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Light Flash and Ionization from Hypervelocity Impacts on Ice
ICARUS
122, 359–365 (1996) 0129
ARTICLE NO.
Light Flash and Ionization from Hypervelocity Impacts on Ice M. J. BURCHELL, M. J. COLE,
AND
P. R. RATCLIFF
Unit for Space Sciences, University of Kent at Canterbury, Kent CT2 7NR, United Kingdom E-mail: [email protected] Received June 9, 1995; revised November 8, 1995
As part of a study of the evolution of icy surfaces in the Solar System and their influence on their surroundings, we have measured in the laboratory hypervelocity impacts (2 to 65 km s21) of micron- and submicron-sized iron particles on low-temperature water ice. We have measured the light flash and ionization arising from the impacts. We find that total light flash energy normalized to projectile mass scales with the impact velocity to the power 3.65 6 0.37. Similarly we find that the directly ionized charge per impact normalized to projectile mass scales with impact velocity to the power 4.12 6 0.41. The total light flash energy and degree of primary ionization are both found to be less than those for similar impacts on a typical metal surface (molybdenum) by factors of (4.62 6 0.93) and (2.00 6 0.40), respectively. 1996 Academic Press, Inc.
I. INTRODUCTION
There are many icy bodies in the Solar System, particularly in the outer regions. Indeed, the two Voyager spacecraft have indicated that water ice is very abundant in the outer Solar System. For small icy bodies lacking appreciable atmospheres, the surface is modified not only by heating or the occasional impact of large projectiles, but also by the hypervelocity impacts of a continual flux of small particles. The study of the properties of impacts (both large and small) is thus important for determining the evolution of an icy surface and the properties of the surrounding region of space. Various authors have studied impact cratering in ice in the laboratory in order to explain the observed large impact craters on, and large-scale disruption of, the satellites of Jupiter and Saturn [e.g., at low velocity, Kawakami et al. (1983), Lange and Ahrens (1987), and Kato et al. 1995, and at hypervelocities, Croft (1981)]. More recently attention has turned to the effects of hypervelocity impacts on ice of smaller particles. Frisch (1992) directly measured crater sizes for impacts of 18 to 124-em glass beads on ice. Eichhorn and Gru¨n (1993) bombarded ice with micron- and submicron-sized projectiles and measured excavated crater volumes indirectly by means of a pressure surge in their (evacuated) target chamber.
The influence on the surrounding volume of space of ejecta or ionized material produced during impacts on ice has also been considered [e.g., plasma production in Saturn’s rings; see Pospieszalska and Johnson (1991), and Timmermann and Gru¨n (1991)]. Such studies will be further encouraged by new space missions to the icy bodies. The Cassini/Huygens probe to Saturn and its moons (to be launched in 1997) will clearly excite increased interest in this field. Also the surface of any comet is exposed to hypervelocity impacts of the small-particle population in space; this will process the surface independent of any solar irradiation. The international Rosetta mission to Comet P/Wirtanen will serve to increase the attention paid to fine details of comet surface structure, and thus laboratory studies of impacts on icy surfaces will be of interest. We have thus studied in the laboratory the hypervelocity impacts of micron- and submicron-sized iron particles on lowtemperature water ice. The temperature of a body warmed solely by solar illumination will depend on many factors (distance from the Sun, albedo, etc.) but will, at saturnian distances, typically be in the range 90 to 120 K. Therefore laboratory data on impacts should, if possible, be taken in the same temperature range to permit more easy application to physical situations; hence our use of water ice at low temperatures. The density of small particles in space is usually assumed to be much lower than that of iron. However, our accelerator is optimized for iron; hence its use in this work. Observation of impact flash has been one of the earliest tools to help probe hypervelocity impacts (Atkins 1955). The light flash in hypervelocity impacts of micron-sized particles on metals has been observed previously (Eichhorn 1975, 1976). In his work, Eichhorn mainly used iron projectiles (accelerated in a similar Van de Graaff facility) and gold or tungsten targets. He also made use of tungsten, carbon, and aluminum projectiles to some degree. The range of velocities covered in his work was 1 to 35 km s21, and he observed the light flash by use of photomultiplier tubes. In Eichhorn (1975) the energy of the light flash (E) is found to be related to projectile mass (m) and velocity
359 0019-1035/96 $18.00 Copyright 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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(v) by E 5 cmav b, where c is a constant, a 5 1.25, and b ranges from 2.5 to 3.0 (depending on the materials used). In his later paper with more data (Eichhorn 1976) a 5 1.0 is used, i.e., E/m 5 cv b, and b ranges from 3.2 to 3.5, depending on projectile and target composition. It was also found (Eichhorn 1976) that the absolute magnitude of the flash was dependent on the pressure of the surrounding atmosphere, but was a constant below approximately 1023 mbar (indicating that part of the light emission can arise from interactions between the surrounding atmosphere and the expanding cloud of gas or plasma liberated during the impact). Impact ionization has also been studied for many years. The early results for impacts of iron on metal (e.g., Friichtenicht and Slattery 1963) showed that ionization (I) was related to projectile mass and velocity by I/m 5 cv b, where b 5 3.0 (c is a constant of proportionality). Later work found similar results with slight variations; e.g., Dietzel et al. (1973) found b 5 3.55 6 0.2 for iron impacting tungsten or gold and copper impacting tungsten. However, theoretical attempts to explain impact ionization have favored I 5 cmav b, adopting a 5 0.8 (e.g., see Kissel and Krueger 1987) and b left free to characterize particular projectile/ target combinations. Despite this, the most recent extensive experimental investigation of hypervelocity impacts on a variety of metals (Go¨ller and Gru¨n 1989) found that a P 1.0 was the best description of the data. Go¨ller and Gru¨n (1989) also noted that for iron impacts on metals there was a change in b at approximately 6.5 km s21, with a lower value at lower impact velocities. This was explained (for their experimental geometry) in terms of a change between the dominant contribution to the signal arising at low velocities from ionization caused by ejecta reimpacting and at high velocities from ions directly produced in the impact. Hypervelocity impacts of micron-sized iron particles on low-temperature water ice are reported in Timmermann and Gru¨n (1991). They measure the direct ion yield (I) in impacts (above v 5 8 km s21) and find that I 5 cm0.8v2.45. They also find that the ionization from ice is less (factor of 100) than that from similar impacts on gold. There has thus clearly been a variety of work on both impact flash and ionization in general. In this paper we present the first investigations of the light flash arising from hypervelocity impacts of micron-sized iron particles on low-temperature water ice. We have also simultaneously measured the ionization that occurs during such impacts. II. EXPERIMENTAL METHOD
The work reported herein was carried out at the Hypervelocity Impact Facility of the University of Kent at Canterbury (UK) (see Burchell et al. 1994). A 2-MV Van de Graaff machine was used to electrostatically accelerate charged particles of iron. The accelerating
FIG. 1. Mass vs velocity for iron particles used as projectiles.
voltage (V) varied between 1.2 and 1.8 MV, and was continually monitored. The accelerated particles traveled into a target chamber maintained at a vacuum of 1026 mbar [note that this is well below the level—1023 mbar— noted by Eichhorn (1976) where the ambient pressure affects light generation]. During flight particles pass through two conducting cylinders, the induced charges on which were measured. The magnitude of the induced signals was used to determine the charge (q) on the accelerated particle. The time separation between the signals provided the particle velocity (v).1 We determine the particle mass (m) from 0.5 mv 2 5 qV.
