interplanetary dust particles have contributed quantities of 3He, He,:øNe

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. E4, PAGES , APRIL 25, 1997 The contribution by interplanetary dust to noble gases in the atmosphere of Mars G. J. Flynn Department of Physics, State University of New York at Plattsburgh Abstract. Interplanetary dust particles accumulate significant concentrations of noble gases, implanted by the solar wind and solar energetic particles, while they are in space. When these dust particles enter the atmosphere of a planet they are heatexl and decelerated, depositing some of the implanted noble gases directly into the atmosphere and, if the particle survives atmospheric entry, eraplacing the remainder on the surface of the planet, where it can be released by particle dec mposition or by episodic surface heating. Because of the low mass of the atmosphere of Mars, compared to the atmospheres of Venus and Earth, the atmosphere of Mars is more severely influenced by the addition of noble gases by interplanetary dust. The contribution of noble gases to Mars has been modeled using estimates of the interplanetary dust flux at Mars and measurements of the noble gas contents of interplanetary dust collected from the atmosphere of the Earth. Over the past 3.6 b.y., interplanetary dust particles have contributed quantities of 3He, He,:øNe and Ne comparable to the current total atmospheric inventories of these isotopes. In the present era, the rates of addition of 3He, :øne, : Ne, 3 Ar, and 3SAr to the atmosphere of Mars by interplanetary dust range from about 2 % to 20 % of the modeled rates of addition by planetary outgassing [Pepin, 1994; Krasnopolsky and Gladstone, 1996], and the isotopic compositions of the He and Ne added by interplanetary dust are distinctly different from those assumed for planetary outgassing' 3He/nile -2.8x10 'n in the interplanetary dust [Nier and $chlutter, 1992] versus almost pure nile for outgassing of U and Th decay products, and 2øNe/22Ne in the interplanetary dust [Nier, 1994] versus 2øNe/22Ne = 13.7 assumed for planetary outgassing Pepin's [ 1994] model. Since the actual outgassing rate of Mars in the present era is not well constrained by observations, the contributions of Ne and Ar to the atmosphere of Mars by the interplanetary dust must serve, at least, as a lower limit on the current sources of these noble gases. rate of accretion of IDPs onto Mars and calculated that these Introduction IDPs carried a significant quantity of solar noble gases to Mars. Pepin [1991] suggested the accretion of solar-wind-rich IDPs or Interplanetary dust particles (IDPs)- 10 lain in size typically the direct interception of the solar wind by the planet could spend 30,000 to 100,000 years in space [Flynn, 1989]. During supply the inferred solar wind component used in his model of this time, noble gas ions from the solar wind and solar energetic the atmosphere of Mars. However, Pepin's [1994] current particles (SEP) are implanted into the surfaces of these IDPs. model of the noble gas sources and sinks on Mars suggests that, Many IDPs recovered from Earth's stratosphere contain high when averaged over the past 3.7 b.y., the contribution from concentrations of solar noble gases [Rajan et al., 1977; Hudson planetary outgassing significantly exceeds the contribution by et al., 1981; Nier, 1994], frequently comparable to the the IDPs. concentrations measured in the f'me fractions of the lunar soils. The planetary outgassing function for Ar and Ne e nployed m These IDPs can add their noble gases to the atmosphere at a Pepin's [1994] model peaks b.y. ago, then it decreases later date if they are destroyed by planetary surface activity. exponentially with time, roughly following the pattern observed Some IDPs collected from Earth's stratosphere contain lower for volcanic resurfacing of Mars. However, Pepin [1994] abundances of noble gases, but the noble gas release patterns suppressed the planetary outgassing rate significantly below the measured on these IDPs suggest they were outgasseduring the volcanic resurfacing rate from 1.8 b.y. ago to the present heating pulse experienced on Earth atmospheric entry [Nier, because the addition of nonnegligible quantities of volatiles in 1994], indicating these IDPs added noble gases directly to the the present era produced isotopic ratios inconsistent with atmosphere during deceleration. Larger IDPs melt or vaporize measured values. He assumed that the volatile-rich reservoir on atmospheric entry, also adding their implanted noble gases within Mars was largely exhausted b.y. ago. If so, then directly to Earth's atmosphere. although the time averaged contribution of noble gases to the There is a continuous, planetwide accretion of interplanetary atmosphere of Mars by the accretion of IDPs may be small, the dust onto Mars as well. Flynn and McKay [1990] modeled the IDP conthbution in the present era could be significant compared to other noble gas sources on Mars. Copyright 1997 by the American Geophysical Union. Since the abundances and isotopic ratios of the noble gases in the atmosphere of Mars are used to constrain the extent and Paper number 96JE /97/96JE duration of planetary outgassing and the efficiency of 9175

