The Viking Lander Terminal Descent Engine (3 per vehicle) was a ~600 lbf monopropellant hydrazine engine with an unusual exhaust system of 18 small nozzles. Multiple sources state that the reason for this design was to prevent exhaust plume erosion of the Martian surface below the lander, to prevent degradation of the science results.
(all annotations mine in these images)
What is the purpose of this metal tube connected to the center of the aft dome of the reactor chamber?
This paper shows a cutaway drawing of the engine, but doesn't label the tube.