(1)
During data taking the accelerating voltage was uncertain to 4% (independent of voltage). The particle velocity (charge) was, on average, accurate to 1% (4%) at low velocities, rising to 10% (30%) at 65 km s21. Figure 1 shows the mass–velocity plot for data reported below. It can be seen that the velocity covered 2 to 65 km s21, and the mass range 10213 to 10219 kg, with a strong correlation between mass and velocity. The latter is a standard feature of such Van de Graaff accelerators. Iron dust of the type used in the accelerator has been examined under a scanning electron microscope and found to be spherical to a high degree. Accordingly the mass range used here corresponds to iron particles between 0.01 and 3.0 em in diameter. The cold target used in the experiments is shown schematically in Fig. 2. It was made of stainless steel. The operating procedure was as follows: the cold target was held vertically and the internal reservoir (capacity 47 cm3) filled with liquid nitrogen (LN2). After the target cooled 1
All velocities given in this paper are absolute and are not the component normal to the surface.
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FIG. 2. Schematic of the cold target. T1 and T2 are thermocouples.
it was inverted and the lower part placed in a LN2 bath to keep it cold. A grid was wetted and placed in the cold target above the disk-shaped water cup. Distilled water was then dropped into the water cup through the wetted grid. The water froze rapidly upon coming in contact with the cold surface. The amount of water used was such that upon freezing the surface layer was in good contact with the grid. The grid was mounted on a ring which isolated it electrically from the stainless-steel surround. The grid was made of molybdenum wire with a mesh spacing of 169 em (150 lines per inch) and a transparency of 85%. A second grid (made of the same material) was then mounted on the shroud 2.1 6 0.1 mm above the first grid. The assembly was then placed horizontally in the vacuum chamber which serves as the Van de Graaff target area. A pipe from the cold target’s internal reservoir was connected to an external reservoir of LN2 via a tube through a flange in the wall of the chamber. A height difference between the two reservoirs enabled the interior to fill with LN2 under gravity flow. However, to ensure consistent flow a flexible narrow-diameter pipe was inserted into the interior reservoir through the feed pipe. This narrow pipe was then connected to a vacuum pump which drew the LN2 into the reservoir. The temperature of the cold target was monitored at two points by thermocouples. Point 1 was in the stainless-steel wall of the cold target close to the ice; point 2 was at the far end of the stainless-steel shroud (which acted to prevent heating of the ice by radiation from the walls of the surrounding chamber). During data taking the chamber was maintained at a vacuum of 0.8 to 1.0 3 1026 mbar and the supply of LN2 was sufficient to maintain the temperature of thermocouple 1 between 115 and 125 K. During operation a photomultiplier (pm) tube (type EMI6097) was positioned next to a viewport in the vacuum chamber wall and was used to obtain a sideview of the light flash from an impact. The pm tube photocathode was 19 cm away from the ice. The pm tube was operated in a charge collection mode with a decay constant (65 esec) greater than the duration of a hypervelocity impact flash (a few microseconds maximum). Thus the total signal read out for an event was a measure of the total light emission
during the impact (i.e., from both the impact crater and any plasma generated above the surface). The response of the tube was calibrated by observing a chopped signal from a calibrated tungsten lamp (a correction being made for the variation of the response of the tube with wavelength). Both linearity and absolute response at the light intensities observed in impacts were checked. No correction is made for variations in the sensitivity of the photomultiplier response (which rapidly falls off above 650 nm). The ionization was measured by applying a voltage (500 V) to the grid in the surface of the ice. The second grid was held at earth, and charge flowing between the two during an impact was fed to an external amplifier and recorded. The amplifier was calibrated by injection of known charges, and thus its response gave the charge liberated in an event. It should be noted that during operation, for reasons of access to the chamber, all data were taken for impacts on the target at 368 to the normal. III. RESULTS
Data were take both with ice present in the cold target and with an empty target. This permitted a cross check to establish if the signals observed were indeed due to impacts in the ice. In this respect, note that when ice was present impacts could occur on the ice or the wire grid. Data taking occurred in runs spread over 3 months. All features of the data repeat in all the runs (i.e., the scatter in the observed data is present in every data subset and is not the result of combining data subsets). According we do not distinguish between separate runs when presenting the results. Events were recorded for analysis when a particle was detected in the beam line and a light flash or ionization pulse observed in coincidence with it. The accuracy of the light flash and ionization measurements is combined with the errors on mass and velocity when plotting data. In Fig. 3a we show for empty target data the total light flash energy (E) normalized to projectile mass (m) vs projectile velocity (v). A clear power law behavior can be seen. This has been fitted (solid line in Fig. 3a) to E/m 5 cv b. We find that c 5 4.94 6 0.99 and b 5 3.65 6 0.18 (E/m in J kg21 and v in km s21). Changing the viewing conditions (i.e., the viewing angle and distance from the target) does not affect the results. In Fig. 3b we similarly show the light flash data for runs when ice was present. The solid curve shown is the fit to the empty target data. Clearly this overlaps with part of the ice-filled target data, but there is also a component of the ice-filled target data in poor agreement with this fit. Thus the ice-filled target data contains two components, one for impacts in the target itself and the other for impacts in the ice. To better establish the two components in our ice data we show Fig. 4a. Here the data have been transformed
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FIG. 3. Light flash energy/projectile mass vs projectile velocity. (a) Empty target; (b) ice-filled target. Solid curve shown in both (a) and (b) is a fit to (a).
into a coordinate space where we measure the minimum distance from each data point to the curve fit to the empty target data (note that both the quantities being plotted in Fig. 3 have a nonnegligible error, so in any fit the combined residual on each quantity has to be considered). The units
FIG. 4. Residuals of light flash data: (a) shows both empty target data (dotted line) and ice-filled target data (solid line); (b) shows a fit (solid line) to the ice-filled target residuals (dotted lines are the individual Gaussians in this fit as given in the text).
