A new study published today in the Proceedings of the National Academy of Sciences reveals that the circulation in fresh water or salty oceans on terrestrial exoplanets would influence their temperatures.
This artist’s concept depicts a super-Earth. Image credit: NASA / Ames / JPL-Caltech.
Until recently, computer simulations of habitable climates on terrestrial planets have mainly focused on their atmospheres.
But studying their oceans is vital for understanding climate stability and habitability – as on our own Earth.
Not only this, but until now, scientists had not considered that the seas on Earth-like exoplanets might not be quite like ours – they might be significantly more or less salty than Earth’s oceans.
“The number of planets being discovered outside our Solar System is rapidly increasing,” said co-author Prof. David Stevens, of the University of East Anglia.
“Our research helps to answer whether or not these planets could sustain alien life.”
“We think that many planets may be uninhabitable because they are either too close or too far from their Sun.”
“A planet’s habitable zone is based on its distance from the Sun and temperatures at which it is possible for the planet to have liquid water.”
Oceans play a key role in sustaining life and also have an immense capacity to control planetary climate.
But previous studies on ocean circulation on extrasolar planets have made the assumption that fundamental ocean properties – such as the salinity and depth of water – would be similar to that on our planet.
“We wanted to find out what might be happening on other planets which might appear superficially similar to Earth, but where conditions such as salinity are radically different to Earth,” Prof. Stevens said.
He and his colleagues used computer models of ocean circulation on exoplanets to see what would happen when their oceans had different salinity levels to Earth.
The team considered oceans with very low salinity (salinity similar to the average value of Earth’s oceans) and high salinity (similar levels to the Dead Sea).
“On Earth, we have a circulation where warm water moves towards the poles at the surface, before being cooled, then sinking at high latitudes and traveling towards the equator at depth,” said co-author Dr. Manoj Joshi, also from the University of East Anglia.
“Our research shows that oceans on other planets with a much higher salinity could circulate in the opposite direction – with polar water flowing towards the equator at the surface, sinking in the tropics and traveling back towards the poles at depth.”
“We also found a similar pattern emerging for freshwater oceans.”
“These circulation patterns are the opposite of what happens on Earth, and would result in a dramatic warming in the polar regions.”
“Such a circulation scenario might extend the planet’s range of habitability.
“Of course, on any given exoplanet, many other properties are likely to differ from their Earth-like values, some of which may also have a significant influence on ocean circulation – such as tidal forces, planetary rotation, ocean depth and the location of continents,” said study lead author Jodie Cullum, also from the University of East Anglia.
“But this is important work which will help us better-understand the habitability of distant planets in more accurate detail than ever before.”
_____
Jodie Cullum et al. Importance of ocean salinity for climate and habitability. PNAS, published online April 4, 2016; doi: 10.1073/pnas.1522034113
