Noble gases and earth's accretion

Science

Volumn 273, Issue 5283, 1996, Pages 1814-1818

  Harper Jr , Charles L ^a   Jacobsen, Stein B ^a  

^a HARVARD UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

INERT GAS;

ARTICLE; ASTRONOMY; ATMOSPHERE; PRIORITY JOURNAL;

EARLY EARTH HISTORY; NOBLE GAS; PLANETARY ACCRETION;

EID: 0029664035     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.273.5283.1814     Document Type: Article

References (97)

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4. 27 g). Very large uncertainties attend such models when applied to the real solar system. However, the range in these predicted time scales implies that most of the planetary accretion could have occurred before the gas from the nebula had cleared [C. Hayashi, K. Nakazawa, Y. Nakagawa, in Protostars and Planets II, D. C. Black and M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1985), pp. 319-340; T. Nakano, Mon. Not. R. Astron. Soc. 224, 107 (1987); K. Ohtsuki, Y. Nakagawa, K. Nakazawa, Icarus 75, 552 (1988)].
5. 27 g). Very large uncertainties attend such models when applied to the real solar system. However, the range in these predicted time scales implies that most of the planetary accretion could have occurred before the gas from the nebula had cleared [C. Hayashi, K. Nakazawa, Y. Nakagawa, in Protostars and Planets II, D. C. Black and M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1985), pp. 319-340; T. Nakano, Mon. Not. R. Astron. Soc. 224, 107 (1987); K. Ohtsuki, Y. Nakagawa, K. Nakazawa, Icarus 75, 552 (1988)].
6. 27 g). Very large uncertainties attend such models when applied to the real solar system. However, the range in these predicted time scales implies that most of the planetary accretion could have occurred before the gas from the nebula had cleared [C. Hayashi, K. Nakazawa, Y. Nakagawa, in Protostars and Planets II, D. C. Black and M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1985), pp. 319-340; T. Nakano, Mon. Not. R. Astron. Soc. 224, 107 (1987); K. Ohtsuki, Y. Nakagawa, K. Nakazawa, Icarus 75, 552 (1988)].
7. 27 g). Very large uncertainties attend such models when applied to the real solar system. However, the range in these predicted time scales implies that most of the planetary accretion could have occurred before the gas from the nebula had cleared [C. Hayashi, K. Nakazawa, Y. Nakagawa, in Protostars and Planets II, D. C. Black and M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1985), pp. 319-340; T. Nakano, Mon. Not. R. Astron. Soc. 224, 107 (1987); K. Ohtsuki, Y. Nakagawa, K. Nakazawa, Icarus 75, 552 (1988)].
8. 27 g). Very large uncertainties attend such models when applied to the real solar system. However, the range in these predicted time scales implies that most of the planetary accretion could have occurred before the gas from the nebula had cleared [C. Hayashi, K. Nakazawa, Y. Nakagawa, in Protostars and Planets II, D. C. Black and M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1985), pp. 319-340; T. Nakano, Mon. Not. R. Astron. Soc. 224, 107 (1987); K. Ohtsuki, Y. Nakagawa, K. Nakazawa, Icarus 75, 552 (1988)].
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11. M. Sekiya, K. Nakazawa, C. Hayashi, Earth Planet. Sci. Lett. 50, 197 (1980); Prog. Theor. Phys. 64, 1968 (1980); M. Sekiya, C. Hayashi, K. Nakazawa, ibid. 66, 1301 (1981); J. C. G. Walker, Precambrian Res. 17, 147 (1982); K. J. Zahnle and J. C. G. Walker, Rev. Geophys. Space Sci. 20, 280 (1982); S. Sasaki and K. Nakazawa, Earth Planet. Sci. Lett. 89, 323 (1988).
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14. M. Sekiya, K. Nakazawa, C. Hayashi, Earth Planet. Sci. Lett. 50, 197 (1980); Prog. Theor. Phys. 64, 1968 (1980); M. Sekiya, C. Hayashi, K. Nakazawa, ibid. 66, 1301 (1981); J. C. G. Walker, Precambrian Res. 17, 147 (1982); K. J. Zahnle and J. C. G. Walker, Rev. Geophys. Space Sci. 20, 280 (1982); S. Sasaki and K. Nakazawa, Earth Planet. Sci. Lett. 89, 323 (1988).
