
2.3 A simple application
2.4 Spectroscopy for asteroids
2.4.1 Reflectance versus emission
2.4.2 Spectral features
II TECHNIQUES FOR ASTEROID SPECTROSCOPY
3 Observing techniques
3.1 IRTF Telescope and the SpeX instrument
3.2 Planning the observations
3.3 Data reduction procedures
4 Spectral analysis techniques
4.1 Interpretation
4.1.1 Taxonomy
4.1.2 Spectral comparison – Comparative planetology
4.1.3 Space weathering effects
4.1.4 Band parameters
4.2 Algorithms
4.2.1 Taxonomic classification
4.2.2 Curve matching
4.2.3 Computing the space weathering effects
4.2.4 Application of the Cloutis model
5 M4AST – Modeling of Asteroids Spectra
5.1 Spectral database
5.1.1 Structure of M4AST database
5.1.2 The content
5.1.3 M4AST database via the Virtual Observatory
5.2 The interface
5.2.1 Database interface
5.2.2 Modeling tool interface
5.2.3 Updating the database
5.3 Testing of M4AST
5.3.1 Results
5.3.2 Discussions regarding misinterpretations of spectra
III OBSERVATIONS AND RESULTS
6 Spectral properties of near-Earth asteroids
6.1 Log of observations
6.2 S-type Near-Earth Asteroids
6.2.1 (1917) Cuyo
6.2.2 (8567) 1996 HW1
6.2.3 (16960) 1998 QS52
6.2.4 (188452) 2004 HE62
6.2.5 2010 TD54
6.2.6 (164400) 2005 GN59
6.3 Spectral properties of two primitive NEAs
6.3.1 (5620) Jasonwheeler
6.3.2 2001 SG286
6.4 Discussion
7 Spectral properties of Main Belt Asteroids
7.1 Log of observations
7.2 (9147) Kourakuen – a V-type asteroid outside Vesta family
7.3 A binary asteroid: (854) Frostia
7.4 1333 and 3623 – two asteroids with large amplitude lightcurves
7.4.1 (1333) Cevenola
7.4.2 (3623) Chaplin
7.5 Asteroid pairs: (10484) Hecht, (31569)1999 FL18
IV CONCLUSIONS AND PERSPECTIVES
8 Conclusions and perspectives
A The GuideDog and the BigDog interfaces
B List of publications
B.1 First Author
B.2 Co-Author
B.3 Conferences and Workshops
