AIM Science with a Thermal IR Imager Simon Green Planetary & Space Sciences Department of Physical Sciences The Open University AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green

Why is Thermal Emission important? Diameter and albedo Thermophysical properties – temperature, – thermal inertia, – surface roughness, – regolith particle size – regolith production Composition Dynamics and Physical Evolution – The Yarkovsky effect – The YORP effect AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green [O. Groussin] 2

Fundamental properties of asteroid population - Large IR surveys (IRAS, Akari, Spitzer, WISE) AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Diameter and Albedo Large and dark? Small and bright? Wavelength (μm) Reflected Emitted Flux (W/m 2 /μm) Wavelength (μm) Reflected Emitted Flux (W/m 2 /μm) For AIM: Spatially resolved - Albedo variation (surface heterogeneity?) - Thermal images for shape model and navigation? 3

Observed flux from surface element (image pixel) proportional to ε(λ,φ,θ) B(λ,T(t)) Temperature (T) of a surface element i depends on: - heliocentric distance (r) - bolometric Albedo (A) - rotation period (P) - direction of rotation axis (λ P, β P ) - orientation of surface (n) i.e. large scale shape - thermal inertia (Γ = √(kρC)) - surface roughness (on scales smaller than the thermal skin depth) Emissivity (ε) depends on: - composition - viewing/illumination angles (shape and surface roughness) AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green φθ n Ti(t)Ti(t) Thermophysical properties 4

5/12 Thermophysical properties Total Incident Flux Heat Conducted Radiated Energy Direct sunlight Scattered Sunlight Thermal emission Conduction Thermal emission Conduction Surface facet i Surface facet j 5

Temperature derived from observed flux Brightness temperature unreliable because of beaming Need to measure spatially resolved thermal flux at: - at least 2 wavelengths λ - range of times i.e. range of illumination angles θ (function of pole orientation) - range of emission angles φ AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Thermophysical properties Temperature Constraints on surface physical properties Significant impact on S/C energy budget Didymoon r = 1.03 AU Assumed: A = 0.07 ε = 0.9 β P = 90° P = P orb = 11.9 h [M. Delbo] Γ = 50 Γ =

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Equatorial surface element; r = 1AU; P = 3.5 hr; β P = 90°; roughness = 0 [M. Delbo] Thermal inertia Γ = √(kρC) Moon: 50, rock: ~2500 (J m -2 K -1 s -0.5 ) Largest variation is in thermal conductivity (k), for regoliths Function of temperature Thermophysical properties 7

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green [Delbo et al., Asteroids IV, Univ. Arizona Press, p107, 2015] Thermal inertia (normalised to temperatures at 1 AU) Didymoon? Thermophysical properties Didymain? 8

Thermophysical properties Γ for different thermal skin depths (l s = √(kP/2πρC) i.e. for P rot Didymoon P ecl Didymoon and P rot Didymain P ecl Didymain AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Eclipses r = AU [Michel et al. Adv. Space Res. submitted.] 9

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Angular dependence of thermal emission due to surface roughness on scales smaller than the facet size (but larger than thermal skin depth ~cm) Thermophysical properties Sunlight Thermal Emission Lambertian Flat Surface Sunlight Beamed Rough Surface Thermal Emission Surface roughness 10

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Thermophysical properties Observer above Sun behind observer Sun In front of observer Local morning illumination Observed thermal emission highly dependant on viewing angle Surface roughness 11

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Crater floor is warmer than surrounding flat terrain Composition Surface roughness MidnightMorning Midday Degeneracy between Γ and roughness in thermophysical models Surface roughness included in models by adding hemispherical section craters to each facet of global shape model Degree of roughness specified by fraction of surface covered with artificial craters 12

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Landing site selection Distinction between solid rock & porous rockregolith Thermophysical properties ISS Rough terrain “Smooth”terrain – How smooth? Itokawa Regolith particle size related to thermal inertia Trend to larger grain size for smaller asteroids [Gundlach & Blum, Icarus 223, 479, 2013] 13

Evidence from Moon Lunar Diviner night-time observations indicate boulder coverage in crater ejecta.  implies shorter than expected boulder survival rates. [Ghent et al. Geology 42, 1059, 2015] Areal fraction of rocks Regolith production by thermal fracturing Thermophysical properties AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green [Delbo et al., Nature 508, 233, 2014] 14

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Thermal IR region more diagnostic than Visible/Near IR Composition Spitzer spectrum of 956 Elisa. [Lim et al. Icarus 213, 510, 2011] 15

