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In-situ studies of interstellar dust from spacecraft
In-situ studies of interstellar dust from spacecraft Ingrid Mann a* and Hiroshi Kimura b aESA Space Science Department at ESTEC, Noordwijk, The Netherlands bMax-Planck-Institut f'tir Aeronomie, Katlenburg-Lindau, Germany Comparison of the present in-situ measurements of interstellar (IS) dust within the solar system to dust models based on astronomical observations can give clues about the evolution of IS dust. Hence, further in-situ measurements in the solar system are expected to provide a better understanding of IS dust properties. Nevertheless, measurements within the solar system cover the large size end of the IS dust distribution, while the majority of the very small IS dust can only be measured from spacecraft that cross the heliopause. 1. INTRODUCTION The study of interstellar (IS) dust particles is important for understanding elemental and isotope abundances in the interstellar medium (ISM) as well as for understanding the formation and evolution of grains. Astronomical observations reveal average optical properties of IS dust along the line of sight, but are biased to the sizes that yield the main contribution to the observed quantities, such as small dust particles determining the extinction of the ISM. The presence of IS dust in the solar system, on the other hand, provides the opportunity of retrieving ISM conditions with in-situ measurements. So far the in-situ measurements provide data of the mass density, mass distribution and flux rate of IS dust entering the solar system [1], which allow crude estimates of the basic properties. 2. EXPERIMENTAL RESULTS IS dust particles have been identified with measurements aboard Ulysses [2], Galileo [3] and Hiten [4] as well as radar meteor observations indicate larger particles to enter the solar system from interstellar space [5]. The flux of IS dust at solar distance 1.8 < r < 5.4 AU was derived to be 1.5 x 10 -4 m -2 s -1 from the Ulysses measurements between 1992 and 1995 [2]. The derived mass density of 2.8 x 10 -23 kg m -3 agrees with densities derived for the average ISM but is beyond the densities derived for the local ISM [1 ]. The mass distribution of IS dust differs for measurements made within and beyond a distance of 3 AU from the Sun, measurements at r < 3 AU showing a dip in the distribution around m ~ 10 - 1 7 kg. The dust impact rate for small masses has decreased since the beginning of 1996 [6] as a result of deflection in the solar magnetic field. While the different in-situ measurements of IS dust mentioned before are comparable within the detection limits, measurements beyond 5 AU on Pioneer [7] and on *Also at Instimt f'tirPlanetologie, Westf~ilischeWilhelms-Universit~itMttnster, Germany.
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I. Mann and H. Kimura Voyager [8] are controversial and may indicate a component of dust that is variable in space and time. Moreover, the detected grains cannot be clearly identified as interstellar [1 ], but may stem from comets or from dust production related to the Kuiper belt objects [9]. This points to the problem of separating IS dust from other dust components measured within the solar system. 3. DYNAMICS AND SEPARATION OF INTERSTELLAR DUST The separation of IS dust particles within a data set is based on their impact speed and direction which is different from solar system dust. The uncertainties of the speed and the direction measurements, as well as the dynamical effects that deflect the grains from their initially monodirectional stream hamper the separation. The identification based on statistical arguments and based on the comparison to other dust components seems, however, reliable. Particles at the large end of the size spectrum are influenced by solar gravitational force Fgrav and solar radiation pressure force Frad. If gravity is the dominant force, i.e., the ratio - - F r a d / F g r a v < 1, particles are in hyperbolic orbits focusing in interstellar downwind direction. This is the case for masses m > 10 -15 kg, for smaller grains with [3 > 1, particles are repelled in hyperbolic orbits. Particles with masses m < 10 -18 kg are deflected at the heliopause [10], while particles with m < 10 -17 kg can enter the solar system but are deflected from their original orbits in the solar magnetic field [11 ]. The mass distribution derived from Ulysses measurements between 1992 and 1995 is shown in Figure 1 as a histogram with error bars derived from Poisson statistics (see [ 12]). Two effects possibly reduce the amount of IS dust identified within the data at the small size part of the distribution compared to the actual dust flux: For one, the detection efficiency of the instrument is reduced for impacting dust with small masses. And secondly small grains that are deflected from the initial stream are not identified as interstellar when applying the velocity criterion. The measured distribution is assumed to better reflect the flux of IS dust within the solar system for masses m > 2.5 x 10-17 kg, shown in a hatched histogram in Figure 1 which allows a discussion of the properties of these large IS dust particles. 