Introduction to Icarus special issue 'From Mars Express to ExoMars'

Context. In February 2018, the international community working on the investigation and exploration of the atmosphere and surface of Mars met at ESAC near Madrid for a few days conference, organized and funded by the project UPWARDS (“Understanding Planet Mars With Advanced Remote- ensing Datasets and Synergistic Studies”, www.upwards-mars.eu) of the EU Horizon 2020 program. This project, and that conference, were devoted to promote the scientific exploitation of Mars Express data and to revisiting and sharing results from this mission, in preparation for the upcoming new European mission to Mars, the ExoMars-2016. The meeting approximately coincided in time with the end of the aerobraking phase of the Trace Gas Orbiter (TGO), which is the orbital element of ExoMars-2016. The TGO science phase started on 21 April 2018. This special issue of Icarus contains eleven papers which report original research results presented at the meeting or based on those presentations. This is obviously not an exhaustive representation of what was presented in Madrid, as other results have been published elsewhere. But they all share the motto of the meeting - from MEX to TGO - by presenting highlights of almost 15 years of Mars EXpress science legacy and new tools and methods for data analysis, thus paving the way for TGO observations and collaboration between two spacecraft. We think this is a valuable approach which, if properly and timely coordinated, can be very fruitful and should be promoted on every new space mission. The UPWARDS project could be considered as a bridge facilitating transition between two ESA missions, building team connections and enhancing science return from both missions. The second goal of the UPWARDS project, now more in the scientific than in the programmatic realm, was to exploit synergies between different teams in a cross-disciplinary approach, looking for an integral vision of the planet in an attempt to unveil couplings between different regions. In the meeting mentioned above, there were investigations from the subsoil and the surface, to the lower atmosphere's composition and dust, to the water cycle and up to the thermospheric structure and escape to space. In this framework, this issue presents its 11 contributions ordered in this bottom-to-top vision of the atmosphere.
We want to acknowledge ESA for successful operations of the Mars Express and the TGO spacecrafts. The project UPWARDS-633127 was funded by the European Union's Horizon 2020 research and innovation Programme.