Five Things Learned from the Antarctic Search for Meteorites
A half century ago, in 1969, the Japanese Antarctic Research Expedition (JARE-10) found nine meteorites that were several hundred meters to a few kilometers apart, on bare ice that was upstream from the Yamato Mountains. A few years later, at the annual Meteoritical Society meeting, the JARE team reported these nine Antarctic finds to the planetary science community. It turned out that the nine specimens were individual meteorites that were distinct from one another, i.e., they were not just nine pieces of one meteorite that broke apart above the ice or on impact with the ice. Furthermore, four of the nine meteorites were of exceedingly rare class and/or petrographic type — exceedingly rare in the fact that if you found 100 meteorites, only one out of that 100 might be any of those classes or types.
William (Bill) Cassidy, from the University of Pittsburgh, was in the audience for that presentation, and he immediately surmised that four rare meteorite finds out of a total of only nine in a relatively small area was extremely fortuitous, maybe even incomprehensible. Unless — and this was Cassidy’s “Eureka!” moment — there were thousands of meteorites on the bare ice areas of Antarctica! Cassidy spent the next few years convincing the National Science Foundation (NSF) to fund a U.S. team to search for meteorites in Antarctica, and finally in 1976–1977 the first Antarctic Search for Meteorites (ANSMET) team recovered nine meteorites from the huge expanses of bare (blue) ice in the Allan Hills area of Antarctica. At present, Cassidy’s legacy, the ANSMET program, is a collaboration between NSF, NASA, and the Smithsonian Institution, and has completed its forty-third field season. In those 43 seasons, ANSMET has recovered more than 23,000 meteorites from approximately 65 separate locations (which often include multiple icefields) in East Antarctica.
The continuing goal of the ANSMET program is to recover a complete and representative sample of the extraterrestrial materials falling to Earth and make them available for research. Here are several things we’ve learned about meteorite recovery in Antarctica and what it means to planetary science.
Meteorites are Found on Slow-Moving or Stagnant Blue Ice that is Being Slowly Ablated Away
Our basic understanding of the genesis of Antarctic meteorite concentration sites (i.e., icefields) is that meteorites have been raining down on the East Antarctic Plateau for several millions of years. They are then buried by snow, and are eventually incorporated into glaciers that flow down through the Transantarctic Mountains (TAMS) and eventually empty into the Ross Sea. With general ice sheet thinning over the last 20,000 years, previously unobstructed and rapidly flowing glaciers have been redirected, trapped, and stranded by exposed and subsurface barriers (i.e., the TAMS). Simply put, free-flowing ice has now been pinched off, slowed, and ablated — allowing deep blue ice to be exposed, and meteorites trapped in that blue ice to be exhumed at the surface and accumulate like a lag deposit. Additionally, these blue ice areas have most likely remained stable for tens to hundreds of thousands of years, so they also have “caught” and preserved meteorites that have fallen on them over those extremely long time periods. We call that process direct infall, and it undoubtedly adds to the concentration of meteorites in blue ice areas.
A classic example of a meteorite concentration site where slow-moving ice is being ablated away is the Miller Range icefields (Fig. 1a). The Miller Range icefields are located in the Beardmore Glacier region of Antarctica and are composed of three fairly geographically distinct blue ice areas: the northern, middle and southern icefields. ANSMET teams have spent part or all of eight field seasons searching the three icefields and have recovered more than 3000 meteorites from them — and we still have at least one more season of work there! During the 2005–2006 field season, Gordon Osinski and the rest of the ANSMET team set up an ice movement and ablation study, which consisted of installing two dozen steel posts into the ice at separate locations throughout the area. The idea was to document the post’s GPS location and also mark the surface level of the ice on the post. Then, after a few years had passed, we would remeasure the post’s location and height of the ice surface level. The results of that study reveal that ice at Miller is moving very slowly, at a speed of less than ~1 meter per year in most cases. And just to give these rates some context, ice in the Nimrod and Marsh glaciers (not shown, but just off the image to the west and east, respectively), is briskly moving along at speeds of tens to hundreds of meters per year. Figure 1b shows that the slow moving ice is being slowly ablated away at an average rate of about 3.5 centimeters per year by scouring katabatic winds, sublimation, and perhaps rare melting events. This combination of slow ice and steady ablation at Miller leads to an optimal setting for meteorite concentration.
Check the Moraines Too
Historically, ANSMET teams have mainly searched for meteorites on the bare, blue ice areas of icefields following a transect-sampling procedure. In this procedure the field team forms a line, each member a few tens of meters to several tens of meters apart (usually on a snowmobile). The team then proceeds to drive across the blue ice in a direction perpendicular to this line, scanning for meteorites as they go. Meteorites are pretty easy to spot — black rocks on light blue ice, and they are also easily distinguished from the small numbers of terrestrial rocks scattered across the blue ice. Searching on blue ice is speedy and relaxed.
