Jonathan Tucker AST 330: Moon Darby Dyar 12 December 2009

The first lunar meteorite: critical summary of papers describing ALHA 81005

On January 18 of 1982, a team in Antarctica discovered ALHA 81005, the first lunar meteorite. This find changed the face of the field meteoritics forever. Small pieces of the specimen were sent to laboratories around the world for analysis, and the results were first presented at the 14th Lunar and Planetary Science Conference, and published as a series of 18 short papers in an issue of Geophysical Research Letters in September 1983. This summary critically examines some of those papers directly as well as the entire coordinated analysis and reporting process for this hugely significant 3 cm piece of the Moon. The first paper in the series (Marvin 1983) gives a good history of the discovery of ALHA 81005 (as well as an explanation of how meteorites are named). The discoverers recognized it as an "anorthositic breccia". When one hears these words associated with extraterrestrial origin, one's thoughts immediately turn to the Moon. In fact, the first scientists to examine the specimen recognized a lunar origin as highly possible, simply because it looked like other Moon rocks. It seems somewhat obvious to us now, but imagine what it must have been like when the first human laid eyes on this rock. Someone trained in meteoritics would have immediately recognized this sample as different from all other known meteorite types, but of course the paper does not report what that first person thought or said. These GRL papers construct a convincing story of lunar origin, but tout the lunar origin well before any evidence is provided. This removes all the mystery and suspense that might have existed between the time of discovery and the analyses. This is in contrast to the story of the SNC meteorites, which were very early recognized as different from other types of meteorites, but whose martian origin was not positively identified for many decades. The petrographic description of the texture of the whole specimen given in this paper is very clear. There is a photograph of the whole sample in the preceding editorial introduction (Bogard, 1983), but it is black-and-white, and not reproduced in sharp detail and contrast in the printed journal. The description given in this paper is a "heterogeneous array of rock, mineral, and glass fragments, and glass spherules embedded in a dark brown glassy matrix". Even without a photograph, a fairly untrained petrologist would have a good idea of what the sample looks like. Many times, including in many of the following papers, petrographic descriptions are more technically precise by using more specific petrographic vocabulary, but because of that they become less descriptive. In general, the petrographic descriptions in this paper take a top-down approach, starting with overviews before getting down to the nitty-gritty. Finally, we arrive at the first evidence of the origin of this meteorite. This paper gives three main lines of evidence: the anorthositic composition, Fe/Mn ratios, and the presence of spherules and agglutinates. The third line of evidence is argued as "clear evidence" and "diagnostic". However, it does not explain how or why. The first two lines of evidence, as described by this paper, are merely "consistent" with Apollo 16 samples, but not necessarily proof. The paper also mentions the high content of siderophile elements and the presence of trapped solar gasses, but does not go into why these might be significant or important. Some of these lines of evidence are repeated almost as a mantra in many of the following papers. The most important conclusion of this paper has nothing to do with the Moon, but rather with Mars. It was speculated, but with little evidence, that the SNC meteorites could be from Mars long before that was confirmed. But the discovery of a lunar meteorite strongly allows for the possibility of martian meteorites. Positive identification of martian meteorites only happened after multiple landed missions sent back information on the chemistry of the martian atmosphere and lithosphere. In a similar vein, this paper mentions for the sake of curiosity whether this sample could have been identified if it we had not brought back samples from the Moon. The verdict is that it probably could have been done based on remote sensing information, but it would have been much more difficult. Another interesting thought not discussed in this or any of the papers is if it were discovered and identified in the 1960s instead of the 1980s, how this 3 cm rock could have affected the planning and execution of the Apollo missions. For example, before the Ranger and Apollo missions, we had little idea about nature of the surface we were landing on. But one piece of brecciated regolith in our possession could have dramatically changed the early years of lunar exploration, the science and politics of the late 20th century, and even the entire Cold War. This discussion raises an issue that is addressed only slightly in these papers. Prior to the discovery of this meteorite, it was believed that the reason no lunar meteorites were found was because it was too difficult for meteorites to be ejected from the Moon and make it to the Earth. But based on the discussions in these papers, this reasoning seems very circular: because no lunar meteorites were found, it is unlikely that meteorites could have come from the Moon; and because of the difficulties in getting meteorites from the Moon to the Earth, no lunar meteorites had been found. It seems that the physical models of impacts, transport, and entry were based on the empirical observation that lunar meteorites are very rare, at best. But it is incredible how a single discovery has the power to change all of that. With a single discovery, it is was then supposed that lunar (and martian) meteorites are not nearly as rare as previously though, an the physics of impact, transport, and entry would be reformulated. This serves to remind us that all of science is inherently empirical. Physical models can work well, but only until there is a single 3 cm piece of evidence to contradict it. It is not until the second paper (Warren et al. 1983), that a more systematic approach to identifying the origin of this meteorite is described. Texturally, the sample is "indistinguishable from Apollo regolith breccias". Mineralogically, the abundance of plagioclase limits the origin to the Moon, Mercury, or a previously unknown type of achondrite asteroid. But asteroidal origin is ruled out on the basis of Fe/Mn ratios, and Mercury is ruled out on the basis of oxygen isotopes and noble gas contents. This paper points out that a lunar origin can only be concluded on the basis of all the evidence taken together; there is no single piece of evidence that confirms a lunar origin and rules out all other possibilities. However, this paper makes one somewhat irresponsible claim. It states, "dynamical problems are more severe for deriving meteorites from Mercury than from the Moon," and cites only a personal communication. As stated above, in 1983, it was thought the reason no lunar meteorites had been found yet was because dynamical problems for deriving meteorites from the Moon were too severe. But with the discovery of this meteorite, that argument is no longer valid, and similar arguments regarding Mercury might also not be valid. The third paper in the series (Treiman & Drake, 1983) makes a very interesting point regarding the discovery of the lunar meteorite. "Because the meteorite has, in effect, provided us with another mission to the Moon, it is important to determine what the meteorite's source was like