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Detail of one of the lander's three terminal descent engines; this particular engine is located at the under the surface sampler boom.

NASA's Viking Mission to Mars Fact Sheet contains the following description of the engine:

Three terminal descent engines provided attitude control and reduced the lander’s velocity after parachute separation. The 2,600-newton (600-pound) throttleable engines were located 120 degrees apart on the lander’s sidebeams. They burned hydrazine monopropellant.

The engines used an advanced exhaust design that wouldn't alter the landing site environment. An unusual grouping of 18 small nozzles on each engine would spread engine exhaust over a wide angle that wouldn't alter the surface or unduly disturb the chemical and biological experiments.

Two spherical titanium tanks, attached to opposite sides of the lander body beneath the RTG wind covers, fed the descent engines from an 85-kilogram (188 pound) hydrazine propellant supply.