Habitability: Tides Tell the Tale

by Paul Gilster on October 14, 2008

How tides affect habitability has become a sub-genre within exoplanetary studies, a theme pushed hard by the gifted trio of Brian Jackson, Rory Barnes and Richard Greenberg (University of Arizona). You may want to browse through earlier Centauri Dreams entries on their work, especially this fascinating take on habitability around M dwarfs, in which the authors consider the possibility that Gliese 581 c was once a relatively benign place, but is now in an orbit that renders life impossible. Orbital evolution is the broad issue, sustained complex life demanding planets with low eccentricities. And orbital evolution can take a lot of time to operate.

Now I see that Brian Jackson has presented new work on tides and habitability at the 40th annual meeting of the Division of Planetary Sciences in Ithaca, NY. Here we push into interesting questions about planets already inside a habitable zone that are nonetheless too hellish to support life, and planets outside that zone that seem too cold to sustain life, but may be able to do so because of tidal effects. Planets in elongated orbits (unlike those in our Solar System, where orbits are relatively circular) undergo tidal stretching when near their star, an effect that diminishes as they move away from it. The result: Friction, generating internal heat that keeps the planet geophysically active.

What we find is that we shouldn’t be too quick to make judgments based upon the presence of liquid water at the surface. Take a planet up to ten times as massive as the Earth — a ‘super Earth’ — and consider the effects of tides upon conditions there. A likely prospect is extreme volcanic activity, which can render a planet already within a star’s habitable zone more like Jupiter’s moon Io than our mild and pacific Earth. This can occur even at relatively low eccentricities. Here’s Jackson on what we may discover when we study the first extrasolar terrestrial planets:

Given the wide range of masses and eccentricities that potentially give rise to extreme volcanism, we might expect that many terrestrial planets will be too volcanically active for life… [T]he most massive terrestrial planets may also be the most heated and thus the most volcanically active. Since the first extra-solar terrestrial planet that is likely to be confirmed will probably be much more massive than the Earth, we might expect it will be volcanically active. Such volcanic activity may be recognizable in the planet’s atmospheric transmission spectrum, similar to Io, whose tenuous atmosphere is largely made of sulfur…

This is drawn from the paper on the team’s work, which also examines the flip side of these effects, that a planet that might otherwise be too cold to sustain life may benefit from tidal effects, which would cause the outgassing of volatiles that could keep its atmosphere viable. For that matter, planets with an ocean under a crust of ice — we can think about Europa as the nearest analog of this process — could maintain warmer water temperatures than would otherwise be possible. Throw in plate tectonics, which can stabilize planetary atmospheres and surface temperatures, and you may have generated what you need to produce a functional biosphere.

And here’s an interesting scenario: A planet whose tidal evolution makes it pass through alternating periods of heating and cooling, such that it may go through an early habitable period, possibly including the development of life. And then, after a long period in which life has been extinguished because of volcanism, the same world may once again become habitable when its orbit circularizes. Thus two separate epochs for the emergence of life may occur on the same planet, although occurring billions of years apart.

The range of outcomes is extensive, as the paper’s summation suggests:

Even with the many simplifying assumptions employed here, these results suggest a wide range of geophysical scenarios. As advancement is made in the understanding of the processes of tidal evolution, in modeling of the geophysics of hypothetical planets, and eventually in the discovery and characterization of actual terrestrial-type planets, these calculations will need to be revisited. In any case, the calculations here show that tidal heating has the potential to be a major factor in governing the internal structures, surfaces and atmospheres of extra-solar terrestrial planets. Accordingly, the effects of tidal heating must be given consideration when evaluating the habitability of such planets.

