Multi-instrument analysis of 67P/Churyumov-Gerasimenko coma particles: COPS-GIADA data fusion

Abstract

Context. The European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko has offered scientists the opportunity to study a comet in unprecedented detail. Four instruments of the Rosetta orbiter, namely, the Micro-Imaging Dust Analysis System (MIDAS), the Grain Impact Analyzer and Dust Accumulator (GIADA), the COmetary Secondary Ion Mass Analyser (COSIMA), and the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) have provided information on cometary dust particles. Cross-instrument comparisons are crucial to characterize cometary dust particles beyond the capabilities of individual sensors, as they are sensitive to different dust components. Aims. We present the first comparison between detections of the ROSINA COmet Pressure Sensor (COPS) and GIADA. These two instruments are complementary as the former is sensitive solely to volatiles of icy particles, while the latter measured the dust particle as a whole, including refractories and condensed (semi)volatiles. Our goal is to correlate the particles detected by COPS and GIADA and to assess whether they belong to a common group. Methods. We statistically analyzed the in situ data of COPS and GIADA by calculating Pearson correlation coefficients. Results. Among the several types of particles detected by GIADA, we find that COPS particles are significantly correlated solely with GIADA fluffy agglomerates (Pearson correlation coefficient of 0.55 and p-value of 4.6 × 10−3). This suggests that fluffy particles are composed of both refractories and volatiles. COPS volatile volumes, which may be represented by equivalent spheres with a diameter in the range between 0.06 µm and 0.8 µm, are similar to the sizes of the fractal particle’s subunits identified by MIDAS (i.e., 0.05–0.18 µm).

Main Results from the ISSI International Team “Characterization of 67P Cometary Activity”

“…To trace back the dust particles' motion, we made the following assumptions, based on dust dynamical models: (a) radial motion; (b) constant acceleration up to 11 km height and constant speed above that height. According to numerical models (e.g., [33,36]), these assumptions are valid for low heights (a threshold of about 70 km was determined a posteriori), because at higher altitudes, dust particles tend to deviate from the radial trajectory, due to the increasing effect of solar radiation pressure.…”

“…The parameter Iv characterizes the efficiency of entrainment of the particle within the gas flow (i.e., the ability of a dust particle to adjust to the gas velocity) and Fu characterizes the efficiency of gravitational attraction; their definition is given in Equations ( 15) and ( 16) in [33], respectively. For a given gas flow field, if two dust particles have the same Iv and Fu parameters (see [36,66] for details), their translational motion is similar. If, furthermore, they have the same R n /a d , where R n is the radius of the nucleus and a d is the size of the particle, then their rotational motion is similar as well.…”