Mars Sample Return: Goals for Geology

I. Mars geologic history: process and relative timing

This section would be a brief overview of the major events in Mars' history using a chart showing their relative timing, significance (e.g., areal extent, intensity, etc.), and relation to the impact record.

    A. Noachian Period (oldest)
      1. Ancient crust
      2. Impact basins
      3. Early volcanism
      4. Global dichotomy
      5. Difficulties in interpreting the record

    B. Hesperian Period
      1. Ridged plains formation: inferred flood volcanism
      2. Highland patera (first "central" volcanism)
      3. Valley networks: liquid water or mass wasting?
      4. Elysium volcanics
      5. Outwash channels
      6. Tharsis volcanism; Valles Marineris

    C. Amazonian Period (youngest)
      1. Continued volcanism
      2. Northern plains
      3. Polar deposits
      5. Current activity
      5. Etc.

II. "Absolute" timing: calibration of crater counts

    A. In the absence of radiometric data, impact size frequency distributions are the principal means to correlate from region-to-region, and planet-to-planet.

    B. Uncertainties arise from extrapolating lunar data to Mars

      1. Scaling uncertainties (gravity, target properties, etc.)
      2. Adjustments from Moon to Mars for impacts from NEOs, main belt objects, comets, etc.
      3. Effect of Jupiter on impacts on Mars
      4. Different approaches/techniques in crater counting (count only primaries vs. count everything, etc.

    C. Several models have been developed, depending on how uncertainties (B, above) are treated.
      1. Time span of three martian periods and their events spread "uniformly" over crater history, versus Mars more Earthlike in geological history.

    D. Determining the "absolute" time scale for Mars is critical for interpreting the climate, volcanic history, magmatic history, and general evaluation of the interior.

III. Processes

    A. Mars is geologically diverse; variety of processes have operated through time.

    B. Knowledge is derived mostly from remote sensing and limited lander results; will be improved with MGS, Pathfinder, Mars-96, and MS '98/MVACS.

    C. Many unanswered geological questions will remain:

      1. Has magmatic differentiation occurred; if so where, when, etc.?
      2. Are the ridged plains (marelike units) volcanic in origin?
      3. Did oceans exist on Mars?
      4. Did glaciation occur?
      5. Are the valley networks the result of surface water or mass wasting?

    D. Samples of key units could address some of these questions:
      1. Memnonia Fossae Formation has been proposed to be:
        a. polar deposits, or
        b. aeolian deposits, or
      2. Ridged plains
        a. volcanic (marelike, or
        b. sedimentary
      3. Daedalia, Syrtis Major, Highland Patera, etc.

IV. Sampling approaches for geology

    A. More than 50 major units have been mapped on Mars; cannot sample all

    B. Sampling to address petrologic diversity: the "grab-bag" approach (e.g., Mars Pathfinder site)

      1. Has potential to sample wide variety (age and type) of rocks in local area
      2. No guarantee of (1)!
      3. Context is not known; could be inferred, but without much confidence
      4. Could lead to some interesting debates!

    C. Sampling to calibrate the cratering record
      1. Focus on key stratigraphic horizons
      2. Homogeneous, wide-spread, easily identified
      3. "Clean" crater counts (clearly superposed craters; surface not eroded, mantled, nor exhumed)
      4. High probability of obtaining valid rock sample from which radiogenic age of unit can be obtained
      5. Hesperian ridged plains
        a. Marks the base of the Hesperian System
        b. Satisfies much of (C) above
        c. However, it is inferred to be volcanic based on mare ridges, but could be of other origins
        d. Perhaps precursor mission (lander) could address origin before sample return

    D. Sampling to address some key events in martian history
      1. Youngest volcanics (determine age and petrology); probably easiest to do
      2. Oldest crust (determine age and petrology); probably very difficult to locate site with guaranteed access
      3. Determine age(s) of outflow channels: need dateable samples which bracket channel formation; some alternatives:
        a. Dateable impact ejecta for crater interleaved between channels
        b. Pillow basalts in channel (?? none identified, but something to seek)
        c. Dateable sediments (a long shot?)

    E. The case for sand (not dust!) samples
      1. Micro-textures are indicative of environment (glacial, fluvial, aeolian, etc.)
      2. SEM analysis of samples could address some of the fundamental questions (did glaciers ever exist?)
      3. Require careful sampling and good knowledge of context

V. The ideal sample site for geology

Ron Greeley's Vugraphs:

Mars Sample Return: Goals for Geology

Summary for geology

Calibration of Crater Counts

Understanding Surface Processes and History

Some Key Units for Sampling

Sampling Strategies for Geology

Scenario 1: The "GrabBag" Approach

Scenario 2: Crater Count "Calibration"

Scenario 3: Some Key Events in Martian History

Daedalia: A Potential Site

Western Daedalia Planum: Sampling Rationale

Geologic Exploration/Development on Earth

The Field Geology Approach to Sampling


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