2 9176 FLYNN: NOBLE GASES FROM INTERPLANETARY DUST PARTICLES ON MARS atmospheric loss mechanisms, a detailed understanding of all of for the smallest radar meteors [Southworth and Sekanina, 1973] the significant sources and sinks is essential. The low mass of and determined the velocity distribution at Mars using the the current atmosphere of Mars (- 200 kgtm2), compared to that transformation developed by Zook (described by Morgan et al. of Venus (-q,100,000 kgtm 2) or Earth ( -10,300 kgtm2), makes [1980]). the atmosphere of Mars particularly susceptible to perturbations Flynn and McKay [1990] calculated the IDP flux at Mars to from external sources, such as the noble gases carded by the be 0.17 times the flux at Earth. The present calculation follows that of Flynn and McKay [1990] except that new values for the The current contribution of noble gases to the atmosphere of interplanetary dust flux and size-frequency distribution at Mars by the IDPs must serve as at least a lower limit on the Earth, measured by impact pits on the Long Duration Exposure current sources of noble gases for Mars. Simple modeling, Facihty (LDEF) spacecraft [Love and Brownlee, 1993] have assuming IDPs of all sizes have the same concentration of noble been employed. The direct measurements of the flux of gases as the - 10 gm IDPs recovered from Earth's stratosphere, interplanetary dust-q0 gm in size in interplanetary space at 1 indicates that IDPs could contribute significant amounts of Ne ALl and 1.5 AU by the Pioneer 10 and 11 spacecrar [Humes, and Ar to the atmosphere of Mars [Flynn, 1996a]. However, 1980] are in accord with the values modeled in the first two since the noble gases in IDPs are believed to be dominated by steps. Thus the only significant uncertainty the current IDP implanted solar wind, the gas content should be correlated with flux at Mars comes from modeling the flux enhancement due to surface area, resulting a concentration (grams of gas/gram gravitational focusing by Mars, which depends on the modeled particle) which decreases with increasing particle size. The velocity distribution. This model gives an accretion rate of purpose of this study was to model the rate of addition and the 5.2x109 gtyr of IDPs onto Mars, compared to a terrestrial isotopicomposition of noble gases accreted onto Mars with the accretion rate of 3.0x10 lø gtyr inferred from the LDEF impact IDPs, using the measured noble gas contents of- 10 gm IDPs, craters [Love and Brownlee, 1993]. scaling this concentration with particle size as appropriate for The resulting accretion rates onto Mars for IDPs ranging surface correlated gas, and using the accretion rate of IDPs onto from 10 to 460 gm in diameter are given in of Table 1. As is Mars initially modeled by Flynn and McKay [1990]. the case for Earth, the bulk of the mass is contributed by IDPs ranging from 100 to 400 gm in diameter. Most IDPs >100 gm Accretion Rate of IDPs Onto Mars in size are melted or vaporized on Earth atmospheric entry [Flynn and McKay, 1990], so the noble gas contents of particles No spacecraft has yet measured the flux of interplanetary in this size range must be inferred from measurements dust incident on Mars. Thus, this flux can only be estimated performed on smaller IDPs which survive Earth atmospheric from measurements of the flux at Earth and modeling of the entry, or from the larger but more severely heated orbital dynmnics to estimate the Mars/ arth flux ratio. micrometeorites recovered from the polar ices. Flynn and McKay [1990] modeled the present rate of accretion of interplanetary dust onto Mars using the method Noble Gas Contents of IDPs developed by Zook to model the interplanetary dust accretion onto Mercury (described in the work by Morgan et al. [1980]. Bradley and Brownlee [1988] have observed amorphous This method proceeds in three steps: (1) calculation of the rims, a feature produced by radiation damage, on the surfaces of interplanetary flux at 1 AU from the flux measured at the top of many 5 to 20 gm IDPs recovered from Earth's stratosphere. Earth's atmosphere by removing the near-earth enhancement The observation of these rims demonstrates that the present resulting from gravitational focusing, (2) transformation of the surfaces of these IDPs were exposed in space. Thus solar wind flux at 1 AU to 1.53 AU using measurements of the falloff of and SEP ions should be implanted into these IDPs during their Zodiacal Light with increasing heliocentric distance, and (3) exposure in space. The extraction of solar noble gases from determination of the flux at the top of the Martian atmosphere IDPs recovered from Earth's stratosphere provided the first by incorporatingravitational focusing at Mars. The clear indication that these particles were extraterrestrial [Rajan adjustments for gravitational focusing require a knowledge of et al., 1977; Hudson et al ]. the IDP velocity distributions at Earth and Mars. Flynn and Of the noble gases, He is present in the highest abundance in McKay [1990] used the velocity distribution measured at Earth the solar wind and SEP spectra, with progressively lower Table 1. Interplanetary Dust Accretion Onto Mars Particle Particle Accretion Mass Diameter' Rate g um 109 gtyr 10 ' ' '7 46 O ' Total 5.2 Accretion Fraction Direct Deposition of 4He Heated to 4He, cm3stp/yr >600øC cm3stp/yr 6.4x x x x x x x x x x xl xl x x107 Assumes spherical particle of density 2.0 g/cm.

