“What keeps things stable on earth?” I ask Google.
Got good answers back.
First was gravity. Holds the atmosphere and water right here pretty much. In that stability, there’s the cycle we got.
The cycle begins with volcanoes spewing CO2 into the atmosphere, which helps keep the planet warm, thanks to the greenhouse effect. This warmth allows seawater to evaporate, forming clouds and rain. As the rain contains dissolved CO2 it is slightly acidic and so it reacts with surface rocks to dissolve carbon-containing minerals into the water.
This mixture is then washed out to sea, where the minerals build up and eventually precipitate out to form new carbon-containing rocks on the seabed. Sooner or later, plate tectonics carries these rocks into a subduction zone, where CO2 is baked out of them by heat of the Earth’s interior and later returns to the atmosphere via volcanoes.
This cycle turns out to be an extremely effective thermostat. When the planet is warm, rainfall increases, speeding the rate of atmospheric CO2 removal and cooling the planet. When it is cold, rainfall decreases, allowing volcanic gases to build up in the atmosphere, warming the planet.
Venus and Mars probably had similar thermostats early on. Venus, though,
If you need direction, you need gravity. Without it, you don’t know how to differentiate. It’s not so much that gravity holds us “down”: it shows us the way to go “up”.