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Roger Weller, geology instructor
Geologic Life Support
by Tressa Mackin
Geologic Conditions Required for
Harboring Life on Other Planets
Image Credit: hubblesite.org/gallery/album/star/pr2012001d/xlarge_web/
The existence of over eight hundred and fifty planets outside our solar system
has been confirmed within the past seventeen years . Given this, an
immediate question arises: Which of these planets, if any, has the potential to
support life? The complete answer requires information beyond the scope of both
this paper and current available data. Nonetheless, recent advances in
technology have yielded enough information to rule out many of these newly
discovered planets as inhabitable.
Since life, as we know it, appears rare in the universe, the geologic
characteristics conducive to habitable conditions can best be identified by
examining our own planet.
It should be noted that our knowledge and definition of life is limited by our own experience. Should sulfur or nitrogen based life forms exist, water might not be necessary for metabolic processes, and so our search criteria would no longer include liquid water, for example.
Based on this limited interpretation of “life” it is reasonable to assume that planets geologically similar to Earth could harbor life. Therefore the search for life on other worlds has been narrowed to those planets which are close to the surface temperature, density, and atmospheric and composition of Earth.
Exoplanets: A brief interlude
Extrasolar planets (or “exoplanets”) have been discovered by monitoring different aspects of light received from distant stars. Due to the gravitational tug of one body on another, a star warbles slightly when it is orbited by a planet. This “warble” can be observed in spectroscopic measurements in the form of a Doppler shift . Using Kepler’s laws, Newtonian physics, and the known mass of the star, the mass of the planet can be determined.
the orbital plane of such a planet intersects with the line of observation from
Earth, a portion of the star is covered and some of its light is blocked. (Of
course, anything which impedes the light from a star could be interpreted as a
planet or some other object orbiting a star, so each object of this kind is
subject to rigorous scrutiny before categorized as an exoplanet.) The light
curve observed during this type of event can be very informative .
The slope of the light curve reveals the eccentricity, or degree
of similarity to an ellipse, of the planet’s orbit . The planet’s relative
size can be determined by examining the amount of light blocked and the size and
temperature of the star . The radial velocity measurement, or the “warble,”
coupled with some information given by the light curve, gives the planet’s mass
. Day and night temperature of the planet can be determined from the light
flux recorded during transit. Since some light emitted by the host star is
absorbed by the atmosphere of the planet during transit, the light received from
the star is slightly different. From this difference, the atmosphere of the
planet can be analyzed .
Of these planets, several are within the “Habitable Zone,” or have the potential
to support life according to surface temperature estimates. However there are
additional requirements necessary for harboring life, such as liquid water, a
molten interior and plate tectonics.
Earth’s position relative to our sun is just right for retaining liquid water.
If Earth were farther away, our oceans would surely freeze. Any closer and total
evaporation would occur. This equilibrium defines what has been called the
“Goldilocks Zone” or the “Habitable Zone.” These terms can be misleading,
however, since a planet within the Habitable Zone is not necessarily right for
life. For additional criteria we look to Earth’s composition and structure.
Although Earth’s complete interior structure remains a mystery (rock is a very
affective visible light blocker), it is known that Earth is differentiated into
a solid crust, hot liquid mantle and outer core, and rocky metallic core. This
makeup is essential to life as we know it.
The movement of material in Earth’s liquid outer core and molten mantle, coupled
with movement in the ionosphere, produces a regenerative magnetic field .
This magnetic field deflects harmful bursts of energy from our sun as well as
other sources in space which could drive away our atmosphere. Without such a
protective layer, rays from the sun would ultimately halt organic reproduction,
and life forms would either seek refuge in Earth’s crust or die out entirely.
Mars’ current state is said to have resulted from the hardening of its interior,
weakening of its magnetic field, and eventual evaporation of its oceans .
A fluid interior allows for plate tectonics as well. Crustal movement acts as a
means of recycling materials by restoring nutrients needed by various organisms.