3 FLYNN: NOBLE GASES FROM INTERPLANETARY DUST PARTICLES ON MARS 9177 Table 2. Average Noble Gas Contents of IDPs From Earth's Stratosphere Number of Samples Size, }am Noble Gas Concentration, 10 '4 cm3stp/g 3He 4He ZONe ZiNc ZZNe 6Ar SAr l ZXe 10a 6 to 28 group of 13 b 5 to c d c f 5 to ' Rajan et al. [ 1977]. Hudson et al. [1981]. Nier and $chlutter [1990]. d Nier and $chlutter [1992]. Nier and $chlutter [1993]. f Brownlee et al. [1993]. abundances of Ne, Ar, Kr, and Xe. Rajan et al. [1977] likely to be representative of intact IDPs. All 20 of the IDPs extracted He from 10 individual IDPs, each in the mass range from 0.21 to 24 ng (about 6 to 30 gm in diameter). Hudson et al. [1981] combined 13 IDPs, each-10 gm in diameter, in a included in the Nier and Schlutter [1992] study are also fragments of cluster IDPs, but this set has a significantly higher noble gas content. Some of the particles included in the other single analysis and extracted Ne, Ar, and Xe. Nier and noble gas studies listed in Table 2 may also be fragments of Schlutter [1990, 1992, 1993] and Brownlee et al. [1993] have extracted He, and in some cases Ne, from 82 individual IDPs. The noble gas abundances measured in these experiments are larger IDPs, and this may reduce the inferred average concentrations of noble gases from the true value. A second effect which reduces the noble gas concentrations summarized in Table 2. The measurement of approximate solar in the IDPs recovered from Earth's stratosphere is the heating ratios of noble gas isotopes, e.g., 3He/4He, is generally in the these IDPs experience when they decelerate during atmospheric range from 2 to 5x10-4, demonstrating that these particles have entry. IDPs having the lowest entry velocities, the lowest experienced little or no contamination with atmospheric He densities, the smallest sizes, and the shallowest entry angles are (3He/nile- 10- ). Nonetheless, 3He/4He and 2øNe/22N½ heated the least severely [Fraundorf, 1980; Flynn, 1989], but isotopic ratios measured in the IDPs are not equal to the solar all IDPs are heated to some extent. Depending on the release wind ratios, the presumed composition of the gases contributed temperature of the implanted solar ions, and the degree of entry to the atmosphere of Mars by planetary outgassing [Pepin, 1994], but plot on a mixing line between the solar wind and the SEP compositions [Nier, 1994]. Thus IDPs will contribute He and Ne to Mars which is isotopically distinct from the presumed solar isotopic composition contributed by planetary outgassing. The concentrations of 4He and of 2ONe averaged over sets of from 13 to 24 IDPs span more than a factor of 20 (see Table 2). heating experienced by a particular IDP, any given IDP may have a noble gas content ranging from fully outgassed to almost fully retained after atmospheric entry. Thus measurement of the noble gas content of an IDP collected from Earth' s stratosphere after atmospheric entry provides only a lower limit on the preentry gas content of that particle. Nier and Schlutter [1992, 1993] and Brownleet al. [1993] performed stepped heating experiments, and demonstrated that, The results for individual particles, reported in the references in most cases, those IDPs with the highest noble gas release cited, span an even greaterange of concentrations. In the study temperatures also had the lowest noble gas concentrations. of 10 particles by Rajan et al. [1977], three had 4He contents Nier and Schlutter [1992] performed a two-step experiment: below detection limits and the remaining seven had 4He first heating an IDP in steps from 100øC up to 600øC, concentrations spanning 2 orders of magnitude, from 20xl 0 '4 to allowing the particle to cool, then reheating it in steps from 2500x10-4 cm3stp/g. Nier and Schlutter [1990] found no 100øC to 1100øC. In the second heating, almost no He was detectable 4He in one of 16 IDPs analyzed, and the 4He released at temperatures below the final step of the first heating concentrations ranged from 8x10 '4 to 2820x10-4 cm3stp/g in sequence [Nier and Schlutter, 1993]. They interpreted this as the remaining 15 IDPs. The average He concentration in the 10 particles analyzed by Rajan et al. [1977], 790xl 0 '4 cm STP/g, is an indication that those IDPs with low noble gas concentrations and high noble gas release temperatures had been partially consistent with the range of average concentrations determined outgassed by the thermal pulse experienced on atmospheric for the four sets of IDPs analyzed by Nier and Schlutter [1990, entry. If so, then the noble gas concentrations of those IDPs 1992, 1993; Brownlee, 1993]. The lowest average He and Ne concentrations were having the lowest gas release temperatures will more accurately reflect the concentrations prior to Earth atmospheric entry. measured on a group of 24 particles by Nier and Schlutter This suggests we should take the noble gas concentrations of [1993]. However, all 24 of the particles included this study those particles having low He release temperatures, or, in were fragments of "cluster IDPs," larger IDPs which broke up experiments where only the total