of these residuals are thus not purely J kg21 or km s21, but a combination of the two; for convenience we use an arbitrary scale of units in Fig. 4. The solid curve in Fig. 4a is for the ice data, and the dashed curve is for the empty target data. A clear difference is seen between the two, with the ice data possessing an apparent contribution similar to the empty target data plus an extra contribution. It is this latter component which we consider to arise from impacts on ice. To illustrate this point further we show Fig. 4b. Here a fit to the residual data for the ice-filled target run is shown. This is a two-component fit. The first component is a Gaussian whose mean and sigma are fixed as the result of fitting a Gaussian to the residual distribution of the empty target data; only the overall normalization of this Gaussian is left free in the fit to Fig. 4b. The second Gaussian is fitted with a free mean, sigma, and normalization, and represents the additional contribution to the data when ice is present. This second component in the data is clearly seen. Further we find that the mean does not shift significantly when we divide the residuals by impact velocity. We conclude that this signal represents impacts on ice, and that these ice impacts observe a power law behavior for E/m vs v with a similar power to the empty target data (but a different normalization, obtained from the position of the mean of the second Gaussian in the residual plot). We thus find for the impacts on ice that E/m 5 cv 3.656.37, where c 5 1.07 6 0.21 (E/m in J kg21 and v in km s21). The given accuracy of the slope is less than that in the fit to empty target data. This is because the variation in slope that can be accommodated by the data is 10% (i.e., the small shift in the position of the second ‘‘ice’’ Gaussian’s mean when the data is subdivided by velocity). We find that the ice impacts produce a smaller light signal at any projectile mass and velocity than do the impacts on the empty target, by a factor of 4.62 6 0.93. As stated earlier, no significant variation in the results was obtained by varying the viewing geometry, so the errors given are obtained from the data and the analysis described above. When observing the ionization, we find that the signal comprises two components. The first is a fast rising signal (order 100 nsec), corresponding to ionization occurring directly in the impact (primary ionization IP). There is also a second, slower, signal (IS) which can be associated with ionization caused by impacts of high-speed ejecta on surrounding surfaces. The rise time of IP is dictated by the electric field applied between the grids. The 500 V used was chosen to ensure that IP and IS would be separable in our data. The magnitude of IP and IS for impacts when the target is empty is shown in Figs. 5a and 5b, respectively. We begin by fitting the data with I 5 mav b. We carry out the fit by fixing a at discrete values in the range 0.7 to 1.2, while b is left free. We find that for IP , the fits where a is less than 1.0 have a significantly worse correlation, while from a 5 1.0 to 1.2 no great change in the fit likelihood
HYPERVELOCITY IMPACTS ON ICE
FIG. 5. Ionization/projectile mass vs projectile velcoity. For empty target data, (a) is primary and (b) is secondary ionization. For ice-filled target data, (c) is primary and (d) is secondary ionization.
occurs with a and b varying against each other. Since for a of 1.0 or above no significant improvement in fit quality occurs, we follow the results of Go¨ller et al. (1986) and Go¨ller and Gru¨n (1989) and fix a 5 1.0 in all the results presented below. We thus obtain I/m 5 cv b, where for primary ionization c 5 (7.89 6 1.58) 3 1023 and b 5 4.12 6 0.21, and for secondary ionization c 5 (33.8 6 6.8) 3 1023 and b 5 3.39 6 0.17 (C kg 21 and km s21). We can thus see that the primary ionization dominates above 7.4 km s21, in good agreement with Go¨ller and Gru¨n (1989). The slope of 4.12 6 0.21 is slightly higher than the 3.5 reported by Dietzel et al. (1973), although we note that they fit their data from 2 km s21 upward, and will thus average over total ionization. The magnitude of the total ionization is within the spread of total ionization values obtained for iron impacts on various metals at similar angles of incidence (e.g., see Go¨ller and Gru¨n 1989). This treatment of the data with a single value for b over the whole velocity range is tested by fitting the low- and high-velocity regions separately. The values thus found for b are not significantly different from the value found for the whole velocity range within the obtained errors. In Figs. 5c and 5d we show the primary and secondary ionization data when ice is present in the target. The solid curves shown are the fits to the respective empty target data. In both cases it can be seen that the ice data partially overlaps the empty target fit. As for the light flash analysis we transform to a space where we obtain the minimum residual to the empty target fit. This is plotted in Fig. 6a (primary ionization) and 6b (secondary ionization). It can be seen that the ice-filled target data again contain a component absent from the empty target data. Proceeding as for the light flash data, we find the component of the data not compatible with the empty target signal and ascribe this to impacts on ice. There is no clear correlation of the
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residuals of the ice component of the data with projectile velocity. We thus describe the ionization for impacts on ice by I/m 5 cv b, using b from the appropriate empty target data set. For primary ionization we find that c 5 0.0040 6 0.0008 and b 5 4.12 6 0.41, and for secondary ionization we have c 5 0.012 6 0.002 and b 5 3.39 6 0.34 (I/m in C kg21 and v in km s21). As before we find that a possible 10% change in the value of b for impacts on ice can be accommodated by dividing the data into velocity subsets. Accordingly we take this as the error on b. We find that, independent of velocity, the average ionization due to the impacts in ice is less than that due to the impacts in the empty target. For primary ionization this ratio is measured as 2.00 6 0.40, and for secondary ionization as 2.82 6 0.56. When comparing our results to Timmermann and Gru¨n (1991), we find that their total ionization signal from ice is smaller than ours by a factor of typically two orders of magnitude. In Timmermann and Gru¨n (1991) there is not much discussion of the experimental procedure, nor a detailed presentation of the results. It is therefore impossible to reproduce their experimental conditions. However, we note that our results are obtained from several separate (but mutually compatible) runs, and that our signals from the impacts on metal agree in magnitude with previous results for impacts on metal (Go¨ller and Gru¨n 1989). We thus have confidence in our experimental setup and the results obtained. As stated, the signal for secondary ionization can be
FIG. 6. Residuals of ice-filled target data to curves fit to empty target data. (a) Primary ionization; (b) secondary ionization. Empty target (icefilled) target data is dashed (solid) line.