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Emissivity features relative to thermal model continuum Composition [Lim et al. Icarus 213, 510, 2011] Features sensitive to: - Silcate mineralogy - Crystallline/amorphous - Regolith particle size - Orientation 16 AIM requirement: λ/Δλ ~ Selected filters give some composition diagnostics

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Yarkovsky effect Diurnal asymmetry of thermal emission - change in orbital semi-major axis - 1/D size dependence: Important for bodies up to ~40 km. - Critically depends on thermal inertia, obliquity and shape Effect of seasonal asymmetry dominates for small fast rotators 17

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Yarkovsky effect Moves asteroids into Earth crossing orbits (vis transport into main belt resonances) 18 ? Disperses asteroid families Makes PHA asteroid orbits difficult to predict AIM can potentially provide ground truth for Yarkovsky models (measured Γ, shape and rotation properties) if Yarkovsky drift is detected for Didymain. Semi-major axis drift da/dt yields bulk density independently. [B. Rozitis]

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green YORP effect Non-radial thermal (and reflected) radiation induces torque - 1/D 2 size dependence: Important for km-sized bodies - changes spin period and obliquity - depends critically on shape and roughness BYORP - radiation-induced acceleration of asymmetric synchronous satellite - can limit satellite lifetime - but under certain conditions results in binary stability [Jacobson & Scheeres, AJ Lett. 736, L19, 2011] 19

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green YORP effect 20 Fast Slow Produces fast and slow rotators in short timescales Deforms the asteroid shape Can cause asteroid to lose material and form binary Didymain is close to the strengthless rubble pile spin limit with “Yorpoid” shape indicative of mass transport - May show variation of thermophysicl properties with latitude?

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green AIM Science Requirements for Thermal IR Science: S.TIR.3.p1: to discriminate between bare rock and rough surfaces. This requires the measurement of the brightness temperature distribution over the surface from a single observation geometry for a rough estimate of thermal inertia. S.TIR.3.s1: to characterize the surface temperature to an accuracy of 5K (goal 1K) at a spatial resolution of a few metres. S.TIR.3.s2: to derive the thermal inertia at a spatial resolution of a few metres through observations at a range of local times and phase angles. S.TIR.9.s3: to characterize the surface composition through spectral mapping of the surface with a resolution λ/Δλ of 200. S.TIR.7.s4; to observe the evolution of the plume of dust ejected by DART’s impact on Didymoon S.TIR.6.s5: to measure the thermal properties of Didymain. Technology: T.TIR.4.p1; To demonstrate the use of an IR instrument to support the asteroid rendezvous phase, by enabling the acquisition of complementary data for Flight Dynamics analyses. 21

Minimum requirement (for primary objective S.TIR.p1): Brightness temperature at one solar geometry Ambiguities in thermal inertia/roughness ≥ 2 wavebands gives more diagnostic colour temperature Range of solar phase angles 5 equatorial positions + 2 ~polar? (e.g. 0° ±45-60° ±90-120°) Images at least every 30° of rotation for Didymain and Didymoon  characterisation of beaming and roughness Additional imaging of impact plume TIRI Observation strategy ? ? ? ? AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green 22

For Imaging Assume 640x480 pixel ULIS (Grenoble) uncooled bolometer array (larger arrays available at lower TRL) At X 1 = 10 km: FoV = ~11°  3 m/pix AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Required TIRI characteristics For slit spectrometer: For full coverage of Didymoon, repeated scans over rotation At least two solar phases R p + R s + a orb Range, X 1 θ Margin 2as2as X2X2 ~ pix ~46-63 pix θ At X 2 = 1 km:  Full disk coverage of Didymoon, 30 cm/pix 23

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Impact plume observations? Secondary Science Objective Enhanced spectral features from dust compared with regolith Dust albedo, size distribution constraints Limitations: - Low signal (optically thin) - constrained observation geometry (s/c safety) - low resolution (100 km range) - Short timescale (limits integration) Possible post impact surface alteration detectable? NASA/JPL-Caltech/C. Lisse [Johns Hopkins Univ./Univ. Maryland] 24

AIM Science Meeting, ESAC, 1-2 March 2016 S.F. Green Conclusions Thermal IR spectrophotometry can discriminate nature of surface … bare rock, porous/regolith, particle size critical for momentum enhancement Thermophysical properties: - changes as a result of DART impact (crater + ejecta field) - constrain models to interpret Yarkovsky and YORP effects - provide ground-truth for modelling of disk-integrated data Spectroscopy provides additional compositional information Diagnostic plume measurements may be possible Multiple wavelengths and observing geometries are required 25