release was determined, those into many fragments on impact into the collection surface [Nier with the highest noble gas contents, as representative of the and Schlutter, 1993]. The low noble gas concentrations in most pre-entry gas concentrations. of these 24 particles may be explained because they are The three highest concentrations of 4He found in the study of fragments of larger particles, and thus parts of their surfaces 10 IDPs by Rajan et al. [1977] were 2500x10-4 cm3stp/g, were not exposed to solar ions in space. Since all the particles 2400x10 '4 cm3stp/g, and 1490x10-4 cm3stp/g, while the in the Nier and Schlutter [1993] data set are known to be average 4He concentration for the set was 790x10 4 cm3stp/g. fragments of larger IDPs, their noble gas concentrations are not Similarly, Nier and Schlutter [1990] found high values of

4 9178 FLYNN: NOBLE GASES FROM INTERPLANETARY DUST PARTICLES ON MARS 2820x10 '4 cm3stp/g and 1860x10 '4 cm3stp/g in a set of 16 solar noble gases to Mars. This is because of the sharp peak in IDPs that averaged 540xl 0 '4 cm3stp/g. Both the Rajan et al. the interplanetary dust flux at 200 gm in diameter. The [1977] and the Nier and Schlutter [1990] experiments measured calculated contributions by IDPs to the other noble gas isotopes only the total gas release. Nier and Schlutter [1992] measured in the atmosphere of Mars are summarized in Table 3. a high of 2000xl 0 '4 cm3stp/g of 4He in one IDP from a set of 20 IDPs with an average 4He concentration of 760x10 '4 cm3stp/g. Stepped heating was performed on this set, but Deposition of Noble Gases in the Atmosphere of Mars interpretation of the stepped heating results is difficult because these particles are fragments of cluster IDPs. The IDP which Interplanetary dust accreted onto Mars can deposit solar released 2000xl 0 '4 cm3stp/g of 4He had its peak 4He release in noble gases into the atmosphere by two mechanisms: direct the 640øC temperature step, while a particle whose 4He release deposition as the IDP outgasses on atmospheric entry, and peaked in the 420øC temperature step released only 800x10 '4 indirect deposition if an IDP cames the implanted noble gases cm3stp/g. Brownleet al. [1993] summarized the noble gas to the surface and the IDP is subsequently heated or measurements on 22 additional IDPs, ranging from 5 to 10 gm decomposes. in diameter, all selected because they were not believed to be fragments of larger particles. This set had a significantly higher noble gas content, with an average 4He concentration of Direct Deposition 1700x10 '4 cm3stp/g [Brownlee et al., 1993], and a high value Solar wind and SEP ions, which were well studied in the of xl 0 '4 cm3stp/g in a single particle [Brownlee, 1995]. soils returned from the Moon, penetrate only tens to hundreds These results suggest that the pre-atmospheric saturation of nanometers into solid material, and these ions are easily concentration of 4He in- 15 Ixm diameter IDPs is at least released by heating. In stepped heating experiments on IDPs 2500xl 0 '4 cm3stp/g, about 3 times the average value of about recovered from Earth's stratosphere, Nier and Schlutter [1992] 700x10 '4 cm3stp/g measured in sets of IDPs recovered from observed that the greatest release of 4He occurred at the stratosphere, and that smaller IDPs (such as the 5 to 10 grn temperatures near 600øC, with more than 50% of the total gas diameter ones reported by Brownlee et al., [1993] contain even typically being released over a 200øC temperature range. To larger He concentrations. model the direct deposition of noble gases into the atmosphere The noble gas abundances measured in-q0 gm diameter of Mars it was assumed that any IDP heated above 600øC on IDPs could be accumulated with exposures of 10 to 100 years atmospheric entry is completely outgassed. Flynn and McKay [Rajan et al., 1977; Hudson et al., 1981]. Since orbital [1990] used their calculated velocity distribution of IDPs evolution modeling indicates that 10 gm IDPs spend 30,000 to accreted onto Mars coupled with the atmospheric entry heating 100,000 years in space before reaching Earth [Flynn, 1959], the model developed by Fraundorf [1980] to model the peak low concentrations of noble gases suggesthat noble gases temperature distribution of IDPs in each size range. Using the quickly reach their saturation concentrations in IDPs. Since results from Flynn and McKay [1990], Flynn [1996b] larger IDPs have even longer orbital evolution times [Flynn, detemfined the fraction of IDPs in each size range which are 1989], virtually all IDPs should contain their saturation heated to 600øC or higher. These results, given in Table 1, concentrations of 4He before entering Earth's atmosphere. allow calculation of the amount of 4He directly deposited into Since the solar wind 4He/2øNe ratio is 570 [Geiss et al., 1972], the atmosphere of Mars by the accretion of ldps. About 46% of saturation of solar wind Ne should take <57,000 years, a time the 4He carried to Mars by IDPs is directly deposited into the comparable to the space exposure ages of- 10 gm diameter atmosphere by outgassing on atmospheric entry. Since IDPs