A group of astronomers have derived a formula that tells us whether a subsurface ocean is present in an exomoon — moons beyond the Earth-moon system — and also how deep its murky depths go, according to a new study published in the journal Astronomy & Astrophysics.
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Astronomers derive equation for subsurface oceans
The search for extraterrestrial life has until recently focused on planets at an equitable distance from their host star — where liquid water can form on the surface. But inside the bounds of our solar system, most of the liquid water seems to exist outside of this conventionally-habitable zone. Moons orbiting cold gas giants heat beyond water's melting point from megalithic tidal forces.
The hunt for viable planetary systems thus spans a wider area if we include moons. And researchers from the SRON Netherlands Institute for Space Research and the University of Groningen (RUG) have found a formula that predicts where and how deep subsurface oceans are, reports phys.org.
Formulating the way to extraterrestrial life
In our solar system, moons show more promise than planets. Europa, Enceladus, and roughly six other moons of Jupiter, Saturn, Uranus, and Neptune could harbor subsurface oceans. All of them orbit far beyond the conventionally habitable zone — it's literally below freezing on their surfaces — but tidal interaction with their nearby host planet stretches and crushes the moons, heating up their respective interiors.
With moons under the cosmic microscope, exoplanet hunters like the forthcoming PLATO telescope — a project of SRON's — gain hunting ground in the search for life.
"According to the most common definition, our solar system has to planets with a habitable surface: Earth and Mars," said lead author Jesper Tjoa. "By a similar definition there are about eight moons with potentially habitable conditions below their surface. If you extend that to other planetary systems, there could be four times as many habitable exomoons as exoplanets."
Formulating subsurface oceans on exomoons
With this analysis in mind, Tjoa and his supervisors Migo Mueller (SRON/RUH/Leiden Observatory) and Floris van der Tak (SRON/RUG) derived an innovative formula that calculates a lower limit for ocean depth on exomoons. Relevant factors include the diameter of the moon, its distance from the host planet, the thickness of the gravel layer on the surface, and thermal conductivity of the ice or soil layer underlying the gravel. We can already measure the first two, but — beyond our solar system — the final two conditions require estimation.
While it's more difficult to suss out the status of life underground than it is for surface-dwelling life, it might be possible to catch a hint of life in the depths in the near future. On the subject, Tjoa said: "Observational astronomers study starlight shining through the atmospheres of exoplanets. They can for example identify oxygen. When they point future telescopes at exomoons, they may see geysers like on Enceladus, stemming from subsurface ocean. In principal you could recognize signs of life that way."
As the hunt for life expands with the future of space telescopes like TESS and the forthcoming James Webb Space Telescope, it's interesting to note that the life we seek could be closer than we thought.