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19. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
20. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
21. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
22. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
23. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
24. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
25. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
26. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
27. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
28. 22Ne ratio quite as high as the solar value [13.8 ± 0.1 (7)], but a few measurements are close (Fig. 3).
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34. M. Tang and E. Anders, Geochim. Cosmochim. Acta 52, 1235 (1988); E. Anders and E. Zinner, Meteoritics 28,490 (1993); G. Huss and R. S. Lewis, ibid. 29, 791 (1994); ibid., p. 811; Geochim. Cosmochim. Acta 59, 115 (1995).
35. M. Tang and E. Anders, Geochim. Cosmochim. Acta 52, 1235 (1988); E. Anders and E. Zinner, Meteoritics 28,490 (1993); G. Huss and R. S. Lewis, ibid. 29, 791 (1994); ibid., p. 811; Geochim. Cosmochim. Acta 59, 115 (1995).
36. M. Tang and E. Anders, Geochim. Cosmochim. Acta 52, 1235 (1988); E. Anders and E. Zinner, Meteoritics 28,490 (1993); G. Huss and R. S. Lewis, ibid. 29, 791 (1994); ibid., p. 811; Geochim. Cosmochim. Acta 59, 115 (1995).
37. M. Tang and E. Anders, Geochim. Cosmochim. Acta 52, 1235 (1988); E. Anders and E. Zinner, Meteoritics 28,490 (1993); G. Huss and R. S. Lewis, ibid. 29, 791 (1994); ibid., p. 811; Geochim. Cosmochim. Acta 59, 115 (1995).
38. M. Tang and E. Anders, Geochim. Cosmochim. Acta 52, 1235 (1988); E. Anders and E. Zinner, Meteoritics 28,490 (1993); G. Huss and R. S. Lewis, ibid. 29, 791 (1994); ibid., p. 811; Geochim. Cosmochim. Acta 59, 115 (1995).
39. 3He abundances reported by Mazor et al, range up to more than a factor of 300 higher than the indigenous (planetary) composition because of the addition of solar wind and spallogenic components during regolith exposure on the parent body and space exposure.
40. L. H, Kellogg and G. J. Wasserburg, Earth Planet. Sci. Lett. 99, 276 (1990).
41. J. A. Tyburczy, B. Frisch, T. J. Ahrens, ibid. 80, 201 (1986); T. J. Ahrens, J. D. O'Keefe, M. A. Lange, in Origin and Evolution of Planetary and Satellite Atmospheres, S. K. Atreya, J. B. Pollack. M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1989), pp. 328-385; T. J. Ahrens, in Origin of the Earth, H. E. Newsom and J. H. Jones, Eds. (Oxford Univ. Press, New York, 1990), pp. 211-227.
42. J. A. Tyburczy, B. Frisch, T. J. Ahrens, ibid. 80, 201 (1986); T. J. Ahrens, J. D. O'Keefe, M. A. Lange, in Origin and Evolution of Planetary and Satellite Atmospheres, S. K. Atreya, J. B. Pollack. M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1989), pp. 328-385; T. J. Ahrens, in Origin of the Earth, H. E. Newsom and J. H. Jones, Eds. (Oxford Univ. Press, New York, 1990), pp. 211-227.
43. J. A. Tyburczy, B. Frisch, T. J. Ahrens, ibid. 80, 201 (1986); T. J. Ahrens, J. D. O'Keefe, M. A. Lange, in Origin and Evolution of Planetary and Satellite Atmospheres, S. K. Atreya, J. B. Pollack. M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1989), pp. 328-385; T. J. Ahrens, in Origin of the Earth, H. E. Newsom and J. H. Jones, Eds. (Oxford Univ. Press, New York, 1990), pp. 211-227.
44. D. L. Anderson, Science 261, 170 (1993); C. J. Allegre, P. Sarda, T. Staudacher, Earth Planet. Sci. Lett. 117, 229 (1993); C. J. Allegre, T. Staudacher, P. Sarda, ibid. 122, 249 (1994).
45. D. L. Anderson, Science 261, 170 (1993); C. J. Allegre, P. Sarda, T. Staudacher, Earth Planet. Sci. Lett. 117, 229 (1993); C. J. Allegre, T. Staudacher, P. Sarda, ibid. 122, 249 (1994).
