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Chambersetal2008.Pdf This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Icarus 194 (2008) 623–635 www.elsevier.com/locate/icarus Hydrodynamical and radiative transfer modeling of meteoroid impacts into Saturn’s rings Lindsey S. Chambers a,∗, Jeffrey N. Cuzzi b, Erik Asphaug a, Joshua Colwell c, Seiji Sugita d a Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA b Space Science Division, NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035, USA c Department of Physics, University of Central Florida, Orlando, FL 32816-2385, USA d Department of Complexity Science and Engineering, Graduate School of Frontier Science, University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan Received 17 March 2006; revised 17 September 2007 Available online 15 December 2007 Abstract In a small hypervelocity impact, superheated gas and particles glow brightly with thermal emission for a brief time interval at short wavelengths; this phenomenon is referred to as an impact flash. Over the past decade, impact flashes have been observed on the Moon and in the laboratory in both the IR and visible portions of the spectrum. These phenomena have been used to constrain impactor parameters, such as impact size, velocity and composition. With the arrival of the Cassini spacecraft at Saturn, we embarked on a study of impact flashes in Saturn’s rings. We present results on the feasibility of observing impact flashes and therefore estimating the flux of meteoroids impacting Saturn’s rings using Cassini’s Ultraviolet Imaging Spectrograph (UVIS). Our modeling effort is two-fold. We start by simulating impacts using the CTH hydrodynamical code. Impacts involve an icy ring particle and a serpentine meteoroid, modeled with the ANEOS equation of state. The objects are centimeters to meters − in diameter and collide at 30 to 50 km s 1. We then use the resulting temperatures and densities of the impact plumes in a radiative transfer calculation. We calculate bound–free, free–free, electron scattering and negative ion opacities along a line-of-sight through the center of each impact plume. Our model has shown that impact flashes will not be seen with the UVIS because (1) the plumes are optically thick when their central temperatures are high, with photosphere temperatures too cool to emit observable UV flux and (2) when the plumes become optically thin, even the hottest region of the plume is too cool to observe in the UV. This corroborates the lack of UVIS impact flash detections to date. Impact flashes are not likely to be seen by other Cassini instruments because of the short lifetimes of the plumes. © 2007 Elsevier Inc. All rights reserved. Keywords: Planetary rings; Saturn; Impact processes; Radiative transfer 1. Introduction and background impactors could lead to a new age estimate for Saturn’s rings. This paper investigates whether flash impacts can be observed Impact flashes, short-lived bursts of light from gas or plasma with the Cassini spacecraft and whether a new mass flux of me- clouds generated in high velocity impact events, have been de- teoroids can thus be determined. tected on the Moon and in laboratory experiments. Impact prop- erties such as impactor size and velocity may be estimated by 1.1. The evolution time-scale of the rings considering the light output from an impact flash event (e.g., Bellot Rubio et al., 2000b). This method has been proposed as The age of Saturn’s rings is an enigmatic problem. Sev- one way to determine the mass flux of meteoroids impacting eral methods have determined an evolution time-scale roughly Saturn’s rings (Cuzzi et al., 2002) and hence the rate at which on the order of hundreds of millions of years. This is much the rings are being eroded. A more accurate mass flux of such younger than the age of the Solar System and Saturn itself (∼4.5 × 109 years). In other words, the current state of Saturn’s * Corresponding author. Fax: +1 (202) 478 8821. rings appears to be the result of relatively recent evolutionary E-mail address: [email protected] (L.S. Chambers). processes and is not a remnant from the formation of Saturn. 