and how the source compares with known sites from which lunar samples have been returned." This is a unique way to view the discovery of ALHA 81005; from a scientific point of view, it is a free Apollo mission. And this is just what the paper does; try to place this sample within the geological contexts established for lunar localities from the previously analyzed Apollo and Luna samples. The bulk of the first three papers is a petrographic and geochemical description of three different thin sections. However, the first was without much context; it was mainly petrographic descriptions of a thin section and statements about the meteorite without any connection from one to the other. The next two papers instead describe the thin section specifically in relation to other lunar samples. They both utilize petrographic plots and locate ALHA 81005 among other lunar and planetary analyses showing quite clearly how different planetary bodies are distinguished on the basis of things like Fe/Mn ratios, Cr compared to Fe, Ti, & Mg, pyroxene quadralaterals, etc. These sorts of analyses are the things referred to in the introductions of many of the other papers, and here, finally, is the evidence. It is not until the third paper that the method of analysis (electron microprobe) is mentioned. These papers, as well the others in the series discussing thin sections, generally agree about the petrography of the sample, with one exception. Some report the existence of oxide minerals, and some point out that their thin section is oxide-free. This highlights the importance of taking all the evidence together in understanding the sample in any geophysical problem, and not relying on a single 200 mm2 thin section, as it may not tell the whole story. One thing none of these papers discuss is why different planetary bodies have consistent, predictable, and classifiable geochemical markers. In other words, they never say why samples from the Moon have a distinguishing Fe/Mn ratio, only that they do. The 5th paper (Ryder & Ostertag, 1983) does not provide much new petrographic insight, but it presents some information in a completely different way. It gives a scaled cartoon sketch of the whole thin section (Figure 1), showing the different textural elements in different patterns. Most of the other papers describing thin sections waste a great deal of space with photomicrographs. Just as with the photograph of the whole sample, the photomicrographs are black-and-white, not well focused, and with confusing contrast. It is nearly impossible to see in them what the authors are trying to indicate. The sketch in this paper provides a much more useful representation than all of the photomicrographs. Further, a sketch of this sort provides more than just textural information, it gives the reader a good idea of the modal petrology. This is in contrast to Table 1 in the 4th paper (Simon et al. 1983) which lists the petrological components and their volume %. The sketch gives the same information, but in a graphical as opposed to verbal way. Some of the later papers in the series are focused on specific kinds of analyses. These are the analyses that were referred to as evidence in the early papers. The 7th paper (Mayeda et al. 1983) discusses measurements of oxygen isotopes in the sample. On the basis of oxygen isotopes alone, the sample origin could be narrowed down to the Earth, Moon, or aubrite asteroids. Mineralogy easily rules out aubrites (which are primarily enstatite), and solar wind exposure rules out the Earth. It seems like this would have been an easy way from the beginning to determine the lunar origin, even without any Apollo or Luna samples for comparison. The 8th paper (Bogard & Johnson, 1983) report the total and relative abundances of noble gasses and the abundances of cosmogenic isotopes. It also makes an interesting observation, "ALHA81005 is the first meteorite to be decisively identified with a specific parent body." The 9th paper (Tuniz et al., 1983) gives irradiation history based on 26Al and 10Be. This paper concludes that the sample spent less time in space than an asteroidal meteorite. Specifically, it spent on the order of 100,000 years in space and 600,000 years in Antarctica. The 11th paper (Sutton & Crozaz, 1983) calculate the transit time using thermoluminescence and nuclear particle tracks, and find that the sample spent 2,500 years in transit, in strong contrast to the 100,000 years reported above. The 15th paper (Laul et al., 1983; the one I waited patiently through 14 other papers for) finally explains the sample in a way geochemists (and not meteoriticists) are familiar with: rare earth elements, positive Eu anomaly, siderophile patterns, and elemental correlations (V/Cr, Mn/Fe, La/K). Following the comment of Treiman & Drake (1983) that this sample represents another mission to the Moon, a natural question to ask is where exactly it came from. Based on the KREEP (potassium, rare earth elements, phosphorous), Na, Th, and Ti contents of ALHA 81005, it is clear that this lunar meteorite did not come from any of the regions sampled Luna and Apollo. Because of the overall anorthositic composition and basaltic clasts, the specimen is likely to originate from a region proximal to both highlands and maria. The dating techniques also imply that the origin crater must be quite young. Two papers offer specific suggestions: crater Eimmart A (Treiman & Drake, 1983 paper) and crater Giordano Bruno (Ryder & Ostertag, 1983). Ryder & Ostertag give a fairly convincing argument, more so than that given by Treiman & Drake, however it is based on very little evidence, and suffers from a lack of global data. Pieters et al. (1983) give a much more thorough discussion of possible origins based on all the available data. They rule out most locations on the nearside, and suggest that ALHA 81005 probably comes from the nearside limb or the farside. However, they do constrain the source location mineralogically, specifying what signatures from remote sensing would indicate the source region. When global mapping would happen later, this paper would serve as a roadmap. Little time will be spent in this criticism regarding the overall organization in this series of papers. The authors may or may not have had input as to the order of the papers, and I will assume that point to be moot. However, the authors did decide to present their results as a series of 18 short papers, instead of a few long papers. One downside to many short papers is that there is no single good citation for the phrase "ALHA 81005 came from the Moon". Another downside is that multiple papers attack the same issues in different ways (e.g. location of the original crater) or attack different issues using the same techniques (e.g. overall petrology). This can be problematic because in order for a reader to understand all whole story about the origin or the petrology or any other aspect, the reader must read all 18 papers. A further downside is that in many of the early papers that only describe thin sections, many arguments for lunar origin are quoted without any evidence or further discussion. For example, oxygen isotopes are mentioned frequently, but it is not until the 7th paper when the results of the oxygen isotope analysis are actually given. An upside of the short papers is that many of the papers cite each other, vastly inflating the number of times each one is cited. Some of these papers come out of the starting block already cited 10 times. But this gives a disadvantage to a group like the oxygen isotope group, whose results are integral in establishing the lunar origin, yet they will not be the ones cited for the phrase "ALHA 81005 came from the Moon".