46. D. L. Anderson, Science 261, 170 (1993); C. J. Allegre, P. Sarda, T. Staudacher, Earth Planet. Sci. Lett. 117, 229 (1993); C. J. Allegre, T. Staudacher, P. Sarda, ibid. 122, 249 (1994).
47. F. M. Stuart, Earth Planet. Sci. Lett. 122, 245 (1994); H. Craig, Science 265, 1892 (1994); H. Hiyagon, ibid. 263, 1257 (1994).
48. F. M. Stuart, Earth Planet. Sci. Lett. 122, 245 (1994); H. Craig, Science 265, 1892 (1994); H. Hiyagon, ibid. 263, 1257 (1994).
49. F. M. Stuart, Earth Planet. Sci. Lett. 122, 245 (1994); H. Craig, Science 265, 1892 (1994); H. Hiyagon, ibid. 263, 1257 (1994).
50. a lower than 8) are possibly derived from recycled oceanic crust.
51. a lower than 8) are possibly derived from recycled oceanic crust.
52. a lower than 8) are possibly derived from recycled oceanic crust.
53. a lower than 8) are possibly derived from recycled oceanic crust.
54. a lower than 8) are possibly derived from recycled oceanic crust.
55. a lower than 8) are possibly derived from recycled oceanic crust.
56. a lower than 8) are possibly derived from recycled oceanic crust.
57. a lower than 8) are possibly derived from recycled oceanic crust.
58. a lower than 8) are possibly derived from recycled oceanic crust.
59. K. A. Farley, J. H. Natland, H. Craig, Earth Planet. Sci. Lett. 111, 183 (1992).
60. 3He derived from D burning in the present solar wind) and a rough correction of the so-called meteoritic "Q" composition (35) for isotopic fractionation.
61. 3 [E. Anders, D. Heymann, E. Mazor, Geochim. Cosmochim. Acta 34, 127 (1970); P. M. Jeffrey and E. Anders, ibid., p. 1175; J. H. Reynolds, U. Frick, J. M. Neil, D. L. Phinney, ibid. 42, 1775 (1978)].
62. 3 [E. Anders, D. Heymann, E. Mazor, Geochim. Cosmochim. Acta 34, 127 (1970); P. M. Jeffrey and E. Anders, ibid., p. 1175; J. H. Reynolds, U. Frick, J. M. Neil, D. L. Phinney, ibid. 42, 1775 (1978)].
63. 3 [E. Anders, D. Heymann, E. Mazor, Geochim. Cosmochim. Acta 34, 127 (1970); P. M. Jeffrey and E. Anders, ibid., p. 1175; J. H. Reynolds, U. Frick, J. M. Neil, D. L. Phinney, ibid. 42, 1775 (1978)].
64. C. J, Allegre, T. Staudacher, P. Sarda, Earth Planet. Sci. Lett. 81, 127 (1986-1987); R. K. O'Nions and I. N. Tolstikhin. ibid 124, 131 (1994); D. Porcelli and G. J. Wasserburg, Geochim. Cosmochim. Acta 59, 4921 (1995).
65. C. J, Allegre, T. Staudacher, P. Sarda, Earth Planet. Sci. Lett. 81, 127 (1986-1987); R. K. O'Nions and I. N. Tolstikhin. ibid 124, 131 (1994); D. Porcelli and G. J. Wasserburg, Geochim. Cosmochim. Acta 59, 4921 (1995).
66. C. J, Allegre, T. Staudacher, P. Sarda, Earth Planet. Sci. Lett. 81, 127 (1986-1987); R. K. O'Nions and I. N. Tolstikhin. ibid 124, 131 (1994); D. Porcelli and G. J. Wasserburg, Geochim. Cosmochim. Acta 59, 4921 (1995).
67. S. B. Jacobsen, Eos 76 (spring suppl.), 42 (1995).
68. 22Ne = 0.0335 ± 0.0015 [D. C. Black, Geochim. Cosmochim. Acta 36, 347 (1972); ibid., p. 377].
69. 22Ne = 0.0335 ± 0.0015 [D. C. Black, Geochim. Cosmochim. Acta 36, 347 (1972); ibid., p. 377].
