Candidate gas sources
Gas could rise through cracks. Seasonal ground ice might block its escape.
Dust and water chemistry can produce HCl. A model matched two earlier summer profiles, but also predicted gas elsewhere.
Measured from orbit
Sunlight passing through the atmosphere carries the gas signature to the orbiter. The spectrum alone does not identify its source.
The test for either explanation: account for the detections and comparable observations with no detectable HCl.