The Origin of water in the Universe
The Big Bang
An estimated 10 to 20 billion years ago, the Universe was in an extremely dense state.
It exploded in what astronomers call 'The Big Bang', and material was blasted out into empty
space. Imitially the material was extremely hot.
Eventually, the Universe expanded and cooled. The huge collections of gas formed through
gravity into billions of separate galaxies and the stars within them.
Many fundamental particles were formed in the beginning of this process, including the
basic building blocks of all atoms: protons, neutrons, and electrons.
The two lightest elements, hydrogen and helium, were also formed.
Hydrogen consists of one proton with one electron circling it.
Helium consists of two protons and two electrons. (Different isotopes of these elements
were also formed, consisting of different numbers of neutrons within them.)
Current models of the Big Bang predict that hydrogen should have been produced
three times more abundantly than helium. Indeed, this proportion has been found by
astronomers in observations of hydrogen and helium in the Universe.
Some heavier elements were created in the Big Bang, but only in very trace amounts,
e.g., one lithium atom (with 3 protons, 3 electrons) out of every 10 billion atoms.
So how are the heavier elements, such as oxygen, formed? They are synthesized during the
evolution of stars.
Evolution of Stars
Stars like our Sun produce huge amounts of energy from nuclear fusion in their hot cores.
Stars contain mostly hydrogen. The pressure and temperature is so great in the core that
hydrogen is fused together to form helium. The produces a huge amount of energy.
Since the mass of helium is less than that of the hydrogen necessary to create it,
energy is released according to Einstein's formula: E = mc2, where E is the energy,
m is the difference in mass, and c is the speed of light.
90 per cent of a star's lifetime is spent fusing hydrogen into helium.
Once the hydrogen is used up, and new pahse in the life of a star begins as helium begins
fusing and one of the by products of that process is oxygen.
Depending on the mass of the star, all the heavy elements up to iron can be created in
succeeding fusion reactions or nucleosynthesis.
So far all of the heavier elements are produced in stars. How do these elements get to
the planets and other bodies? In tracing how water was formed - how did the oxygen and
hydrogen get together?
The answer lies in the last phase of the evolution of stars - a giantic explosion!
Once iron is formed in the core of these stars, there are no further nuclear reactions
that are stable enough to fuse the iron.
Without, the output of energy to balance the star's inward gravity, the star collapses
upon itself, leading to its destruction in a supernova explosion.
A supernova remnant formed from the exploded star expands outward and eventually all
the elements within it are spread throughout the galaxy and mix into the region between
the stars (the interstellar medium).
Over time, denser regions of the interstellar medium form into giant interstellar
clouds of gas and dust. These clouds are stellar nurseries in which numerous stars
will be born. Around each star, residual gas and dust slowly congregates and forms into
planets.
Therefore, the planets and ourselves, are in fact, all made out of star material.
Now, given the creation of hydrogen in the Big Bang and oxygen in nucleosynthesis in stars,
and the fact that these elements are highly reactive chemically, water should therefore
be fairly common in the Universe. It is common but mostly as a gas. Only at certain
temperatures and pressure, like those we find on Earth, would we expect to find liquid water.
Evidence for Water in the Universe
Detecting Water Beyond the Earth
Spacecraft have at least partially explored all the planets around the Sun except Pluto.
However, an analyses of the chemistry of a sample of the surface or of the atmosphere
of each of the planets has been quite limited.
Therefore, detecting water in the Universe up to now has been done almost entirely remotely.
Fortunately, the composition of a planet's atmosphere and surface can be partially
determined by analyzing the spectrum of light emitted or absorbed by the elements that
compose it.
A spectrum is a display of the intensity of light emitted at each wavelength.
Each type of molecules has a unique spectrum of light. Thus, if the spectrum of
water is found to be present in the full spectrum of light that we observe from a given
planet, we can infer the existence of water on that planet.
Water molecules have been detected in this manner in the atmospheres and the surfaces of
some of the planets and elsewhere in the universe.
Below is a partial list of evidence of the existence of water in the universe,
detected spectroscopically and by other means:
Ice on the Moon: Over the last couple of years, spacecraft orbiting the Moon have
used radar to study its surface. The reflection of the radar signals from craters near
the poles indicates that there may be a large amount of subsurface ice there.
Comets: Comets are chunks of dust and frozen gases including water that are in
highly oblong (elliptical or hyperbolic) orbits around the Sun. They are sometimes
referred to as "dirty snowballs" although they are many kilometers in size. As they
near the Sun, the sunlight melts some of the comet's material which results in a long tail.
Some astronomers have raised the possibility that comets have fed the oceans with water
through numerous collisions with the Earth over the aeons.
Mars: Even the earliest spacecraft photographs of the famous Red Planet show
long jagged structures that appear to be old rivers and canyons. One canyon is as long
as the United States! Photographs taken recently by the Pathfinder lander show stacked
boulders that were probably deposited by raging floods. However, the atmospheric pressure
on Mars is now 100 times less than ours and, therefore, water cannot exist as a liquid
there anymore. It is possible that much of the water exists as subsurface ice. There are
polar ice caps on Mars that get larger during the Martian winter and smaller in the summer.
The ice caps are largely composed of frozen carbon dioxide, but small amounts of water-ice
have also been detected.
Europa: The Galileo spacecraft orbiting Jupiter has photographed its four largest
moons. The surface of one of the moons, Europa, appears cracked with many fissures,
as if it is made of ice that freezes and then thaws repeatedly. There may actually be a
liquid ocean under the ice! Ganymede, another of the four moons, has a similar looking
surface but to a lesser degree.
Interstellar Clouds: The spectrum of water has been detected in interstellar gas/dust
clouds. Water masers have even been detected. Maser stands for Microwave Amplification
by the Stimulated Emission of Radiation. Water molecules in masers in interstellar clouds
are stimulated by the energies of nearby stars. Very powerful masers have also been detected
near the centers of other galaxies.