70. S. Sasaki, Icarus 91, 29 (1991).
71. K. Metzler, A. Bischoff, D. Stöffler, Geochim. Cosmochim. Acta 56, 2873 (1992). Recently reported in situ laser spot analyses of Ne isotopes in Murchison have been interpreted as evidence for the exposure of small objects to the protosolar wind during an early stage of accretion [D. S. Woolum, K. Kehm, C. M. Hohenberg, K. Poelstra, E. Guntalilib, Lunar Planet. Sci. XXVI, 1519 (1995)] but are also consistent with heterogeneous distribution of "Q" Ne (T. Nakamura, K. Nagao, N. Takaoka, ibid., p. 1027). The presence of gas in the nebula would act as a very efficient shield for dust from solar wind irradiation [K. R. Housen and L. L. Wilkening, Proc. Lunar Planet. Sci. Conf. 11, 1251 (1980)].
72. K. Metzler, A. Bischoff, D. Stöffler, Geochim. Cosmochim. Acta 56, 2873 (1992). Recently reported in situ laser spot analyses of Ne isotopes in Murchison have been interpreted as evidence for the exposure of small objects to the protosolar wind during an early stage of accretion [D. S. Woolum, K. Kehm, C. M. Hohenberg, K. Poelstra, E. Guntalilib, Lunar Planet. Sci. XXVI, 1519 (1995)] but are also consistent with heterogeneous distribution of "Q" Ne (T. Nakamura, K. Nagao, N. Takaoka, ibid., p. 1027). The presence of gas in the nebula would act as a very efficient shield for dust from solar wind irradiation [K. R. Housen and L. L. Wilkening, Proc. Lunar Planet. Sci. Conf. 11, 1251 (1980)].
73. K. Metzler, A. Bischoff, D. Stöffler, Geochim. Cosmochim. Acta 56, 2873 (1992). Recently reported in situ laser spot analyses of Ne isotopes in Murchison have been interpreted as evidence for the exposure of small objects to the protosolar wind during an early stage of accretion [D. S. Woolum, K. Kehm, C. M. Hohenberg, K. Poelstra, E. Guntalilib, Lunar Planet. Sci. XXVI, 1519 (1995)] but are also consistent with heterogeneous distribution of "Q" Ne (T. Nakamura, K. Nagao, N. Takaoka, ibid., p. 1027). The presence of gas in the nebula would act as a very efficient shield for dust from solar wind irradiation [K. R. Housen and L. L. Wilkening, Proc. Lunar Planet. Sci. Conf. 11, 1251 (1980)].
74. K. Metzler, A. Bischoff, D. Stöffler, Geochim. Cosmochim. Acta 56, 2873 (1992). Recently reported in situ laser spot analyses of Ne isotopes in Murchison have been interpreted as evidence for the exposure of small objects to the protosolar wind during an early stage of accretion [D. S. Woolum, K. Kehm, C. M. Hohenberg, K. Poelstra, E. Guntalilib, Lunar Planet. Sci. XXVI, 1519 (1995)] but are also consistent with heterogeneous distribution of "Q" Ne (T. Nakamura, K. Nagao, N. Takaoka, ibid., p. 1027). The presence of gas in the nebula would act as a very efficient shield for dust from solar wind irradiation [K. R. Housen and L. L. Wilkening, Proc. Lunar Planet. Sci. Conf. 11, 1251 (1980)].
75. D. N. C. Lin and J. Papaloizou, Astrophys. J. 309, 846 (1986).
76. E. Jagoutz et al., Proc. Lunar Planet. Sci. Conf. 10, 2031 (1979); H Wänke, G. Dreibus, E. Jagoutz, in Archean Geochemistry, A. Kröner et al., Eds. (Springer-Verlag, Berlin, 1984), pp. 1-24; H. E. Newsom, in Origin of the Earth, H. E. Newsom and J. H. Jones, Eds. (Oxford Univ. Press, New York, 1990), pp. 273-288; H. Palme and H. St. C. O'Neill, Geochim. Cosmochim. Acta 60, 1106 (1996). The main observation supporting the scenario is the similar depletion factor of about 11 for several moderately siderophile elements (Fe, Ni, Co, and W) that have a wide range of siderophility.
