The Big Bang

10 to 20 billion years ago, the Universe was in an extremely dense and hot (~10 billion °C !) state that exploded in what astronomers call The Big Bang. Eventually, the Universe expanded and cooled and huge collections of gas formed into billions of separate galaxies, and billions of stars formed within each. 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 deduced 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.

Stellar Evolution

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. 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, 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.

At this point, you might be wondering how the oxygen that formed in the core of stars ever got incorporated into our planet! Well, one rather dramatic way occurs at the end of a very massive star's life. 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. (By the way, a supernova is as bright as an entire galaxy!)

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. Thus, the planets and ourselves, are in fact, all made out of star-stuff!

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. However, only at certain temperatures and pressure, like those we find on Earth, would we expect to find liquid water.

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. A spectroscope splits the light into its components, like a prism which shows the different colors (wavelengths) making up white light.

Only light of specific wavelengths can be emitted by atoms of a given element. Similarly, 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.

Specifically, the following 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. A laser, on the other hand, stands for (visible) Light Amplification ... . 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.

• The Water Hole: The ongoing search for extraterrestrial intelligence (SETI) is based on the proposition that other civilizations are sending radio signals that we can detect. While the radio part of the spectrum is very large and contains billions of channels, one small window of the spectrum may be chosen by intelligent water-based life seeking to contact other intelligent water-based life. This communications window is called the "water hole" because it lies between the 21 cm wavelength of natural radio emission from hydrogen and the 18 cm wavelength of natural radio emission from OH molecules. These wavelengths really have nothing to do with water, but since water is made of two molecules of hydrogen and one of oxygen, the term "water hole" is too inviting to ignore!

Suggested Readings

• "Astronomy" magazine, February 1999, has an article ("Divining on Mars") on the Mars Polar Lander and the search for water there.

• "Sky and Telescope" magazine, February 1999, has an article which includes short summary of water in the universe on page 34.