0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2007.11.017 Author's personal copy 624 L.S. Chambers et al. / Icarus 194 (2008) 623–635 This poses one of the primary problems in Saturn ring science Waite (1984) who estimate material in the C-ring region can be today. lost in ∼3 × 107 years. Goldreich and Tremaine (1982) calculated the torques on Doyle et al. (1989) calculated the timescale for darkening the the A-ring due to several of the ringmoons interior to Mi- B-ring particles to their current visible albedo. They assumed mas (namely, 1980S1 Janus, 1980S3 Epimetheus, 1980S26 initially pure water-ice ring particles, which darkened in time by Pandora, 1980S27 Prometheus and 1980S28 Atlas). These acquiring debris following impacts of extra-Saturn meteorites torques transfer angular momentum between the A-ring and of cometary origin. By their calculation, the rings would darken the moons, causing the moons to move outward from the ring. from an albedo of one (pure water-ice) to their current albedo The timescale for these moons to migrate from the outer edge of ∼50% in ∼1–2 × 108 years. of the A-ring to their current positions is between 2 × 107 and Cuzzi and Durisen (1990) calculated the velocity and inten- 7 × 108 years, assuming a surface density of Σ = 100 g cm−2 sity distribution of meteoroids impacting the rings and showed and that no gaps open in the rings (if resonances cause gaps that accretion of such material causes the ring particles to drift to open, timescales range from 8 × 107 to 3 × 109 years). radially inward. They estimated that the C-ring would collapse Borderies et al. (1984) repeated this calculation for 1980S26 into Saturn’s atmosphere in 108 years. Pandora and 1980S27 Prometheus, using Σ = 50 g cm−2, Durisen et al. (1992) calculated the ballistic transport of de- and estimated the timescale of these ringmoons’ migration at bris resulting from impacts of meteoroids with ring particles. 4 × 106 to 7 × 107 years. Poulet and Sicardy (2001) suggested They estimate the evolution time-scale of the inner edges of the that frequent disruptive collisions or resonance captures be- A- and B-rings to be between 107 and 2 × 108 years, depend- tween the ringmoons might ameliorate this problem for the ing on what assumptions are made for the hypervelocity impact moons but not for the evolution of the rings. yield (Y ) for ice and snow. Morfill et al. (1983) calculated a total meteorite mass flux Building on this earlier work, Cuzzi and Estrada (1998) cal- at 10 AU of (0.25–2.2) × 106 gs−1, assuming an effective sur- culated both impact darkening of the rings and ballistic trans- face area of the rings of 0.64 × 1016 m2. This implies that the port of this darkened ring material resulting from impacts of timescale to erode a 1 cm thick layer of solid ice by impacts of cometary meteoroids with the ring particles. They estimated it micrometeoroids (roughly 100 µm in size) is 3 × 102−3 years, would take on the order of 108 years for the rings to achieve assuming a mean impact velocity of 25 km s−1 and an ejecta their current color profile due to these processes. yield of Y = 3×103 (the mass of ejecta per unit impactor mass, Colwell et al. (2000) investigated the origin of ring systems see Morfill et al., 1983). By this logic, the timescale to erode a by considering the disruption of small moons by comet impacts. 1 m thick layer is 3×104−5 years and to erodea1kmringmoon They determined that disruption lifetimes for the small uranian 3 × 107−8 years. Ip (1984) reevaluated the meteorite mass flux and neptunian moons (with radii < ∼100 km) are less than the at 10 AU and found a meteoroid mass loading rate at Saturn’s age of the Solar System (Colwell et al., 2000, Table 2 and Sec- rings to be 6 × 104 gs−1, a factor of 20 lower than that found tion 4). By analogy, small saturnian moons are also likely to by Morfill et al. (1983). In considering the effect of such mass have disruptive lifetimes less than the age of the Solar System. loading on ring torque, Ip found that in 109 years the angular However, Saturn’s rings are too massive to have been contin- momentum of the rings would change by ∼5%, producing a ually replenished by satellite disruption on a timescale of 108 very pronounced change in ring structure. years (Colwell, 1994). Lissauer et al. (1988) found the proba- Durisen (1984) considered the ballistic transport of impact bility that Saturn’s rings could be the result of the disruption of ejecta and concluded that most erosional ejecta are exchanged a single moon is roughly one in 4 × 109 years, and thus small with other parts of the rings, leading to much less net erosion in the last few 108 years, especially because the projectile flux than that calculated by Morfill et al.
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