Five Things Learned from the Antarctic Search for Meteorites
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FIVE THINGS LEARNED FROM THE ANTARCTIC SEARCH FOR METEORITES Featured Story | From the Desk of Lori Glaze | Meeting Highlights | Opportunities for Students | News from Space Spotlight on Education | In Memoriam | Milestones | New and Noteworthy | Calendar LUNAR AND PLANETARY INFORMATION BULLETIN October 2020 Issue 162 FEATURED STORY Cover photo: Longtime ANSMET mountaineer John Schutt gets a closer look at a meteorite spotted in a moraine on the edge of the blue ice at Davis-Ward. Credit: ANSMET. FIVE THINGS LEARNED FROM THE ANTARCTIC SEARCH FOR METEORITES James M. Karner (University of Utah), Ralph P. Harvey (Case Western Reserve University), John S. Schutt (Case Western Reserve University), and Brian Rougeux (Case Western Reserve University) A half century ago, in 1969, the Japanese of a total of only nine in a relatively small representative sample of the extrater- Antarctic Research Expedition (JARE-10) area was extremely fortuitous, maybe restrial materials falling to Earth and found nine meteorites that were several even incomprehensible. Unless — and make them available for research. Here hundred meters to a few kilometers this was Cassidy’s “Eureka!” moment — are several things we’ve learned about apart, on bare ice that was upstream there were thousands of meteorites on meteorite recovery in Antarctica and from the Yamato Mountains. A few years the bare ice areas of Antarctica! Cassidy what it means to planetary science. later, at the annual Meteoritical Society spent the next few years convincing the meeting, the JARE team reported these National Science Foundation (NSF) to nine Antarctic finds to the planetary fund a U.S. team to search for meteorites Meteorites are Found science community. It turned out that in Antarctica, and finally in 1976–1977 the nine specimens were individual the first Antarctic Search for Meteorites on Slow-Moving or meteorites that were distinct from one (ANSMET) team recovered nine meteor- another, i.e., they were not just nine ites from the huge expanses of bare (blue) Stagnant Blue Ice pieces of one meteorite that broke apart ice in the Allan Hills area of Antarctica. At above the ice or on impact with the ice. present, Cassidy’s legacy, the ANSMET that is Being Slowly Furthermore, four of the nine meteorites program, is a collaboration between NSF, were of exceedingly rare class and/ NASA, and the Smithsonian Institution, Ablated Away or petrographic type — exceedingly and has completed its forty-third field rare in the fact that if you found 100 season. In those 43 seasons, ANSMET Our basic understanding of the genesis meteorites, only one out of that 100 has recovered more than 23,000 of Antarctic meteorite concentration sites might be any of those classes or types. meteorites from approximately 65 (i.e., icefields) is that meteorites have separate locations (which often include been raining down on the East Antarctic William (Bill) Cassidy, from the University multiple icefields) in East Antarctica. Plateau for several millions of years. of Pittsburgh, was in the audience for They are then buried by snow, and are that presentation, and he immediately The continuing goal of the ANSMET eventually incorporated into glaciers that surmised that four rare meteorite finds out program is to recover a complete and flow down through the Transantarctic 2 Issue 162 October 2020 © Copyright 2020 Lunar and Planetary Institute FEATURED STORY tration site where slow-moving searched for meteorites on the bare, ice is being ablated away blue ice areas of icefields following a is the Miller Range icefields transect-sampling procedure. In this pro- (Fig. 1a). The Miller Range cedure the field team forms a line, each icefields are located in the member a few tens of meters to several Beardmore Glacier region of tens of meters apart (usually on a snow- Antarctica and are composed mobile). The team then proceeds to drive of three fairly geographically across the blue ice in a direction perpen- distinct blue ice areas: the dicular to this line, scanning for meteorites northern, middle and southern as they go. Meteorites are pretty easy to icefields. ANSMET teams spot — black rocks on light blue ice, and have spent part or all of they are also easily distinguished from eight field seasons searching the small numbers of terrestrial rocks the three icefields and have scattered across the blue ice. Searching recovered more than 3000 on blue ice is speedy and relaxed. meteorites from them — and we still have at least one more season of work there! During the 2005–2006 field season, Moraines are a different story. They are Gordon Osinski and the rest by definition an accumulation of rocks of the ANSMET team set up that have been carried and deposited by an ice movement and abla- a glacier, therefore