- 4120 - WATER ON PLANETS? Liquid water on Rocky Planets could be 100 times more likely than we thought. It’s easy to think of Earth as a water world, with its vast oceans and beautiful lakes, but compared to many worlds, Earth is not particularly wet.
--------------------- 4120 - WATER ON PLANETS?
- Even the icy moons of Jupiter and Saturn
have far more liquid water than Earth. Earth is unusual not because it has
liquid water, but because it has liquid water in the warm habitable zone of the
Sun.
-
- Water is one of the more common molecules
in the universe. Hydrogen is the most abundant element in the cosmos, and
oxygen is easily produced as part of the stellar CNO fusion cycle.
-
- So we would expect water-rich planets to
be plentiful in stellar systems. But that isn’t to say liquid water will be
plentiful. In our solar system, two kinds of worlds have liquid water. Earth
and gas giant moons.
-
- Like other warm terrestrial planets such as
Venus and Mars, Earth had liquid water in its youth. Mars was too small to
retain its water. Much of it evaporated into space, while some froze into its
surface crust. Venus was large enough to retain water, but its extreme heat
boiled much of it off into its thick atmosphere.
-
- We still aren’t entirely sure how Earth
managed to retain its oceans, but it was likely a combination of a strong
magnetic field and an extra helping of water from asteroids and comets during
the heavy bombardment period.
-
- The icy moons of Jupiter and Saturn are
another story. They were far enough from the Sun that they retained the water
of their formation. They quickly formed a thick layer of ice to prevent water
from evaporating into space. But these moons are small worlds and would have
very quickly frozen solid were it not for the tidal forces exerted by their gas
giant.
-
- Since cold gas planets are likely to have
icy moons, the general thought we would be far more likely to find life on a
Europa-like world than an Earth-like one. But this new study argues that liquid
water is much more likely to be found on super-Earths.
-
- How icy super-Earths could have liquid
water oceans. On the large end, they are likely to be gassy worlds with thick
atmospheres. On the small end, they are likely to be more Earth-like.
-
- Based on the exoplanets we’ve found so far,
super-Earths are by far the most common. And a majority of them are likely to
be outside their star’s habitable zone in the cold regions of the star system.
So they are likely water-rich. But they also aren’t likely to be found orbiting
a gas giant, so it’s generally been assumed that their ice layer would be
mostly frozen solid over time.
-
- The reason has to do with the various
freezing and melting points of ice. The kind of ice we have on Earth melts at
around 0 °C. But this is only true at around Earth’s atmospheric pressure. At
higher pressures, there are several varieties of ice with differing melting
points. Generally at higher pressures
ice can have a much higher melting point. So even if a super-Earth is
geologically active, it might not be warm enough to melt ice.
-
- This new study shows that super-Earths
don’t have to be hot enough to create a deep ocean. Through geothermal and
nuclear heating it can melt a thin layer of water at its surface, and thanks to
fissures and various water phase transitions water can creep up to the layer
just below the frozen surface. This process would be enough to create a rich
ocean layer of liquid water. Since the heat of a super-Earth lasts billions of
years, it could maintain a liquid ocean long enough for life to evolve.
-
- Based on what we know about exoplanets,
super-Earth oceans could be 100x more common than those of Earth-like worlds or
icy moons. And that means life has even more possible homes than we thought.
-
-
August 17, 2023 WATER ON
PLANETS? 4120
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Thursday, August 17, 2023 ---------------------------------
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