ESA Graduate Trainee positions have been advertised at https://jobs.esa.int .

There are >100 vacancies listed all across ESA for 1 year of employment at ESA sites, often extendable to 2 years, for graduates with Masters-level qualification.

Some of the posts relevant to Mars missions include:
Mars Orbiting Missions Operations Unit (ESOC, DE)
Systems Engineering for a Mars Precision Lander (ESTEC, NL)
Planetary Radio Occultation (ESTEC, NL)
ExoMars/Rosalind Franklin Mass Spectrometry Instrument Optimisation (ESTEC, NL)
Machine Learning & Accessibility for Space Science (ESAC, ES)
Positioning, Navigation and Timing for Lunar Mars Exploration (ESTEC, NL)
Human and Robotic Exploration Programme Management Support (ESTEC, NL)
Simulant Research and Development (Vulcan Facility) (ECSAT, UK)
Development and Integration of Tools for Supporting Field Science Activities (ESTEC, NL)
Science Mission Payload Preparation (ESTEC, NL)

More information at https://www.esa.int/About_Us/Careers_at_ESA/Apply_now_to_be_an_ESA_YGT

Application deadline is 7 March.