Merging data from Rosetta GIADA and MIDAS dust detectors to characterize 67P’s activity

Abstract

We studied the dust activity of comet 67P/Churyumov-Gerasimenko (67P), by relating the detections of the Rosetta/ GIADA and MIDAS instruments, obtaining a correlation between flux of ejected mm- and μm-sized dust.

The ESA/Rosetta mission orbited the 67P comet for two years, escorting it through perihelion, occurred on 13th August 2015.

The mission configuration is still permitting a thorough characterization of cometary activity, even 4 years from the end of the mission.

In particular, the 67P’s dust ejection has been detected in different stages of the comet’s orbit by both remote observations (e.g., the OSIRIS camera and the VIRTIS imaging spectrometer) and in-situ measurements performed by detectors as GIADA, MIDAS and COSIMA.

This work merges data from the GIADA (Grain Impact Analyser and Dust Accumulator) [1] dust detector and the MIDAS (Micro-Imaging Dust Analysis System) [2] atomic force microscope. The two instruments detected dust different in size (mm-sized vs μm-sized) and complementary dust properties (porosity and mass vs physical properties and 3D structure). The GIADA and MIDAS data fusion could allow to relate ejection of smaller and larger dust particles as well as to monitor variations of dust properties during Churyumov-Gerasimenko’s orbit and to link them to the morphology of ejecting surface regions.

 

2.1 The GIADA dataset

The GIADA instrument consists of three subsystems: Grain Detection System (GDS), Impact Sensor (IS) and Quartz Crystal Microbalances (QCM). The first two subsystems measured velocity of fluffy particles and momentum of compact (mm-sized) particles, respectively, whereas the QCM measured the cumulative mass of nm-sized dust [3].

To compare GIADA and MIDAS dataset, we considered only the IS data, because MIDAS mostly detected individual compact particles. In particular, we considered only the IS detections acquired in the periods when MIDAS detected dust particles (see next subsection).

[4] developed a procedure to trace back the motion of dust particles detected by GIADA in the coma down to the nucleus surface. This allowed the retrieval of the geomorphological region ejecting each dust particle and the association of dust and surface properties. These results are intended to be extended to the GIADA-MIDAS data fusion to relate dust physical properties and surface morphology.

2.2 The MIDAS dataset

MIDAS collected micron-sized dust particles on several targets, each working in a defined period. Cometary dust was detected on four MIDAS targets: Target 10, Target 12 and Target 14 collected dust in three periods before perihelion (September-November 2014, December2014-February 2015 and February-March 2015, respectively), whereas the dust collected on Target 13 was released in an outburst on 19^th February 2016 (after perihelion).

Except one case of a fluffy agglomerate [5], all the particles detected by MIDAS are compact.

 

3.1 mm- vs μm- sized dust flux

Our first step was to compare the millimetric and micrometric dust flux measured by GIADA-IS and MIDAS, respectively.

For each period corresponding to the collection time of a MIDAS target, we retrieved the number of particles detected by the two instruments, normalized it to the period duration (in days) and to the spacecraft-comet distance (in km). The MIDAS flux required an additional normalization, taking into account the scanned area on the respective target (not constant among targets).

The comparison of dust fluxes measured by GIADA and MIDAS is shown in Figure 1.

Figure 1. Dust fluxes measured by MIDAS and GIADA. Each asterisk corresponds to a MIDAS target, i.e., to a defined orbit period. The red asterisk corresponds to MIDAS Target 14 (which detected dust between February and March 2015).

 

The flux logarithms are linearly related, suggesting a strong correlation between ejection of large (mm-sized) and small (mm-sized) compact dust agglomerates.

The flux corresponding to the MIDAS Target 14 period (February/March 2015), highlighted in red in Figure 1, is slightly outside this linear trend, with GIADA (MIDAS) detecting more (less) particles than expected from the linear trend identified by the other three periods. This can be ascribed to the occurrence of an outburst (detected by GIADA but not by MIDAS) in a moment when the spacecraft had the largest distance to the comet among all herein considered periods. As smaller particles are stronger deviated from their initially radial trajectory than larger ones (due to  the more important role of solar pressure force with respect to nucleus gravity [6]), it is reasonable that fewer small than large particles arrived at the spacecraft position. However, this result needs further refinement regarding a possible falsification of MIDAS particle size distribution caused by particle fragmentation upon collection. Our next steps are to discern parent particles and fragments in the MIDAS data set and to update the relation shown in Figure 1. This will be done by analyzing the spatial distribution of dust particles on the MIDAS targets, by comparing  MIDAS’ dust size distribution to that expected during nominal activity [7] and during outbursts [8] and by taking into account the largest dust size expected to be ejected depending on the nucleus surface temperature [9].

3.2 Dust vs surface properties

By applying the traceback algorithm developed by [4], we obtained that in certain periods dust is mostly ejected from rough or from smooth terrains. Therefore, we can relate properties of particles stemming from different locations at the comet to cometary surface morphology. In addition, dust ejected during outbursts may probe deeper layers of the comet, allowing us to discern dust properties at different depths.

 

Acknowledgements

This research was supported by the Italian Space Agency (ASI) within the ASI-INAF agreement I/032/05/0, by the Austrian Science Fund FWF P 28100-N36, and by the International Space Science Institute (ISSI) through the ISSI International Team “Characterization of cometary activity of 67P/Churyumov-Gerasimenko comet”.

 

References

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