We shouldn’t be too doctrinaire about the exact specifications for a living planet. As our own system shows, even bizarre outliers like Enceladus may eventually show evidence of a second outbreak of life within reachable distance of our own. Who knows how wide a range of living planets we may find as we zero in on exoplanetary systems? The paper is Jackson et al., “Tidal Heating of Terrestrial Extra-Solar Planets and Implications for their Habitability,” accepted for publication in Monthly Notices of the Royal Astronomical Society and available online.

philw1776 October 14, 2008 at 21:27

Habitability indicating simplistic life forms is one thing. The Earth was in that category for 80% plus of its existance but these extreme time varying environments do not allow time enough for the development of complex life forms, what we would call animals and a vegetated landmass, stuff that happened here only ‘recently’.

It’s become clear in our discovery of the wide variation in extrasolar systems that Earthlike planets, planets with mild invariant conditions over several billion years, suitable for the evolution of land animals and plants must be quite uncommon even in a vast galaxy.

Adam October 15, 2008 at 5:34

Hi All

philw1776’s points are sound – if Earth isn’t an outlier. Consider Venus – it’s believed to have begun with a relatively shallow ocean, compared to Earth, but that’s not necessarily a disadvantage. The high hydrogen loss rate at the top of Venus’s atmosphere may well have created an oxygenated planet long before Earth. Thus complex life might well have sprung into being early on Venus, before the planet was dessicated. Such a scenario may well have played out time and time again… but we don’t yet know. Not enough data.

I know Fermi’s long shadow covers all such discussion, but as an argument against complex life it’s still a presumption born from ignorance.

yeti101 October 15, 2008 at 17:30

i’m still hopefull of finding systems like our ours. Roll on the kepler telecope in 2009

philw1776 October 16, 2008 at 10:26

Stating that Earth is an outlier does not make it so, q.v. the vast variety in solar systems only recently known to science and with previously unexpected chaotic characteristics. Ignoring the only data point we have, life’s slow multi-billion year evolution to complexity on Earth and making wild hypotheticals about Venus to establish a philisophical view about life elsewhere is unscientific. The universe is not what you want it to be, it is what it is.

Dave Moore October 16, 2008 at 19:23

I read the paper and one thing the authors didn’t consider was that some types of volcanism are more detrimental to life that others. A majority of Earth’s internal heat is carried away by mid-oceanic crustal formation, which is relatively benign in terms of ecosystem disruption. Basaltic heat-plume volcanism as in the Hawaiian Islands is also fairly benign. Only when you get massive eruptions like the Deccan traps in India does it severely disrupt the world’s ecosystem.

It is the explosive Andersitic lava’s caused by plate subduction that is really destructive, the huge ash showers can lead to global freezing and major extinction events. Tambora in Sumatra 80,000 years ago nearly wiped out the human race.

Given this, ocean planets could tolerate very high rates of volcanism as still maintain habitability. Even planets with continents but without plate tectonics could tolerate a higher rate of volcanism without causing life to go extinct. I also read (skimmed) through the paper on volcanism on massive Earth-like planets, and it is suspected that plate tectonics shuts down on planets with high heat fluxes as the oceanic mantle doesn’t have time to cool and is just too warm and buoyant to turn over.

So while human habitability may be moot on planets with a high rate of volcanism, life may well thrive on such planets.

Yeti101 mentions he’s still hopeful of finding planets like ours. I think we will have no trouble finding Earthlike planets, that is planets with Earth’s approximate mass, composition and stellar flux, but that planets like Earth, such that we could inhabit them will be very rare.


Richard Greenberg October 17, 2008 at 14:33

Thanks for the nice coverage of our work on the role of tides in understanding exoplanets and their potential habitability, as well as for your kind comments about Brian, Rory and me.

I also want to remind you that tides are the key to what makes Europa so special. My new book “Unmasking Europa: The Search for Life on Jupiter’s Ocean Moon” has just come out. It may be of interest to your readers. They can ask for it at their local book store or order it from Amazon.

Administrator October 20, 2008 at 16:11

Unmasking Europa is certainly on my reading list, especially in regard to those tidal forces that make it such a tantalizing place!

Andy November 25, 2008 at 4:17

How do we get back on your website to previous blogs?

Administrator November 25, 2008 at 9:31

Andy, the best bet is to use the archives on the main page. They’re set up by the month. You can also use the search function for specific topics.

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