Galactic Cosmic Ray Attenuation in Dusty Cometary Environments

Introduction

The Standard Radiation Environment Monitor (SREM) aboard the Rosetta spacecraft was designed to record the number of high-energy particles – including galactic cosmic rays (GCRs) – that collided with the spacecraft throughout the mission [1,2]. During the orbital phase of Comet 67P/Churyumov–Gerasimenko (67P), SREM recorded a continuous 8-10% decrease in the number of GCRs that Rosetta encountered when compared to simulated counts based solely off heliocentric distance and solar activity [3]. Three preliminary hypotheses for the 8-10% decrease in SREM-detected GCRs are attenuation via magnetic field fluctuations, outgassing of volatiles, and/or dust grain production by 67P. While not an exhaustive list of possible explanations, these three factors contribute significantly to altering the surrounding environment Rosetta was exposed to. The goal of this work is to investigate whether or not dust grains in the coma of 67P can be a possible explanation for the SREM-detected attenuation. We aim to constrain what types of dust environments are necessary to cause significant levels of cosmic ray attenuation, and whether or not these environments are realistic when compared to that of 67P during its course towards perihelion.

Methods

Methods are divided into two parts. First, the attenuating power of various gaseous, ice, and refractory materials common to the environment surrounding 67P is analyzed using the Multi-Layered Shielding Simulation Software (MULASSIS) [4], a tool built to simulate high-energy particles impacting different shielding materials. ­The geometry of the shielding material is chosen to be a planar slab, with thickness either defined by column density for gas species, or width for refractory species. Column density is varied between 10^-3 – 10² g cm^-2, while refractory thickness is varied within 10^-6 – 10² cm. For each simulation, the attenuation fraction of a spectrum of incident GCRs impacting the material is determined. The total amount of material required to create an 8% energy attenuation matching the SREM detections is then recorded for all species.

Second, simulations of GCR trajectories through comet comae are performed in order to determine the total number of grains that incident GCRs would likely interact with. This provides an estimate for the cumulative grain thickness to compare to our MULASSIS results. To do this we use the Cometary Model for Dust Environments (ComMoDE) [5], a model used to simulate cumulative dust fluence onto an object whose trajectory passes through the material ejected by a comet. Grain size distribution, dust production, and gas production rates are all varied to represent typical cometary environments. For each environment, total accumulated grain thickness is recorded and compared to thicknesses derived by MULASSIS. The compatible environments are then compared to observed environmental conditions of 67P for validation.

Results

MULASSIS: In order to achieve a minimum 8% GCR attenuation, the necessary column densities of H₂O, CO₂, and CO vapor were 7-8 magnitudes higher than that observed by Rosetta, indicating that the primary attenuating agents are not likely gaseous species. Upon testing the various refractory materials, an average slab thickness of 1.36±0.66 cm was found necessary to attenuate 8% of incident GCRs. Attenuation curves for calculated gas species (left) and refractory species (right) are shown in Fig. 1. Depending on the relative abundance ratio of less dense (ice and carbonaceous) to dense (silicate and sulfide) materials present in the coma of 67P, the total required thickness would increase or decrease, respectively. With the majority of the composition of refractory material belonging to the carbonaceous classification [6], it is expected that the attenuating thickness of dust grains lies closer to the upper bound of our reported value.

ComMoDE: We find that 1.36±0.66 cm is an achievable cumulative thickness for two possible cases: GCRs may suffer from attenuation via frequent, low-energy loss collisions with small (sub-µm) grains, or attenuation via few, high-energy loss collisions with large (µm-mm) grains. Under these two cases, the cumulative thickness experienced by an incident flux of GCRs reaches the 1.36±0.66 cm threshold within typical grain size limits (≤10³ µm), shown in Fig. 2. These preliminary simulations indicate that the dusty environment of 67P can be a compatible explanation for the 8% GCR attenuation reported by SREM. It is possible however that attenuation due to dust accounts for only a fraction of the total observed GCR reduction. Other variables not examined in this work, including an induced magnetic field and Forbush decrease events possibly occur as well throughout Rosetta's mission and may account for a percentage of the attenuation.

Conclusion

By studying the absorbing power of different grain materials and modeling GCR trajectories through dusty environments, we investigate a common process that occurs in dense interstellar molecular clouds and apply it to cometary environments. For future work, more complex factors, including mixtures of different grain and ice compositions, non-spherical and porous shapes of grains, as well as GCR trajectories and scattering behaviors are suggested to be added to these preliminary simulations of irradiated grain environments. With the planned launch of the Comet Interceptor mission, a multitude of new opportunities to study grain-GCR interactions will ultimately present themselves in the future.

Fig. 1: MULASSIS GCR fraction attenuation results for primary gas (left) and refractory (right) species. The dashed black line indicates the observed 8% attenuation measured by Rosetta SREM.

Fig. 2: Cumulative thickness for two scenarios in which parameters intersect with 1.36±0.66 cm (green line and shaded area): low power-law index (κ), dust production rate (Q_d), and dust-to-gas mass ratio (χ) (blue), or high κ, Q_d and χ (red). Q_d is measured in kg s^-1.

 

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