Complex sentence: On 21 October 2015, Oxia Planum was chosen as the preferred landing site for the ExoMars rover assuming a 2018 launch, but since the launch was delayed until 2020, Aram Dorsum and Mawrth Vallis will also be considered.
Split into simpler sentences: On 21 October 2015, Oxia Planum was chosen as the preferred landing site for the ExoMars rover assuming a 2018 launch. If the launch is delayed until 2020, Aram Dorsum and Mawrth Vallis will also be considered.