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Hydrology. Originating from the Greek words ‘hydor’ meaning water and ‘logos’ meaning study,
the term ‘hydrology’ came to its current meaning around 1750 and is defined as ‘the study of
water and its movement and distribution across our planet and others’¹. The idea of managing
this movement first occurred five or six thousand years ago, where rivers such as the Indus,
Euphrates, Nile and Hwang Ho had dams, levees, subsurface water conduits and other hydraulic
constructions built on them by the world’s first hydraulic engineers, signifying the origins of
hydrology. These civilisations were also the first to measure rain fall and flow rate. The first
records that provide evidence for a global hydrologic cycle date back to Ancient Greece, where
eminent philosophers including Hippocrates, Plato and Aristotle, seem to have had knowledge
of the basic outline of the system. However, some of their basic understanding of the
hydrologic or water cycle was incorrect, and, in 1500, scientist, artist, and great thinker
Leonardo da Vinci used observations of rivers to deduce that the water in the rivers was a result
of precipitation. This confirmed the Greek’s theoretical mechanisms to be incorrect and they
were either overturned or modified in turn. Then, working in the 17th century, Pierre Perault
and Edme Marriotte are widely accredited with having developed the current scientific
approach to investigating the hydrologic cycle. They supplied further evidence of da Vinci’s
theory, providing investigative results in the form of data and calculations and their work was
then built upon by the English scientist Edmund Halley, who estimated the quantity of water
engaged in the water cycle in the surrounding area to the Mediterranean Sea. During the 18th
century, other areas of science were applied, including hydraulics, mathematics and fluid
mechanics, by eminent scientists including Euler, Chezy and many more Europeans and by
1800, John Dalton, a renowned English physicist and chemist, had finally come to globalise the
current understanding of the hydrologic cycle.
Hydrology is a topic at the forefront of discussion in the Environmental Physics community at
the moment, after significant breakthroughs were recently made at the University of Arizona,
with regard to the hotly debated topic of whether or not there is life on Mars. The presence of
water on Mars would provide some of the strongest evidence yet that life is sustainable there.
Recently NASA’s Mars Reconnaissance Orbiter recorded images of ‘dark tendrils a few metres
wide’², thought to be channels of water, most probably salt water, running down craters,
underneath the planet’s surface. The first supposed sightings of water on Mars was in the 19th
century, when astronomers claimed to have seen canals on the surface of the red planet, but
were, in fact, mistaken by an optical illusion. The trails were found in the Southern Hemisphere
of the planet, and were more pronounced in the warmer times of the year, fading in the winter,
which is in keeping with water that runs during the summer but freezes in the winter. There are
in excess of a thousand trails and, although they do not provide definite proof of the existence
of salt water on Mars, the lowered freezing temperature of water containing salt means that
water with the same salt content as the oceans of earth would melt during the summer
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months, even in the harsh, cold climate of Mars. Frozen carbon dioxide and pure water have
also both been ruled out, as the sun-facing regions of the planet would be at a too high
temperature for the carbon dioxide, and too low a temperature for the pure water. However,
there is a possibility that they are the result of dust or sand avalanches. Images of these
‘seasonal streaks’ were recorded by the most powerful camera yet to orbit the planet³, the
HiRISE, a shortening of High Resolution Imaging Science Experiment, which was built under the
leadership of the Lunar and Planetary Laboratory at the University of Arizona. Its mass is 65 kg
and has an aperture reflecting telescope of 0.5m, the largest of any mission yet to explore deep
space. Its findings were endorsed by another craft, the Phoenix Lander, which found the soil of
Mars to be plentiful in salts.
Seasonal Dark Streaks on Mars (source: apod)
The scientific search for water on planets other than our own has been long and, so far,
reasonably fruitless. However, at the University of Ohio in the late 2000’s, Scott Gaudi, the
assistant professor of astronomy at the University, and his team decided to expand the search
somewhat and discovered very cold planets, on the edges of distant solar systems, known as
‘Super-Earths’, which have the potential to support extraterrestrial life of some sort⁴. Although
they are named very similarly, there are very few similarities between the super-Earths and the
Earth that we know. There are stark contrasts between the two, with the super-Earth having a
considerably denser atmosphere and icy surface, although both surfaces are of a solid
consistency. It is also suggested by experts in the field that up to one third of solar systems
could contain super-Earths. Modelling has been carried out by Gaudi, and other scientists Eric
Gaidos and Sara Seager, to see if it is possible for life-supporting oceans to have formed on
these planets and whether we, on Earth, would be able to identify them from here. They have
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discovered that it may be possible, provided that there is a dense enough atmosphere or if
these super-Earths are either very young, very large, or a combination of the two.
