SUISEI is a suite of computational tools that has been developed over the past three decades and successfully applied to comets; including ComChem, a global, multi-fluid gas dynamics simulation with detailed chemical kinetics of the cometary coma; ComDust, a model of comet dust evolution and interaction with gas; and ComNuc, a 3-D simulation of gas and heat flow within the comet nucleus porous subsurface layers. The combination of these tools have resulted in an improved knowledge of chemical species in the comet’s coma and their relationship to native molecules in the nucleus ices by analyzing space- and ground-based observations and in situ measurements from spacecraft missions. A review of SUISEI is presented and applications are made to two cases: chemical recycling of HCN in comets and the physical conditions of the near-Sun object, 3200 Phaethon.
Comets are believed to be the remnants of the swarm of planetesimals from which the planets formed some 4.6 Gy ago. By investigating in detail the physical and chemical properties of comets, we can characterize the conditions and processes of the Solar System’s earliest epoch. Comets are also thought to hold clues to the origins of life since they contain large inventories of water and organics, including prebiotic molecules, on Earth-crossing orbits.
Modeling is central to understand the important properties of the cometary environment. We have developed a comet model, SUISEI (Chinese 彗星; pronounced “suisei” in Japanese kanji), that self-consistently includes the relevant physicochemical processes within a global modeling framework, from the porous subsurface layers of the nucleus to the interaction with the solar wind and formation of the bow shock and plasma tail. Our goal is to gain valuable insights into the intrinsic properties of cometary nuclei so we can better understand observations and in situ measurements. SUISEI includes a multifluid, reactive gas dynamics simulation of the dusty coma (ComChem) with dust entrainment (ComDust) and transition to free molecular flow, a 3-D code (ComNuc) of gas and heat transport in porous subsurface layers in the interior of the nucleus, and a suite of other coupled numerical simulations. We have successfully applied this model to a variety of comets in previous studies as noted below. After briefly describing these codes, we present two applications of SUISEI to illustrate the chemical recycling of HCN in cometary come and to investigate the thermal environment of the near-Sun object, 3200 Phaethon.
SUISEI is a versatile suite of numerical simulations to model the global comet environment. It consists of several codes that can be linked depending on the requirements of a particular application as shown in
This multifluid gas dynamics model with chemistry is very advanced and has been described by [
The ComChem code incorporates our model of dusty gas flow in a cometary atmosphere (ComDust, [
We have developed a 3-D simulation of gas and heat flow within the comet nucleus subsurface layers (ComNuc, [
The solar radiation field initiates most of the processes that occur in cometary comae. Photons at ultraviolet (UV) wavelengths photo dissociate and ionize the original parent molecules, producing second-generation reactive radicals, ions, and electrons. These ions and radicals can subsequently react with other species to form third-generation species. Photoelectrons are an additional source of ionization (and dissociation) via impact reactions [
CN has been seen in cometary spectra for over a century but its source(s) remains unknown. A CN source must be able to produce highly collimated CN “jets” be consistent with the observed CN parent scale length, have a production rate consistent with the observed CN production, and isotopic ratios of C and N must be consistent. HCN fulfills these conditions in some comets [e.g., 1P/Halley, C/2002 T7 (LINEAR), 17P/Holmes, 73P/S-W 3, 2P/Encke] while it does not in others [e.g., C/1983 H1 (IAA), C/1995 O1 (Hale-Bopp), C/2001 Q4, 8P/Tuttle, 6P/d’Arrest] [
potential source [
The near-Sun object 3200 Phaethon was discovered in 1983 and classified as an asteroid [
providing a clear link to the Geminids. The observed dust activity would traditionally lead to Phaethon’s classification as a comet, e.g., as in 7968-133P/ Elst- Pizarro. However, Phaethon’s orbit has a perihelion of only 0.14 AU, resulting in subsolar surface temperatures > 1000 K, much too hot for water ice or other volatiles to exist near the surface to drive the activity like a traditional comet. At heliocentric distances < 0.2 AU, temperatures are high enough to vaporize surface hydrocarbons and dust, providing a source of gas. This led [
An important question concerns the energy balance at the body’s surface, namely what fraction of incident energy will be conducted into the interior versus that expended through sublimation. It is important to understand whether the interior remains cold and is relatively unaltered during each perihelion passage or is significantly devolatilized. Since Phaethon exhibits a mixture of cometary and asteroidal properties, it may be an extinct or dormant comet nucleus. Time-dependent thermal modeling using ComNuc [
temperature evolution along Phaethon’s orbit at the subsolar point as it undergoes diurnal rotation (period of 3.604 hours). It is noted that the subsolar temperature is consistent with the standard thermal model, STM [
Model results also confirm the likelihood of large thermal stresses in the surface layers of Phaethon, leading to fractures and the release of dust particles as suggested by [
The gas flux of H2O along Phaethon’s orbit for two thermal conductivities (one typical of asteroids, the other typical of comets) is sharply peaked around perihelion. After the initial approach, the water gas flux is under 10−8 kg∙m−2∙s−1 in all cases. At these low gas fluxes, it would take about 2 My to devolatilize a surface layer of 10m depth on Phaethon. The interior of Phaethon may therefore still have a relatively primitive volatile inventory despite repeated close approaches to the Sun. This is due to the low thermal conductivity of the object, making the surface layers an effective thermal insulator. With maximum gas flux at perihelion being a few 10−9 kg∙m−2∙s−1, total water production is about 1 kg∙s−1. This is not sufficient to entrain the observed dust production [
We conclude that: 1) Phaethon may contain primitive volatiles in its interior despite repeated perihelion passages at 0.14 AU during its history in its present orbit, 2) steady water gas fluxes at perihelion and throughout its orbit are likely insufficient to entrain the currently observed dust production, 3) thermal gradients beneath the surface as well as those caused by rotation are consistent with the mechanism of dust release due to repeated thermal fracture, and 4) the large gas release during the first several perihelion passages may be sufficient to produce enough dust to explain the entire meteoroid stream.
Using SUISEI, we have conducted simulations of HCN in comets and found that it undergoes ion-molecule chemistry in the inner coma, including protonation with water-group ions followed by electron dissociative recombination that “recycles” molecules, resulting in its extended range in the coma, formation of HNC, and rotational-vibrational emission bands from highly excited states. Chemical recycling of other molecules with high proton affinities such as, NH3 and H2O, will occur and possibly provide an explanation for spectral observations showing “hot” bands of these molecules and others in the inner coma.
We have investigated the hypothesis that Phaethon may be a dormant or extinct comet nucleus that is still experiencing low level water gas production as the driver of its recently observed dust activity. The model shows the extreme heat cycling that Phaethon experiences during each perihelion passage and its diurnal variation. Our results indicate that although Phaethon may still harbor a volatile reservoir in its interior despite repeated perihelion passages due to its very low thermal conductivity typical of small solar system bodies. The water gas production rates are not sufficient to explain the observed dust production rates. The possibility that Phaethon is a dormant comet nucleus is not ruled out.
SUISEI has proven to be a unique and valuable model to understand the relevant physical processes and properties of small Solar System bodies, including near-Sun comets [
This work was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, Brazil, Grant No. 2015/03176?8) and the National Science Foundation Planetary Astronomy Program (NSF, USA, Grant No. 0908529).
Boice, D.C. (2017) SUISEI―A Versatile Global Model of Comets with Applications to Small Solar System Bodies. Journal of Applied Mathematics and Physics, 5, 311-320. https://doi.org/10.4236/jamp.2017.52028