A recurring question during the development of BepiColombo has therefore been to identify the science issues for which there will be a major delta in knowledge after Messenger.
BepiColombo implements a two spacecraft concept: a nadir pointing, low eccentricity spacecraft, the MPO, for investigating at close range the planet Mercury itself and its inner exosphere, and a high eccentricity spacecraft, the MMO, dedicated to studies of the global exosphere and magnetosphere of the planet. The comprehensive dedicated payload and spinning attitude of the BepiColombo MMO gives it a clear-cut advantage over the 3-axis pointing Messenger spacecraft for studying the environment of the planet.
There are significant deltas between the Bepicolombo MPO and Messenger in terms of payload, in particular a thermal IR imaging spectrometer and a much higher accuracy positioning thanks to a dual uplink- downlink in X and Ka bands.
The strongest assets of the MPO are however its low eccentricity orbit, nadir pointing mode and much higher data rate. The low eccentricity orbit and nadir pointing mode provides global coverage at similar spatial resolutions. The low eccentricity orbit also increases by a large factor the average counting rates of the gamma-ray and X-ray spectrometers compared to the much higher apocenter distance of Messenger. The much higher data rate has a direct impact on effective spatial resolution.
One of the main science asset of BepiColombo will be the simultaneous investigations of the planet and its environment, as important issues such as the intense space weathering expected on Mercury, the impact of the dipolar field on the magnetosphere or the closure of magnetospheric currents at the very surface of the planet for lack of an ionosphere require close cooperation between these two types of investigations.
The conclusion is that there is a lot of science ground to cover for what is by far the least well known of inner planetary bodies with a sequence of two missions with different scopes and resources, Messenger and BepiColombo.