The key point is that the rust that gives Mars its red color was thought to form in a dry period, after mars lost its water:
> Previous studies of the iron oxide component of the martian dust based on spacecraft observations alone did not find evidence of water contained within it. Researchers had therefore concluded that this particular type of iron oxide must be hematite, formed under dry surface conditions through reactions with the martian atmosphere over billions of years – after Mars’s early wet period.
The new research suggests otherwise:
> new analysis of spacecraft observations in combination with novel laboratory techniques shows that Mars’s red colour is better matched by iron oxides containing water, known as ferrihydrite. Ferrihydrite typically forms quickly in the presence of cool water