What is the second major sink for the missing Martian water volume, besides escape to space?

Answer

Chemically bound or absorbed into surface rocks and minerals.

The second major sink is retention on the planet, where a substantial portion of the missing water volume is believed to be chemically bound or absorbed into the surface rocks and minerals through a process called hydration as the planet cooled and dried.

What is the second major sink for the missing Martian water volume, besides escape to space?
MarsSpacewaterplanetevidence