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FIG. 7. Ejecta vs projectile velocity derived from secondary ionization signal rise times. The solid line is the fit described in the text.
interpreted as the observation of ionization caused by impacts of high-speed ejecta on surrounding surfaces. We assume that the typical angle of ejection is 608 to the target surface (see Croft 1981). Ejecta will thus typically impact the grid above the surface after traversing an average path of 2.4 mm. The rise times of the secondary ionization signal are combined with this distance to obtain ejecta velocity (ve) and are shown in Fig. 7 as a function of the velocity of the original projectile (vp). A clear correlation is seen, and we find that ejecta velocity ve 5 0.093v 1.13 p . It is not unreasonable to expect a range of 6108 in the ejecta angle from impact to impact. This would indicate a possible 30% spread in the ejecta velocities obtained; this is slightly less than the observed scatter in the data at any fixed projectile velocity. The velocity of ice ejecta has been measured before (Frisch 1992). For vp in the range 1.8 to 8 km s21, ve was found to vary between 0.01 and 0.7 km s21. No correlation between ve and vp was found in that work. If we examine our data we see that in same range of vp , ve varies between 0.2 and 1.3 km s21. The lower velocities of this range seem rather low for an impact producing ionization, indicating a possible weakness in our assumptions. The range of 0.2 to 1.3 km s21 only partially overlaps the results of Frisch (1992). The difference is explained by noting that Frisch (1992) only observes spallation ejecta. This is held to be the component of ejecta which contains most of the mass (50 to 95%), but is not the highest speed ejecta in an event. Given that we observe the ejecta as a result of impact ionization, which requires high impact speed, we do not expect to be sensitive to spallation ejecta. Further, the dependence of ve on vp only becomes apparent in our work due to the large range of vp over which we observe. We can proceed further and estimate the mass of ejecta observed in our events. This we do by assuming that the secondary ionization signal is dependent on ejecta mass
and velocity in the same way that the primary signal depends on projectile mass and velocity. This requires two separate assumptions. The first is that the iron projectiles can be considered similar to the molybdenum in the grid. Previous work for ionization caused by iron impacts on various metals (Dietzel et al. 1973, Go¨ller and Gru¨n 1989) indicates a spread in total ionization of approximately a factor of 10 as the target metal changes. This sets a limit on any results we obtain below. The second assumption is that impacts from ice on metal can be approximated by impacts of metal on ice. The validity of this is unquantifiable, but it is the simplest approximation that can be made and is thus appropriate in these circumstances. We also assume that ejecta impact the grid to produce the secondary ionization signal. Since the grid has an optical transparency of 85% we assume it only intercepts 15% of the ejecta. From our results we thus find that at a projectile velocity of 5 km s21 the volume of ice ejected from the target is 7.0 3 10210 cm3. Similarly at 10 km s21 it is 2.8 3 10211 cm3. We can compare our measure of the non-spallation ice ejecta volume with estimates by Eichhorn and Gru¨n (1993) of the total volume of ice ejected in hypervelocity impacts. Using their parameterization of crater volume applied to our projectile mass and velocity, we find that at 5 km s21 they predict a total ejecta volume of 1.5 3 1029 cm3, and at 10 km s21 one of 5.9 3 10210 cm3. If we accept these values, the ejecta volumes derived from our results represent 47 and 4.7%, respectively, of the total ice ejected in hypervelocity impacts at 5 and 10 km s21. Frisch (1992) reports that between 50 and 95% of ejecta is spallation (to which we are not sensitive), not incompatible with our results. We note that our results imply a correlation of this fraction with impact velocity, indicating a possible increase in the influence of spallation in total crater volume as the impact velocity increases from 5 to 10 km s21. IV. CONCLUSIONS
In conclusion we have simultaneously measured light flash and ionization for hypervelocity impacts of iron on low-temperature ice. In both cases the intensities of the signals are less than those for similar impacts on a typical metal (molybdenum). We find that the light flash energy for impacts of iron on ice can be described in terms of projectile mass and velocity by E/m 5 (1.07 6 0.21)v 3.6560.37 (E/m in J kg21 and v in km s21). It is remarkable that this relationship holds true over such a large range of impact velocities (2 to 65 km s21). It is normally expected that such a velocity range covers several distinct regions of behavior in terms of, for example, the degree of vaporization of the projectile. Similarly the ionization I (both primary and secondary) obeys power laws in I/m vs impact velocity: IP /m 5 (40 6 8) 3 1023 v 4.1260.41 and IS /m 5 (12 6 2) 3 1023 v 3.3960.34 (I/m in C kg21 and v in km s21).
HYPERVELOCITY IMPACTS ON ICE
Again, given the range of impact velocities used, this is quite remarkable. It should be noted that the power of the velocity is found to only 610%, but this acts as quite a stringent limit on changes of ionization mechanisms and their yields with velocity. ACKNOWLEDGMENTS The hypervelocity impact facilities at the University of Kent at Canterbury are financed by grants from PPARC (the Particle Physics and Astronomy Research Council, UK) and its predecessor, SERC (the Science and Engineering Research Council).
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EICHHORN, G. 1976. Analysis of the hypervelocity impact process from impact flash measurement. Planet. Space Sci. 24, 771–781. EICHHORN, G., AND GRU¨ N 1993. High velocity impacts of dust particles in low-temperature water ice. Planet. Space Sci. 41, 429–433. FRIICHTENICHT, J. F., AND J. C. SLATTERY 1963. Ionization associated with hypervelocity impact. Proc. VIth Symp. Hypervelocity Impacts, 591–612. FRISCH, W. 1992. Hypervelocity impact experiments with water ice targets. In Hypervelocity Impacts in Space (J. A. M. McDonnell, Ed.), pp. 7–14. Unit for Space Sciences, Univ. of Kent, Canterbury, UK. GO¨ LLER, J. R., AND E. GRU¨ N 1989. Calibration of the Gallileo/Ulysses dust detectors with different projectile materials and at varying impact angles. Planet. Space Sci. 37, 1197–1206.
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ATKINS, W. W. 1955. Flash associated with high velocity impacts on aluminium. J. Appl. Phys. 26, 126–127. BURCHELL, M., J A. M. MCDONNELL, M. J. COLE, AND P. R. RATCLIFF 1994. The hypervelocity impact facilities at the University of Kent. Lunar Planet. Inst. Tech. Rep. 94-05 (Houston, TX), 32–35. CROFT, S. K. 1981. Hypervelocity impact craters in icy media. In Lunar and Planetary Science Abstracts XII, pp. 190–192. The Lunar and Planetary Institute, Houston, TX. DALMANN, B. K., E. GRU¨ N, J. KISSEL, AND H. DIETZEL 1977. The ion composition of the plasma produced by impacts of fast dust particles. Planet. Space Sci. 25, 135–147. DIETZEL, H., G. EICHORN, H. FECHTIG, E. GRU¨ N, H-J. HOFFMAN, AND J. KISSEL 1973. The HEOS2 and HELIOS micrometeroid experiments. J. Phys. E. Sci. Instrum. 6, 209–217. EICHHORN, G. 1975. Measurements of the light flash produced by high velocity particle impact. Planet. Space Sci. 23, 1519–1525.
