To amplify on what you said, I think it is important to note that MARIE returns in situ radiation data from Martian *orbit*, not at the surface. Any surficial radiation models based on MARIE data are *extrapolations* based on our current models of Mars' atmospheric density, not "remote sensing" observations of surface radiation from orbit. To be fair, though, the MARIE data on "Radiation on Mars, Hits per Cell Nucleus," which were released at the March 1 press conference, were tagged as "estimates." That said, a familiarization with the instrument and its specs should make clear that the instrument is only reading radiation that enters its detectors from source emitters, and not "reflected" from the Martian surface. Positing the radiation flux at the Martian surface depends on the particular model that one uses for global/local magnetic fields, atmospheric density, etc., and MGS has returned some very good data that constrain these parameters. Therefore, I would not be surprised if the surface readings are close to what the MARIE team "estimates" from cruise and from orbit; however, to be precise one has to realize that these data are NOT surface readings. So, for example, using MARIE data to "infer" atmospheric density at a particular locale is circular reasoning in that atmospheric density is one of the data inputs used for the MARIE surface extrapolation.

As an aside, one might note that under the original Mars Surveyor 2001 mission concept, which consisted of both an orbiter and lander/rover combination,
MARIE had an orbital and surface component that would have enabled simultaneous measurements.

Alex R. Blackwell
University of Hawaii