Moraines are a different story. They are by definition an accumulation of rocks that have been carried and deposited by a glacier, therefore spotting a meteorite among thousands of terrestrial rocks is definitely not easy. It takes intense concentration, patience, and even luck. Searching in moraines is slow (the search is done on foot) and arduous. But after 43 years of searching, it is apparent that the moraines accompanying icefields (lateral, terminal, medial, etc.) hold significant concentrations of meteorites. For example, in its last two field seasons at Davis Nunataks-Mt. Ward (DW), ANSMET recovered ~760 meteorites from the moraines that surround the icefields. That number amounts to almost 50% of the meteorites found in the area over those two seasons. Moraine searching among thousands of terrestrial rocks is decidedly more challenging than spotting meteorites on blue ice, but the potential payoff in extraterrestrial samples makes it worth the effort (see Fig. 2).
Check the Downwind Ice Edge
The downwind border of an icefield is often characterized by blue ice that gives way to a compact snow called firn. The strong katabatic winds in Antarctica are capable of moving rocks of up to 100 grams, and firn has proved to be an excellent trap for wind-blown rocks and meteorites (see Fig. 3). The meteorites found on firn are typically small in size, maybe not much larger than a centimeter or so in longest dimension and weighing less than 10 grams. ANSMET’s policy is to collect all meteorites, no matter what the size, because we often cannot readily recognize small specimens of rare or interesting types in the field. On several occasions ANSMET teams have been pleasantly surprised to find out that a small, non-descript meteorite in the field actually turns out to be a lunar or martian specimen — good things do come in small packages!
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.
Check Areas that have been Previously Searched
For icefields showing a meteorite concentration, ANSMET strives to systematically search all blue ice areas, relevant moraines, and ice edges in order to recover the most meteorites possible. On occasion we have time to revisit areas searched in previous seasons — and we almost always find more meteorites, sometimes a lot more meteorites. An extreme example of this is illustrated in Figs. 4a and 4b. Figure 4a shows an area known affectionately as The Beach, an amazingly meteorite-laden moraine at the foot of Mt. Ward on the eastern side of the icefields at DW. In 2014–2015, the ANSMET team recovered over 200 meteorites from the moraine, and you can see where most of them were found, as a flag marks each individual find. ANSMET returned to DW and The Beach in 2018–2019, and to our surprise, we found more than 100 additional meteorites (Fig. 4b)! Some of these meteorites seemed to be in almost the exact same location as meteorites found four years before. Most additional finds in previously searched areas (especially moraines) are undoubtedly due to human error; we simply missed them the first time around. Other additional finds are most likely the result of wind and/or glaciotectonic redistribution. Still others may be an actual recharge, where continuous ablation, over time, results in “new” meteorites being exposed at the ice surface. We’re leaning toward glaciotectonic redistribution for the paradox of The Beach, but whatever the reason, it is worth having another look in areas where meteorites have been previously found.
ANSMET is Important to the Planetary Science Community, and Vice Versa
The meteorites recovered by ANSMET are vitally important to planetary research. As such, the meteorites recovered by ANSMET are not owned by the principle investigators on the grant, nor by their universities where they are employed, but instead are expressly made available to researchers around the world for scientific study. Kevin Righter, the Antarctic Meteorite Curator at NASA Johnson Space Center (JSC), documents that since 1978 there have been over 3600 requests for samples, and over 1800 peer-reviewed journal articles published using ANSMET meteorites as their major source of data. Current rates of publication predict about 60 peer-reviewed publications and nearly 200 abstracts per year on ANSMET meteorites. ANSMET meteorites are in particular demand because they are a continuous (yearly) supply of new extraterrestrial materials (except for this coming year; COVID has ruined everything fun). Furthermore, speedy initial characterization and classification by NASA JSC and the Smithsonian lead to rapid availability to researchers, and free of charge!
Finally, the planetary science community is vitally important to ANSMET. In a typical season ANSMET-funded personnel (principal investigators, mountaineers) rely on four to five volunteers to fill out the field team. These volunteers willingly give up two months of their time to live in tents in subzero temperatures and tirelessly search for extraterrestrial rocks on snowmobile and foot (with no showers for about six weeks). As of this past season, ANSMET has had ~195 (volunteer) participants on our field teams, and nearly three dozen of those volunteers have served on multiple ANSMET teams. In fact, ANSMET leadership receives approximately 100 letters per year from men and women around the globe who want to serve on an ANSMET team, and for that we are very grateful. Certainly the success and longevity of ANSMET would not be possible without the outstanding support of the planetary science community.
Harvey R. P. (2003) The origin and significance of Antarctic meteorites. Chem. Erde, 63, 93–147.
Righter K., Corrigan C., Harvey R., and McCoy T., eds. (2014) 35 Seasons of U.S. Antarctic Meteorites (1976–2010): A Pictorial Guide to the Collection. Special Publication Series, AGU/Wiley.
Acknowledgments. ANSMET is supported by NASA grant NNH16ZDA001N-SSO6 to Co-P.I. Ralph Harvey at Case Western Reserve University. We thank K. Righter at JSC for the data on ANSMET meteorite requests and publications.
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.