77. E. Jagoutz et al., Proc. Lunar Planet. Sci. Conf. 10, 2031 (1979); H Wänke, G. Dreibus, E. Jagoutz, in Archean Geochemistry, A. Kröner et al., Eds. (Springer-Verlag, Berlin, 1984), pp. 1-24; H. E. Newsom, in Origin of the Earth, H. E. Newsom and J. H. Jones, Eds. (Oxford Univ. Press, New York, 1990), pp. 273-288; H. Palme and H. St. C. O'Neill, Geochim. Cosmochim. Acta 60, 1106 (1996). The main observation supporting the scenario is the similar depletion factor of about 11 for several moderately siderophile elements (Fe, Ni, Co, and W) that have a wide range of siderophility.
78. E. Jagoutz et al., Proc. Lunar Planet. Sci. Conf. 10, 2031 (1979); H Wänke, G. Dreibus, E. Jagoutz, in Archean Geochemistry, A. Kröner et al., Eds. (Springer-Verlag, Berlin, 1984), pp. 1-24; H. E. Newsom, in Origin of the Earth, H. E. Newsom and J. H. Jones, Eds. (Oxford Univ. Press, New York, 1990), pp. 273-288; H. Palme and H. St. C. O'Neill, Geochim. Cosmochim. Acta 60, 1106 (1996). The main observation supporting the scenario is the similar depletion factor of about 11 for several moderately siderophile elements (Fe, Ni, Co, and W) that have a wide range of siderophility.
79. E. Jagoutz et al., Proc. Lunar Planet. Sci. Conf. 10, 2031 (1979); H Wänke, G. Dreibus, E. Jagoutz, in Archean Geochemistry, A. Kröner et al., Eds. (Springer-Verlag, Berlin, 1984), pp. 1-24; H. E. Newsom, in Origin of the Earth, H. E. Newsom and J. H. Jones, Eds. (Oxford Univ. Press, New York, 1990), pp. 273-288; H. Palme and H. St. C. O'Neill, Geochim. Cosmochim. Acta 60, 1106 (1996). The main observation supporting the scenario is the similar depletion factor of about 11 for several moderately siderophile elements (Fe, Ni, Co, and W) that have a wide range of siderophility.
80. The problem of the apparent disequilibrium of the mantle with respect to the core remains unresolved after three decades since it was raised by A. E. Ringwood [Geochim. Cosmochim. Acta 30, 41 (1966)], who concluded that "the mean oxidation state of the mantle is not now, nor has it ever been, in equilibrium with the metallic iron." Studies of the oxygen fugacity of the mantle [H. St. C. O'Neill and V. J. Wall, J. Petrol. 28, 1169 (1987); B. J. Wood and D. Virgo, Geochim. Cosmochim. Acta 53, 1277 (1989)] have confirmed Ringwood's observations and indicate that the present mantle is about 4 log-bar units more oxidizing than the iron-wüstite buffer. It remains unclear, however, whether this might be explained by core partitioning at high temperature or high pressure [C. Ballhaus, Earth Planet. Sci. Lett. 132, 75 (1995)].
81. The problem of the apparent disequilibrium of the mantle with respect to the core remains unresolved after three decades since it was raised by A. E. Ringwood [Geochim. Cosmochim. Acta 30, 41 (1966)], who concluded that "the mean oxidation state of the mantle is not now, nor has it ever been, in equilibrium with the metallic iron." Studies of the oxygen fugacity of the mantle [H. St. C. O'Neill and V. J. Wall, J. Petrol. 28, 1169 (1987); B. J. Wood and D. Virgo, Geochim. Cosmochim. Acta 53, 1277 (1989)] have confirmed Ringwood's observations and indicate that the present mantle is about 4 log-bar units more oxidizing than the iron-wüstite buffer. It remains unclear, however, whether this might be explained by core partitioning at high temperature or high pressure [C. Ballhaus, Earth Planet. Sci. Lett. 132, 75 (1995)].
82. The problem of the apparent disequilibrium of the mantle with respect to the core remains unresolved after three decades since it was raised by A. E. Ringwood [Geochim. Cosmochim. Acta 30, 41 (1966)], who concluded that "the mean oxidation state of the mantle is not now, nor has it ever been, in equilibrium with the metallic iron." Studies of the oxygen fugacity of the mantle [H. St. C. O'Neill and V. J. Wall, J. Petrol. 28, 1169 (1987); B. J. Wood and D. Virgo, Geochim. Cosmochim. Acta 53, 1277 (1989)] have confirmed Ringwood's observations and indicate that the present mantle is about 4 log-bar units more oxidizing than the iron-wüstite buffer. It remains unclear, however, whether this might be explained by core partitioning at high temperature or high pressure [C. Ballhaus, Earth Planet. Sci. Lett. 132, 75 (1995)].
