MIROMIRO Continuum Observations of Asteroid (2867) Steins with the MIRO Instrument Ringberg Castle Tegernsee, Germany February 25-28, 2009 S. Gulkis, S. Keihm, L. Kamp, C. Backus, M. Janssen, S. Lee, B. Davidsson, + MIRO SCIENCE TEAM Jet Propulsion Laboratory California Institute of Technology S. Gulkis 2/22/09 OUTLINE MIRO * MIRO INSTRUMENT OVERVIEW * GEOMETRY OF THE ENCOUNTER * OVERVIEW OF THE OBSERVATIONS * DUAL CONTINUUM DATA * DATA ANALYSIS - SHAPE MODEL - THERMAL MODEL AND RADIATIVE TRANSFER * ERROR ANALYSIS * RESULTS and CONCLUSIONS * ACKNOWLEDGEMENT S. Gulkis 2/22/09 Instrument OvervieOverview MIRO Telescope 30 cm diameter Boresight along z-axis of s/c Receivers (two) Continuum 190 GHz (1.6 mm) 563 GHz(0.5 mm) Spectroscopic (563 GHz) H2O, CO, NH3, CH3OH Spectral resolution (44 kHz) Resolving Power = 1.3 x 10^7 Single linear polarization(crossed) Flip-mirror calibration(warm,cold,sky) Beam Characteristics Submillimeter HPBW-7.5 arc min Millimeter HPBW-23.8 arc min STRUCTURAL THERMAL MODEL S. Gulkis 2/22/09 ROSETTA-STEINS FLYBY GEOMETRY MIRODAY NIGHT S. Gulkis 2/22/09 Overview of ObservationObservations MIRO * MIRO was powered on for approximately 10 hours centeredon CA * Warm and cold calibration targets were observed every 30minutes * Instrument performance was nominal * Thermal emission from Steins was observed with high S/N inboth mm and smm continuum channels * First detection of Steins occurred about 92 s(mm)[44 s smm] before CA * MIRO boresight was significantly displaced from Steins formost of the fly-by - Phase coverage of data was limited [~ 315-90 deg] * Spectroscopic data were obtained but no spectral lines wereobserved S. Gulkis 2/22/09 Predicted Antenna Temperatures for beam centers on target center for beam centers on target center MIRO82.9 K-590 GHz 29.9 K-190 GHz Peak Observed Temperatures 1st detection Last detection For Planning Purposes Only Calibrated Continuum Data MIRO Millimeter Submillimeter S. Gulkis 2/22/09 ANALYSIS MIRO * Shape Model of Steins (NEXT SLIDE) * Important quantities needed in analysis - Sub-spacecraft and Sub-Solar positions - Unit surface normals on a grid around the intersection of boresight and Steins * Intersection of beam boresight with the shape model - Used Walter Sabolo's measurements of Steins in WAC images - Corrected for photometric center of Steins and true body center * Based on shape model and Lambertian model of scattered sunlight - Corrected for geometrical distortion in WAC image plane using a formula providedby Walter Sabolo * Distance from Rosetta obtained from SPK kernals - ORHR_______________00077.BSP for Rosetta - ORHO_______________00077.BSP for Steins * Orientation of asteroid relative to boresight - Used SPK data using calls from Spice This work carried out by Dr. Lucas Kamp (JPL) S. Gulkis 2/22/09 Digital Shape Model used in analysis MIRO * Based on data provided by OSIRIS instrument * Digital Shape Kernel (DSK)steins_ shape_model_ver2a.dsk * Provided by group at Laboratoire d'Astrophysique de Marseille, France (contact Olivier Groussin) * Reference: Lamy et al. 2008, A&A 487,1179 S. Gulkis 2/22/09 MIROThermal Models (Top) Pencil Beam Brightness Temp(Bottom) * Thermal models use Steins period, * * * * 1022 304 87 22 smm mm albedo (0.4 assumed) and heliocentric distance-Temperatures are equatorial. Albedo not well determined Thermal Inertia (mks units) - 22(lunar powder) - 87(lunar fine) - 304(lunar basalt) - 1022(lunar igneous rock) Arrows point to pencil beam zenithemission (blackbody equivalent T) for the two MIRO continuum wavelengths (0.53 mm & 1.58 mm) for thermal inertia = 22 Steins thermal model needs to fit IR(VIRTIS, SPITZER,+) andsubmillimeter data (MIRO) Calculations by Dr. Steve Keihm (JPL) S. Gulkis 2/22/09 MIRO BORESIGHTS NEAR CCA MIROS. Gulkis 2/22/09 11 Estimated Errors MIRO Pointing Random Systematic Range, km +/- 1 arc min All < [Modeling error] [Meas error] [Meas error] +/- 3 K mm +/- 1 K mm < |0.9 K| mm 820.7 +/- 12 K smm +/- 1 K smm < |2.7 K| smm Maximum CA 1412.6 +/- 1 K mm +/- 1 K mm < |0.3 K| mm Local Minimum +/- 1 K smm +/- 1 K smm < |0.2 K| smm 2129.4 +/- 1 K mm +/- 1 K mm < |0.3 K| mm 2nd Maximum +/- 12 K smm +/- 1 K smm < |0.8 K| smm S. Gulkis 2/22/09 MODEL PARAMETERS MIRO MODEL PARAMETER POWDER ROCK Density (g/cm^3) 1.25 2.6 Thermal Conductivity (w/cm K) 6 x 10^-6 6 x 10^-3 Specific Heat (w-s/g K) 0.67 0.67 Thermal Inertia MKS (J/K m^2 s^0.5) 22 1022 