IDPs. Thus, both Ne and He are expected to be nearly reach their peak temperature at an altitude of- 30 km in the saturated in IDPs >10 grn in size. atmosphere of Mars, the bulk of this deposition should occur Since solar implantation is a surface phenomenon, this near 30 kin. saturation concentration (grams gas/gram particle) should It is likely that the heavier noble gases will be released at decrease with increasing particle size. The highest average higher temperatures than the He. However, no stepped-release concentration of 4He was measured by Brownlee et al. (1993) in heating experiments have been performed on the IDPs to the set of IDPs which contained the smallest particles, some determine the release temperatures of the heavier noble gases, down to 5 grn in size, supporting the idea that the solar noble so detailed modeling of the fraction released directly into the gases are surface correlated. atmosphere cannot be performed. If we assume the heavier To model the rate of addition of 4He onto Mars by the noble gases are released at the same temperature as the He, accretion of IDPs, the pre-atmospheric 4He concentration in 15 then we can model the direct deposition of all the noble gas micron IDPs was taken to be 2500x10 cm3stp/g. The noble isotopes into the atmosphere of Mars. The results of these gas concentrations in IDPs of other sizes have been modeled calculations are given in column 3 of Table 3. assuming the He abundance increases linearly with surface area. The rate of addition of each of the other noble gas isotopes modeled using the 2øNe/4He, and 3I-Ie/4He ratios determined in interplanetary dust by Nier and Schlutter (1992) and the ratios of the other noble gas isotopes 2øNe detem ed in interplanetary dust by Hudson et al. [1981]. Although these 4He abundances were measured for IDPs collected at Earth, the short saturation times indicate that IDPs entering the atmosphere of Mars should contain similar noble gas concentrations, at least for He and Ne. The 4He results, shown in column 4 of Table 1, indicate that IDPs in the 100 to 210 micron size range carry the bulk of the Indirect Deposition The collection of IDPs containing significant concentrations of solar noble gases from Earth's stratosphere demonstrates that some IDPs are not completely outgassed on atmospheric entry. For the terrestrial case, Farley [1995] and Farley and Patterson [1995] have demonstrated that He is retained in IDPs deposited into oceanic sediments for periods exceeding 450,000 years. The extent to which those IDPs which are accreted onto Mars with their noble gases intact are subsequently outgassed is not known. These noble gases may be released slowly, if the

5 FLYNN: NOBLE GASES FROM INTERPLANETARY DUST PARTICLES ON MARS 9179 Table 3. Noble Gas Contributions by IDPs lsotol Acorotion Directly Mass Accreted Mass Directly Total Mars Rate, Deposited, Over 3.6 b.y., Deposited Over Atmospheric cmsstp/yr cmsstp/yr g over 3.6 b.y., Inventory,' g g 3He 4.8xl x x lx1010 4He 1.7x xl lx lx x Ne 5.0x x x x x Ne 1. lx x xl x Ne 4.5x xl x x x Ar 5.7x x xl x x Ar 1. lx x x x x Kr <4.3x103 < 1.9x103 <5.9xl 010 <2.6xl lx Xe 1. lx x x x x Xe 1. lx x x x xl 011 ' FromPepin [1989] except 4He fromkrasnopolsky and Gladstone [1996] and 132Xe frompepin [1991]. IDPs decompose, possibly due to UV interactions, or suddenly, if, for example, a large area of the planetary surface is heated the past few hundred thousand years, IDPs are likely to have contributed noble gases to Mars at 2 to 3 times the rates by geological activity such as a lava flow. This latter modeled in Table 3. mechanism could produce the sudden release into the atmosphere of a spike of solar noble gases implanted in those IDPs which had accumulated on the surface of Mars since the The flux of large objects, capable of producing kilometersized craters, was significantly higher during the first billion years of solar system history than it is at present. Little is previous outgassing event. Since the model calculations known about the flux of IDPs in that early era. However, indicate that about 54% of the solar noble gases contributed by IDPs are carried to the surface intact, such spikes could release orders of magnitude more solar noble gas in a short period than the average annual contribution by direct deposition. collisions between the comets and asteroids responsible for the crater production must have produced IDPs. The accretion of meteoritic matter onto the Moon has been fit by a twocomponent model: a rapidly decaying flux (tl/2 = 40 million Time Variation in the Dust Accretion Rate years) exceeding the present flux by an order of magnitude or more 4 b.y. ago, and a relatively constant flux, near the present value, over the past 3.6 b.y. [Wasson et al., 1973]. If we take the flux of IDPs over the past 3.6 b.y. to have been While the current contribution of noble gases to Mars can be modeled using the current terrestrial flux of IDPs, which was well determined by the LDEF experiment [Love and Brownlee, 1993], the relation of the current flux to the long-term flux is less well established. Kyte and Wasson [1986] used