KATO, M., Y. IIJIMI, M. ARAKAWA, Y. OKIMURA, A. FUJIMURA, N. MAENO, AND H. MIZUTANI 1995. Ice-on-ice impact experiments. Icarus 113, 423–441. KISSEL, J., AND F. R. KRUEGER 1987. Ion formation by impact of fast dust particles and comparison with related techniques. Appl. Phys. A 42, 69–85. LANGE, M. A., AND T. J. AHRENS 1987. Impact experiments in lowtemperature ice. Icarus 69, 506–518. POSPIESZALSKA, M. K., AND R. E. JOHNSON 1991. Micrometeorite erosion of the main rings as a source of plasma in the inner saturnian plasma torus. Icarus 93, 45–52. TIMMERMANN, R., AND E. GRU¨ N 1991. Plasma emission from high velocity impacts of microparticles onto water ice. In Origin and Evolution of Interplanetary Dust (A. C. Levasseur-Regourd and H. Hasegawa, Eds.), Proceedings, 126th Colloquium of the International Astronomical Union, Vol. 173, pp. 375–378.
122, 359–365 (1996) 0129
ARTICLE NO.
Light Flash and Ionization from Hypervelocity Impacts on Ice M. J. BURCHELL, M. J. COLE,
AND
P. R. RATCLIFF
Unit for Space Sciences, University of Kent at Canterbury, Kent CT2 7NR, United Kingdom E-mail: [email protected] Received June 9, 1995; revised November 8, 1995
As part of a study of the evolution of icy surfaces in the Solar System and their influence on their surroundings, we have measured in the laboratory hypervelocity impacts (2 to 65 km s21) of micron- and submicron-sized iron particles on low-temperature water ice. We have measured the light flash and ionization arising from the impacts. We find that total light flash energy normalized to projectile mass scales with the impact velocity to the power 3.65 6 0.37. Similarly we find that the directly ionized charge per impact normalized to projectile mass scales with impact velocity to the power 4.12 6 0.41. The total light flash energy and degree of primary ionization are both found to be less than those for similar impacts on a typical metal surface (molybdenum) by factors of (4.62 6 0.93) and (2.00 6 0.40), respectively. 1996 Academic Press, Inc.
I. INTRODUCTION
There are many icy bodies in the Solar System, particularly in the outer regions. Indeed, the two Voyager spacecraft have indicated that water ice is very abundant in the outer Solar System. For small icy bodies lacking appreciable atmospheres, the surface is modified not only by heating or the occasional impact of large projectiles, but also by the hypervelocity impacts of a continual flux of small particles. The study of the properties of impacts (both large and small) is thus important for determining the evolution of an icy surface and the properties of the surrounding region of space. Various authors have studied impact cratering in ice in the laboratory in order to explain the observed large impact craters on, and large-scale disruption of, the satellites of Jupiter and Saturn [e.g., at low velocity, Kawakami et al. (1983), Lange and Ahrens (1987), and Kato et al. 1995, and at hypervelocities, Croft (1981)]. More recently attention has turned to the effects of hypervelocity impacts on ice of smaller particles. Frisch (1992) directly measured crater sizes for impacts of 18 to 124-em glass beads on ice. Eichhorn and Gru¨n (1993) bombarded ice with micron- and submicron-sized projectiles and measured excavated crater volumes indirectly by means of a pressure surge in their (evacuated) target chamber.
The influence on the surrounding volume of space of ejecta or ionized material produced during impacts on ice has also been considered [e.g., plasma production in Saturn’s rings; see Pospieszalska and Johnson (1991), and Timmermann and Gru¨n (1991)]. Such studies will be further encouraged by new space missions to the icy bodies. The Cassini/Huygens probe to Saturn and its moons (to be launched in 1997) will clearly excite increased interest in this field. Also the surface of any comet is exposed to hypervelocity impacts of the small-particle population in space; this will process the surface independent of any solar irradiation. The international Rosetta mission to Comet P/Wirtanen will serve to increase the attention paid to fine details of comet surface structure, and thus laboratory studies of impacts on icy surfaces will be of interest. We have thus studied in the laboratory the hypervelocity impacts of micron- and submicron-sized iron particles on lowtemperature water ice. The temperature of a body warmed solely by solar illumination will depend on many factors (distance from the Sun, albedo, etc.) but will, at saturnian distances, typically be in the range 90 to 120 K. Therefore laboratory data on impacts should, if possible, be taken in the same temperature range to permit more easy application to physical situations; hence our use of water ice at low temperatures. The density of small particles in space is usually assumed to be much lower than that of iron. However, our accelerator is optimized for iron; hence its use in this work. Observation of impact flash has been one of the earliest tools to help probe hypervelocity impacts (Atkins 1955). The light flash in hypervelocity impacts of micron-sized particles on metals has been observed previously (Eichhorn 1975, 1976). In his work, Eichhorn mainly used iron projectiles (accelerated in a similar Van de Graaff facility) and gold or tungsten targets. He also made use of tungsten, carbon, and aluminum projectiles to some degree. The range of velocities covered in his work was 1 to 35 km s21, and he observed the light flash by use of photomultiplier tubes. In Eichhorn (1975) the energy of the light flash (E) is found to be related to projectile mass (m) and velocity
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(v) by E 5 cmav b, where c is a constant, a 5 1.25, and b ranges from 2.5 to 3.0 (depending on the materials used). In his later paper with more data (Eichhorn 1976) a 5 1.0 is used, i.e., E/m 5 cv b, and b ranges from 3.2 to 3.5, depending on projectile and target composition. It was also found (Eichhorn 1976) that the absolute magnitude of the flash was dependent on the pressure of the surrounding atmosphere, but was a constant below approximately 1023 mbar (indicating that part of the light emission can arise from interactions between the surrounding atmosphere and the expanding cloud of gas or plasma liberated during the impact). Impact ionization has also been studied for many years. The early results for impacts of iron on metal (e.g., Friichtenicht and Slattery 1963) showed that ionization (I) was related to projectile mass and velocity by I/m 5 cv b, where b 5 3.0 (c is a constant of proportionality). Later work found similar results with slight variations; e.g., Dietzel et al. (1973) found b 5 3.55 6 0.2 for iron impacting tungsten or gold and copper impacting tungsten. However, theoretical attempts to explain impact ionization have favored I 5 cmav b, adopting a 5 0.8 (e.g., see Kissel and Krueger 1987) and b left free to characterize particular projectile/ target combinations. Despite this, the most recent extensive experimental investigation of hypervelocity impacts on a variety of metals (Go¨ller and Gru¨n 1989) found that a P 1.0 was the best description of the data. Go¨ller and Gru¨n (1989) also noted that for iron impacts on metals there was a change in b at approximately 6.5 km s21, with a lower value at lower impact velocities. This was explained (for their experimental geometry) in terms of a change between the dominant contribution to the signal arising at low velocities from ionization caused by ejecta reimpacting and at high velocities from ions directly produced in the impact. Hypervelocity impacts of micron-sized iron particles on low-temperature water ice are reported in Timmermann and Gru¨n (1991). They measure the direct ion yield (I) in impacts (above v 5 8 km s21) and find that I 5 cm0.8v2.45. They also find that the ionization from ice is less (factor of 100) than that from similar impacts on gold. There has thus clearly been a variety of work on both impact flash and ionization in general. In this paper we present the first investigations of the light flash arising from hypervelocity impacts of micron-sized iron particles on low-temperature water ice. We have also simultaneously measured the ionization that occurs during such impacts. II. EXPERIMENTAL METHOD
The work reported herein was carried out at the Hypervelocity Impact Facility of the University of Kent at Canterbury (UK) (see Burchell et al. 1994). A 2-MV Van de Graaff machine was used to electrostatically accelerate charged particles of iron. The accelerating
FIG. 1. Mass vs velocity for iron particles used as projectiles.
