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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.06.016 Byline: Ryo Ishimaru (a), Hiroki Senshu (a), Seiji Sugita (b), Takafumi Matsui (a) Keywords: Comets; Impact processes; Jupiter, Satellites; Organic chemistry; Satellites, Surfaces Abstract: The synthesis of organic molecules via chemical reactions within impact vapor plumes has been proposed as a mechanism to supply organics on a planet. However, the kinetics of chemical reactions within a rapidly expanding vapor plume or quenching process of the reactions has not been studied extensively. In this study, we constructed a new numerical model that calculates kinetics of the entire chemical reactions within an impact vapor plume. Numerical results revealed that the semi-analytical models proposed so far, in which the final amount of a chemical species was given by the equilibrium abundance at the quenching temperature of the fastest reaction path involving the species, underestimates the yield of organic molecules, such as HCN, by up to a factor of 10. This is because the previously used assumption that a species can achieve equilibrium with the rest of the reaction system via the fastest reaction path involving the species is not necessarily valid. Our analysis of the high-temperature H/C/N/O reaction system suggests that the quenching of slow reactions divides the reaction network into smaller reaction sub-systems isolated from the rest of the reaction system. Then, the fastest reaction path cannot equilibrate an isolated reaction sub-system with the rest of the reaction system. Simulation of this actual disequilibrium mechanism requires a simultaneous numerical calculation of the entire reaction network, which is equivalent to conducting a full kinetic model calculation, such as our model. Our numerical code makes it possible to discuss quantitatively the impact chemistry for various situations, such as the Galilean satellites. In this study, our numerical model is applied to the delivery of organic molecules via cometary impact on the Galilean satellites. Our numerical results indicate that small-particle impacts would produce HCN efficiently. Resulting HCN may freeze out immediately and be deposited on satellite surfaces, where it may be eventually converted into complex organics via irradiation of charged particle. On the other hand, large-size impacts may form transient CH.sub.4-N.sub.2 atmospheres, in which complex organics (tholin) may be formed via energy deposition of UV and/or charged particle. Resulting complex organics may subsequently precipitate on the satellite surfaces without clear correlation with the locations of impact craters. Such distribution of complex organics created by chemical reactions within vapor plumes due to cometary impacts may explain an absorption (4.57[mu]m) on Galilean satellites nonassociated with observable (moderate- and large-size) impact craters. Author Affiliation: (a) Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino City, Chiba 275-0016, Japan (b) Department of Complexity Science and Engineering, Graduate School of Frontier Science, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan Article History: Received 3 February 2010; Revised 10 May 2010; Accepted 12 June 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.04.015 Byline: Sho Fukuzaki (a), Yasuhito Sekine (a), Hidenori Genda (b), Seiji Sugita (a), Toshihiko Kadono (c), Takafumi Matsui (d) Keywords: Titan; Atmospheres, Evolution; Impact processes; Satellites, Atmospheres Abstract: Chemical reactions and volatile supply through hypervelocity impacts may have played a key role for the origin and evolution of both planetary and satellite atmospheres. In this study, we evaluate the role of impact-induced N.sub.2 production from reduced nitrogen-bearing solids proposed to be contained in Titan's crust, ammonium sulfate ((NH.sub.4).sub.2SO.sub.4), for the replenishment of N.sub.2 to the atmosphere in Titan's history. To investigate the conversion of (NH.sub.4).sub.2SO.sub.4 into N.sub.2 by hypervelocity impacts, we measured gases released from (NH.sub.4).sub.2SO.sub.4 that was exposed to hypervelocity impacts created by a laser gun. The sensitivity and accuracy of the measurements were enhanced by using an isotope labeling technique for the target. We obtained the efficiency of N.sub.2 production from (NH.sub.4).sub.2SO.sub.4 as a function of peak shock pressure ranging from [approximately equal to]8 to [approximately equal to]45GPa. Our results indicate that the initial and complete shock pressures for N.sub.2 degassing from (NH.sub.4).sub.2SO.sub.4 