| アブストラクト |
It is essential for constraining the thermal history of the Moon to determine flow volumes of mare basalt according to age. However, there is no precedent study of direct estimation of basalt unit volume that is the total amount of lavas erupted during a series of magmatisms. At present, the vertical structures under the lunar maria are directly investigated using sounder observations. The Lunar Radar Sounder (LRS) onboard Kaguya (SELENE) detected widespread horizontal reflectors under nearside maria. The presumed depths of the detected reflectors are up to several 100s of meters. Previous studies suggest that these subsurface reflectors are interfaces between mare basalt units. We estimate volumes of each lithofacies with in the ages of about 3 to 4 Ga in the nearside maria. We use SAR-processed LRS data for these analyses. The SAR processing enhances the echoes from the subsurface reflectors and removes clutter noises from surface topography. Comparison between the subsurface reflectors and surface age maps allow us to discuss the volume of each basalt unit and its space and time variation. Estimated unit thicknesses are from several 10s to a few 100s of meters and correlate with their ages. Estimated single unit volume is an order of 1,000-10,000 cubic kilometers and clearly decreases with time with in a mare. This volume range is consistent with flow volumes derived from numerical simulations of a thermal erosion model for sinuous rilles formation and also those in Archean komatiite eruptions on the Earth. It suggests that the scale of lunar magma chambers of the nearside in this period (approximately 3 to 4 Ga) is comparable to that of Archean terrestrial magma chambers. |
