Observations of the subsurface geology of the Moon help advance our understanding of lunar origin and evolution. Radar sounding from the Kaguya spacecraft has revealed subsurface layers at an apparent depth of several hundred meters in nearside maria. Comparison with the surface geology in the Serenitatis basin implies that the prominent echoes are probably from buried regolith layers accumulated during the depositional hiatus of mare basalts. The stratification indicates a tectonic quiescence between 3.55 and 2.84 billion years ago; mare ridges were formed subsequently. The basalts that accumulated during this quiet period have a total thickness of only a few hundred meters. These observations suggest that mascon loading did not produce the tectonics in Serenitatis after 3.55 billion years ago. Global cooling probably dominated the tectonics after 2.84 billion years ago.

[1] We constructed an empirical model of the electron density profile with solar zenith angle (SZA) dependence in the polar cap during geomagnetically quiet periods using 63 months of Akebono satellite observations at solar maximum. The electron density profile exhibits a transition at ∼2000 km altitude only under dark conditions. The electron density and scale height at low altitudes change drastically, by factors of 25 (at 2300 km altitude) and 2.0, respectively, as the SZA increases from 90°to 120°. The SZA dependence of the ion and electron temperatures is also investigated statistically on the basis of data obtained by the Intercosmos satellites and European Incoherent Scatter (EISCAT) Svalbard radar (ESR). A drastic change in the electron temperature occurs near the terminator, similarly to that in the electron density profile obtained by the Akebono satellite. The sum of the ion and electron temperatures obtained by the ESR (∼6500 K at ∼1050 km altitude under sunlit conditions and ∼3000 K at ∼750 km altitude under dark conditions) agrees well with the scale height at low altitudes obtained from the Akebono observations, assuming that the temperature is constant and that O + ions are dominant. Comparisons between the present statistical results (SZA dependence of the electron density and ion and electron temperatures) and modeling studies of the polar wind indicate that the plasma density profile (especially of the O + ion density) in the polar cap is strongly controlled by solar radiation onto the ionosphere by changing ion and electron temperatures in the ionosphere during geomagnetically quiet periods.

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[1] Characteristics of photoelectron flows and presence of a field-aligned potential drop on the open magnetic field lines in the polar cap are systematically investigated using the data obtained by the FAST satellite during geomagnetically quiet periods in July 2002. We found high occurrence frequencies of the potential drop larger than $10 V, reaching $90% (small field-aligned current (FAC) case) and $83% (all data). A typical magnitude of the potential drop above $3800 km altitude is $20 V. This value is significantly larger than the potential drop below $3800 km altitude (probably $1-3 V), although the typical potential drop is smaller by a factor of $2-3 in comparison to the modeling results that suggested presence of a field-aligned potential jump at several earth radii. The net escaping electron number flux negatively correlates with the upward electron number flux and with the magnitude of the potential drop. This relation is contrary to expectation from photoelectron-driven polar wind models that an increase in the photoelectrons drives the larger polar wind flux, since the net escaping electron number flux balances the flux of polar wind ions under zero net FAC conditions. An increase in the upward backscatter of reflected electrons with an increasing potential drop may explain the negative correlations. A potential drop at high altitudes would provide a polar wind system regulated by a negative feedback, and the most appropriate balance for polar wind ions would be achieved near the median of the reflection potential.