Experimental Phase Functions of Millimeter-sized Cosmic Dust Grains

Abstract

We present experimental phase functions of three types of millimeter-sized dust grains consisting of enstatite, quartz and volcanic material from Mount Etna, respectively. The three grains present similar sizes but different absorbing properties. The measurements are performed at 527 nm covering the scattering angle range from 3 to 170 degrees. The measured phase functions show two well defined regions i) soft forward peaks and ii) a continuous increase with the scattering angle at side-and back-scattering regions. This behavior at side-and back-scattering regions are in agreement with the observed phase functions for the Fomalhaut and HR 4796A dust rings. Further computations and measurements (including polarization) for millimeter sized-grains are needed to draw some conclusions about the fluffy or compact structure of the dust grains.

Photometry of Particles Ejected From Active Asteroid (101955) Bennu

“…The particle phase functions are consistent with laboratory measurements of nearly centimeter‐sized terrestrial rocks by Muñoz et al (2017). The measured terrestrial sample was a basaltic rock from Mount Etna with a 0.7‐cm volume‐equivalent sphere diameter with a dark, rough surface.…”

“…The phase functions of the particles are unlike those seen for asteroid surfaces. The phase coefficient of 0.013 mag/deg is consistent with terrestrial millimeter-to centimeter-scale rocks (Muñoz et al, 2017). The particles are among the smallest discrete objects observed above Earth's atmosphere.…”

“…To derive H V values, the phase coefficient 0.013 ± 0.005 mag/deg is extrapolated to 0°phase angle. Our observations and the laboratory work of Muñoz et al (2017) could not produce a measurement of any opposition effect or nonlinearity due to back scattering at small phase angles. Telescopic observations at low phase angles of dark carbonaceous asteroids found a shallow opposition effect of 0.0 to 0.3 magnitudes but these objects have diameters of tens of kilometers or larger and may not experience an opposition effect similar to centimeter sized particles (Muinonen et al, 2010;Shevchenko et al, 2008).…”

“…The measured terrestrial sample was a basaltic rock from Mount Etna with a 0.7‐cm volume‐equivalent sphere diameter with a dark, rough surface. We used the Muñoz et al (2017) results available at the Amsterdam‐Granada light scattering database (Muñoz et al, 2012) to convert their photometric data from flux to astronomical magnitudes and from scattering angle to phase angle. Between phase angles of 70° and 120°, where the majority of Bennu particles were observed, the phase coefficient of the Etna sample was 0.006 mag/deg.…”

Synthesis of the morphological description of cometary dust at comet 67P/Churyumov-Gerasimenko

“…In a complementary modelling attempt, Markkanen et al (2018) could reproduce the OSIRIS phase function at different times using aggregates in the 5 -100 µm size range, consisting of submicrometre-sized organic grains and micrometre-sized silicate grains. Indication for macroscopic particles (in contrast to dispersed sub-micrometre monomers) is also provided from laboratory analogous experiments by Muñoz et al (2017). Overall, there are indications for particles smaller that the best OSIRIS resolution in OSIRIS data but interpretation and detailed studies are still ongoing.…”