Information on Origin Description

Impactor or origin classification is based on residue signals present (or not) in or near the impact feature (the latter already pointing to the problem of pre-, in-, and/or postflight contamination). The list below should familiarize you with the assumptions and general approach that was used for impact residue classification:

• The images were corrected for CCD and/or target shifts
• The SIMS data acquisition process is classified using instrument settings, notebook entries and other information not always archivable in an electronic format. Each of the almost 3500 SIMS images was then classified using values 1, 2 and 3 with:
  • 1 meaning species either positively identified using element standards, EDS spectra or the species being a substrate constituent,
  • 2 meaning that all procedures, settings and conditions indicate that the mass image may be tagged as a certain species, with the only information missing being the explicit use of an element standard,
  • 3 meaning that procedures, settings and conditions of the analyses do not allow a species classification. Other possible reasons for this classification as "unknown species" include a mass resolution not high enough to resolve an interferent being present most likely, explicit notes towards interference influences not explicitly stored in this archive, and other information.
  • In addition, black images are not displayed and classified. With no surface material of this particular species present; it is impossible to resolve a nonexisting signal from the signal of an interfering species.
• The next step involved a selection of the analysed feature by drawing a box either
  • around the central crater part of a MOS discharge feature, if distiguishable, since impactor residue is expected to be present there, even though the crater formation process for the MOS detectors is driven by a capacitor discharge process,
  • around the discharge zone where the aluminum deposit had been vaporized, if residue seemed to be present in this zone,
  • around the crater lips for impacts into aluminum and gold,
  • in the crater bottom for elliptic impacts in aluminum in order to avoid shadowing signal effects,
  • around what appeared to be impactor debris sprays including or excluding the crater, depending on the debris pattern,
  • around the spall zone for impacts on the germanium witness plates, trying to avoid contamination spots present on these surfaces,
  • usually not around isolated signals rather distant from the impact features or, in case of the MOS detectors, right at the edge of the vaporization zone, because:
    • isolated features may well be paint flakes or other flakes that probably contaminated many of the surfaces during the LDEF mission,
    • impactor residue should also be present near the center of the impact site, not just in one isolated spot,
    • signals well away from the feature have a higher probability of being a contamination of any sort, bleed-in signals from mass signals adjacent to the signal being imageg or being caused by any SIMS anomaly, such as channel plate asymmetry and others, depending on the shape of the signal.