RADIO ASTRONOMY INVESTIGATIONS OF THE MOON
Introduction

This web page is devoted to a summary of my radio astronomy investigations of the moon, conducted during the 1960s and 1970s.  The principal result of these investigations is the finding that the uppermost several inches of the moon's "soil" is very similar over the entire moon's front surface.  This adds confidence that certain Apollo in situ findings can be extrapolated to essentially the entire moon; in other words, Apollo findings relating to the thermophysical nature of the lunar soil can be viewed as having global significance.  Finally, the moon maps show that one dark region, Mare Tranquilitatis, is more absorbing of microwaves, and this is most easily accounted for by hypothesizing that it contains more titanium-containing mineral ilmenite.

The set of maps of the moon's microwave thermal radiation remain unequaled to this date.  Also unsurpassed to the present are the measurements and interpretation of the amplitude and shape of the moon's brightness temperature over a lunar month, and the dependence of this variation with wavelength.  The interpretation of this "lunation data" was greatly aided by the contributions of my able moon collaborater Stephen Keihm.

The moon is a difficult object to observe in spite of its large signal, which is due to several factors unique to the moon.  First, the location of features on the moon are constantly changing due to the earth's rotation (this is called "lunar libration").  Second, the moon's coordinates in the sky are always changing fast, due both to its orbital motion and the changing position of an observer on the earth's surface with respect to the nearby moon.  Another difficulty is the large angular size of the moon, which requires corrections for effects related to antenna pattern sidelobes, which are not necessary for almost all other radio sources.  Finally, quality moon maps require that corrections be made for atmospheric absorption and its changes during the observing session.

I devised a method for mapping the moon using a radio telescope that overcomes some of these problems; it's called "spokes of a wheel" scanning.  All scans go through the moon center, each at a different scan direction orientation, or clock angle.  Since all scans go through the center of the moon, which can be assumed to have the same brightness temperature during the observing session, compensation for changes in atmospheric absorption are easily accomplished.  Moreover, a plot of the moon center brightness temperature versus air mass allows a determination to be made of the average atmospheric absorption during the observing session, and also allows a check to be made of absorption trends during the session.  Also, sidelobe effects can be deduced from spokes of a wheel scans much easier than from the traditional raster scan mapping procedure since the "off-moon" scan path is always perpendicular to the moon's edge.

Interest in studying the moon "waxes and wanes" with decade timescales.  Since the "Man to the Moon" Apollo program ended, lunar studies have waned.  When humans begin to establish settlements there, initially for scientific purposes, interest will wax.  However, by then much of the techniques for studying the moon from earth will have been forgotten.  There may someday be a measureable merit in preserving some of this expertise, and that is one of my purposes in creating the following web pages.

The remainder of this web site consists of links to articles.

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1. "Radiometric Mapping of the Moon at 3 Millimeters Wavelength"
Bruce Gary, Joseph Stacey, and Frank Drake
The Astrophysical Journal, Supplement Series, Nr. 108, pp 239-262, November, 1965.

Full moon map of 3-mm microave brightness temperature (physical temperature at about 1 inch below surface), showing that hottest location is not at the "noon" longitude but is at about the "2 pm" longitude.

Link to first page of this article, including the abstract.

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2. "Results of a Radiometric Moon Mapping Investigation at 3 Millimeters Wavelength"
Bruce Gary
The Astrophysical Journal, Vol. 147, No. 1, January, 1967

Full moon image with areas subjected to separate regolith properties model solutions.

Link to first page of this article, including the abstract.

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3.  "Interpretation of Ground-Based Microwave Measurements of the Moon Using a Detailed Regolith Properties Model"
Bruce L. Gary and Stephen J. Keihm
Proc. Lunar Planet. Sci. Conf. 9th (1979), p. 2885-2900

Spectrum of lunar regolith microwave absorption coefficient deduced from lunation and eclipse measurements.

Link to first page of this article, including the abstract.

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4.  "Comparison of Theoretical and Observed 3.55 cm Wavelength Brightness Temperature Maps of the Full Moon"
Stephen J. Keihm and Bruce L. Gary
Proc. Lunar Planet. Sci. Conf. 10th (1979), p. 2311-2319

Anomaly map at full moon, showing large positive Tb anomaly centered on Mare Tranquilitatis.

Link to first page of this article, including the abstract.

Return to calling web page.

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This site opened:  July 26, 2001 Last Update:  June 23, 2007