are [approximately equal to]10 and [approximately equal to]25GPa, respectively. These results suggest that cometary impacts on Titan (i.e., impact velocity v.sub.i [approximately equal to]8km/s) produce N.sub.2 efficiently; whereas satellitesimal impacts during the accretion (i.e., v.sub.i Author Affiliation: (a) Department of Complexity Science and Engineering, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8561, Japan (b) Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo 113-0033, Japan (c) Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita 565-0871, Japan (d) Planetary Explorations Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino 275-0016, Japan Article History: Received 23 November 2009; Revised 21 April 2010; Accepted 21 April 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.04.019 Byline: Satoru Yamamoto (a), Olivier S. Barnouin-Jha (b), Takashi Toriumi (c), Seiji Sugita (d), Takafumi Matsui (e) Keywords: Impact processes; Cratering; Collisional physics Abstract: The present paper describes observations of crater growth up to the time of transient crater formation and presents a new empirical model for transient crater growth as a function of time. Polycarbonate projectiles were impacted vertically into soda-lime glass sphere targets using a single-stage light-gas gun. Using a new technique with a laser sheet illuminating the target [Barnouin-Jha, O.S., Yamamoto, S., Toriumi, T., Sugita, S., Matsui, T., 2007. Non-intrusive measurements of the crater growth. Icarus, 188, 506-521], we measured the temporal change in diameter of crater cavities (diameter growth). The rate of increase in diameter at early times follows a power law relation, but the data at later times (before the end of transient crater formation) deviates from the power law relation. In addition, the power law exponent at early times and the degree of deviation from a power law at later times depend on the target. In order to interpret these features, we proposed to modify Maxwell's Z-model under the assumption that the strength of the excavation flow field decreases exponentially with time. We also derived a diameter growth model as: d(t)a[1-exp(-[beta]t)].sup.[gamma], where d(t) is the apparent diameter of the crater cavity at time t after impact, and [beta] and [gamma] are constants. We demonstrated that the diameter growth model could represent well the experimental data for various targets with different target material properties, such as porosity or angle of repose. We also investigated the diameter growth for a dry sand target, which has been used to formulate previous scaling relations. The obtained results showed that the dry sand target has larger degree of deviation from a power law, indicating that the target material properties of the dry sand target have a significant effect on diameter growth, especially at later times. This may suggest that the previously reported scaling relations should be reexamined in order to account for the late-stage behavior with the effect of target material properties. Author Affiliation: (a) Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba 305-8506, Japan (b) The Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723, USA (c) Institute of Physical Education, Keio University, Yokohama, 223-8521, Japan (d) Graduate School of Frontier Sciences, University of Tokyo, Chiba, 277-8561, Japan (e) Planetary Exploration Research Center, Chiba Institute of Technology, Chiba, 275-0016, Japan Article History: Received 6 October 2008; Revised 2 April 2009; Accepted 6 April 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.08.031 Byline: Yasuhito Sekine (a), Hiroshi Imanaka (b)(c), Takafumi Matsui (d), Bishun N. Khare (e)(f), Emma L.O. Bakes (c)(e), Christopher P. McKay (e), Seiji Sugita (d) Keywords: Titan; Organic chemistry; Satellites; atmospheres; Atmospheres; chemistry Abstract: In Titan's atmosphere consisting of N.sub.2 and CH.sub.4, large amounts of atomic hydrogen are produced by photochemical reactions during the formation of complex organics. This atomic hydrogen may undergo heterogeneous reactions with organic aerosol in the stratosphere and mesosphere of Titan. In order to investigate both the mechanisms and kinetics of the heterogeneous reactions, atomic deuterium is irradiated onto Titan tholin formed from N.sub.2 and CH.sub.4 gas mixtures at various surface-temperatures of the tholin ranging from 160 to 310 K. The combined analyses of the gas species and the exposed tholin indicate that the interaction mechanisms of atomic deuterium with the tholin are composed of three reactions; (a) abstraction of hydrogen from tholin resulting in gaseous HD formation (HD recombination), (b) addition of D atom into tholin (hydrogenation), and (c) removal of carbon and/or nitrogen (chemical erosion). The reaction probabilities of HD recombination and hydrogenation are obtained as I*.sub.abst=1.9([+ or -]0.6)x10.sup.