Jonathan Tucker AST 330: Moon Darby Dyar 16 November 2009 Apollo 13 Lunar Heat Flow Experiment Langseth et al. 1970. Critical Summary This paper describes the equipment, measurement techniques, and interpretation of a heat flow experiment that was to be placed on the Moon on the Apollo 13 mission. Obviously, this experiment was never performed--this paper was published April 10, 1970, and "Houston, we've had a problem" occurred four days later. There will be two focuses of this criticism: first to explore why such a detailed description of an instrument and experiment is necessary prior to deployment, and second to discuss the utility of a paper describing an experiment that was never performed. In terms of background, the paper presents the physics of planetary heat flow with special regard to comparative planetology. Heat flow is a surprisingly simple concept in geophysics, and the mathematics presented in this paper are very straightforward. Although these physics are hardly representative of the whole field of planetary heat flow measurement and interpretation, this paper is not the place for a lengthy discussion of geophysics. As it is, this paper in Science provides a very good background for a non-geophysicist of these topics. The paper in fact goes much further than an introduction to planetary heat flow measurements. It also discusses a reasonable range of expected results, and what the interpretation of those results would mean in relation to the internal structure of the Moon. Planetary heat flow on Earth is one of a few ways including gravimetric, resistivity, and seismic measurements to probe the near subsurface. These kinds of measurements are usually made over as wide an area as possible, as area covered is related to the depth probed. The astronauts on the Apollo 13 mission would not have had the time to set up the two heat flow probes at multiple distances, so these geophysicists and engineers designing the experiment had to employ a very clever probe design to be able to make more than just a single measurement. It is not clear whether these authors actually invented the probe design, but regardless they recognize and are forced to address a problem that would not even give geophysicists on Earth pause. This could be a reason a paper such as this is published in Science. It represents the first time heat flow measurements are to be made on another planetary body. On Earth, geophysicists have the luxury of relying on a suite of measurements, all of which are relatively easy to make. But future missions that go to any planetary body might not be able to set up resistivity or gravimetric or seismic experiments, and might only be able to set up two permanent heat flow probes. Thus the problems faced by this group will undoubtedly be faced by many others in the future. It is uncommon currently for an instrument or experiment to be described in such detail in a prestigious journal like Science before the mission is launched. But especially with more lunar landings planned at the time of this paper's publication, the future applicability of this work is clear. It is always useful to have a single reference explaining an instrument or experiment prior to deployment, so that papers presenting results and similar experiments and instruments in the future can cite a single all-encompassing reference. The paper actually goes farther than describing this one experiment, it also defends lunar heat flow measurement. As mentioned above, heat flow is one of many measurements that canprobe the subsurface structure. But the astronauts' time on the lunar surface is extremely valuable and setting up this experiment requires drilling two ten foot boreholes, placing the probes in the holes carefully, connecting the electronics, and testing them. Why, then, are heat flow measurements a good use of the the astronauts' time, especially since the surficial heat flow on Earth can vary widely? The paper answers this particular question by explaining how the bulk of these variations are due to processes unique to the Earth, such as plate tectonics, and thus a restricted measurement on the Moon is expected to be much more representative of the whole Moon than an equivalent measurement on the Earth. Unlike other kinds of measurements, with this setup the scientists have the ability to perform heat flow measurements long after the astronauts have left the Moon--up to a year, they say. Additionally, they have the ability to bring back samples from this location to validate the heat flow measurements and further constrain some of the unknown variables. This discussion will end with a brief reflection on an "orphaned" scientific paper. Obviously this particular experiment was never performed, as Apollo 13 never landed. However, it was probably known at the time that this, or very similar experiments would be done during the other Apollo landings. This paper could have been a guide or roadmap for all future extraterrestrial heat flow measurements. But it did not become that. This paper is cited few if any times. A few years later, the authors published a very similar paper in the journal Earth, Moon, and Planets (Langseth et al. 1972). That paper is nearly identical to this, just replace "Apollo 13" with "Apollo 15". That paper, which does not even cite this one, is the one that became the roadmap for extraterrestrial heat flow measurements. The importance of those measurements is clear, as the results of that experiment (Langseth et al. 1976) are cited ubiquitously. References: Langseth., M.G., A.E. Wechsler, E.M. Drake, G. Simmons, S.P. Clark., and J. Chute. (1970). Apollo 13 Lunar Heat Flow Experiment, Science, 168, 211-217. Langseth, M.G., S.P. Clark, J.L. Chute, S.J. Keihm, and A.E. Wechsler (1972). The Apollo 15 lunar heat-flow measurement, Earth, Moon, and Planets, 4, 390-410. Langseth, M.G., S.J. Keihm, and K. Peters (1976). Revised lunar heat-flow values, Proc. Lunar Sci. Conf. 7th, Houston, TX,