Do Lunar Pyroclastic Deposits Contain the Secrets of the by jdo13922


									Lunar Reconnaissance Orbiter Science Targeting Meeting (2009)                                                                    6014.pdf

       Do Lunar Pyroclastic Deposits Contain the Secrets of the Solar System?
       David S. McKay, Mail Code KA, NASA Johnson Space Center, Houston TX 77058,

          Introduction: A new lunar exploration program should          chemistry may have changed over time as different
          have a broad scientific basis, going much beyond the          populations of impactors became dominant or disap-
          lunar focus of the Apollo program. Part of the new pro-       peared. It is entirely possible that meteorites generated
          gram should include regolith studies, but these studies       on early Earth may be preserved in ancient regolith
          should have much broader implications beyond under-           pockets, and even meteorites from Venus, Mercury, the
          standing the development of the lunar regolith. The           satellites of Jupiter and Saturn may be present on the
          concept of using the lunar regolith as a tape recorder        Moon. Ejecta from large impact may have left lunar
          containning a broad but complex record of solar system        ray material or marker beds preserved in ancient re-
          history can be the basis for some fundamental science.        golith. Whether or not such ejecta can be traced to spe-
          Answers to a number of important questions may be             cific craters, age dating of ejecta from larger impacts
          preserved in the lunar regolith, some of which have sig-      should still provide us with statistically useful impact
          nificance for the evolution of the Earth and even for the     data. How frequent were large impacts? What was the
          development of life on Earth:                                 relative frequency of large to small impactors and how
                                                                        did that ratio vary over time? Do early preserved re-
                    o    How long did planetary objects bombard         golith samples have agglutinates? Or are agglutinates a
                         the Moon and Earth the rate of bom-            characteristic of much younger regoliths? The lunar re-
                         bardment decrease finally allowing life        golith also contains a history of the sun and possibly
                         to catch hold on Earth?                        other stars and supernova in the form of implanted and
                    o    Did life start independently on Earth and      trapped solar wind, solar flare materials, and radiation
                         can we find still preserved Earth-derived      damage and cosmogenic isotope changes resulting from
                         meteorites from the earliest billion years     energetic gamma rays.
                         or so, a record destroyed on the Earth by
                         geologic processing and resurfacing?           This impact history is preserved as mega-regolith layers
                    o    Or did life come via meteorite transport       dating back to the lunar cataclysm or earlier, ejecta lay-
                         from a more quiet Mars where impacts           ers, impact melt rocks, ancient impact breccias, and
                         were less severe and life could start ear-     perhaps most valuable, buried and well-preserved an-
                         lier, allowing Mars life to become im-         cient regolith. A major goal of the developing lunar ex-
                         planted on Earth in welcoming nutrient-        ploration program should be to find and sample existing
                         rich oceans and ponds?                         fragments of that taped record in the form of carefully
                    o    Did the solar system undergo occasional        located and sampled pockets of preserved ancient re-
                         bursts of severe radiation from the sun,       golith. Burial of existing regolith by a hot basalt flow, a
                         from other stars, supernovas, black holes,     hot impact-generated melt flow, or even an impact-
                         or from unknown mysterious sources?            produced base surge of impact debris is one set of pos-
                    o    Were these this radiation bursts strong        sibilities, and such burial from any of these events could
                         enough to kill early life on Earth or to       preserve ancient regolith. However, for each of these
                         blast away the ozone layer causing major       scenarios the upper exposed regolith layers may be me-
                         mutations in the development of life?          chanically and thermally disrupted by the basalts and
                    o    If so, is a record of these strong radiation   impact melt or debris flows. By contrast, it is likely
                         bursts still preserved in ancient regolith     that coverage of ancient regolith by fine-grained pyro-
                         samples on the Moon?                           clastic deposits was the most gentle and least destruc-
                                                                        tive process for sealing off and preserving the detailed
          The answer to these and other solar system history            history of the moon and the solar system up to the
          questions may already be recorded in the lunar regolith       moment in time of the pyroclastic eruption event. On
          and be waiting for us to develop the tape reader allow-       earth, deposits of pyroclastic ash falling from an erup-
          ing this history channel to be played. The lunar regolith     tion may preserve the underlying soil and stratigraphy
          has, in theory, recorded the history of the early Moon,       in complete detail and without significant disruption.
          the early Earth, and the entire solar system back through     The equivalent lunar sandwich filling, bounded on top
          time. While much of that record has been destroyed by         and bottom by precisely datable lunar pyroclastic de-
          continuous impact bombardment, some may still pre-            posits, has likely been trapped and removed from fur-
          served in pockets of ancient regolith, fortuitously buried    ther processing at many places and constitutes a buried
          and preserved by subsequent protective deposits. This         time capsule perhaps containing well-preserved ancient
          recording may consist of impact remains from large and        regolith, including recoverable records of solar wind,
          small meteorites that can be identified by their chemis-      giant solar flares, supernova events, and ejection debris
          try and possible remnant fabrics, textures, and mineral-      layers or marker beds from impacting bodies such as
          ogy just as they are on Earth. The meteorite type and         comets and asteroids. It is entirely possible that some
Lunar Reconnaissance Orbiter Science Targeting Meeting (2009)          6014.pdf

          areas contain pyroclastic deposits from multiple erup-
          tion events and that these deposits were each exposed to
          space weathering for short periods of time until they
          were covered by a subsequent ash deposits. This sce-
          nario would create multiple time capsules or snapshots,
          each preserving a unique slice of the record of the moon
          and solar system. Detailed study of materials in a cross
          section or trench through such deposits should show us
          how the solar system has evolved and changed over
          time and provide us with detailed snapshots of specific
          portions of solar system history.

          Large areas of presumed pyroclastic dark mantle are
          found on the front side of the moon, and small dark
          mantle regions have been identified on the back side.
          The Aristarchus Plateau is one of the most studied ar-
          eas, and estimates for the thickness of those deposits
          range from 10s of meters to 100s of meters. If these
          thicknesses represent multiple eruptions extending over
          significant time, the probability is that layers of space-
          weathered pyroclastics have been encapsulated and pre-
          served by each subsequent pyroclastic deposit. Close
          examination by LRO might spot evidence for multiple
          eruptions separated by layers of space-weathered re-

          While we should not expect large expanses of pristine
          impact ejecta layers or marker beds, or pristine or even
          continuous pyroclastic deposits to be preserved intact,
          discontinuous fragments of such layers may be found in
          many places protected from destruction or gardening by
          burial by subsequent pyroclastic deposits. In general,
          the thicker the covering blanket, the greater the chance
          that covered ancient regolith will be preserved intact
          and escape subsequent gardening, and the relatively
          thick large dark mantle deposits such as those on the
          Aristarchus Plateau may be the best place to look.

          The lunar regolith is the only readily accessible place
          in the solar system for finding and revealing the de-
          tailed record of solar system history. The tape re-
          corder aspect of the moon may ultimately turn out to
          be its single most valuable scientific asset, and we
          need to focus a major exploration effort in this direc-
          tion. This focus is readily understood by the public
          and can be the underlying scientific basis of much of
          our lunar program. The recovery, sample return, and
          decoding of pristine ancient regolith may be one of
          the most important scientific discoveries of the new
          lunar era.

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