4. SIZE DISTRIBUTION AND MODELS OF IS DUST Comparing the results to ISM studies, the detected grains are clearly larger than grains of "typical" ISM dust models which describe the size distribution as dn/da , ~ a -3"5 with a dropo f f b e y o n d 10 -16 kg (radius a, a = 0.25 #m). The Ulysses results, in contrast, are approximated with a size distribution dn/da ~ a -2"65 for compact grains of constant density [1,12]. The derived size interval covers particle radii, a, 0.015 < a < 4.1 #m, when assuming spherical grains. Also IS dust particles found in meteorites indicate the existence of larger grains: RowanRobinson [13] could explain the observed millimetre emission from cold dust near the galactic plane with grains of 30 #m radius. Moreover composite grains of 0.003 to 3 #m in size are suggested by a recent model to account for revised abundances of the heavy elements in the ISM [14]. The large dust particles can be interpreted as results of the coagulation of grains in the ISM. The suggested size distribution is shown in Figure 1. Although it is different from the distribution derived from Ulysses measurements, both results point to the existence of large grains in the ISM and the in-situ measurements provide the opportunity of estimating particle properties for this part of the size distribution. Large particles predominantly influenced by solar
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In-sire studies of interstellar dust from spacecraft
Figure 1. The number density of IS dust as function of mass: Ulysses measurements [12] (a histogram with error bars); composite grain model [14] (solid curve).
Figure 2. Calculated ~ ratios for large composite grains (solid curve) and for coremantle particles (dotted curve from [15]). Insitu results require condition 1.4 < [3 < 3.1 for m ,,~ 10 -17 kg.
gravity and radiation pressure force can be studied for instance with detailed measurements near 1 AU. Already the present data allow for estimating the acting radiation pressure force. 5. MODELS OF THE RADIATION PRESSURE FORCE We estimate the radiation pressure force that could explain the measured mass distribution of grains and its gap by repulsion of in-falling IS dust (see [1,6,12]). The [3 ratios required to lead to this repulsion are 1.4 < ~max < 3.1 for m ~ 10 -17 kg. We compare this to calculations of the [3 ratio for different model assumptions of IS dust as shown in Figure 2. Assuming compact particles with a silicate core and a mantle of an ice-dust mixture produced by surface condensation, the model calculation leads to a maximum 13value of 0.8 at a mass of 10 -16 kg [15]. These particular calculations [ 15] for core-mantle particles is based on the assumption that the mantle consists of water ice. The lifetime of the ice mantles, however, is limited by sublimation, so that these model assumptions are not suitable to describe the detected dust particles. We further assume properties of dust produced by coagulation growth [14] as suggested in the above mentioned model for large dust in the ISM. The model assumptions for composite grains meet the conditions in terms of the maximum [3 value as well as in terms of the mass at which the [3 value maximizes, if the grains are assumed to be of low porosity (45%) as shown in the figure. 6. DISCUSSION We conclude that at least a fraction of IS dust is accessible to in-situ studies in the solar system. In-situ measurements, even only of the mass distribution and the flux of IS dust, allow for some estimates of the particle properties. The conditions of the entry into the solar system are however depending on the properties of IS dust, which has to be taken into account for an analysis. Dedicated in-situ measurements would provide information about the size, structure
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1. Mann and H. Kimura
and composition of some IS dust particles. The data lead towards a better understanding of the physics of dust in the ISM, but require a discussion as to how representative they are for the average ISM dust properties. Measurements would certainly benefit from improved detector capabilities, such as a better measurement of the dust velocities as well as from measurements of the dust composition. Since experimental data obtained in the solar system will cover a selected range of particle sizes, compositions and properties, measurements on spacecraft that leave the heliosphere are a further step. Such measurements could provide a larger sample of different IS grains and also reveal the properties and the behaviour of smaller dust particles that are expected to be most abundant in the ISM and prevented from entering the heliosphere. Moreover, the data would allow for a study of the entry conditions and hence would also increase the scientific return of the measurements carried out within the solar system.
