Solid rocket motors are made up of a casing, typically steel, filled with a solid propellant.
According to the NASA Jet Propulsion Laboratory, the Mars Sample Return campaign is a “proposed mission to return samples from the surface of Mars to Earth. The mission would use robotic systems and a Mars ascent rocket to collect and send samples of Martian rocks, soils and atmosphere to Earth for detailed chemical and physical analysis.”
— Scott MacFarlane (@MacFarlaneNews) September 17, 2019
It also describes the MAV as a “two-stage vehicle,” requiring “two solid rocket motors of varied design.” The motors would need to withstand “environments of launch, space transport, Entry, Descent and Landing and residence on the Martian surface for a period of time (approximately up to one year) prior to activation.” NASA also outlines specific characteristics it is seeking for its rocket motors.
NASA also notes that the procurement request is only for planning purposes at this point in time.