New techniques are also being discovered for identifying potentially life-supporting planets and
exoplanets, planets that orbit a star outside of our solar system. One of the latest, and the one
favoured by Gaudi and his team, is gravitational microlensing. It originates from the work of the
likes of Newton, von Soldner, Einstein and Orest Chwolson, but the current theoretical
framework is accredited to Yu Klimov, Sidney Liebes and Sjur Refsdal. It allows stars a great
distance away to be magnified, using the gravitational field of a nearby collinear star. Much of
the research using gravitational microlensing is being focused in what is known as the
‘habitable zone’ of other solar systems, places that are the ideal distance away from a star to
create the correct temperature for liquid water. However, there is a far greater proportion of
water outside of this ‘habitable zone’, although it is mostly found on frozen moons, in the core
of gas planets, and on super-Earths, as ice, rather than flowing or liquid water. Gaudi and his
team, therefore, took the approach to look for super-Earths, which would already have water
already in place upon formation, rather than planets that, like Earth, are warmer, but which
obtained their water at a point after formation. The team then looked at the possibility that
these super-Earths manage to retain a core warm enough from its formation to keep water in a
liquid state underneath the ice. They concluded that it was possible for a planet approximately
ten times the size of our Earth to form a liquid ocean under the ice in this way. Gaudi also
discovered that, using microlensing, it would be possible to detect these faraway planets, and
what was, in fact, the optimum distance for a microlense.
Despite the often fruitless nature of searches for water on other planets, and the possibilities
arising from this, including the opportunity for life, the last few years have seen many great
breakthroughs in the field. In 2007, water vapour was found in the spectrum of the planet
HD189733b, by NASA’s Spitzer Space Telescope, the first time water had been detected on any
exoplanet⁵. Then, in 2008, astronomers used the Hubble Telescope to reveal the first signs of
an organic molecule on an exoplanet, by again analysing planet HD189733b⁶. Further
discoveries are being made almost daily on the Kepler Mission, a mission for habitable planets
launched in 2009, which is able to observe thousands of stars at any one time, and is looking for
planets similar to our own, including the presence of water. The results will reveal just how
many planets there are in our galaxy similar to earth.⁷ The Kepler Mission has already
discovered 2,362 planet candidates, as of 5 December 2011⁸, but it is unclear yet whether they
are truly habitable. Its principal investigator, Bill Borucki, has said ‘Certainly in three years, we
will give you the first good answers. And after that they will be even better’⁹.
It is a very exciting time for space exploration into the hydrology of other planets, and therefore
the possibility of life elsewhere than Earth. It is a tantalizing idea, one that has tormented
mankind since almost the beginning of time, and produced the world-famous, mythical alien,
who has gone on to be the star of countless novels, films, and even advertising campaigns.
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Space exploration such as this has also provided fuel for some of the best loved fictional
characters, such as those in the hit 1960’s TV series ‘Star Trek’. It is an area of science saturated
with mystery and intrigue, still very much a dark corner, each discovery awaited with huge
anticipation and abundant media attention. It is an exciting time for humankind, teetering on
the brink of discoveries that could prove categorically whether or not we are alone in this
Universe, a discovery that could change the world, and whatever lies beyond, forever.
Natasha Mulley
Sources:
¹ The Encyclopaedia of Earth: http://www.eoearth.org/article/History_of_hydrology - Article on History of
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Hydrology, Lead Author Jason A. Hubbart Ph.D., published February 11 2008. Jason A. Hubbart is an
assistant professor of forest hydrology at University of Missouri, Columbia. First paragraph informed from
here. Citation: Jason A. Hubbart (Lead Author);Jim Kundell (Topic Editor) "History of hydrology". In:
Encyclopaedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition,
National Council for Science and the Environment). [First published in the Encyclopaedia of Earth February
11, 2008; Last revised Date February 11, 2008; Retrieved August 23, 2011
<http://www.eoearth.org/article/History_of_hydrology>
² From an article written by David Batty for The Guardian, from page 16 of the main section of the Friday 5
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August 2011 issue, and also online at guardian.co.uk at 22:07 on Thursday 4 August 2011
(http://www.guardian.co.uk/science/2011/aug/04/strongest-evidence-yet-water-mars)
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³ From an online article in the magazine ‘Science’, published by Richard A. Kerr on August 4 2011 at 2:06 PM
(http://news.sciencemag.org/sciencenow/2011/08/is-mars-weeping-salty-tears.html)
⁴ Ohio State University (2008, December 16). Ocean-bearing Planets: Looking for Extraterrestrial Life in All
the Right Places. ScienceDaily. Retrieved September 2, 2011, from
(http://www.sciencedaily.com/releases/2008/12/081215091011.htm)
⁵,⁶,⁷ NASA PlanetQuest Historical Timeline, Jet Propulsion Laboratory, California Institute of Technology
(http://www.nasa.gov/externalflash/PQTimeline/)
⁸ The Kepler Mission Official Website, Ames Research Centre (http://kepler.nasa.gov/Mission/discoveries/)
⁹ Bill Borucki Podcast interview with Scientific American, July 18 2011
(http://www.scientificamerican.com/podcast/episode.cfm?id=kepler-searches-for-habitable-plane-11-07-18)
Picture: NASA Astronomy Picture of the Day, August 8 2011 (http://apod.nasa.gov/apod/ap110808.html)
Natasha Mulley, The Leys School, Cambridge
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