REFERENCES 1. 2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
I. Mann and H. Kimura, J. Geophys. Res. 105 10317 (2000). E. Grtin, B. Gustafson, I. Mann, et al., Astron. Astrophys. 286 915 (1994). E. Grtin, P. Staubach, M. Baguhl, et al., Icarus 129 270 (1997). H. Svedhem, R. Mtinzenmayer and H. Iglseder, In Physics, Chemistry, and Dynamics of Interplanetary Dust, B.A..S. Gustafson and M.S. Hanner (eds.), pp. 27, ASP, San Francisco (1996). A.D. Taylor, W.J. Baggaley and D.I. Steel, Nature 380 323 (1996). M. Landgraf, J. Geophys. Res. 105 10303 (2000). D.H. Humes, J. Geophys. Res. 85 5841 (1980). D.A. Gurnett, J.A. Ansher, W.S. Kurth and L.J. Granroth, Geophys. Res. Lett. 24 3125 (1997). S. Yamamoto and T. Mukai, Astron. Astrophys. 329 785 (1998). H. Kimura and I. Mann, Astrophys. J. 499 454 (1998). G.E. Morrill and E. Grtin, Planet. Space Sci. 27 1283 (1997). H. Kimura, I. Mann and A. Wehry, Astrophys. Space Sci. 264 213 (1998). M. Rowan-Robinson, Mon. Not. R. Astron. Soc. 258 787 (1992). J.S. Mathis, Astrophys. J. 472 643 (1996). M. Wilck and I. Mann, Planet. Space Sci. 44 493 (1996).
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I. Mann and H. Kimura Voyager [8] are controversial and may indicate a component of dust that is variable in space and time. Moreover, the detected grains cannot be clearly identified as interstellar [1 ], but may stem from comets or from dust production related to the Kuiper belt objects [9]. This points to the problem of separating IS dust from other dust components measured within the solar system. 3. DYNAMICS AND SEPARATION OF INTERSTELLAR DUST The separation of IS dust particles within a data set is based on their impact speed and direction which is different from solar system dust. The uncertainties of the speed and the direction measurements, as well as the dynamical effects that deflect the grains from their initially monodirectional stream hamper the separation. The identification based on statistical arguments and based on the comparison to other dust components seems, however, reliable. Particles at the large end of the size spectrum are influenced by solar gravitational force Fgrav and solar radiation pressure force Frad. If gravity is the dominant force, i.e., the ratio - - F r a d / F g r a v < 1, particles are in hyperbolic orbits focusing in interstellar downwind direction. This is the case for masses m > 10 -15 kg, for smaller grains with [3 > 1, particles are repelled in hyperbolic orbits. Particles with masses m < 10 -18 kg are deflected at the heliopause [10], while particles with m < 10 -17 kg can enter the solar system but are deflected from their original orbits in the solar magnetic field [11 ]. The mass distribution derived from Ulysses measurements between 1992 and 1995 is shown in Figure 1 as a histogram with error bars derived from Poisson statistics (see [ 12]). Two effects possibly reduce the amount of IS dust identified within the data at the small size part of the distribution compared to the actual dust flux: For one, the detection efficiency of the instrument is reduced for impacting dust with small masses. And secondly small grains that are deflected from the initial stream are not identified as interstellar when applying the velocity criterion. The measured distribution is assumed to better reflect the flux of IS dust within the solar system for masses m > 2.5 x 10-17 kg, shown in a hatched histogram in Figure 1 which allows a discussion of the properties of these large IS dust particles. 4. SIZE DISTRIBUTION AND MODELS OF IS DUST Comparing the results to ISM studies, the detected grains are clearly larger than grains of "typical" ISM dust models which describe the size distribution as dn/da , ~ a -3"5 with a dropo f f b e y o n d 10 -16 kg (radius a, a = 0.25 #m). The Ulysses results, in contrast, are approximated with a size distribution dn/da ~ a -2"65 for compact grains of constant density [1,12]. The derived size interval covers particle radii, a, 0.015 < a < 4.1 #m, when assuming spherical grains. Also IS dust particles found in meteorites indicate the existence of larger grains: RowanRobinson [13] could explain the observed millimetre emission from cold dust near the galactic plane with grains of 30 #m radius. Moreover composite grains of 0.003 to 3 #m in size are suggested by a recent model to account for revised abundances of the heavy elements in the ISM [14]. The large dust particles can be interpreted as results of the coagulation of grains in the ISM. The suggested size distribution is shown in Figure 1. Although it is different from the distribution derived from Ulysses measurements, both results point to the existence of large grains in the ISM and the in-situ measurements provide the opportunity of estimating particle properties for this part of the size distribution. Large particles predominantly influenced by solar
- 362 -
In-sire studies of interstellar dust from spacecraft
Figure 1. The number density of IS dust as function of mass: Ulysses measurements [12] (a histogram with error bars); composite grain model [14] (solid curve).
