VIRTIS flyby of Steins M-IR Spectral analysis Virtis Rosetta TM, Paris 10-12/05/2010  Stéphane Erard & Virtis-H team stephane.erard@obspm.fr

Virtis Steins geometry Version presented at the previous TM (computed February 2009) Uses: • Refined S/C attitude kernel from Osiris images (Oct 2008) • PC6 pointing info for Virtis H • Steins figure from early Osiris estimates (ellipsoid 2.87 x 2.45 x 2.30 km) Next version, May/October 2009: Osiris plate model implemented in our geometric code • Then using the Spice development routines for plate models (DSK) Uses L Jorda model with L Kamp spice routines & F Tosi advices, interfaced with GeoRos by X Bonin + some improvements in GeoRos (H/M coregistration better than 1 px)

M data projection Disk orientation as seen from S/C 3D animation : I1_00179260351.wrl Before start of acquisition At end of acquisition Rotation during acquisition ~40° Osiris 18:38:00 Osiris 18:39:18 See animation for acquisition scheme: PI895I31_2a.gif

M cube projections Angle values are fully consistent with the data OK within ~1 pixels, with reduced accuracy at the limb Correspondence between observed and retrieved craters. Limitations are assumed to be related to model orientation Estimated intercepts: From plate model From spheroid (previous results) Incidence angle Emergence angle Measured I/F From DTM Measured I/F

1- M vs H features Example M spectrum M-IR contains a systematic “bump” at 4.87 µm 2 absorption bands on the sides? Not a calibration effect 4.87 µm Band depth       Reflectance

M vs H features H status: • Steins acquisition perturbed by on-board software (fixed for ESB3) => oscillations at short wavelengths • FOV generally not filled by the target => grating not entirely illuminated Early studies : no 4.87 µm feature detected (except a single cold pixel)

M vs H features H acquisition: From FOV projection on plate model: 22 spectra with H FOV entirely filled => can be used to study detailed spectral features

M vs H features H: 22 pixels with FOV filled by Steins - preliminary calibration M: 104 pixels far from limb H synchro issue M filter defect

M vs H features H preliminary calibration: No feature detected from these 22 spectra H TF to be improved

M vs H features No discrepancy on ESB3 (Earth night side) => No systematic calibration issue

M vs H features Conclusions: • H channel does not confirm the 4.88 µm feature from M channel • H transfer function still to be improved, but TF is unlikely to conceal an existing feature H TF to be improved

2- Spectral modeling (updated) • Simultaneous inversion for T and spectral reflectance • Requires a photometric model to relate reflectance and emissivity • Results based on Lambertian model L-S provides similar results with slightly lower T

Spectral inversion: using plate model geometry Provides realistic T and reflectance T is slightly lower than with basic geometry 4.8 µm feature always appears as a peak in reflectance (emitted contribution is smaller) whatever the model used, and for every pixel Upper twist at long wavelengths => mix of temperatures? => T is too low?

Inversion: Temperature map Pixels near the limb may not be entirely filled From plate model Reflectance     Temperature map   Temperature map      (limb excluded)     (all pixels included) Temperatures histogram

Temperature variations Lambertian surface: T  cos(i)1/4 r  cos(i) Good correlation (>0.8) observed far from limbs Temperatures histogram limb pixels in blue

Inversion: average spectrum & dispersion Error bar = ±5 s • 4.8 µm feature is a peak in reflectance (emitted contribution is smaller) • Mix of temperatures? (either lateral or sub-surface gradient)

Analysis of variations Reflectance analysis based on ICA, limb excluded — main 3 components 1st comp (33% contrib) 2nd comp (23% contrib) 5th comp (21% contrib) Three main sources of variation: signal magnitude filter crossings / hot pixels + low frequency variations / slope near limbs filter pattern around 3 µm Reflectance 1 µm

Spectral inversion: fitting upper envelop The 4.87 µm dip is expected to occur between 2 bands => thermal emission should be larger With surface T ~ 6 K higher • Suppresses upward twist • “Band” profile does not look like a natural absorption band?

Conclusions Geometry is now OK • Target intercept is OK within ~1 M-px • Correspondence between measured signal and retrieved geometry is not perfect, though (craters - DSK version?) => Geometry files are available on Otarie server No confirmation of the 4.87 µm M-feature by Virtis-H • Calibration is being updated, but no feature is expected Spectral / Temperature inversion on M-IR • Temperature map + reflectance spectrum & variability Slight improvement relative to ellipsoid projection • On M-IR, one single feature observed: the bump at 4.87 µm (previously suspected features are artifact: 3.46 µm is hot pixel, 4.32 µm is a filter) • Surface follows the Lambertian model in good approximation