Article

Space Science Reviews

, Volume 128, Issue 1, pp 363-381

First online:

Ptolemy – an Instrument to Measure Stable Isotopic Ratios of Key Volatiles on a Cometary Nucleus

  • I. P. WrightAffiliated withPlanetary and Space Sciences Research Institute, Open University
  • , S. J. BarberAffiliated withPlanetary and Space Sciences Research Institute, Open University Email author 
  • , G. H. MorganAffiliated withPlanetary and Space Sciences Research Institute, Open University
  • , A. D. MorseAffiliated withPlanetary and Space Sciences Research Institute, Open University
  • , S. SheridanAffiliated withPlanetary and Space Sciences Research Institute, Open University
  • , D. J. AndrewsAffiliated withPlanetary and Space Sciences Research Institute, Open University
  • , J. MaynardAffiliated withPlanetary and Space Sciences Research Institute, Open University
  • , D. YauAffiliated withPlanetary and Space Sciences Research Institute, Open University
  • , S. T. EvansAffiliated withPlanetary and Space Sciences Research Institute, Open University
    • , M. R. LeeseAffiliated withPlanetary and Space Sciences Research Institute, Open University
    • , J. C. ZarneckiAffiliated withPlanetary and Space Sciences Research Institute, Open University
    • , B. J. KentAffiliated withSpace Science Department, Rutherford Appleton Laboratory
    • , N. R. WalthamAffiliated withSpace Science Department, Rutherford Appleton Laboratory
    • , M. S. WhalleyAffiliated withSpace Science Department, Rutherford Appleton Laboratory
    • , S HeysAffiliated withSpace Science Department, Rutherford Appleton Laboratory
    • , D. L. DrummondAffiliated withSpace Science Department, Rutherford Appleton Laboratory
    • , R. L. EdesonAffiliated withSpace Science Department, Rutherford Appleton Laboratory
    • , E. C. SawyerAffiliated withSpace Science Department, Rutherford Appleton Laboratory
    • , R. F. TurnerAffiliated withSpace Science Department, Rutherford Appleton Laboratory
    • , C. T. PillingerAffiliated withPlanetary and Space Sciences Research Institute, Open University

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Abstract

A fundamental goal of cometary studies is to determine the exact relationship between these bodies and the Solar System – the question(s) can be summarised as follows: did comets originate during the same events that spawned the Sun and planets, are they more primitive bodies that record a pre-solar history, or are they interstellar materials collected in relatively more recent times? Now, whatever the origin of comets, it is entirely possible that they could, in part, contain interstellar or pre-solar components – indeed, it seems rather likely in light of the fact that primitive meteorites contain such entities. These particular components are likely to be refractory (dust, macromolecular organic complexes, etc.). Of more relevance to the issues above are the volatile constituents, which make up the bulk of a comet's mass. Since these materials, by their very nature, volatilise during perihelion passage of a comet they can, in some instances, be detected and measured spectroscopically. Perhaps the most useful species for isotopic investigations are C2, HCN and CN. Unfortunately, spectroscopic measurements can only currently be made with accuracies of ±10 to ±20%. As such it is very often not practical to conclude anything further than the fact that isotopic measurements are compatible with ‘`solar’' values, which tends to imply an origin from the margins of the solar accretion disk. But there is another problem with the spectroscopic measurements – since these are made on gaseous species in the coma (and relatively minor species at that) it is impossible to be certain that these represent the true nuclear values. In other words, if the processes of sublimation, active jetting, and photochemistry in the coma impart isotopic fractionation, the spectroscopic measurements could give a false impression of the true isotope ratios. What is required is an experiment capable of measuring isotopic ratios at the very surface of a comet. Herein we describe the Ptolemy instrument, which is included on the Philae lander as part of the Rosetta mission to 67P/Churyumov-Gerasimenko. The major objective of Ptolemy is a detailed appraisal of the nature and isotopic compositions of all materials present at the surface of a comet.

Keywords

Rosetta Philae comets stable isotope ratios gas chromatography mass spectrometry ion trap