References:

Marvin, U. B. (1983). The Discovery and Initial Characterization of Allan Hills 81005: The First Lunar Meteorite, GRL, 9, 775-778.

Bogard, D. (1983). A Meteorite from the Moon. Editorial. GRL, 9, 773.

Warren, P. H., G. J. Taylor, and K. Keil (1983). Regolith Breccia Allan Hills A81005: Evidence of Lunar Origin, and Petrogrophy of Pristine and Nonpristine Clasts, GRL, 9, 779-782.

Treiman, A. H. and M. J. Drake (1983). Origin of Lunar Meteorite ALHA 81005: Clues from the Presence of Terrae Clasts and a Very Low-Titanium Mare Basalt Clast, GRL, 9, 783-786.

Ryder, G. and R. Ostertag (1983). ALHA 81005: Moon, Mars, Petrography, and Giordano Bruno, GRL, 9, 791-794.

Simon, S.B., J. J. Papike, and C. K. Shearer (1983). Petrography of ALHA81005, The First Lunar Meteorite, GRL, 9, 787-790.

Mayeda, T. K., R. N. Clayton, and C. A. Molini-Velsko (1983). Oxygen and Silicon Isotopes in ALHA 81005, GRL, 9, 799-800.

Bogard, D. D. and P. Johnson (1983). Trapped Noble Gases Indicate Lunar Origin for Antarctic Meteorite, GRL, 9, 801-803.

Tuniz, C., D. K. Pal, R. K. Moniot, W. Savin, T. H. Kruse, G. F. Herzog (1983). Recent Cosmic Ray Exposure History of ALHA 81005, GRL, 9, 804-806.

Sutton, S. R. and G. Crozaz (1983). Thermoluminescence and Nuclear Particle Tracks in ALHA-81005: Evidence for a Brief Transit Time, GRL, 9, 809-812.

Laul, J. C., M. R. Smith, and R. A. Schmitt (1983). ALHA 81005 Meteorite: Chemical Evidence for Lunar Highland Origin, GRL, 9, 825-828.

Pieters, C. M., B. R Hawke, M. Gaffey, L. A. McFadden (1983). Possible Lunar Source Areas of Meteorite ALHA81005: Geochemical Remote Sensing Information, GRL, 9, 813-816.