There is a relationship between the salinity of Earth's oceans and its climate. Salinity can have a dramatic effect on the climate of any Earth-like planet orbiting a Sun-like star. But what about exoplanets orbiting M-type dwarf stars?
Each planet has a measurable amount albedothe percentage of starlight that is reflected back into space. It is measured on a scale from 0, which would be a black object that reflects no light, to 1.00, an object that reflects all light. Since a higher albedo reflects more starlight, it has a cooling effect on an object's climate. In our Solar System, Saturn's moon Enceladus has the highest albedo. Enceladus is covered in bright, reflective ice that reflects most of the sunlight that hits it. (Note that there are different measurements for albedo and they can be quite different, leading to some confusion.)
Mercury has the lowest albedo because it is mostly covered in dark rock. (Objects such as comets can have even lower albedos.)
Earth's albedo is about 0.3, largely due to our planet's cloudy atmosphere. Ice from Antarctica, Greenland, and seasonal Arctic ice also contribute. Earth's albedo changes over the seasons as ice expands and retreats. In short, Earth's albedo helps regulate the planet's climate.
Ocean salinity levels affect how much sea ice forms, and in turn affect Earth's albedo. The more salt there is, the lower the freezing point falls, making it harder for ice to form. Higher salinity means less ice, which means the albedo is lower and less sunlight is reflected back into space.
But how would ocean salinity affect exoplanets orbiting stars other than our Sun? That is the question behind a new study titled “Climate effects of ocean salinity on M dwarf exoplanets.The lead author is Kyle Batra of Purdue University's Department of Earth, Atmospheric and Planetary Sciences. Batra is also a member of NASA's Ocean Worlds Network exooceanography team.
Dwarves M They are also called red dwarfs and their light is different from that of the Sun. Much research has been done on ocean salinity and its overall effect on Earth's climate. According to the authors, research is lacking when it comes to red dwarf exoplanets. “However, how ocean composition affects climate under different conditions, such as around different types of stars or at different positions within the habitable zone, has not been investigated,” the authors write.
M dwarf exoplanets are particularly important when it comes to the study of exoplanets and their potential habitability. M dwarfs are low-mass stars that have extremely long and stable lifespans, which is beneficial for their potential habitability. M dwarfs are also the most abundant type of star, so logic dictates that they host the most rocky planets, and observations show that they host few gas giants.
The researchers worked with several key variables in their models, including how instellation changes over a star's lifetime.
The researchers used an oceanic atmosphere general circulation model (GCM) to investigate how M dwarf and G-type stars like our Sun respond to ocean salinity. The results show that stars like our Sun respond most dramatically to changes in ocean salinity. “We find that increasing ocean salinity from 20 to 100 g/kg in our model results in nonlinear ice reduction and warming on G-type planets, sometimes causing abrupt transitions to different climate states,” they write.
As on real Earth, simulations of G-type stars showed that sea ice was restricted to high latitudes and that its coverage decreased as salinity increased. Coverage went from 19.5% at 35 grams of salt per kg to 3.5% at 100 grams per kg. This is an abrupt transition.
The transitions were less abrupt on M dwarf planets. “In contrast, sea ice on M dwarf planets responds more gradually and linearly to increasing salinity,” they write.
The researchers also determined how salinity and ice cover affected surface temperatures. On Earth, the average surface temperature increased from 8 °C to 14 °C as salinity increased from 35 to 100 grams/kg. M dwarf planets did not show a similar increase in surface temperature.
“Moreover, reductions in sea ice on M dwarf planets are not accompanied by significant surface warming as on G planets,” they explain.
Planets in habitable zones around M dwarfs share another characteristic. Since the habitable zone around an M dwarf is much closer to the star than around a Sun-like star, many of the planets are expected to be tidally locked. That affects everything about their climates.
“In this scenario, sea ice is even less coupled to planetary albedo than in our Earth-like simulations because nightside ice would not interact with incoming radiation,” the authors explain.
In a tidal locking scenario, oceanic and atmospheric mixing has more dynamic variables. “Therefore, under different rotation and circulation regimes, climate sensitivity to salinity may differ,” the researchers explain. They leave the investigation of such scenarios to future research.
These results are very interesting, but unfortunately we won't have the chance to compare them with observations in the near future, because we can't even remotely detect ocean salinity. In fact, we're not even sure that what look like exoplanets with oceans have oceans. But at least this work shows what effect ocean salinity can have on the abundant rocky planets orbiting M dwarf galaxies.
“This is an encouraging result suggesting that uncertainties regarding exoceanic salinity are less of a concern for understanding the climates and habitability of M dwarf planets compared to G planets,” they conclude.
