a) In what discernible way does a molecule of water based on this 'different' hydrogen atom (with both a proton and a neutron in its nucleus) differ from a 'normal' water molecule?

b) What name is given to the 'different' hydrogen atom?

Deuterium is rare on Earth. One atom of deuterium exists for every 7,000 or so of normal hydrogen atoms. However, deuterium in the water of three comets that have been studied is twice as common. This observation is having a significant impact in discussions about the origin of Earth's water. According to many scientists, Earth has more water than expected for a planet so close (93 million miles) to the Sun. A prevailing explanation/theory has been that Earth's water came from bombardment by icy comets over the billions of years of the planet's evolution. If this were a true explanation one would expect more of the deuterium isotope to be present on Earth. New theories about the origin of Earth's water are having to take the deuterium situation into consideration.

[See Science News, March 23, 2002 pages 184-186 Vol. 161, No. 12.]