-3xexp(-300/T) and I*.sub.hydro=2.08([+ or -]0.64)xexp(-1000/T), respectively. The chemical erosion process is very inefficient under the conditions of temperature range of Titan's stratosphere and mesosphere. Under Titan conditions, the rates of hydrogenation > HD recombination a' chemical erosion. Our measured HD recombination rate is about 10 times (with an uncertainty of a factor of 3-5) the prediction of previous theoretical model. These results imply that organic aerosol can remove atomic hydrogen efficiently from Titan's atmosphere through the heterogeneous reactions and that the presence of aerosol may affect the subsequent organic chemistry. Author Affiliation: (a) Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan (b) Department of Chemistry, University of Arizona, 1306 E. University Blvd., Tucson, AZ 85721-0041, USA (c) SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, USA (d) Department of Complexity Science & Engineering, Graduate School of Frontier Science, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 227-8562, Japan (e) Space Science Division, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA (f) The Carl Sagan Center for the Study of Life in the Universe, SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, USA Article History: Received 9 February 2007; Revised 1 August 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.01.009 Byline: Olivier S. Barnouin-Jha (a)(b), Satoru Yamamoto (b), Takashi Toriumi (b), Seiji Sugita (b), Takafumi Matsui (b) Keywords: Impact processes; Cratering Abstract: An experimental technique to measure crater growth is presented whereby a high speed video captures profiles of a crater forming after impact obtained using a vertical laser sheet centered on the impact point. Unlike previous so called 'quarter-space experiments,' where projectiles were launched along a transparent Plexiglas sheet so that growth of half a crater could be viewed, the use of the laser sheet permits viewing changes in crater shape without any physical interference to the cratering process. This technique indicates that for low velocity impacts (45[degrees]) are also seen, and could be due to the low friction angle of the target. Significant crater modification, which is rarely seen in 'quarter-space experiments,' is also observed and appears to be controlled by the dynamic angle of repose of the target. These latter observations indicate that differences in target friction angles may need to be considered when determining near rim ejecta-mass distributions and large-scale crater modification processes on the planets. Author Affiliation: (a) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USA (b) Department of Complexity Sciences and Engineering, University of Tokyo, Kashiwa, Chiba 277-8561, Japan Article History: Received 17 January 2006; Revised 24 October 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.02.002 Byline: Satoru Yamamoto (a), Koji Wada (b), Norihisa Okabe (a), Takafumi Matsui (a) Keywords: Cratering; Impact processes; Collisional physics Abstract: We experimentally studied the formation and collapse processes of transient craters. Polycarbonate projectiles with mass of 0.49 g were impacted into the soda-lime glass sphere target (mean diameters of glass spheres are [approximately equal to]36, 72, and 220 [mu]m, respectively) using a single-stage light-gas gun. Impact velocity ranged from 11 to 329 mas.sup.-1. We found that the transient crater collapses even at laboratory scales. The shape (diameter and depth) of the transient crater differs from that of the final crater. The depth-rim diameter ratios of the final and transient craters are 0.11-0.14 and 0.26-0.27, respectively. The rim diameter of both the transient and final crater depends on target material properties; however, the ratio of final to transient crater diameter does not. This suggests that target material properties affect the formation process of transient craters even in the gravity regime, and must be taken into account when scaling experimental results to planetary scales. By observing impacts into glass sphere targets, we show that although the early stage of the excavation flow does not depend on the target material properties, the radial expansion of the cavity after the end of vertical expansion does. This suggests that the effect of target