voltage (V) varied between 1.2 and 1.8 MV, and was continually monitored. The accelerated particles traveled into a target chamber maintained at a vacuum of 1026 mbar [note that this is well below the level—1023 mbar— noted by Eichhorn (1976) where the ambient pressure affects light generation]. During flight particles pass through two conducting cylinders, the induced charges on which were measured. The magnitude of the induced signals was used to determine the charge (q) on the accelerated particle. The time separation between the signals provided the particle velocity (v).1 We determine the particle mass (m) from 0.5 mv 2 5 qV.
(1)
During data taking the accelerating voltage was uncertain to 4% (independent of voltage). The particle velocity (charge) was, on average, accurate to 1% (4%) at low velocities, rising to 10% (30%) at 65 km s21. Figure 1 shows the mass–velocity plot for data reported below. It can be seen that the velocity covered 2 to 65 km s21, and the mass range 10213 to 10219 kg, with a strong correlation between mass and velocity. The latter is a standard feature of such Van de Graaff accelerators. Iron dust of the type used in the accelerator has been examined under a scanning electron microscope and found to be spherical to a high degree. Accordingly the mass range used here corresponds to iron particles between 0.01 and 3.0 em in diameter. The cold target used in the experiments is shown schematically in Fig. 2. It was made of stainless steel. The operating procedure was as follows: the cold target was held vertically and the internal reservoir (capacity 47 cm3) filled with liquid nitrogen (LN2). After the target cooled 1
All velocities given in this paper are absolute and are not the component normal to the surface.
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FIG. 2. Schematic of the cold target. T1 and T2 are thermocouples.
it was inverted and the lower part placed in a LN2 bath to keep it cold. A grid was wetted and placed in the cold target above the disk-shaped water cup. Distilled water was then dropped into the water cup through the wetted grid. The water froze rapidly upon coming in contact with the cold surface. The amount of water used was such that upon freezing the surface layer was in good contact with the grid. The grid was mounted on a ring which isolated it electrically from the stainless-steel surround. The grid was made of molybdenum wire with a mesh spacing of 169 em (150 lines per inch) and a transparency of 85%. A second grid (made of the same material) was then mounted on the shroud 2.1 6 0.1 mm above the first grid. The assembly was then placed horizontally in the vacuum chamber which serves as the Van de Graaff target area. A pipe from the cold target’s internal reservoir was connected to an external reservoir of LN2 via a tube through a flange in the wall of the chamber. A height difference between the two reservoirs enabled the interior to fill with LN2 under gravity flow. However, to ensure consistent flow a flexible narrow-diameter pipe was inserted into the interior reservoir through the feed pipe. This narrow pipe was then connected to a vacuum pump which drew the LN2 into the reservoir. The temperature of the cold target was monitored at two points by thermocouples. Point 1 was in the stainless-steel wall of the cold target close to the ice; point 2 was at the far end of the stainless-steel shroud (which acted to prevent heating of the ice by radiation from the walls of the surrounding chamber). During data taking the chamber was maintained at a vacuum of 0.8 to 1.0 3 1026 mbar and the supply of LN2 was sufficient to maintain the temperature of thermocouple 1 between 115 and 125 K. During operation a photomultiplier (pm) tube (type EMI6097) was positioned next to a viewport in the vacuum chamber wall and was used to obtain a sideview of the light flash from an impact. The pm tube photocathode was 19 cm away from the ice. The pm tube was operated in a charge collection mode with a decay constant (65 esec) greater than the duration of a hypervelocity impact flash (a few microseconds maximum). Thus the total signal read out for an event was a measure of the total light emission
during the impact (i.e., from both the impact crater and any plasma generated above the surface). The response of the tube was calibrated by observing a chopped signal from a calibrated tungsten lamp (a correction being made for the variation of the response of the tube with wavelength). Both linearity and absolute response at the light intensities observed in impacts were checked. No correction is made for variations in the sensitivity of the photomultiplier response (which rapidly falls off above 650 nm). The ionization was measured by applying a voltage (500 V) to the grid in the surface of the ice. The second grid was held at earth, and charge flowing between the two during an impact was fed to an external amplifier and recorded. The amplifier was calibrated by injection of known charges, and thus its response gave the charge liberated in an event. It should be noted that during operation, for reasons of access to the chamber, all data were taken for impacts on the target at 368 to the normal. III. RESULTS
Data were take both with ice present in the cold target and with an empty target. This permitted a cross check to establish if the signals observed were indeed due to impacts in the ice. In this respect, note that when ice was present impacts could occur on the ice or the wire grid. Data taking occurred in runs spread over 3 months. All features of the data repeat in all the runs (i.e., the scatter in the observed data is present in every data subset and is not the result of combining data subsets). According we do not distinguish between separate runs when presenting the results. Events were recorded for analysis when a particle was detected in the beam line and a light flash or ionization pulse observed in coincidence with it. The accuracy of the light flash and ionization measurements is combined with the errors on mass and velocity when plotting data. In Fig. 3a we show for empty target data the total light flash energy (E) normalized to projectile mass (m) vs projectile velocity (v). A clear power law behavior can be seen. This has been fitted (solid line in Fig. 3a) to E/m 5 cv b. We find that c 5 4.94 6 0.99 and b 5 3.65 6 0.18 (E/m in J kg21 and v in km s21). Changing the viewing conditions (i.e., the viewing angle and distance from the target) does not affect the results. In Fig. 3b we similarly show the light flash data for runs when ice was present. The solid curve shown is the fit to the empty target data. Clearly this overlaps with part of the ice-filled target data, but there is also a component of the ice-filled target data in poor agreement with this fit. Thus the ice-filled target data contains two components, one for impacts in the target itself and the other for impacts in the ice. To better establish the two components in our ice data we show Fig. 4a. Here the data have been transformed
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FIG. 3. Light flash energy/projectile mass vs projectile velocity. (a) Empty target; (b) ice-filled target. Solid curve shown in both (a) and (b) is a fit to (a).
