Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 16

masses through accretion, and eventually became the young Earth, within
which volcanism and impacts allowed the degassing of water vapor.
Figure 2 Infrared absorption spikes indicating the presence of water in the Orion nebula (from
the Harvard-Smithsonian Center for Astrophysics).
Comets may also have played an important role in bringing water to
Earth. Indeed, ice is a major component of comets, as confi rmed by ISO on
the Hale-Bop comet (Figure 3). The Rosetta probe, launched in 2004, should
reach the Churuymov-Gersimenko comet in 2014 and provide information
on its composition. Comets can also sometimes collide with planets, as was
observed during the impact of Shoemaker-Levy 9 on Jupiter in 1994. They
can therefore contribute to the water supply. However, the isotopic ratio of
deuterium to hydrogen is higher in the comets that have been studied than
on Earth, indicating that they only provide a partial contribution.
Three remarkable properties enable the presence of liquid water on
the Earth's surface:
• the Earth is at a distance from the Sun which allows the existence of
water in all three of its states;
• the presence of the atmosphere causes a greenhouse effect without
which the average surface temperature would be -15°C;