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Ocean Worlds
 
Earth. A world dominated by water. Trillions of gallons flow freely across the
surface of our blue-green planet. While we once thought oceans made our planet
unique, we're now coming to realize that `ocean worlds' are all around us.
 
https://www.youtube.com/watch?v=gw_bX0ZQOy0
 
Our planet retains its atmosphere and in turn its abundance of liquid water
thanks, in part, to a very strong magnetic field, which provides protection
from the solar wind. Without this magnetic field our atmosphere would be
stripped away leaving the pale blue dot looking more like. one of our
neighbors? Scientists believe Venus' early oceans evaporated. With no water
left on the surface, carbon dioxide built up in the atmosphere, leading to a
runaway greenhouse effect that created present conditions. Likewise, Mars
appears to have had oceans long ago. But the Red Planet's global magnetic field
decayed, leaving it vulnerable to atmospheric erosion by the solar wind.
 
Farther from the sun in the sub-freezing temperatures of the outer solar
system, it may seem impossible for liquid water to exist. However, not only
does it exist, but it some places it may be more abundant than on Earth!
Jupiter is orbited by at least three moons that contain oceans; Europa,
Ganymede, and Callisto.
 
Europa is crisscrossed by cracks and semi-rectangular features that look like
ice rafts frozen into the surface. Below those immobile icebergs, researchers
believe there is a vast ocean just below the icy crust warmed by the tidal
forces of Jupiter, containing about twice as much water as is found on Earth.
In 2014 and 2016, the Hubble Space Telescope has observed what appear to be
water vapor plumes coming out of cracks near the south pole.
 
Photos of Ganymede from the Galileo space probe show ancient ice flows frozen
into its surface.  Moreover, Hubble has looked at Ganymede's auroras and seen
signs suggesting an ocean's worth of salt water. Hiding beneath a thick crust
of ice, the ocean on Ganymede could actually harbor as much as four times more
water than all of Earth's oceans combined. Callisto also seems to contain a
salt water ocean beneath the icy crust, betraying its presence by the effects
of Callisto on Jupiter's overlying magnetic field.
 
In 2005, NASA's Cassini spacecraft found Saturn's moon, Enceladus, busily
puffing plumes of water vapor and organic compounds out through fissures (now
known as "tiger stripes") in its frozen carapace. Cassini flew through the
plumes frequently since then, and has discovered nanosilica grains and the
presence of molecular hydrogen, both suggesting the movement of heated water on
the seafloor of the icy moon. Other evidence provided by Cassini has convinced
researchers that Enceladus has a global subsurface ocean spewing into space
through these tiger stripes. Saturn's E Ring, the planet's second outermost
ring, was actually formed from this water and ice!
 
Saturn's largest moon, Titan, has a landscape dotted with lakes and seas. The
liquid on Titan's surface is not, however, H2O. Researchers believe the fluid
sculpting Titan's surface is a mixture mostly of methane with smaller amounts
of ethane, and other hard-to-freeze hydrocarbons.
 
Other bodies around the solar system also show signs of liquid water.
 
In 2014, scientists using the European Space Agency led Herschel space
observatory detected water vapor coming from two regions of the dwarf planet
Ceres. NASA's Dawn probe reached Ceres in 2015 and while the water vapor had
subsided, there were other signs of water. Ahuna Mons is an ice mountain
apparently formed from repeated eruptions of salty muddy water. Also,
widely-reported bright spots in Ceres' crater Occator are thought to be
deposits of salt left behind by the escape and sublimation of briny water from
below.
 
Even distant Pluto may be an ocean world. As revealed by the recent flyby of
New Horizons, Pluto's strangely molded surface features suggest the presence of
a liquid underground.
 
As astronomers look beyond our solar system they are finding exoplanets of
sizes and distances from their stars that could have oceans. And based on our
solar system these exoplanets could potentially have moons with oceans as well.
The locations of water within the diverse environments of our own planetary
system will guide and inform the search for oceans beyond our solar system.
 
Next time you look out over the ocean, think about our neighboring worlds. They
may have more in common with our own ocean world than we once believed was
possible.
 
For more news about oceans-at home and abroad-stay tuned to science.nasa.gov
 
 
Regards,
 
Roger

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