Criteria for the selection of Exobiology Sites

The possibility that life may once have evolved on Mars is strongly linked to the availability of liquid water over long time periods. Thus, the selection of sites for exobiology exploration is related to photo-geologic evidence of water activity. Here is a table which will help you in interpreting the Viking orbiter images of the martian landscape, especially fluvial features and in judging the length of time that the water may have persisted.

GEOMORPHIC CRITERIA FOR EVALUATING THE DURATION OF FLUVIAL-LACUSTRINE ENVIRONMENTS ON MARS

Feature                                Duration


                                Shorter                 Longer



Channel :                           Straight                    Meandering
Floodplain :                        Absent                      Broad
Drainage networks :                 Simple                      Dendritic
Drainage basin :                    Small, many divides         Large, few divides
Tributary :                         None to first order         Second to higher order
Stream terrace :                    Single level                Multiple level, incised meanders
Delta :                             Absent or small             Large, multi-lobed
Other :                             Only young, crisp           Craters show various degree of
                                    featured craters present        erosion

Although the Viking images are of extraordinary quality, their resolution generally does not allow us to see features as small as a football field. Nor, at this time do we have much information about surface composition. This inability to identify small geological features and to identify surface compositions of Mars limits our ability to select sites of exobiology importance. This explains the need for high resolution orbital imaging in future missions. A minimum resolution of 75 m is required to recognize most geological features critical for site selection. Even better resolution may be required to recognize such potentially exciting features as deposits from ancient hydrothermal springs.

Here are some clues to give priority levels to sites for exobiology exploration.

MARS SITES FOR EXOBIOLOGY : SAMPLE SCORING SHEET

Features/deposits                           Weighting factors

                                       Visibility          Process      Value for
                                       of feature          uniqueness    Exobiology
                                          (0-3)                (1-3)          (1-3)


Fluvial
       Dendritic drainage networks
       Channel widens downslope
       Floodplains
       Channel meandering
       Stream terraces
       Visible layering
Score subtotals

Lacustrine
      Basin fed by channels
      Shoreline terraces
      Delta at stream mouth
      Lava/aoelian cover
      Channels on basin floor
      Impact craters
Score subtotals

Thermal spring
      Simple channel morphology
      Channel head a point source
      Channels branch downslope
      Associated with volcanic center
      Possible subsurface heat source
Score subtotals

Ground ice
      Polygonal terrain
      Irregular depressions
      Mounds (possible pingos)
      Lobate crater ejecta
      Debris flows (possible rock glaciers)
      Laminated terrain
      Latitude
Score Subtotals

Total Score

Comments

1. a "Visibility" score ranging from 0 to 3 (0 = not visible, 3 = highly visible) ;
2. a "Value to Exobiology" score ranging from 1 to 3 (1 = low significance , 3 = high significance), which evaluate the perceived importance of the feature for meeting the stated goals of exobiology (e.g. search for extant life, search for extinct life, and the search for organic molecules or repositories of biologically-important minerals) ; and
3. a "Process Uniqueness" score ranging from 1 to 3 (1 = not unique, 3 = unique), which evaluated the degree to which a certain kind of geologic feature can be associated with a particular mode of formation (some features, especially if seen at poor resoluion, can be mistaken for similar looking features of quite different origin e.g. a score of 1 applies to features that are similar in appearance but which can originate by different geological processes).