We studied the influence of emerging angle on emissivity spectra measured in air and in vacuum, with particular attention to asteroids-like conditions

D'Amore, Mario

Ferrari, Sabrina

Helbert, Jörn

Maturilli, Alessandro

Institute of Transport Research:Publications

ON THE EFFECT OF EMERGING ANGLE ON EMISSIVITY SPECTRA: APPLICATION TO SMALL BODIES. A. Maturilli1, J. Helbert1, M. D’Amore1, S. Ferrari1, 1Institute for Planetary Research, German Aerospace Center DLR, Rutherfordstr. 2, 12489 Berlin, Germany – alessandro.maturilli@dlr.de  Introduction:  Dependence of  laboratory meas-ured emissivity spectra from the emerging angle is a subject that still needs a lot of investigations to be fully understood. Most of the precedent works are based on measurements of flat, solid surfaces (mainly metals) and they are not easily usable for the analysis of re-mote sensing data. Especially small bodies (c.f. aster-oid Itokawa, the target of JAXA Hayabusa mission) have a very irregularly shaped surface, hence the spec-tra from those regions are very difficult to compare with laboratory spectra, where the observing geometry is always close to “nadir”. At the Planetary Emissivity Laboratory (PEL) of  the German Aerospace Center (DLR) we have set-up a suite of measurements to investigate this problem, by dividing the experiment in 4 separate sessions. As a first step, we measured emissivity for the same quartz sample shaped in 5 different ways: flat, oblique, con-cave, waves, and “fairy castle” configuration. Those measurements were performed in air, at moderate sample temperature (150°C). Then we measured emissivity of two endmembers and their binary mixture (50% each in volume) in air, at moderate sample temperature (150°C), for a very small emerging angle (2.3°), for the smaller (<25µm) and the larger (125-250µm) of our typical grain size fractions. Successively, we measured the emissivity of two (very different in color and grain size) endmem-bers and their binary mixture (50% each in volume) in vacuum at moderate temperature (100°C as typical of asteroid daily temperature), for 0°, 5°, 10°, 15° emerg-ing angles. The last experiment, still undergoing, con-sists of measuring two asteroid analog materials (me-teorite Millbillillie and a synthetic enststite) in vacu-um, at a typical near-earth asteroid dayside tempera-ture (100°C), for emerging angles 0°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, 80°. Complementary reflectance measurements were taken for all the experiments. Experimental set-ups:  At PEL we use two in-struments equipped with external chambers to measure emissivity. The Bruker VERTEX 80V Fourier Trans-form Infrared-Spectrometer (FTIR) can be operated under vacuum to remove atmospheric features from the spectra. An external evacuable chamber is used to measure emissivity: it contains a motor-driven carousel for up to 12 samples in the same working session, thermal sensors for each sample and a webcam for monitoring the activities inside the chamber. Induction heating system allows to heat up the samples (in stain-less steel cups) from 50°C to higher than 800°C. Bruker A513 accessory is used to obtain biconical reflectance with variable incidence angle i and emis-sion angle e between 13° and 85° at room temperature, under purge or vacuum conditions, in the 1 to 100 µm spectral range.  The Bruker IFS 88 FTIR spectrometer has an at-tached emissivity chamber for measurements at low to moderate temperatures [1, 2], that can be cooled down to 0° C. Samples can be heated from room temperature to 150°C in a purging environment. A Harrick Sea-gullTM variable-angle reflection accessory allows measurement of the biconical reflectance at room tem-perature, under purging conditions in the extended spectral range from 0.4 to 55 µm, for i and e from 5° to 85°. For the measurements in air and in vacuum with small emerging angles (<15°, first three experiment described above) we used aluminum wedges (see Fig-ure 1), for the measurements in vacuum with all the emerging angles from 0° to 80° (step 10°) we used custom-designed stainless steel wedges.  Figure 1. Aluminum wedges with 5° and 10° degree inclina-tion.  Emissivity vs emergence angle:  Figure 2 shows emissivity for the same quartz sample whose surface was shaped in 5 different modes. Appreciable changes can only be observed in the Reststrahlen region (8-10µm) and in the region below 5µm. Those results are very close to the one obtained in [3].  Figure 2. Quartz 45-63µm sample with 5 surface shapes.  In Figure 3, we show the results of the second ex-periment. From this set of data we can conclude that in air, for low inclination angles (2.3°) no significant differences are observed between plane and inclined spectra. Surface scattering effect seems to influence more the larger grain size separates.   Figure 3. Deconvolution of a binary mixture (upper <25µm, lower 125-250µm) by using plane and inclined measure-ments.  Even when increasing the inclination angles in vacuum, the spectra still are not showing differences. We measured the endmembers and the mixture at 0°, 5°, 10°, 15° inclination and we could hardly see a change in the spectra.   Figure 4. Emissivity of a binary mixture with increasing inclination angles.  Figure 5. Deconvolution of a 15° inclined binary mixture by using endmembers measured with increasing inclination angles.  This is confirmed from the deconvolution we show in Figure 5, showing that we get the same results from linear deconvolution when using the 0°, 5°, 10°, or 15° inclined endmembers. In this case the linear deconvo-lution method shows its limit, because we mixed a bright material having deep and intense spectral fea-tures with a dark and spectrally flat one; the grain sizes were also different, so instead of finding a 50%+50% composition, the linear deconvolution algorithm pro-vides results around 25%+75%. Summary:  The PEL laboratory is already provid-ing emissivity measurements for asteroid analogs in view of future space missions to asteroids [4]. To un-derstand the influence of emerging angles on emissivi-ty spectra we built up a series of experimental meas-urements. We found spectral variations in the emissivi-ty of the same quartz sample whose surface was shaped in 5 different ways. We measured emissivity in air and in vacuum, for surface temperatures close to asteroids daily temperature, and we found that for small emerging angles (<15°) the spectra still show no significant differences. This result is already important because it means that for quasi-flat surfaces the analy-sis of remote sensing emissivity spectra can be suc-cesfully carried on with laboratory spectra with 0° inclination. We are currently performing the last set of measurements, in vacuum and at 100°C surface tem-perature for two asteroid analogs with inclination angle from 0° to 80° with a 10° stepping Figure 6).  Figure 6. Some of the wedges that will be used to measure inclined emissivity of asteroid analog samples.  References:  [1] Maturilli A. et al. (2006) PSS, 54, 1057. [2] Maturilli A. et al. (2008) PSS, 56, 420. [3] Wald A. E. and Salisbury J. W. (1995) JGR, 100, 24665–24675. [4] Maturilli A..et al. (2014), LPS, XXXXV, this meeting. 

On the Effect of Emerging Angle on Emissivity Spectra: Application to Small Bodies

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