Solid-solid hydration and dehydration of Mars-relevant chlorine salts: Implications for Gale Crater and RSL locations

•We perform low-temperature laboratory studies of perchlorate & chloride hydration.•Under certain lab conditions, salts can hydrate and dehydrate.•Salts at Gale Crater, Mars unlikely to both hydrate and dehydrate diurnally.•A different diurnal H2O vapor exchange mechanism is likely occurring at Gale.•Hydrated salts observed at recurring slope lineae on Mars may require recent brines.