material properties is specifically important in the later part of the crater excavation and collapse. Author Affiliation: (a) Graduate School of Frontier Sciences, University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8562, Japan (b) Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan Article History: Received 1 June 2005; Revised 31 January 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.10.002 Byline: Koji Wada (a)(b), Hiroki Senshu (c), Takafumi Matsui (a) Keywords: Cratering; Impact processes; Computer techniques Abstract: A new numerical code based on the Distinct Element Method (DEM) is developed to study the impact cratering processes on granular material. This code has a potential advantage to simulate the cratering process on granular material, since the movement of discrete particles can be treated. To show the physical plausibility of this code, we conduct 3-D numerical simulations of vertical impact into granular material targets that consist of 384,000 particles, and compare the results with those from experimental studies. It is shown that the excavation stage of cratering derived from experimental studies is represented well by our simulation: the size of the crater cavity, and the ejecta velocity and angle distributions are consistent with those obtained in laboratory experiments. The impact simulation code developed in this study is thus suggested to be useful for the analysis of the impact cratering process on granular material. Author Affiliation: (a) Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan (b) Institute of Low Temperature Science, Hokkaido University, N19-W8, Kita-ku, Sapporo 060-0819, Japan (c) Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15, Natsushima-cho, Yokosuka 237-0061, Japan Article History: Received 28 February 2005; Revised 6 September 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.04.007 Byline: Satoru Yamamoto (a), Toshihiko Kadono (b), Seiji Sugita (a), Takafumi Matsui (a) Keywords: Impact processes; Cratering; Collisional physics; Regoliths Abstract: We measured the velocity distributions of impact ejecta with velocities higher than [approximately equal to]100 mas.sup.-1 (high-velocity ejecta) for impacts at variable impact angle [alpha] into unconsolidated targets of small soda-lime glass spheres. Polycarbonate projectiles with mass of 0.49 g were accelerated to [approximately equal to]250 mas.sup.-1 by a single-stage light-gas gun. The impact ejecta are detected by thin aluminum foils placed around the targets. We analyzed the holes on the aluminum foils to derive the total number and volume of ejecta that penetrated the aluminum foils. Using the minimum velocity of the ejecta for penetration, determined experimentally, the velocity distributions of the high-velocity ejecta were obtained at [alpha]=15[degrees], 30[degrees], 45[degrees], 60[degrees], and 90[degrees]. The velocity distribution of the high-velocity ejecta is shown to depend on impact angle. The quantity of the high-velocity ejecta for vertical impact ([alpha]=90[degrees]) is considerably lower than derived from a power-law relation for the velocity distribution on the low-velocity ejecta (less than 10 mas.sup.-1). On the other hand, in oblique impacts, the quantity of the high-velocity ejecta increases with decreasing impact angle, and becomes comparable to those derived from the power-law relation. We attempt to scale the high-velocity ejecta for oblique impacts to a new scaling law, in which the velocity distribution is scaled by the cube of projectile radius (scaled volume) and a horizontal component of impactor velocity (scaled ejection velocity), respectively. The high-velocity ejecta data shows a good correlation between the scaled volume and the scaled ejection velocity. Author Affiliation: (a) Mailbox 408, Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8562, Japan (b) IFREE, JAMSTEC, Kanagawa 273-0061, Japan Article History: Received 7 August 2004; Revised 22 February 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.03.016 Byline: Yasuhito Sekine (a), Seiji Sugita (b), Takafumi Shido (c), Takashi Yamamoto (c), Yasuhiro Iwasawa (c), Toshihiko Kadono (d), Takafumi Matsui (b) Keywords: Fischer-Tropsch catalysis; Titan; Cassini-Huygens mission; Circum-planetary subnebula; Methane Abstract: Fischer-Tropsch catalysis, which converts CO and H.sub.2 into CH.sub.4 on the surface of iron catalyst, has been proposed to produce the CH.sub.4 on Titan during its formation process in a circum-planetary subnebula. However, Fischer-Tropsch reaction rate under the conditions of subnebula have not been measured quantitatively