I was clicking through YouTubes and came across the NASA Video ISS Expedition 53 Landing in Kazakhstan (also here). I don't recall seeing such prominent and continuous smoke from returning Soyuz capsules before. Is this in fact smoke? What exactly is the source (smoldering heat shield, steam?), and is this phenomenon standard/typical?
It's a purge of remnant hydrogen peroxide from the "secondary control system".1 "On the “ОТСТРЕЛ ЛОБОВОЙ ТЕПЛОЗАЩИТЫ” (Bottom Shield Jettison) all УРМД thruster valves are opened and all the peroxide residuals are drained out."3
A secondary guidance, navigation and control system in the Descent Module enables the crew to maneuver the vehicle after the Instrumentation/Propulsion Module has been jettisoned. The Soyuz commander can pilot the module using a rotational hand controller that manages the firing of eight hydrogen peroxide thrusters on the vehicle's exterior. This system is deactivated 15 minutes before landing, when the parachutes are deployed. 2