Albedo asymmetry of Iapetus


Voyager images of Saturn's moon, Iapetus1,2, confirm deductions made from Earth-based observations dating back to 1671 of a very dark leading hemisphere and a very bright trailing hemisphere3–5. Figure 1 displays contours of surface albedo from three Voyager images. The darkest area is at the apex of orbital motion, with a pronounced (10×) increase in albedo towards the antapex, constituting the greatest interhemispheric albedo contrast known in the Solar System. The poles are brighter still. Figure 1 also shows that the albedo distribution resembles the calculated areal variation of the trans-saturnian impact flux6 remarkably closely. Dark areas correspond to regions with the highest calculated flux. We propose here that the dark areas contain organic chromophores produced in situ by UV irradiation of CH4-rich ice, and that the albedo pattern results from ballistic redistribution of surface material in response to the impact flux gradient. Where the impact flux is high, net ablation will cause exposure of CH4-rich darkenable ice, creating a dark surface. Where the flux is low, net accumulation of non-darkenable icy regolith that has lost CH4 through repeated impact volatilization and evaporation, will create a bright surface.

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  1. 1

    Smith, B. A. et al. Science 212, 163–191 (1981).

    ADS  CAS  Article  Google Scholar 

  2. 2

    Smith, B. A. et al. Science 215, 504–537 (1982).

    ADS  CAS  Article  Google Scholar 

  3. 3

    Cassini, J. D. Phil. Trans. R. Soc. 8, 51–78 (1671).

    Google Scholar 

  4. 4

    Widorn, Th. Der Lichtwechsel des Saturn Satelliten Japetus im Jahre 1949, Osterr. akad. Wissenschaften Abt. IIa 159, 186–199 (1950).

    Google Scholar 

  5. 5

    Morrison, D., Jones, T. J., Cruikshank, D. P. & Murphy, R. E. Icarus 24, 157–171 (1975).

    ADS  Article  Google Scholar 

  6. 6

    Cook, A. F. & Franklin, F. A. Icarus 13, 282–291 (1970).

    ADS  Article  Google Scholar 

  7. 7

    Cruikshank, D. P. et al. Icarus 53, 90–104 (1983).

    ADS  CAS  Article  Google Scholar 

  8. 8

    Soter, S. Pap. at IAU planet. Satellite Conf., Cornell University, Ithaca (1974).

  9. 9

    Pollack, J. B., Grossman, A. S., Moore, R. & Graboske, H. C. Icarus 29, 35–48 (1976).

    ADS  CAS  Article  Google Scholar 

  10. 10

    Lebofsky, L. A. Icarus 25, 205–217 (1975).

    ADS  CAS  Article  Google Scholar 

  11. 11

    Gault, D. E., Horz, F., Brownlee, D. E. & Hartung, J. B. Proc. 5th Lunar Sci. Conf, 2365–2386 (1974).

  12. 12

    Gault, D. E. & Heitowit, E. D. Proc. 6th Hypervelocity Impact Symp. 2, 419–456 (1963).

    Google Scholar 

  13. 13

    Breslau, D. J. geophys. Res. 75, 3987–3999 (1970).

    ADS  Article  Google Scholar 

  14. 14

    O'Keefe, J. D. & Ahrens, T. J. Proc. 7th Lunar planet. Sci. Conf., 3007–3026 (1976).

  15. 15

    Housen, K. R., Wilkening, L. L., Chapman, C. R. & Greenberg, R. Icarus 39, 317–351 (1979).

    ADS  Article  Google Scholar 

  16. 16

    Squyres, S. W. & Sagan, C. Proc. 14th Lunar planet. Sci. Conf., 739–740 (1983).

  17. 17

    Khare, B. N. & Sagan, C. in Molecules in the Galactic Environment (eds Gordon, M. & Snyder, L.) 399–408 (Wiley, New York, 1973).

    Google Scholar 

  18. 18

    Hagen, W., Allamondola, L. J. & Greenberg, J. M. Astrophys. Space Sci. 65, 215–240 (1979).

    ADS  CAS  Article  Google Scholar 

  19. 19

    Greenberg, J. M. in Comets (ed. Wilkening, L.) 131–163 (University of Arizona Press, 1982).

    Google Scholar 

  20. 20

    Khare, B. N., Sagan, C., Zumberge, J. E., Sklarew, D. S. & Nagy, B. Icarus 48, 290–297 (1981).

    ADS  CAS  Article  Google Scholar 

  21. 21

    Bar–Nun, A., Bar-Nun, N., Bauer, S. H. & Sagan, C. Science 168, 470–473 (1970).

    ADS  Article  Google Scholar 

  22. 22

    Duxbury, T. C. Pap. at. Int. Colloq. on the Saturn System, Tucson (1982).

    Google Scholar 

  23. 23

    Andersson, L. E. thesis Indiana Univ. (1974).

  24. 24

    Brown, R. H., Cruikshank, D. P. & Morrison, D. Nature 300, 423–425 (1982).

    ADS  Article  Google Scholar 

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Squyres, S., Sagan, C. Albedo asymmetry of Iapetus. Nature 303, 782–785 (1983).

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