A habitable (human could survive in its atmosphere without suits) desert planet which used to have large oceans is quite common in ScFi. I was thinking about the processes which could have been responsible for the loss of water.
I want to leave biological processes—like sandworms from Dune, out. Likewise, I'm aware of technological solutions, like orbital rings, but I want something natural. Also, water swallowing holes in the ground don't work out either and I want real water loss, not just deeper oceans and more land.
So I came up with with 3 ways how a planet might lose a significant portion of its surface water. For all discussions, I want to use an earth twin (0.02% by mass surface water) as reference, unless mass or orbital parameters need to be altered to make something happen. The definition for desert planet I'm gonna go by is less than 10% surface coverage by water.
- Atmospheric stripping and subsequent ocean loss, like it happened on Mars. But this destroys the atmosphere and further outgassing would bring up water again.
- Stripping via impacts. To achieve this, we need to hit the planet with a rock (which has at least the mass of the oceans) at escape velocity. Due to several rather obvious issues the true figure is going to be several orders of magnitude higher. Also, I aim to create a habitable desert world and not a lava planet/asteroid belt.
- Break up the water bonds with strong radiation (from the star or an nearby event). But aren't the odds of survival on that world terrible during such an event? Furthermore I am aware that unicellular organisms deep within the crust could survive all of those events, but I want my survivors to be macroscopic and multicellular.
So how does such a transition happen in the least destructive way?