Water is a precious resource here on
Earth. Although the surface of our planet is 71% water, only a tiny
fraction of that is available for human needs, such as drinking, food
production, and sanitation. In fact, a massive 97.5% of Earth’s
total stock of water is saline, leaving only 2.5% freshwater, and 70%
of that freshwater is locked frozen in the polar ice caps.1
From Earth’s perspective, water is scarce.
However, water may in fact be prevalent
throughout the universe. NASA estimates that the Jovian moon of
Europa may hold 2 to 3 times the amount of Earth’s water. Venus and
Mars are also thought to have once had liquid surface water. It is
possible that Mars may still have liquid surface water, although this
is currently under debate.2
Since water is widely considered to be
a requirement for the existence of life, finding water beyond Earth
is of great interest to scientists conducting research in the field
of astrobiology, the study of life in space. Finding incontrovertible
evidence of extraterrestrial liquid water will bolster the idea that
life is widespread throughout the universe.
So far, Earth is the only planet known
for sure to have stable surface bodies of liquid water. However,
water molecules have been detected in the atmospheres of recently
discovered extrasolar planets (planets that orbit stars other than
the Sun), and there is evidence that Enceladus, one of the moons of
Saturn, may have liquid oceans beneath its frozen exterior.3
Our own surface water exists as a result of atmospheric pressure and
our stable orbit around the Sun. Other planets would probably have to
have similar conditions for liquid surface water to be present.
There are a number of ways to confirm
the presence of water vapor and ice on (or in the atmosphere of)
other astronomical bodies, the most accurate being absorption
spectroscopy and geochemistry. Detecting liquid water is a lot
harder, though. To ascertain whether an astronomical body has liquid
surface (or subsurface) water, scientists use a toolkit of detection
technques, making use of data pertaining to habitability,
gravitational pull, tidal theory, planetary differentiation and
radiometry.4
Water vapor is sometimes used as an
indicator of surface water, with some scientists inferring the
presence of liquid oceans from atmospheric water signatures, while
others have conjectured that the presence of subsurface water bodies
may also be inferred from this; geysers have recently been detected
on Enceladus, for instance, throwing out water vapor that may
originate in subsurface oceans.
Some planets and moons, therefore, are
likely to hold liquid water. But comets can also carry water, albeit
most commonly as water ice.5 It is possible that at least
some of the water on Earth may have been brought here by collisions
with small astronomical bodies such as comets. Dust samples from
comet Wild-2 have yielded evidence that liquid water may exist inside
some comets. Saturn’s rings also contain large amounts of ice, a
percentage of which is ejected there from the geysers on Enceladus.
However, the very existence of water on
some planets may be hindering researchers from finding it. A recent
paper argues that small, warm planets may commonly develop cloudy
atmospheres, which can unfortunately thwart attempts to observe the
atmospheres themselves, by scattering light and reducing the ability
to measure spectral absorption and thereby identify chemical
species.6
Given the amount of water sources already discovered within our own
solar system, and the rate at which water vapor is being detected in
the atmospheres of newly discovered exoplanets,7 it is
likely that the universe is literally awash with water. Perhaps one
day, when we have a self-sustaining space economy, we may be able to
make use of off-world water sources. Until then, though, water on
Earth is a precious commodity, and one which we must protect and
conserve.
By Jon Fern
References
1. Uncredited, 2006. Human
Appropriation of the World's Fresh Water Supply. University of
Michigan.
http://www.globalchange.umich.edu/globalchange2/current/lectures/freshwater_supply/freshwater.html
Accessed 24th September 2014.
2.
McEwen, A., et al., 2011. Seasonal Flows on Warm Martian
Slopes. Science.
http://www.sciencemag.org/content/333/6043/740.abstract
Accessed 24th September 2014.
3. O’Neill, C., et
al., 2010. The role of episodic overturn in generating the
surface geology and heat flow on Enceladus. Nature Geoscience.
http://www.nature.com/ngeo/journal/v3/n2/abs/ngeo731.html
Accessed 24th September 2014.
4. Feltman, R., 2014. Scientists hit
new milestone in search for water on planets outside our solar
system. Washington Post.
http://www.washingtonpost.com/news/speaking-of-science/wp/2014/09/24/scientists-hit-new-milestone-in-search-for-water-on-planets-outside-our-solar-system/
Accessed 24th September 2014.
5. Cyr, K., et al., 2014.
Distribution and Evolution of Water Ice in the Solar Nebula:
Implications for Solar System Body Formation. Icarus.
http://www.sciencedirect.com/science/article/pii/S0019103598959590
Accessed 24th September 2014.
6. Fraine, J., et al., 2014. Water
vapour absorption in the clear atmosphere of a Neptune-sized
exoplanet. Nature.
http://www.nature.com/nature/journal/v513/n7519/full/nature13785.html
Accessed 24th September 2014.
7. Clavin, W. et al., 2014. NASA
telescopes find clear skies and water vapor on exoplanet.
http://www.jpl.nasa.gov/news/news.php?release=2014-322
Accessed 24th September 2014.
Image credit: "Drinking the
Universe" by Melzzeny on deviantART
This article was originally published by Save the Water.
