Lunar radar sounder observations of subsurface layers under the nearside maria of the Moon

Details Lunar radar sounder observations of subsurface layers under the nearside maria of the Moon Lunar radar sounder observations of subsurface layers under the nearside maria of the Moon

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p13e..03k&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #P13E-03

Computer Science

Sound

5470 Surface Materials And Properties, 6250 Moon (1221), 6994 Instruments And Techniques (1241)

Scientific paper

The Lunar Radar Sounder (LRS) on-board the Kaguya (SELENE) spacecraft started the lunar surface and subsurface soundings since November 2007 in order to understand the origin and evolution of the Moon. Kaguya is in circular orbit with an altitude of 100 km and an inclination of 90 degrees. Orbital period is about 2 hours. The LRS system transmits a radar signal modulated from 4 MHz to 6 MHz with a pulse width of 200 microsecond and a peak power of about 800 Watts. The range resolution of LRS is 75 m in free space. The pulse repetition frequency of pulse transmission is 20Hz (Ono and Oya, 2000; Ono et al., 2008). After the operation for 10 month, the radar sounder observation covered almost whole area of the lunar surface. Based on the observations performed by Apollo Lunar Sounder Experiment (ALSE), it was reported that there are two reflectors at depths of ~1km and ~2km, or apparent depths of ~3km and ~6km, in Mare Serenitatis (Peeples et al., 1978). Therefore, we have checked LRS data obtained in Mare Serenitatis. However, we could not find such reflectors in the LRS data. Instead, we have discovered prominent reflectors lying at the apparent depths of a few hundred meters. Because the range resolution of ALSE was ~400 m, or ~1200 m in free space, it could not distinguish shallow reflectors as found by LRS. It should be noted that similar peak structures can be also formed by range sidelobes of surface echoes. However, we can conclude the peaks seen in the LRS data are not by range sidelobes. If they are sidelobes, (i) the power ratio of range sidelobe to mainlobe should be constant and (ii) the frequency offset of range sidelobe from mainlobe should also be constant. Neither of them is true in the LRS data. The comparison of B-scan images along parallel orbits also supports the conclusion. We also found that most of neaside maria have subsurface stratifications at depths of several hundred meters as seen in Mare Selenitatis. It suggests that they are common geologic structures in the lunar maria. Based on the intensity of echoes, we can also discuss the electric permittivity and loss tangent of surface medium.

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