yet. In this study, we conduct laboratory experiments to determine CH.sub.4 formation rate and also conduct theoretical calculation of clathrate formation to clarify the significance of Fischer-Tropsch catalysis in a subnebula. Our experimental result indicates that the range of conditions where Fischer-Tropsch catalysis proceeds efficiently is narrow (T[approximately equal to]500-600K) in a subnebula because the catalysts are poisoned at temperatures above 600 K under the condition of subnebula (i.e., H.sub.2/CO=1000). This suggests that an entire subnebula may not become rich in CH.sub.4 but rather that only limited region of a subnebula may enriched in CH.sub.4 (i.e., CH.sub.4-rich band formation). Our experimental result also suggests that both CO and CO.sub.2 are converted into CH.sub.4 within time significantly shorter than the lifetime of the solar nebula at the optimal temperatures around 550 K. The calculation result of clathration shows that CO.sub.2-rich satellitesimals are formed in the catalytically inactive outer region of subnebula. In the catalytically active inner region, CH.sub.4-rich satellitesimals are formed. The resulting CH.sub.4-rich satellitesimals formed in this region play an important role in the origin of CH.sub.4 on Titan. When our experimental data are applied to a high-pressure model for subnebula evolution, it would predict that there should be CO.sub.2 underneath the Iapetus subsurface and no thick CO.sub.2 ice layer on Titan's icy crust. Such surface and subsurface composition, which may be observed by Cassini-Huygens mission, would provide crucial information on the origin of icy satellites. Author Affiliation: (a) Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan (b) Department of Complexity Science and Engineering, Graduate School of Frontier Science, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 227-8562, Japan (c) Department of Chemistry, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan (d) Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan Article History: Received 12 August 2004; Revised 4 March 2005

Titan, the largest satellite of Saturn, has a thick nitrogen/methane atmosphere with a thick global organic haze. A laboratory analogue of Titan's haze, called tholin, was formed in an inductively coupled plasma from nitrogen/methane = 90/10 gas mixture at various pressures ranging from 13 to 2300 Pa. Chemical and optical properties of the resulting tholin depend on the deposition pressure in cold plasma. Structural analyses by IR and UV/VIS spectroscopy, microprobe laser desorption/ionization mass spectrometry, and Raman spectroscopy suggest that larger amounts of aromatic ring structures with larger cluster size are formed at lower pressures (13 and 26 Pa) than at higher pressures (160 and 2300 Pa). Nitrogen is more likely to incorporate into carbon networks in tholins formed at lower pressures, while nitrogen is bonded as terminal groups at higher pressures. Elemental analysis reveals that the carbon/nitrogen ratio in tholins increases from 1.5-2 at lower pressures to 3 at 2300 Pa. The increase in the aromatic compounds and the decrease in C/N ratio in tholin formed at low pressures indicate the presence of the nitrogen-containing polycyclic aromatic compounds in tholin formed at low pressures. Tholin formed at high pressure (2300 Pa) consists of a polymer-like branched chain structure terminated with -C[H.sub.3], -N[H.sub.2], and -C[equivalent to]N with few aromatic compounds. Reddish-brown tholin films formed at low pressures (13-26 Pa) shows stronger absorptions (almost 10 times larger k-value) in the UV/VIS range than the yellowish tholin films formed at high pressures (160 and 2300 Pa). The tholins formed at low pressures may be better representations of Titan's haze than those formed at high pressures, because the optical properties of tholin formed at low pressures agree well with that of Khare et al. (1984a, Icarus 60, 127-137), which have been shown to account for Titan's observed geometric albedo. Thus, the nitrogen-containing polycyclic aromatic compounds we find in tholin formed at low pressure may be present in Titan's haze. These aromatic compounds may have a significant influence on the thermal structure and complex organic chemistry in Titan's atmosphere, because they are efficient absorbers of UV radiation and efficient charge exchange intermediaries. Our results also indicate that the haze layers at various altitudes might have different chemical and optical properties. Keywords: Titan; Haze; Tholin; Spectroscopy; Organic chemistry; Nitrogen-containing polycyclic aromatic compounds