Arecibo radar mapping of the lunar poles: A search for ice deposits

Science

Volumn 276, Issue 5318, 1997, Pages 1527-1530

  Stacy, N J S ^a   Campbell, D B ^b   Ford, P G ^c  

^a DEFENCE SCIENCE AND TECHNOLOGY ORGANISATION   (Australia)

^b Department of Astronomy   (United States)

^c MASSACHUSETTS INSTITUTE OF TECHNOLOGY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ARTICLE; GEOGRAPHIC DISTRIBUTION; IMAGE ANALYSIS; MOON; POLARIZATION; PRIORITY JOURNAL; RADIOFREQUENCY RADIATION; TELECOMMUNICATION;

ICE; LUNAR POLE; RADAR MAPPING;

EID: 0030617997     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.276.5318.1527     Document Type: Article

References (25)

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3. K. Watson, B. C. Murray, H. Brown, J. Geophys. Res. 66, 3033 (1961); J. R. Arnold, ibid. 84, 5659 (1979); J. D. Burke, in Lunar Bases and Space Activities of the 21st Century, W. W. Mendell, Ed. (Lunar and Plane-tary Institute, Houston, TX, 1985), pp. 77-84.
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14. B. A. Campbell, R. E. Arvidson, M. K. Shepard, J. Geophys. Res. 98, 17099 (1993), figure 11.
15. The incidence angle is the angle between the incident radiation and the local vertical (mean surface normal). Near the lunar poles, the incidence angle for Earth-based observations approaches 90°. An imaging radar system forms a map of the surface backscatter from range-Doppler measurements of backscatter. The projection of range-Doppler coordinates onto a surface at high incidence angles is similar to a regular grid in surface coordinates, resulting in a "plan view" of the surface backscatter.
16. G. L. Gutschewski, D. C. Kinsler, E. Whitaker, Atlas and Gazetteer of the Near Side of the Moon (NASA, Washington, DC, 1971).
17. E. M. Shoemaker, M. S. Robinson, E. M. Eliason, Science 266, 1851 (1994).
18. The two senses of circular received polarizations contain the OC and SC components of the backscatter signal. The OC component is the circular polarization sense expected from a single reflection with a plane interface, and the SC component is the orthogonal circular polarization. The main contributions to the backscatter signal are from single quasi-specular reflections (caused by mirrorlike reflection from smooth facets possibly many wavelengths in size) and diffuse scattering mechanisms (arising from wavelength-size surface and near-surface structures). These effects contribute to the OC signal and to both received polarizations, respectively. Backscatter from two successive quasi-specular reflections is not expected to contribute appreciably to the SC signal because of the low intrinsic reflectivity of the lunar surface.
19. In a pulsed-radar experiment, a short burst of energy is transmitted and the echo is received during the transmitter off time. The fraction of the time spent transmitting (the duty cycle) varied from 2% to 15% for our lunar observations.
20. T. Hagfors, Radio Sci. 5, 189 (1970).
21. Double-bounce scattering has been identified in radar observations of the crater Carlini (33.7°N, 24.0°W) in Mare lmbrium where forward scatter from the front inner rim is reflected a second time from the radar-facing inner rim (1), and in observations of terrace structures in the rim of the crater Copernicus.
22. 1/2.
23. Small features with high CPRs were identified as regions of adjacent pixels (in the up-down and left-right directions, not at 45°) with CPRs > 1.2 from the four-look data. If such a region was found to contain 10 or more pixels, then it was encompassed by a rectangle and the mean OC and SC backscatter of all the pixels within the rectangle and above a signal-to-noise ratio threshold were calculated (a threshold of 15 dB for the OC polarization was used for the south pole data analysis). The CPR for the region was calculated from the ratio of the rectangle mean SC and OC backscatter values. If the cumulative F distribution for the region polarization ratio had a value greater than 0.9773, then the region was accepted as a candidate high-CPR feature. (The cumulative F distribution was used here to test whether a CPR was significantly different from a value of 1.)
24. Most of the absolute backscatter uncertainty is attributable to systematic errors, which apply equally to both the SC and OC cross-section measurements. Consequently, the uncertainty in the CPR (= SC/OC) is almost entirely attributable to the statistical uncertainties in the SC and OC cross sections.
25. We thank P. Perillat, A. Crespo, A. Hine, and other support staff at the Arecibo Observatory who helped to make these lunar observations possible. Supported in part by NASA grant NAGW 3985 from the Planetary Geology and Geophysics program. The National Astronomy and Ionosphere Center is operated by Cornell University under a cooperative agreement with NSF, and is also supported by NASA.