the Ir concentration in Pacific sediments to infer the long-term accretion rate of interplanetary matter onto Earth. Excluding the spikes from major impact events, Kyte and Wasson [1986] found a relatively constant mass influx of about 78,000 tons/yr, more than 2.5 times the current mass influx of 30,000 tons/yr measured by Love and Brownlee [1993] using impacts on the LDEF. constant and equal to the current flux determined by LDEF impacts, then we can calculate the total accretion of solar noble gases over the past 3.6 b.y., and compare the IDP contribution with the modeled noble gas contribution from outgassing employed by Pepin [1994] over a similar time period. The calculated noble gas contributions are given in columns 4 and 5 of Table 3. As discussed above, the actual long-term average flux is likely to have been a factor of 2 to 3 higher than the current flux, which is used in this model. Thus the long-term contribution of noble gases to the atmosphere of Mars by the IDPs may be larger, by a factor of 2 to 3, than is calculated. The current total atmospheric inventories of the noble gases Farley [1995] and Farley and Patterson [1995] inferred the are given in Table 3. Comparison of the current atmospheric accretion rate of interplanetary dust recorded by the 3He inventories with the contributions by the IDPs indicates that the concentrations in oceanic sediments. They reported evidence for factor of 3 variations in the flux with time. Farley and Patterson [1995] found evidence for a periodicity in the terrestrial interplanetary dust flux of about 100,000 years. Combining their results with data reported by Marcantonio et al. [ 1995], Farley and Patterson [1995] suggest that minima in the flux occurred about 100,000, 190,000, 280,000, and 385,000 years ago. This, coupled with the 100,000 year periodicity they infer [Farley and Patterson, 1995], suggests the current flux of interplanetary dust would be near a minimum, and that the flux determined by Love and Brownlee [1993] from LDEF impacts should be increased by a factor of 2 to 3 to provide a long-term average flux. This correction would bring the LDEF flux into better agreement with the long-term flux inferred by Kyte and Wasson [1986], suggesting that, even over amount of 4He contributed by the IDPs over the past 3.6 b.y. significantly exceeds the current atmospheric inventory, and the amounts of 2øNe and 22Ne added to Mars by IDPs over the past 3.6 b.y. are comparable to the current atmospheric inventories of those isotopes. The IDP contributions to Ar, Kr, and Xe are much smaller fractions of the total atmospheric inventories. Going back farther in time, the Wasson et al. [1973] model of the Lunar flux suggests that from 3.6 to 4.0 b.y. ago an amount of IDPs equal to about 30% of that from 3.6 b.y. ago to the present was accreted. The IDP contribution rises more rapidly before 4.0 b.y. ago. In the 200 m.y. interval from 4.2 to 4.0 b.y. ago the IDP contribution would have been 5 times that from 3.6 b.y. to the present, and in the 300 m.y. interval from 4.5 to 4.2 b.y. ago the IDP contribution would exceed that from 3.6 b.y. to present by greater than a factor of 100.

6 9180 FLYNN: NOBLE GASES FROM INTERPLANETARY DUST PARTICLES ON MARS Implications of IDP Accretion for the Atmosphere source, about 20% of the IDPs analyzed contain He with a of Mars 3He/4He >10-3. If these 16 IDPs had been included in the average, the 3He/4He in the IDPs would have increased to The long-term accretion of IDPs contributes noble gases to 1.4xl 0 '3, and the inferred contribution of 3He to the atmosphere the atmosphere of Mars, though these quantities are of Mars by IDPs would increase to 32 g/yr, about 25% of the significantly lower than the modeled noble gas contributions current loss rate. However, since 11 of the 16 IDPs which had from planetary outgassing [Pepin, 1994]. However, Pepin's high 3HeftHe ratios were from a single stratospheric collector, it [1994] model supposes an exponential decline in the rate of is not clear if this was an atypical group of ldps. planetary outgassing beginning 3.3 b.y. ago. Since the IDP Krasnopolsky and Gladstone [1996] have modeled the direct contribution is thoughto have been relatively constant over the capture of solar wind He by Mars, and find that if the capture past 3.6 b.y. [Wasson et al., 1973], in the present era the efficiency is 0.2 to 0.4 the capture of solar wind He is adequate contribution by IDPs to the noble gases in the atmosphere of to balance the He loss. For the terrestrial case this capture Mars will be more significant compared to the contribution efficiency is much lower, but the value for Mars has not been from planetary outgassing than over the 3.6 b.y. average. established. The high 3HeftHe ratios in the IDPs, compared to the typical Helium He outgassed by the Earth, suggests that the addition of He by IDPs, although it contributes little to the 4He in the atmosphere, Helium was not detected (detection limit 100 ppm) in the may perturb the atmospheric 3HeftHe ratio. This addition of a atmosphere of Mars by the Viking spacecraft [Nier and 3He rich component suggests