Heck, Mars is so oxidized it has soils that are 0.5% perchorate: https://en.wikipedia.org/wiki/Perchlorate#On_Mars
and that stuff's so oxidizing it's not even common on Earth outside of fireworks and rocket propellants
What are the current thoughts on what caused oceans on Mars to disappear (with some remainder concentrating in ice caps)? Was it some impact of a really big meteor/comet? It is accepted that Mars used to have liquid water oceans and rivers in the past, correct? You can see geological remains of rivers and oceans, for example, quite clearly on Mars.
It seems increasingly likely that there was originally quite a bit of liquid water on Mars during the Noachian (https://en.wikipedia.org/wiki/Noachian) and Hesperian (https://en.wikipedia.org/wiki/Hesperian) -- the Martian geological time periods more or less contemporaneous with what we call the Archean Eon on Earth. It's not clear that this would have been quite as extensive as "oceans" yet, but it's definitely possible.
As to why the water was lost, it basically all boils down to "Mars is too small". Too small to have a strong gravitational field that makes escape velocity hard for gas molecules to achieve, too small to have enough primordial+radiogenic heat driving a magnetic field to protect from solar wind, and too small to have primordial+radiogenic heat to drive plate tectonics and the silicate weathering feedback (https://doi.org/10.1029/JC086iC10p09776) that stabilizes liquid water on Earth on billion year timescales
As for how: ions and molecules reaching escape velocity. For water in particular, a lot of this would have been through hydrogen escape, which actually happens on Earth too (just slower). If you photolyze H2O, it's very easy to lose the H because it's so light, thermally or by solar wind, etc. There's more info on the how here: https://en.wikipedia.org/wiki/Atmospheric_escape
That's why those schemes for terraforming Mars seem so unrealistic. In order to sustain an atmosphere over the long term we'd have to keep dropping ice comets down the gravity well. There's no way to make it self sustaining.
oh, forgot about this paper (https://www.nature.com/articles/nature05873) which makes a pretty reasonable argument for paleo-shorelines on the northern basin of the Martian dichotomy. If this is correct then it'd be entirely reasonable to call the body of water responsible an "ocean"
Yep! Plus the lack of a magnetic field, which is largely also due to Mars being too small, and losing heat too fast to have any liquid metal convecting in the core anymore
I wonder if it's the solar wind or just the escape velocity vs. the termal escape.
I'm sure in a couple of centuries humans would be able to build superconducting magnets powerful enough to shield Mars from the solar wind (those would double as energy storage as well), but we can't do anything about the fundamental properties of gases and escape velocity.
So how can we Terraform Mars to make it like Earth then? It seems impossible due to the gravity being so small. Even if we melt ice caps and get liquid water running again, it will slowly evaporate to space? How could a colony on Mars then be self sustaining with this problem?
As the other reply mentioned, this is a millions of years sort of problem. But there are two straightforward solutions. First, in the new atmosphere make sure there's enough oxygen to breath so that an ozone layer forms. That's something we want to do anyways. Also, give Mars a magnetic field. That's a stupendously large but straightforward engineering problem, sort of like terraforming is in the first place but smaller.
Heck, Mars is so oxidized it has soils that are 0.5% perchorate: https://en.wikipedia.org/wiki/Perchlorate#On_Mars and that stuff's so oxidizing it's not even common on Earth outside of fireworks and rocket propellants