Dielectric Constant 2.34 5.03 Thermal Skin Depth (cm) 0.22 4.89 Electrical Skin Depth (cm )(smm-mm) .24-.76 .04-.12 Loss Tangent .017 .036 S. Gulkis 2/22/09 Observed and Model Ant TemperatureAnt Temperatures MIRO RANGE TA(OBS) TA(M) TA(OBS) TA(M) KM smm smm mm mm Powder 820.7 82.9 92.4 29.9 25.1 Rock 84.5 24.9 Powder 1013.9 18.6 19.9 15.5 11.3 Rock 19.1 11.4 Powder 1412.6 5.3 1.6 9.2 4.9 Rock 1.6 5.0 Powder 1722.1 11.3 8.0 10.3 6.1 Rock 8.6 6.3 Powder 2129.4 25.7 25.2 9.7 6.2 Rock 28.3 6.4 Powder 2640.1 17.4 17.9 7.2 4.0 Rock 20.4 4.2 S. Gulkis 2/22/09 Pointing based on photographic measurements Brightness Temperature Models for Steins at MIRO Wavelengths (0.53 & 1.58 mm) MIRO Calculations by Dr. Steve Keihm S. Gulkis 2/22/09 Surface temperature for various model assumptions (Latitude = 0) assumptions (Latitude = 0) MIRO Calculations by Dr. Steve Keihm S. Gulkis 2/22/09 CONCLUSIONS MIRO * Both high and low thermal inertia models, similar to those used for the moon, can fit the MIRO data within the uncertainties of the data; * Principal source of errors is the modeling pointing error, not the measurement errors; for the peak antenna temperature measured, the error is estimated to be 12 K (smm) and 3 K (mm) for a 1 arc min pointing error; * For low thermal inertia models, the ratio of thermal to smm electrical skin depths is of the order of unity; the thermal wave is attenuated by 1/e at the depth of penetration * For high thermal inertia models, the thermal skin depth is considerably larger than the smm electrical skin depth and the surface temperature is measured * Thermal models for Steins need to fit VIRTIS, SPITZER, and MIRO data; the high surface temperatures reported by VIRTIS and SPITZER require a low thermal inertia regolith; * There is a suggestion that the emissivity of a low thermal inertia model needs to be less than that calculated from the dielectric constant itself. We estimate that emissivity could be as low as .79 but probably not lower. Reduced emissivity could be produced by subsurface scattering. * Reducing the loss tangent doesn't lower the temperature significantly S. Gulkis 2/22/09 ACKNOWLEDGEMENT MIRO The MIRO team would like thank- ESOC and ESAC teams for their technical support -OSIRIS, VIRTIS and ALICE teams for sharing their pre- published data -U. Keller, P. Lamy, O. Groussin, W. Sabolo, M. Fulchignoni, and A. Barucci for information about their Steins observations and models S. Gulkis 2/22/09 MIROMIRO Backup Figures S. Gulkis 2/22/09 DOPPLER SHIFT AND ANG DIAM MIRO Phase Angle (|Rosetta-Steins-Sun|) MIRO Angular Diameter SteinDiameter Steins MIRO MIRO TIMELINE AT STEINAT STEINS MIRO Min from CA EVENT -329 -------Power on -307 -------CTS/dual continuum -37 -------Start s/c roll -36.5 -------End s/c roll -16 -------Dual continuum -11 -------Asteroid mode -1.9 -------~ 0 solar phase angle -1.5 -------First mm detection -0.7 -------First smm detection 0 -------CA 0.5 -------Peak mm signal (29.9 K) 0.6 -------Peak smm signal (82.9 K) 1.7 -------s/c over morning terminator 10.6 -------Loss of signal 10.6 -------Dual continuum 13 -------CTS/dual continuum 302 -------Disable science acquision S. Gulkis 2/22/09 Pointing Error Sensitivity MIROS. Gulkis 2/22/09 Range, km Millimeter +/- 1 arc min Sub-Millimeter +/- 1 arc min 820.7 Maximum near CA +/- 3 K +/- 12 K 1412.6 Local Minimum +/- 1 K +/- 1 K 2129.4 2nd Maximum +/- 1 K +/- 12 K Brightness Temperature Models for Steins at MIRO Wavelengths (0.53 & 1.58 mm) MIRO CW CCW k (w/cm-K) rho (gr/cm^3) Model computed by Steve Keihm (JPL) S. Gulkis 2/22/09 Science Objectives MIRO * Constrain thermal and electrical properties of Asteroid Steins - k Thermal conductivity (W/cm K) - c Specific heat - *r Dielectric constant - * Density - Tan(*) Loss tangent = 2*/*r * * Observation related quantities - Thermal skin depth (2k/**c)^0.5 (thermal wave damping to 1/e ) - Thermal inertia (k*c)^0.5 * Assist in the determination of regolith properties * Detect or set upper limit on abundance of water vapor around Asteroid Steins S. Gulkis 2/22/09 Thermal and Electrical Skin Depth Estimates for MIRO Estimates for MIRO MIRO * Thermal Skin Depth (amplitude for thermal wave 1/e surface value) - Thermal skin depth = 2k "#c * 2 mm * Electrical Skin Depth - d= " 2#tan($)% - * 5 * for fine powder (loss tangent = .017) - d(smm) = 2.4 mm * 1 thermal skin depth - d( mm) = 7.5 mm * 3 thermal skin depths S. Gulkis 2/22/09