Figure 2. Calculated ~ ratios for large composite grains (solid curve) and for coremantle particles (dotted curve from [15]). Insitu results require condition 1.4 < [3 < 3.1 for m ,,~ 10 -17 kg.
gravity and radiation pressure force can be studied for instance with detailed measurements near 1 AU. Already the present data allow for estimating the acting radiation pressure force. 5. MODELS OF THE RADIATION PRESSURE FORCE We estimate the radiation pressure force that could explain the measured mass distribution of grains and its gap by repulsion of in-falling IS dust (see [1,6,12]). The [3 ratios required to lead to this repulsion are 1.4 < ~max < 3.1 for m ~ 10 -17 kg. We compare this to calculations of the [3 ratio for different model assumptions of IS dust as shown in Figure 2. Assuming compact particles with a silicate core and a mantle of an ice-dust mixture produced by surface condensation, the model calculation leads to a maximum 13value of 0.8 at a mass of 10 -16 kg [15]. These particular calculations [ 15] for core-mantle particles is based on the assumption that the mantle consists of water ice. The lifetime of the ice mantles, however, is limited by sublimation, so that these model assumptions are not suitable to describe the detected dust particles. We further assume properties of dust produced by coagulation growth [14] as suggested in the above mentioned model for large dust in the ISM. The model assumptions for composite grains meet the conditions in terms of the maximum [3 value as well as in terms of the mass at which the [3 value maximizes, if the grains are assumed to be of low porosity (45%) as shown in the figure. 6. DISCUSSION We conclude that at least a fraction of IS dust is accessible to in-situ studies in the solar system. In-situ measurements, even only of the mass distribution and the flux of IS dust, allow for some estimates of the particle properties. The conditions of the entry into the solar system are however depending on the properties of IS dust, which has to be taken into account for an analysis. Dedicated in-situ measurements would provide information about the size, structure
- 363 -
1. Mann and H. Kimura
and composition of some IS dust particles. The data lead towards a better understanding of the physics of dust in the ISM, but require a discussion as to how representative they are for the average ISM dust properties. Measurements would certainly benefit from improved detector capabilities, such as a better measurement of the dust velocities as well as from measurements of the dust composition. Since experimental data obtained in the solar system will cover a selected range of particle sizes, compositions and properties, measurements on spacecraft that leave the heliosphere are a further step. Such measurements could provide a larger sample of different IS grains and also reveal the properties and the behaviour of smaller dust particles that are expected to be most abundant in the ISM and prevented from entering the heliosphere. Moreover, the data would allow for a study of the entry conditions and hence would also increase the scientific return of the measurements carried out within the solar system.
REFERENCES 1. 2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
I. Mann and H. Kimura, J. Geophys. Res. 105 10317 (2000). E. Grtin, B. Gustafson, I. Mann, et al., Astron. Astrophys. 286 915 (1994). E. Grtin, P. Staubach, M. Baguhl, et al., Icarus 129 270 (1997). H. Svedhem, R. Mtinzenmayer and H. Iglseder, In Physics, Chemistry, and Dynamics of Interplanetary Dust, B.A..S. Gustafson and M.S. Hanner (eds.), pp. 27, ASP, San Francisco (1996). A.D. Taylor, W.J. Baggaley and D.I. Steel, Nature 380 323 (1996). M. Landgraf, J. Geophys. Res. 105 10303 (2000). D.H. Humes, J. Geophys. Res. 85 5841 (1980). D.A. Gurnett, J.A. Ansher, W.S. Kurth and L.J. Granroth, Geophys. Res. Lett. 24 3125 (1997). S. Yamamoto and T. Mukai, Astron. Astrophys. 329 785 (1998). H. Kimura and I. Mann, Astrophys. J. 499 454 (1998). G.E. Morrill and E. Grtin, Planet. Space Sci. 27 1283 (1997). H. Kimura, I. Mann and A. Wehry, Astrophys. Space Sci. 264 213 (1998). M. Rowan-Robinson, Mon. Not. R. Astron. Soc. 258 787 (1992). J.S. Mathis, Astrophys. J. 472 643 (1996). M. Wilck and I. Mann, Planet. Space Sci. 44 493 (1996).
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