into a coordinate space where we measure the minimum distance from each data point to the curve fit to the empty target data (note that both the quantities being plotted in Fig. 3 have a nonnegligible error, so in any fit the combined residual on each quantity has to be considered). The units
FIG. 4. Residuals of light flash data: (a) shows both empty target data (dotted line) and ice-filled target data (solid line); (b) shows a fit (solid line) to the ice-filled target residuals (dotted lines are the individual Gaussians in this fit as given in the text).
of these residuals are thus not purely J kg21 or km s21, but a combination of the two; for convenience we use an arbitrary scale of units in Fig. 4. The solid curve in Fig. 4a is for the ice data, and the dashed curve is for the empty target data. A clear difference is seen between the two, with the ice data possessing an apparent contribution similar to the empty target data plus an extra contribution. It is this latter component which we consider to arise from impacts on ice. To illustrate this point further we show Fig. 4b. Here a fit to the residual data for the ice-filled target run is shown. This is a two-component fit. The first component is a Gaussian whose mean and sigma are fixed as the result of fitting a Gaussian to the residual distribution of the empty target data; only the overall normalization of this Gaussian is left free in the fit to Fig. 4b. The second Gaussian is fitted with a free mean, sigma, and normalization, and represents the additional contribution to the data when ice is present. This second component in the data is clearly seen. Further we find that the mean does not shift significantly when we divide the residuals by impact velocity. We conclude that this signal represents impacts on ice, and that these ice impacts observe a power law behavior for E/m vs v with a similar power to the empty target data (but a different normalization, obtained from the position of the mean of the second Gaussian in the residual plot). We thus find for the impacts on ice that E/m 5 cv 3.656.37, where c 5 1.07 6 0.21 (E/m in J kg21 and v in km s21). The given accuracy of the slope is less than that in the fit to empty target data. This is because the variation in slope that can be accommodated by the data is 10% (i.e., the small shift in the position of the second ‘‘ice’’ Gaussian’s mean when the data is subdivided by velocity). We find that the ice impacts produce a smaller light signal at any projectile mass and velocity than do the impacts on the empty target, by a factor of 4.62 6 0.93. As stated earlier, no significant variation in the results was obtained by varying the viewing geometry, so the errors given are obtained from the data and the analysis described above. When observing the ionization, we find that the signal comprises two components. The first is a fast rising signal (order 100 nsec), corresponding to ionization occurring directly in the impact (primary ionization IP). There is also a second, slower, signal (IS) which can be associated with ionization caused by impacts of high-speed ejecta on surrounding surfaces. The rise time of IP is dictated by the electric field applied between the grids. The 500 V used was chosen to ensure that IP and IS would be separable in our data. The magnitude of IP and IS for impacts when the target is empty is shown in Figs. 5a and 5b, respectively. We begin by fitting the data with I 5 mav b. We carry out the fit by fixing a at discrete values in the range 0.7 to 1.2, while b is left free. We find that for IP , the fits where a is less than 1.0 have a significantly worse correlation, while from a 5 1.0 to 1.2 no great change in the fit likelihood
HYPERVELOCITY IMPACTS ON ICE
FIG. 5. Ionization/projectile mass vs projectile velcoity. For empty target data, (a) is primary and (b) is secondary ionization. For ice-filled target data, (c) is primary and (d) is secondary ionization.
occurs with a and b varying against each other. Since for a of 1.0 or above no significant improvement in fit quality occurs, we follow the results of Go¨ller et al. (1986) and Go¨ller and Gru¨n (1989) and fix a 5 1.0 in all the results presented below. We thus obtain I/m 5 cv b, where for primary ionization c 5 (7.89 6 1.58) 3 1023 and b 5 4.12 6 0.21, and for secondary ionization c 5 (33.8 6 6.8) 3 1023 and b 5 3.39 6 0.17 (C kg 21 and km s21). We can thus see that the primary ionization dominates above 7.4 km s21, in good agreement with Go¨ller and Gru¨n (1989). The slope of 4.12 6 0.21 is slightly higher than the 3.5 reported by Dietzel et al. (1973), although we note that they fit their data from 2 km s21 upward, and will thus average over total ionization. The magnitude of the total ionization is within the spread of total ionization values obtained for iron impacts on various metals at similar angles of incidence (e.g., see Go¨ller and Gru¨n 1989). This treatment of the data with a single value for b over the whole velocity range is tested by fitting the low- and high-velocity regions separately. The values thus found for b are not significantly different from the value found for the whole velocity range within the obtained errors. In Figs. 5c and 5d we show the primary and secondary ionization data when ice is present in the target. The solid curves shown are the fits to the respective empty target data. In both cases it can be seen that the ice data partially overlaps the empty target fit. As for the light flash analysis we transform to a space where we obtain the minimum residual to the empty target fit. This is plotted in Fig. 6a (primary ionization) and 6b (secondary ionization). It can be seen that the ice-filled target data again contain a component absent from the empty target data. Proceeding as for the light flash data, we find the component of the data not compatible with the empty target signal and ascribe this to impacts on ice. There is no clear correlation of the
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residuals of the ice component of the data with projectile velocity. We thus describe the ionization for impacts on ice by I/m 5 cv b, using b from the appropriate empty target data set. For primary ionization we find that c 5 0.0040 6 0.0008 and b 5 4.12 6 0.41, and for secondary ionization we have c 5 0.012 6 0.002 and b 5 3.39 6 0.34 (I/m in C kg21 and v in km s21). As before we find that a possible 10% change in the value of b for impacts on ice can be accommodated by dividing the data into velocity subsets. Accordingly we take this as the error on b. We find that, independent of velocity, the average ionization due to the impacts in ice is less than that due to the impacts in the empty target. For primary ionization this ratio is measured as 2.00 6 0.40, and for secondary ionization as 2.82 6 0.56. When comparing our results to Timmermann and Gru¨n (1991), we find that their total ionization signal from ice is smaller than ours by a factor of typically two orders of magnitude. In Timmermann and Gru¨n (1991) there is not much discussion of the experimental procedure, nor a detailed presentation of the results. It is therefore impossible to reproduce their experimental conditions. However, we note that our results are obtained from several separate (but mutually compatible) runs, and that our signals from the impacts on metal agree in magnitude with previous results for impacts on metal (Go¨ller and Gru¨n 1989). We thus have confidence in our experimental setup and the results obtained. As stated, the signal for secondary ionization can be
FIG. 6. Residuals of ice-filled target data to curves fit to empty target data. (a) Primary ionization; (b) secondary ionization. Empty target (icefilled) target data is dashed (solid) line.
