In October 2014, the close encounter between Mars and comet Siding Spring produced a transient ionized layer in the upper atmosphere composed primarily of Mg⁺ ions. The layer was detected by instruments on three spacecraft, including the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on Mars Express. Analyses of the MARSIS data indicated the transient layer persisted up to ~19 hours after the comet’s closest approach. We report MARSIS observations that suggest the transient layer lasted at least 7 days – and potentially as long as 32 days – after closest approach. During this period, the transient layer was mostly confined to a narrow latitude range between 20°N-60°N and a longitude range spanning 275°E to 95°E. Since this period coincided with a highly active Sun, we discuss how solar flares may have contributed to the layer’s prolonged lifetime.