that precise measurement of the McElroy, 1977], but the He abundance has recently been 3He/4He ratio in the atmosphere of Mars might be used as a measured using the Extreme Ultraviolet Explorer satellite monitor of the contribution of He by IDPs to the current [Krasnopolsky and Gladstone, 1996], who found a He mixing atmosphere of Mars. ratio in the lower atmosphere of Mars of 4:1:2 ppm. This implies a total current atmospheric inventory of 1.0x10 3 g of 4He. Over the past 3.6 b.y. the accretion of interplanetary dust is modeled to have deposited 1.1x10 TM g of 4He onto Mars, The modeled contribution of 2øNe to Mars by IDPs over the about 10 times the current atmosphericontent measured by past 3.6 b.y. is 1.6xl 0 3 g, an amount comparable to the current Krasnopolsky and Gladstone [ 1994]. atmospheric inventory of 2øNe of 2.8xl 0 3 g. However, Pepin's He is rapidly lost from the atmosphere of Mars. [1994] baseline model (for fiz/t = 0.40) of the evolution of the Krasnopolsky et al. [1994] calculate a loss rate of 1.8x105 He atmosphere of Mars results in an outgassing of 7.9x104 g of atoms/c s or 2.6x1023 He atoms/s planetwide. This 2øNe per gram planet over the past 3.7 b.y.. This corresponds to a loss of 5.4x10? g/yr of He from Mars. The to a total of--5xl 015 g of 2øNe being outgassed from Mars over modeled rate of accretion onto Mars of He from IDPs, 1.7x108 the past 3.7 b.y., over 340 times the contribution from IDPs cm3stp/yr (see Table 1), corresponds to 3.1x104 g/yr. Thus over that time interval. Thus, if Pepin's [1994] model is Krasnopolsio, et al. 's [1994] calculation requires a much larger correct, then, over the long term the IDPs contribute an source of He than is provided by the IDPs to maintain the insignificant fraction of the 2øNe in the atmosphere of Mars. atmosphere at its current He abundance. However, Pepin's [1994] baseline model employs a For Earth the major He resupply mechanism is likely to be planetary outgassing function which decreases exponentially outgassing of the planet. If planetary outgassing is also the with time, with a time constant of 0.73 b.y., from a peak -3.3 major He resupply mechanism on Mars, then much of the Mars- b.y. ago to a very low value in the present era. The modeled derived He in the present atmosphere may be from radioactive planetary outgassing of 2øNe in the current era is 5.4x10 4 g/yr decay and should be dominated by 4He. The terrestrial (R.O. Pepin, Personal Communication, 1996). The 2øNe atmosphere has a 3I-Ie/4He ratio of 1.4x10 ', while the solar He contribution by the IDPs is calculated to be 4.5x10 3 g/yr, about implanted in most of the IDPs has a muchigher 3He/4He ratio, 8% of Pepin [1994] modeled planetary outgassing rate of averaging 2.4xl 0-4 to 5.6xl 0-4[Nier, 1994] and quite similar to 2øNe. However, the current outgassing rate of Mars is not well the ratio of 4.3x10-4 measured in the solar wind [Geiss et al., constrained by observations. dakosky et al. [1994] proposed a 1972]. model fitting the planetary outgassing with a gaussian, rather V.A. Krasnopolsky (Personal communication, 1996) models than the exponential employed by Pepin [1994]. Thus, in the a 3He/4He loss rate ratio of 2.3. If we assume that planetary present era, 2øNe contributed to the atmosphere of Mars by IDPs outgassing contributes He of the same isotopic composition to is significant and must be taken as a lower limit on the current the atmosphere of Mars as it does to Earth, then the loss rate of sources of Ne on Mars. 3He from the atmosphere of Mars would be 130 g/yr. Taking Pepin's [1994] model assumes that outgassing of Mars the 3I-Ie/4He ratio in IDPs to be 2.8x10 4 INlet and Schlutter, contributes Ne of solar isotopic composition, 2øNe/22Ne = 13.7, 1992] results in the deposition of about 6.5 g/yr of 3He onto to the atmosphere of Mars. Direct measurements of the Mars by the IDPs, about 5% of the modeled loss rate of 3He. 2øNe/2eNe ratio in the IDPs recovered from However, Nier and Schlutter [1993] excluded 16 IDPs (20% show values of [Nier and Schlutter, 1990] and of the 82 they analyzed) which were significantly enriched in [Nier, 1994], suggesting a mixture of solar wind and solar 3He (3He/4He > 10-3) when they computed their 3I-Ie/4He average. energetic particle Ne in the IDPs. The 3He/4He in these 16 IDPs averaged 5.7x10 '3(Nier and A second source of Ne in IDPs is spallation, which produces Schlutter, 1993). Nier (1994) indicated this 3He enrichment all three isotopes in approximately equal abundances. The might have resulted from spallation, which produces similar effect of adding a spallogenicomponent to solar ble is to abundances of 3He and 4He or from the trapping a He enrich 2 Ne relative to the other isotopes. Spallogenic 2 Ne has component having an unusual isotopic rat/.o. Whatever its not been detected in --10 gm IDPs, because their orbits evolve corresponds Earth'stratosphere