spotting a meteorite tion study, which consisted among thousands of terrestrial rocks of installing two dozen steel is definitely not easy. It takes intense Fig. 1a. Ice motion at the Miller Range. This multi-spectral satellite mosaic shows the blue ice areas of the Northern posts into the ice at separate concentration, patience, and even luck. and Middle Icefields in relation to the exposed peaks of the locations throughout the area. Searching in moraines is slow (the search Miller Range. The dark-colored areas are exposed moun- The idea was to document the is done on foot) and arduous. But after tains, whereas the white areas are snow. General ice motion post’s GPS location and also 43 years of searching, it is apparent that is south to north (from the bottom of the image up); i.e., the ice is flowing down off the polar plateau to the south and mark the surface level of the the moraines accompanying icefields running into the Miller Range peaks. The colored vectors ice on the post. Then, after a (lateral, terminal, medial, etc.) hold denote the specific ice motion and rates measured at each few years had passed, we significant concentrations of meteorites. location. Note that most of the ice in the area is moving at a would remeasure the post’s For example, in its last two field seasons rate of less than 1 meter per year. Credit: ANSMET. location and height of the ice at Davis Nunataks-Mt. Ward (DW), surface level. The results of that ANSMET recovered ~760 meteorites from Mountains (TAMS) and eventually empty study reveal that ice at Miller is moving the moraines that surround the icefields. into the Ross Sea. With general ice sheet very slowly, at a speed of less than ~1 That number amounts to almost 50% of thinning over the last 20,000 years, pre- meter per year in most cases. And just to the meteorites found in the area over viously unobstructed and rapidly flowing give these rates some context, ice in the those two seasons. Moraine searching glaciers have been redirected, trapped, Nimrod and Marsh glaciers (not shown, among thousands of terrestrial rocks is and stranded by exposed and subsur- but just off the image to the west and east, decidedly more challenging than spotting face barriers (i.e., the TAMS). Simply put, respectively), is briskly moving along meteorites on blue ice, but the poten- free-flowing ice has now been pinched at speeds of tens to hundreds of meters tial payoff in extraterrestrial samples off, slowed, and ablated — allowing per year. Figure 1b shows that the slow makes it worth the effort (see Fig. 2). deep blue ice to be exposed, and moving ice is being slowly ablated away meteorites trapped in that blue ice to be at an average rate of about 3.5 centime- exhumed at the surface and accumulate ters per year by scouring katabatic winds, Check the Downwind like a lag deposit. Additionally, these sublimation, and perhaps rare melting blue ice areas have most likely remained events. This combination of slow ice and Ice Edge stable for tens to hundreds of thousands steady ablation at Miller leads to an opti- of years, so they also have “caught” and mal setting for meteorite concentration. The downwind border of an icefield is preserved meteorites that have fallen often characterized by blue ice that gives on them over those extremely long time way to a compact snow called firn. The periods. We call that process direct infall, Check the strong katabatic winds in Antarctica are and it undoubtedly adds to the concen- capable of moving rocks of up to 100 tration of meteorites in blue ice areas. Moraines Too grams, and firn has proved to be an excellent trap for wind-blown rocks and A classic example of a meteorite concen- Historically, ANSMET teams have mainly meteorites (see Fig. 3). The meteorites 3 Issue 162 October 2020 © Copyright 2020 Lunar and Planetary Institute FEATURED STORY Furthermore, the downwind ice edge can be used to gauge whether or not a specific icefield contains a meteorite concentration: abundant small meteorites, probably a concentration upwind; few small meteorites, probably no concentration upwind. Fig. 3. The downwind ice edge at DW with flags marking dozens of recovered meteorites that were fetched up on firn. Credit: ANSMET. Check Areas surface. We’re leaning toward glacio- tectonic redistribution for the paradox of that have been The Beach, but whatever the reason, it is worth having another look in areas where Previously meteorites have been previously found. Searched ANSMET is Important For icefields showing a mete- orite concentration, ANSMET to the Planetary strives to systematically search Fig. 1b. Ice ablation rates at the Miller Range. The red num- all blue ice areas, relevant Science Community, bers denote the ice ablation rates in centimeters per year moraines, and ice edges in for the specific locations (black dots). The ASTER satellite imagery used in Figs.