83. The problem of the apparent disequilibrium of the mantle with respect to the core remains unresolved after three decades since it was raised by A. E. Ringwood [Geochim. Cosmochim. Acta 30, 41 (1966)], who concluded that "the mean oxidation state of the mantle is not now, nor has it ever been, in equilibrium with the metallic iron." Studies of the oxygen fugacity of the mantle [H. St. C. O'Neill and V. J. Wall, J. Petrol. 28, 1169 (1987); B. J. Wood and D. Virgo, Geochim. Cosmochim. Acta 53, 1277 (1989)] have confirmed Ringwood's observations and indicate that the present mantle is about 4 log-bar units more oxidizing than the iron-wüstite buffer. It remains unclear, however, whether this might be explained by core partitioning at high temperature or high pressure [C. Ballhaus, Earth Planet. Sci. Lett. 132, 75 (1995)].
84. A time scale for gas dissipation in the neighborhood of 1 million years is consistent with observational constraints on the dissipation of gas in residual disks [M. F. Skrutskie et al., Astron. J. 102, 1749 (1991); B. Zuckerman, T. Forville, J. H. Kastner, Nature 373, 494 (1995)]. The oldest observed massive (∼0.1 solar mass) gas disk surrounding a near-solar mass T-Tauri star (GM Aurigae, ∼2 million years old) implies that gas disks may survive up to a few million years [D. W. Koerner, A. I. Sargant, S. V. W. Beckwith, Icarus 106, 2 (1993)].
85. A time scale for gas dissipation in the neighborhood of 1 million years is consistent with observational constraints on the dissipation of gas in residual disks [M. F. Skrutskie et al., Astron. J. 102, 1749 (1991); B. Zuckerman, T. Forville, J. H. Kastner, Nature 373, 494 (1995)]. The oldest observed massive (∼0.1 solar mass) gas disk surrounding a near-solar mass T-Tauri star (GM Aurigae, ∼2 million years old) implies that gas disks may survive up to a few million years [D. W. Koerner, A. I. Sargant, S. V. W. Beckwith, Icarus 106, 2 (1993)].
86. A time scale for gas dissipation in the neighborhood of 1 million years is consistent with observational constraints on the dissipation of gas in residual disks [M. F. Skrutskie et al., Astron. J. 102, 1749 (1991); B. Zuckerman, T. Forville, J. H. Kastner, Nature 373, 494 (1995)]. The oldest observed massive (∼0.1 solar mass) gas disk surrounding a near-solar mass T-Tauri star (GM Aurigae, ∼2 million years old) implies that gas disks may survive up to a few million years [D. W. Koerner, A. I. Sargant, S. V. W. Beckwith, Icarus 106, 2 (1993)].
87. W. R. Ward, Icarus 106, 274 (1993).
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90. 3He by two to three orders of magnitude by addition of a solar component.
91. N. Grevesse and A. Noels, in Origin and Evolution of the Elements, N. Prantzos, E. Vangioni-Flam, M. Casse, Eds. (Cambridge Univ. Press, Cambridge, 1993), pp. 15-25. Solar abundances for noble gas index isotopes normalized to
92. M. N. Rao, D. H. Garrison, D. D. Bogard, G. Badhwar, A. V. Murali, J. Geophys. Res. 96, 19321 (1991).
93. D. Heymann and E. Mazor, ibid. 72, 2704 (1967).
94. C. M. Hohenberg, P. K. Davis, W. A. Kaiser, R. S. Lewis, J. H Reynolds, Proc. Apollo 11 Lunar Sci. Conf. 2, 1283 (1970).
95. 22Ne ratios for MORB sample CH98-DR11 (12) and a Zaire diamond (9).
96. As shown in Fig. 3, the isotopic composition of air Ne looks like a simple mixture between solarlike degassed Ne from the mantle and planetary-like Ne from a late volatile-rich veneer (atmospheric Kr and Xe are also isotopically mass fractionated compared to their solar and meteoritic isotope compositions, possibly as a result of fractionation in a cometary ice component of the late veneer).
97. We thank the reviewers for their thoughtful comments on this manuscript. This work was supported by NSF (EAR9220020) and NASA (NAGW-2858 and NAGW-1598).