Comments on ?lunar thermal anomalies and internal heating? by J. M. Saari

Abstract

Saari makes three principal points in his article: (1) The linear thermal anomaly is not real; (2) The anomalous thermal behavior of Mare Humorum as a whole cannot be explained by internal heat flow; and (3) even vigorous internal heat flow will produce so small a AT at the surface that it could not be detected radiometrically. With regard to point number one, Saari suggests that our instrument sensed only a gradient. In our article, we acknowledged a potential gradient problem, but the presence of the linear thermal feature in the lunar afternoon, when there is no gradient at the appropriate location, led us to conclude that the anomaly is real. Saari suggests, on the contrary, that thermal scans across Mare Humorum during the lunar afternoon (shown in his Figure 2) do display a significant thermal gradient. Indeed, a steep gradient is seen on his scan 32F, but it is in the wrong direction - i.e. down, rather than up. Our instrument can differentiate between a cold gradient and a warm one, and in fact, did detect the gradient in Saari's curve. It represents the cold face of the cliffs on the western edge of the Mare Humorum basin, as can be seen in Figure 1. The linear anomaly lies further to the east, as can also be seen in Figure 1. In short, Saari's gradient is in the wrong location and in the wrong direction to be our linear anomaly, and is clearly displayed on our images for what it is. Consequently, we must again conclude that our linear thermal anomaly is real. With regard to point number two, we did not suggest that the entire Mare Humorum thermal anomaly is caused by internal heat. Like Saari, we doubt that it is. With regard to point number three, we did suggest that the linear thermal anomaly might be caused by internal heat, and we question Saari's statement that the detection of lunar internal heat flow comparable to that of "the most anomalous region on the Earth" is "far beyond the detection limit of modern infrared detectors". This statement seems questionable when one remembers that internal heat flow is routinely detected on earth from aircraft and even satellites (Friedman and Williams, 1968). The sharp discrepancy between what is clearly possible on earth and what Saari calculates is possible on the Moon can be traced directly to the assumptions upon which his calculations are based. First, he assumes a maximum heat flow that is two orders of magnitude below that measured for warm spring areas in Iceland (Bodvarsson, 1960), and five orders of magnitude below that measured for an erupting volcano. Second, and much more important, he assumes negligible thermal contact (a layer