As nutrient poor, and dense, material sinks into the mantle, other areas are
pushed upward exposing “newer” and nutrient rich material. Plate tectonics
recycle carbon which has been stripped from the atmosphere and deposited in the
soil in a process known as the carbon cycle . Bacteria then digest the carbon
as it sinks into the mantle, and volcanism ejects carbon gas back into the
atmosphere through a process known as outgassing.
Plate tectonics require liquid water in order to weaken rock. This means that
where there is movement in the crust, there is liquid water . It seems that
two components of life go hand in hand.
The internal composition of Earth has been deduced from seismic imaging, a
process which uses the properties of sound waves to determine density at a
particular depth, and chemical analysis. Waves will travel at velocities related
to the temperature and elasticity of the medium through which it travels. Using
known densities and bulk sound velocities of elements and common compounds, the
composition of Earth has been determined on a general scale . Since
densities of exoplanets can be determined, the composition can be inferred.
However, in order to determine whether an exoplanet’s interior is differentiated
and/or molten, new technology must be developed.
Earth is a dense, rocky planet comprised mostly of silicates with an iron core.
Of the exoplanets discovered thus far, there are more dense rocky planets than
hot gassy Jovian type planets (which would be easier to find due to their size)
. However, these dense planets are not necessarily within the habitable
Image Credit: The Habitable Exoplanets Catalog- Planetary Habitability
Laboratory @ UPR Arecibo, 2012
The ultimate discovery of an Earth-twin has not yet been achieved, but a planet orbiting Alpha Centauri B has given astrophysicists hope. This planet is much closer to its star than Earth is to our sun, but its size and mass are very close to that of Earth. Alpha Centauri B exhibits behavior similar to our sun, and there exists a strong possibility of it being host to other planets which could orbit in the habitable zone .
The question of Earth’s uniqueness has been a subject of pure speculation until very recently. Since the first exoplanet discovery in 1995, an entirely new science has been developed. Astronomers, astrophysicists and planetary geologists are now able to derive information about worlds orbiting stars in the far reaches of our galaxy that can be used to determine habitability. We are living in an era when questions of science fiction are being seriously considered.
Life could exist elsewhere, but elsewhere is a big place. As far as geology is
concerned, a high density, molten interior, and plate tectonics are essential to
the production and support of life as we know it. These criteria narrow the
“elsewhere” in question, but it will be a few years before planetary geologists
and astrophysicists will be able to infer the interior structure of exoplanets.
Therefore the mystery remains and the search continues.
Cosmic Jackpot, Paul Davies, 2007, Houghton Mifflin Company, NY
Exoplanet Mass Calculated Directly
Fifty New Exoplanets Discovered by HARPS
The Search for Exosolar Planets
The Transit Light Curve
 The Habitable Exoplanets Catalog- Planetary Habitability Laboratory @ UPR Arecibo, 2012
 Lindergren, L., Dravins, D., 2003, ESO, Astronomy & Astrophysics, 1185-1188
 Holman, M. J., et al., 2007, The Astrophysical Journal, ed. 655:1103-1109
 Kipping, D., 2008, Mon. Not. R. Astron. Soc., ed. 389:1383-1390
 Mandel, K., Agol, E., 2002, The Astrophysical Journal, ed. 580:L171-L175
 Rogers, L. A., Seager, S., 2010, The Astrophysical Journal, ed. 712:974-991
 Sabaka, T. J., Olsen, N., Langel, R. A., 2000, NASA/TM-2000-209894
 Gibson, E. K., et al., 2010, Early Mars: A warm wet niche for life, Astrobiology Science Conf., 5062.pdf
 Paulen, L., 2009, Plate Tectonics Could be Essential for Life, Astrobiology Magazine, http://www.astrobio.net/exclusive/3039/plate-tectonics-could-be-essential-for-life
 McDonough, W. F., 1995, The Composition of the Earth, Department of Earth and Planetary Sciences, Harvard University, 5-21
 Udry, S., Pepe, F., Lovis, C., Mayor, M., 2009, Astrophysics and Space Science Proceedings, Science with the VLT in the ELT Era, ed. A. Moorwood