We start with a puzzle.
While Earth is about 4.56 billion years old, 30% to 50% of its water may be 9 billion years old. Not only is the water older than earth, it is also older than our Sun.
What is the evidence?
Water is abundant on Earth and although it seems counter-intuitive, it is also found throughout our solar system, in comets, in asteroids, on the moons of Jupiter and Saturn, in shady places on Mercury; it is also found locked into minerals and even imprisoned in tiny minerals from Martian volcanoes. (Cleeves et al., 2014)
In the model used to describe the early evolution of the universe, water was formed in what cosmologists call the "dense interstellar medium," the cloud of dust, atomic particles and elements where cold temperatures, oxygen, and a molecular hydrogen (H2) ionizing source (cosmic rays) resulted in the formation of water in the form of ice clinging to the particles of dust.
The formation of the water in the early universe was facilitated by the high energy radiation in the interstellar medium. Water molecules produced under these conditions contain a richer proportion of water molecules that are composed of deuterium rather than hydrogen.
Deuterium is an isotope of hydrogen that has a neutron in the nucleus. Water composed of deuterium is called "heavy water." According to the Vienna Standard Mean Ocean Water (VSMOW), the water in the Earth's oceans has a 2H/1H (deuterium to hydrogen) ratio is 1 part per approximately 6420 parts. This deuterium to hydrogen ratio is the fingerprint that this water dates from the early universe. (Cleeves et al., 2014)
Is this ancient water?
The question arises because energy from the developing star radiates the materials that are rotating around it, causing the ice to vaporize and then for the water molecules to split into their component parts. The splitting and reformation would erase the deuterium to hydrogen ratio.
"Until now, researchers were unsure how much of the old water would be spared in this process." If, during the process of solar system formation, most or all of the water was destroyed, any modern water with the same or similar 2H/1H ratio would have to be the result of a water formation process similar to that in the early universe. (Gibney, 2014)
In a 2014 report in the journal Science (Cleeves et al., 2014), a team led by astrochemist Ilse Cleeves investigated the specific question of whether the much smaller scale interaction in the protoplanetary disk among high energy radiation from the new sun and from the cosmic background would be able to form water. The evidence would be that the water formed in the "protoplanetary disk" would have the same deuterium to hydrogen ratio found in the ancient water formed in the "dense interstellar medium."
Cleeves' work is based on "the theoretical modeling" of early planetary system. Her research focus is on the "chemical and physical processes, such as ionization, in protoplanetary disks."
Her team created a mathematical model of conditions in protoplanetary disks and simulated the interactions taking place. Would there be sufficient radiation to create water with the telltale deuterium to hydrogen ratio? The simulation was run using a variety of possible scenarios. The investigation found that the conditions in the protoplanetary disk were not powerful enough to form 2H/1H ratio water like that formed in the early universe in the quantities needed to account for the amount of 2H/1H ratio water in modern Earth oceans.
The result provided evidence to support the argument that 30% to 50% of the water in the oceans was indeed ancient water and not water reformed after the solar system's formation.
This finding has deep implications for astrobiology since Earth's water helped to facilitate the emergence of life. Since ancient water from the interstellar cloud survived the emergence of the sun and was available on Earth, this may mean that other planets forming around new stars similar to our sun may have water available to help create the conditions for the emergence of life. On the other hand, had conditions in the protoplanetary disk permitted the formation of the deuterium ratio water, it might mean that the ancient water had been destroyed, meaning that whether water is available or not depends on local conditions.
While the findings are, of course, theoretical, there is other evidence to support the "ancient water" argument.
Recent data from the radio telescope Atacama Large Millimeter/submillimeter Array reported in April of 2015, astronomers found a protoplanetary disk around million year old star MWC 480 that “is brimming with methyl cyanide, a complex carbon-based molecule that provides “a hint that prebiotic chemistry is universal.” This is precisely the conclusion reached by Cleeves’s team.
Additional evidence supporting the Cleeves study was reported in a 2015 study led by Lydia Hallis, a planetary scientist at the University of Hawaii (and now at the University of Glasgow).
Hallis and her team looked at water trapped in Earth’s mantel some 1,800 miles deep that emerged as a lava outflow in the Canadian arctic in 1985. The depth mattered because as Hallis said “We needed an undisturbed source of mantle from Earth’s formation.” Water on Earth’s surface would have been affected by a wide range of changes over the 4 billion + years of Earth history and this would have affected the deuterium/hydrogen ratio. But the materials in the deep mantle would be unchanged. “Hydrogen is everywhere…Hallis said, ‘…its difficult to tell if what you’re measuring isn’t hydrogen from contamination. You’re measuring parts per million in a piece of rock so small you can’t see it.”
The Hallis team found that the deuterium/hydrogen ratio in the water from the deep mantle source supported the contention that it was interstellar cloud formed “water-saturated dust [that was]…embedded in the rocky clumps that eventually took shape as Earth.” (Weisberger, 2015)
The implication of this finding that the ancient water from the interstellar cloud is available to developing planetary systems means that there are many, many more systems that have abundant water, which combined with other conditions (right distance from the star, gravity, etc.) might facilitate the conditions for life.
If the work of the Cleeves’s team is thought of as “project-based” science, its example provides a kind of caution. In project-based learning, the emphasis is often on the “building” of the project. In the Cleeves’s example, a huge proportion of the work went into the pre-project work: the theory, the body of research, other related sources of knowledge. Perhaps we need a more descriptive name for what we call project-based learning, one that includes all of the learning “around” the project.
Cleeves, L. I., Bergin, E. A., Alexander, C. M. O., Du, F., Grainiger, D., Oberg, K. I., & Harries, T. J. (2014). The ancient heritage of water ice in the solar system. Science, 345(6204), 1590-1593.
Gibney, E. (2014). Earth has water older than the Sun. Nature: International weekly journal of science.
Weisberger, M. (2015). Earth’s Oldest Water May Have Come from Ancient H2O-Filled Dust. livescience.
Dr. John Holton
Dr. John Holton joined the S²TEM Centers SC in July of 2013, as a research associate with an emphasis on the STEM literature including state and local STEM plans from around the nation.