Investigating Lunar Ionosphere Using GRAIL Radio Science Signals

The nature of the dense lunar ionosphere is controversial; the maximum electron density values in observed vertical profiles obtained from previous Lunar missions vary by two orders of magnitude. NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission consisted of an identical pair of spacecraft approximately 100 km apart in a circular polar orbit around the Moon at a mean altitude of 55 km during the science phase in 2012. The two GRAIL spacecraft conducted various radio science observations to determine the lunar gravity field. As a serendipitous consequence of these primary observations, the one-way spacecraft - Earth radio occultation observations of the lunar ionosphere were acquired using a carrier-only X-band radio signal from the Radio Science Beacon referenced to an onboard ultrastable oscillator (USO). GRAIL’s X-band Radio Science beacon (RSB) data provide applicability for the radio occultation of the lunar electron density profiles with the uncertainty of frequency residual measurement ~ 1 mHz corresponding to ~  2x10⁸ m^-3 electron density uncertainties. We will present our analysis of the electron density profiles retrieved from GRAIL Radio Science Beacon data to understand the formation and variations of the moon ionosphere. The findings of GRAIL results will improve our understanding of how those variations are spatial (e.g., latitude, longitude, solar zenith angle) or temporal (e.g., responses to external factors, such as meteor impacts and solar winds) during the GRAIL mission period.