46th Lunar and Planetary Science Conference (2015) 1781.pdf

MODELING THE NEAR-SUN OBJECT, 3200 PHAETHON. Daniel C. Boice1,2 and J. Benkhoff3, 1Scientific Studies and Consulting, 9410 Harmon Dr., San Antonio, TX 78209 USA ([email protected]), 2Trinity Univer- sity, Dept. of Physics, 1 Trinity Place, San Antonio, TX 78212 USA ([email protected]), 3ESA-ESTEC/RSSD, Noordwijk, The Netherlands ([email protected]).

Introduction: Physico-chemical modeling is cen- Belt Comets”) necessitates a revision of how we un- tral to understand the important physical processes in derstand and classify these small asteroid-comet transi- small solar system bodies. We have developed a com- tion objects [9]. puter simulation, SUSEI, that includes the physico- Results: The time-dependent thermal results ob- chemical processes relevant to comets within a global tained in our calculations show the temperature evolu- modeling framework. Our goals are to gain valuable tion along Phaethon’s orbit at the sub-solar point as it insights into the intrinsic properties of cometary nuclei undergoes diurnal rotation (period of 3.604 hrs). It is so we can better understand observations and in situ noted that the subsolar temperature is consistent with measurements. SUISEI includes a 3-D model of gas the standard thermal model (STM [10]) and that diur- and heat transport in porous sub-surface layers in the nal variations are extreme at perihelion, resulting in interior of the nucleus. We have successfully used this temperature changes in excess of 700K in 1.8 hours, model in previous studies of comets at normal helio- and are consistent with the near-Earth asteroid thermal centric distances [1,2]. model (NEATM [11]) that assumes very low thermal SUISEI has been adapted to model near-Sun ob- inertia and/or nonrotation. The average surface tem- jects to reveal significant differences in the chemistry perature, integrated over the entire surface are consis- and dynamics of their comae (atmosphere) with comets tent with the so-called fast-rotating model (FRM [12]) that don’t closely approach the Sun. At small heliocen- that assumes a very high thermal inertia and/or very tric distances, temperatures are high enough to vapor- fast rotation. Ohtsuka et al. (2009) obtained similar ize surface materials and dust, forming a source of gas. results [13]. Another important question concerns the energy bal- The gas flux of H2O along the orbit for two dif- ance at the body’s surface, namely what fraction of ferent thermal conductivities (high typical of NEAs incident energy will be conducted into the interior ver- and low typical of comets as well as a model with high -2 -1 sus that used for sublimation. This is important to un- thermal inertia for H2O gas flux in units of kg m s . derstand if the interior remains cold and is relatively We note a sharply peaked rise in gas flux around unaltered during each perihelion passage or is signifi- perihelion. In all cases the water gas flux is under 10-8 cantly devolatilized. This also bears upon the regimes kg m-2 s-1. where sublimation and ablation due to ram pressure Conclusions and Discussion. Our results show dominate in the erosion or eventual destruction of sun- that at these low gas fluxes, it will take about 2 Myr to grazers. The resulting model is an important tool for devolatilize a surface layer of 10 meters depth on studying sun-grazing comets and other near-Sun ob- Phaethon assuming an average gas flux of 5x10-14 kg jects. m-2 s-1. We conclude that the interior of Phaethon may 3200 Phaethon: We present results on the applica- still have a relatively pristine volatile inventory despite tion of SUISEI to the near-Sun object, Phaethon. Dis- repeated close approaches to the Sun. This is due to the covered in 1983 and classified as an asteroid [3,4], it low thermal conductivity of Phaethon, making the sur- has recently exhibited an active dust coma [5,6,7]. face layers an effective thermal insulator. With maxi- Phaethon has long been associated as the source of the mum gas flux at perihelion being a few 10-9 kg m-2 s-1, Geminids meteor shower [8] so the dust activity pro- that total water production is on the order of 1 kg s-1. vides a clear link to the meteor shower. The observed This is not sufficient to entrain the observed dust pro- dust activity would traditionally lead to Phaethon being duction [5] so we rule out the possibility of steady wa- also classified as a comet (e.g., 2060-95P/Chiron, ter sublimation as the driver of the dust activity. How- 133P/Elst-Pizarro). This is unusual since the orbit of ever, it is possible that an impulsive gas outburst, trig- Phaethon has a perihelion of 0.14 AU, resulting in sur- gered by pockets of subsurface gas suddenly bursting face temperatures of more than 1025K, much too hot through the surface, may be responsible, similar to for water ice or other volatiles to exist near the surface outbursts the of comets Holmes and 1P/Halley seen at and drive the activity, like a traditional comet. This has large heliocentric distances. lead [7] to suggest thermal or desiccation cracking as The model results also confirm the likelihood of the driving mechanisms and calling Phaethon an active large thermal stresses in the surface layers of Phaethon, asteroid. This situation (and others such as the “Main leading to fractures and release of dust particles as 46th Lunar and Planetary Science Conference (2015) 1781.pdf

suggested previously [7]. Thermal cycling from about 1000K to about 300K at perihelion due to diurnal ef- fects probably leads to disintegration of surface rocks which form the observed dust coma and material for the meteor stream. We also see extreme temperature gradients into the surface, especially at perihelion, which will fuel the dust release also. Thus, we conclude the following for Phaethon: 1. It is likely to contain relatively pristine volatiles in its interior despite repeated near perihelion passages of 0.14 AU during its history in its present orbit, 2. Steady water gas fluxes at perihelion and throughout its orbit are insufficient to entrain the cur- rently observed dust production, 3. Thermal gradients into the surface as well as those caused by diurnal rotation are consistent with the mechanism of dust release due to thermal fracture, 4. The initial large gas release during the first peri- helion passage may be sufficient to produce enough dust to explain the entire meteor stream. References: [1] Boice D. C. et al. (1995) Earth, Moon and Planets, 71, 235-236. [2] Benkhoff J. and Boice D. C. (1996) Planet. Space Sci., 44, 665. [3] Green S. and Davies J. (1983) IUAC 3878. [4] Kowal C. (1983) IAUC 3878. [5] Jewitt D. et al. (2013) As- trophys. J. Lett., 771, L36. [6] Li J. and Jewitt D. (2013) Astron. J., 145, 154. [7] Jewitt D. and Li J. (2010) Astron. J., 140, 1519. [8] Whipple F. (1983) IAUC 3881. [9] Boice D. (2012) IAU GA, C15 Busi- ness Meeting Minutes. [10] Lebosky L. et al. (1986) Icarus, 68, 239. [11] Harris A. (1998) Icarus, 131, 291. [12] Lebosky L. and Spencer J. (1989) Asteroids II, 128. [13] Ohtsuka K. et al. (2009) Publ. Astron. Soc. Japan, 61, 1375. Acknowledgements: We greatly appreciate sup- port from the NSF Planetary Astronomy Program and the ESA/ESTEC Visiting Scientist Program.