Why are there no terrestrial planets with a subsurface ocean?

  • Subsurface oceans in satellites are pretty common: Europa, Enceladus, Ganymede, Callisto, maybe Pluto... This is due to tidal heating of their host planet, Jupiter and Saturn, which heats up the inner ice of those satellites. However, planets don't exhibit this inner ice layer, so they don't usually have subsurface oceans (except Pluto or Ceres, if you can call them "planets"). Why is that? Only small bodies like satellites present this inner layer of ice? Is there any Earth-type exoplanets that exhibit this inner layer of ice that could potentially melt down to liquid water? And if there is, why some cold planets have inner ice layers and others don't?

    Earth apparently has a subsurface reservoir of water greater than all the oceans combined; https://www.newscientist.com/article/dn25723-massive-ocean-discovered-towards-earths-core/

  • James K

    James K Correct answer

    2 years ago

    The terrestrial planets are Mercury, Venus, Earth and Mars. Mercury and Venus are too hot for liquid water to exist at any level, Mars has lost nearly all its water and Earth has a surface ocean, not a subsurface one. The inner planets lost most of their volatiles (including water) as they formed, the water on Earth was provided by later icy asteroid impacts.

    So none of the terrestrial planets have a sub-surface ocean. The other planets are gas and ice giants. Uranus and Neptune likely have liquid layers surrounding their cores, composed of water, ammonia and other "ices"

    To get a subsurface ocean you need a planet that is beyond the frost line (the distance from the sun at which ice is stable in space) and in our solar system the planets beyond the frost line are either dwarfs or giants.

    In a sense, the Earth does have a subsurface ocean, only it isn't a water ocean, it is an ocean of molten iron. The outer core of the Earth is highly fluid; it's no more viscous than water.

    Among exoplanets, there are several candidate ice planets. Wikipedia lists OGLE-2005-BLG-390Lb, OGLE-2013-BLG-0341L b and MOA-2007-BLG-192Lb. (The principle way of discovering small planets that orbit far from their host star is by microlensing events, hence many of the candidate planets were found by the Optical Gravitational Lens Experiment, or OGLE)

    So what you say is, the formation of planets/satellites with inner ice layers is posible behind the snow line? Because Mars actually didn't lose all the subsurface ice, actually it's believed that there is some subterranean ice, not a layer. Maybe the reason that Mars doesn't have an ice layer is because when it was formed, the ice couldn't sustain its solid form...

    Yes, there is a little subsurface water on Mars, but Mars is not cold enough to have a thick ice shroud, like Ceres, and not big enough to hold onto what water it had after its formation, like Earth. exoplanets like OGLE-2005-BLG-390Lb, are distanct enough from their stars that they must be icy.

    @Carlos There's some info about the amount of water / ice on Mars in my answer to this question: https://astronomy.stackexchange.com/q/32484/16685

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