7 FLYNN: NOBLE GASES FROM INTERPLANETARY DUST PARTICLES ON MARS 9181 Table 4. 2 Ne From Cosmic Ray Production Total Particle Mass, g Orbital Evolution, Spallation 21Ne, 21Ne Addition Rate, yr cm3stp/g cm3stp/yr lx105 4x10 '10 7x10-3 2x105 9x10 '10 lx10-1 4x105 2x10 '9 8x10 '1 lx106 4x10 ' x106 8x10 ' x106 2x10 ' so rapidly under the influence of Poynting-Robertson radiation drag that the typical -10 micron IDP from a main-belt asteroid wind ratio. Because of the long exposure of the idps to the solar wind, it seems reasonable to assume that the IDPs contain or an active comet spends only-105 years in space before 84Kr/nøAr in roughly the sola ratio, which suggests the IDP accretion onto Earth [Flynn, 1989]. However, the space contribution to the atmosphere is approximately one-tenth the exposure duration of a particle whose orbit is evolving under upper limit shown in Table 2. Poynting-Robertson drag varies linearly with particle diameter and with particle density. Thus, larger IDPs should accumulate XeHoH significantly higher concentrations of spallogenic 2 Ne because of their longer exposure ages. Table 4 gives the calculated 2 Ne The Xe content of the set of 13 IDPs measured by Hudson et concentrations in particles exposed in space for the time al. [1981] was over an order of magnitude higher than that intervals required for orbital evolution from 4 AU to 1.5 AU expected from the solar wind, and the isotopic composition under the influence of Poynting-Robertson radiation drag. suggested a substantial planetary contribution [Hudson et al., Olinger [1990] measured the Ne isotopic contents of 1981]. The modeled >Xe and mxe contributions to the micrometeorites having masses from 1 to 20 grn collected from atmosphere of Mars by the IDPs were calculated assuming a the polar ices. Olinger [1990] found spallogenic 2 Ne surface correlated Xe component, with its concentration varying concentrations ranging from 0.6 to 24x10 '8 cm3stp/g, with 1/radius. If the Xe is planetary, then it should be volume consistent with these model calculations. The cm3stp/yr correlated. Taking the concentration of 32Xe to be - 10'7 of spallogenic 2 Ne contributed by IDPs from 10 '9 to 10 '4 g cm3stp/g, the value in the set of 13 IDPs, from 5 to 20 grn in constitutes only 1% of the total 2 Ne carried to Mars by IDPs in diameter, measured by Hudson et al. [1981], in all IDPs over this size range. Thus IDPs are not expected to imprint a the entire mass range from 10 '9 to 10 '4 g, the IDPs would distinctive spallogenic 21Ne signature on the atmosphere of contribute 1.1x10 ø g of 32Xe and 1.1x109 g of 3øXe to Mars Mars unless they have long near-surfac exposures increasing over the past 3.6 b.y. their spallogenic Ne contents. No evidence for such exposures was found in the polar micrometeorites analyzed by Olinger [1990]. Conclusions The total mass of the atmosphere of Mars is substantially Ar on less that of the atmosphere of Earth or of Venus; thus the Hudson et al. [1981] detected 36Ar and 3BAr but no 4øAr in contribution of solar gases delivered by interplanetary dust particles is more significant on Mars than on Earth or Venus. the set of 13 IDPs they analyzed. The modeled addition of 36Ar to the atmosphere of Mars by IDPs over the past 3.6 b.y. is The continuous accretion of noble-gas-rich interplanetary 3.3x10 2 g, about 2% of the current total atmospheric inventory dust particles onto Mars results in the direct deposition of about of 36Ar. Thus, over the long-term, another source is required to 46% of the solar implanted gases they contain due to heating on atmospheric entry and deceleration. The remainder of the noble contribute most of the Ar presently observed in the atmosphere of Mars. gases contained these IDPs is initially deposited into the soil, However, as was the case for Ne, in the present era the but may be released if the IDPs decompose or the surface is modeled contribution of 36At by the IDPs is 9.3x102 gtyr, about heated. Episodic heating of large areas of the surface could 20% of the current planetary outgassing rate of 4.6x103 gtyr for release spikes of solar gas brought to Mars by the IDPs. 36Ar in the model proposed by Pepin [1994]. The 38Ar/36Ar The 3He/4He ratio in the IDPs is significantly greater than that of the He outgassed by Earth, and the 3He contributed to ratio of 4).2 measured in the IDPs by Hudson et al. [1981] is the atmosphere of Mars by the IDPs is likely to perturb the somewhat lower than the ratio of 0.24 [Pepin, 1991] in the current atmosphere of Mars, thus the IDP contribution to 38At is atmospheric 3He/4He ratio. The Ne implanted in the IDPs is a somewhat smaller. mixture of solar wind and SEP Ne, enriched in 22Ne compared to the solar composition Ne which Pepin [1994] assumes the outgassed component. Krypton Although the contribution of noble gases delivered over the Krypton was below the detection limit in the set of 13 IDPs long term may be small compared to the contribution from analyzed by Hudson et al. [1981]. They set an upper limit on planetary outgassing, the present era the delivery rates of 84Kr/36Ar which was an order-of-magnitude above the solar 2øNe and 3eAr to the atmosphere of Mars by interplanetary dust

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Res., (Received June 3, 1996; revised December 5, 1996; 101, 15,765-15, 772, accepted December 13, 1996.)