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, firstname.lastname@example.org 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- golith. 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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