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FIG. 7. Ejecta vs projectile velocity derived from secondary ionization signal rise times. The solid line is the fit described in the text.
interpreted as the observation of ionization caused by impacts of high-speed ejecta on surrounding surfaces. We assume that the typical angle of ejection is 608 to the target surface (see Croft 1981). Ejecta will thus typically impact the grid above the surface after traversing an average path of 2.4 mm. The rise times of the secondary ionization signal are combined with this distance to obtain ejecta velocity (ve) and are shown in Fig. 7 as a function of the velocity of the original projectile (vp). A clear correlation is seen, and we find that ejecta velocity ve 5 0.093v 1.13 p . It is not unreasonable to expect a range of 6108 in the ejecta angle from impact to impact. This would indicate a possible 30% spread in the ejecta velocities obtained; this is slightly less than the observed scatter in the data at any fixed projectile velocity. The velocity of ice ejecta has been measured before (Frisch 1992). For vp in the range 1.8 to 8 km s21, ve was found to vary between 0.01 and 0.7 km s21. No correlation between ve and vp was found in that work. If we examine our data we see that in same range of vp , ve varies between 0.2 and 1.3 km s21. The lower velocities of this range seem rather low for an impact producing ionization, indicating a possible weakness in our assumptions. The range of 0.2 to 1.3 km s21 only partially overlaps the results of Frisch (1992). The difference is explained by noting that Frisch (1992) only observes spallation ejecta. This is held to be the component of ejecta which contains most of the mass (50 to 95%), but is not the highest speed ejecta in an event. Given that we observe the ejecta as a result of impact ionization, which requires high impact speed, we do not expect to be sensitive to spallation ejecta. Further, the dependence of ve on vp only becomes apparent in our work due to the large range of vp over which we observe. We can proceed further and estimate the mass of ejecta observed in our events. This we do by assuming that the secondary ionization signal is dependent on ejecta mass
and velocity in the same way that the primary signal depends on projectile mass and velocity. This requires two separate assumptions. The first is that the iron projectiles can be considered similar to the molybdenum in the grid. Previous work for ionization caused by iron impacts on various metals (Dietzel et al. 1973, Go¨ller and Gru¨n 1989) indicates a spread in total ionization of approximately a factor of 10 as the target metal changes. This sets a limit on any results we obtain below. The second assumption is that impacts from ice on metal can be approximated by impacts of metal on ice. The validity of this is unquantifiable, but it is the simplest approximation that can be made and is thus appropriate in these circumstances. We also assume that ejecta impact the grid to produce the secondary ionization signal. Since the grid has an optical transparency of 85% we assume it only intercepts 15% of the ejecta. From our results we thus find that at a projectile velocity of 5 km s21 the volume of ice ejected from the target is 7.0 3 10210 cm3. Similarly at 10 km s21 it is 2.8 3 10211 cm3. We can compare our measure of the non-spallation ice ejecta volume with estimates by Eichhorn and Gru¨n (1993) of the total volume of ice ejected in hypervelocity impacts. Using their parameterization of crater volume applied to our projectile mass and velocity, we find that at 5 km s21 they predict a total ejecta volume of 1.5 3 1029 cm3, and at 10 km s21 one of 5.9 3 10210 cm3. If we accept these values, the ejecta volumes derived from our results represent 47 and 4.7%, respectively, of the total ice ejected in hypervelocity impacts at 5 and 10 km s21. Frisch (1992) reports that between 50 and 95% of ejecta is spallation (to which we are not sensitive), not incompatible with our results. We note that our results imply a correlation of this fraction with impact velocity, indicating a possible increase in the influence of spallation in total crater volume as the impact velocity increases from 5 to 10 km s21. IV. CONCLUSIONS
In conclusion we have simultaneously measured light flash and ionization for hypervelocity impacts of iron on low-temperature ice. In both cases the intensities of the signals are less than those for similar impacts on a typical metal (molybdenum). We find that the light flash energy for impacts of iron on ice can be described in terms of projectile mass and velocity by E/m 5 (1.07 6 0.21)v 3.6560.37 (E/m in J kg21 and v in km s21). It is remarkable that this relationship holds true over such a large range of impact velocities (2 to 65 km s21). It is normally expected that such a velocity range covers several distinct regions of behavior in terms of, for example, the degree of vaporization of the projectile. Similarly the ionization I (both primary and secondary) obeys power laws in I/m vs impact velocity: IP /m 5 (40 6 8) 3 1023 v 4.1260.41 and IS /m 5 (12 6 2) 3 1023 v 3.3960.34 (I/m in C kg21 and v in km s21).
HYPERVELOCITY IMPACTS ON ICE
Again, given the range of impact velocities used, this is quite remarkable. It should be noted that the power of the velocity is found to only 610%, but this acts as quite a stringent limit on changes of ionization mechanisms and their yields with velocity. ACKNOWLEDGMENTS The hypervelocity impact facilities at the University of Kent at Canterbury are financed by grants from PPARC (the Particle Physics and Astronomy Research Council, UK) and its predecessor, SERC (the Science and Engineering Research Council).
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EICHHORN, G. 1976. Analysis of the hypervelocity impact process from impact flash measurement. Planet. Space Sci. 24, 771–781. EICHHORN, G., AND GRU¨ N 1993. High velocity impacts of dust particles in low-temperature water ice. Planet. Space Sci. 41, 429–433. FRIICHTENICHT, J. F., AND J. C. SLATTERY 1963. Ionization associated with hypervelocity impact. Proc. VIth Symp. Hypervelocity Impacts, 591–612. FRISCH, W. 1992. Hypervelocity impact experiments with water ice targets. In Hypervelocity Impacts in Space (J. A. M. McDonnell, Ed.), pp. 7–14. Unit for Space Sciences, Univ. of Kent, Canterbury, UK. GO¨ LLER, J. R., AND E. GRU¨ N 1989. Calibration of the Gallileo/Ulysses dust detectors with different projectile materials and at varying impact angles. Planet. Space Sci. 37, 1197–1206.
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