The next time you raise a glass of water, take a moment to look at its contents. That water is probably older than the Sun itself, according to the results of a new study.
Water is one of the essential ingredients of life as we know it and is present almost everywhere we look on our planet. It covers over 70 percent of the Earth’s surface and is the major constituent of the human body. Beyond the confines of our wet world we find that water is ubiquitous in the rest of the Solar System as well—as ice on comets and on Mercury, Mars, and the Moon, and as water vapor on many planetary atmospheres—and as evidence suggests, it might also be present in large quantities in the underground oceans of many outer Solar System moons.
One of the biggest puzzles of modern astrophysics is the abundance of all this water in the first place. According to the currently accepted model of planetary formation, the planets were the end result of the coalescence of microscopic dust grains inside the protoplanetary disk of material that surrounded the proto-Sun approximately 4.6 billion years ago. The study of comets and asteroids, which are considered to be the left-over primordial material from the time when the Solar System formed, has shown that the composition of the protoplanetary disk was similar to that of the Sun (mainly hydrogen and helium with trace amounts of heavier elements). In the inner parts of the protoplanetary disk, temperatures were such that permitted only the heavier rocky and metallic particles to condense, leading to the formation of the terrestrial planets like Earth. Further out where temperatures were cooler, lighter volatile elements like hydrogen and helium could solidify, forming icy particles that eventually gave rise to the gas giant planets. This region far from the Sun was also rich in water ices, which eventually found their way to the surfaces and interiors of many worlds throughout the Solar System, including Earth. Yet scientists have been debating for many years about the exact origin of this primordial water ice. Was it already present in dust particles in the interstellar molecular cloud that eventually gave rise to the Solar System, or was it created by chemical processes inside the protoplanetary disk that surrounded the early Sun instead?
In a new paper that appeared in the 26 September issue of the journal Science, an international team of astronomers led by L. Ilsedore Cleeves, a Predoctoral Fellow at the University of Michigan, provides an answer to this long-standing question by reporting on the results of their studies of the deuterium to hydrogen ratio in the water ice that was present in the protoplanetary disk during the formation of the Solar System. Deuterium is an isotope of hydrogen, an atom similar to the latter but contains one proton and one neutron in its nucleus instead of just one proton, which gives it slightly different properties like slightly higher boiling and freezing points. Like hydrogen, deuterium is believed to have been created in very small amounts in the first minutes after the Big Bang, which gave rise to the Universe 13.7 billion years ago. Past studies have determined that the average cosmic ratio of deuterium relative to hydrogen is approximately 20 parts per million. Since deuterium is destroyed relatively easily, its total amount in the Universe should be decreasing with time. Yet the study of Solar System bodies during the last 50 years, as well as more recent spectroscopic observations of our galactic neighborhood from space-based telescopes like NASA’s Far Ultraviolet Spectroscopic Explorer, or FUSE, have shown that the amounts of deuterium in the Sun’s planetary family are above the solar values.
A possible source of this excess deuterium are the denser parts of the interstellar medium, which is characterized by very low temperatures in the range of a few dozen Kelvin above absolute zero while exhibiting a constant flux of hydrogen-ionising cosmic rays—conditions that are just right for the formation of deuterium-rich water ices. Another possible explanation was that the water ice was indigenous to the protoplanetary disk around the newborn Sun, being created during the earliest phases of the Solar System’s formation. Yet earlier studies had indicated that the solar wind from the Sun would quickly deflect the galactic cosmic rays coming from interstellar space during that epoch, thereby inhibiting the disk from forming any significant amounts of deuterium-rich water. In order to check the latter hypothesis, Cleeves’ team constructed various computer models that simulated the initial conditions inside the protoplanetary disk, to check whether they would result in the formation of a deuterium to hydrogen ratio similar to that observed in the Solar System today. In their models, the researchers reset the amount of water in the disk to zero and ran the simulations in order to see whether the deuterium-rich water would reappear in the course of 1 million years. The results showed that the protoplanetary disk failed to produce sufficient amounts of water consistent with those found in the Solar System today, indicating that a substantial fraction of up to 50 percent of the water found on the Earth’s oceans and most of that found on comets must have an interstellar origin instead. “In summary, using a detailed physical ionization model, updated treatment of oxygen-bearing ice chemistry, and a simplified deuterium chemical network, we find that chemical processes in disks are not efficient at producing significant levels of highly deuterated water,” conclude the researchers in their study. “Thus a significant fraction of the Solar System’s water predates the Sun. These findings imply that some amount of interstellar ice survived the formation of the solar system and was incorporated into planetesimal bodies. Consequently, if the formation of the solar nebula was typical, our work implies that interstellar ices from the parent molecular cloud core, including the most fundamental life-fostering ingredient, water, are widely available to all young planetary systems.”
Besides their importance in potentially solving one of the most significant mysteries in astrophysics—that of the origin of water in the Solar System—these findings have very interesting implications for astrobiology as well. “The implications of these findings are pretty exciting,” says Cleeves. “If water formation had been a local process that occurs in individual stellar systems, the amount of water and other important chemical ingredients necessary for the formation of life might vary from system to system. But because some of the chemically rich ices from the molecular cloud are directly inherited, young planetary systems have access to these important ingredients.”
“Chemistry tells us that Earth received a contribution of water from some source that was very cold—only tens of degrees above absolute zero, while the Sun being substantially hotter has erased this deuterium, or heavy water fingerprint”, adds Dr. Edwin Bergin, a professor of astronomy at the University of Michigan and advisor to Cleeves. “Based on our simulations and our growing astronomical understanding, the formation of water from hydrogen and oxygen atoms is a ubiquitous component of the early stages of stellar birth. It is this water, which we know from astronomical observations forms at only 10 degrees above absolute zero before the birth of the star, that is provided to nascent stellar systems everywhere.”
Despite being intriguing, these results are mostly theoretical for the time being. Future observations, like the ones scheduled for next year with the Atacama Large Millimeter/submillimeter Array of radio telescopes in Chile, should allow astronomers to further check the predictions of theoretical models, by studying the deuterium-rich water ice distribution in the protoplanetary disks of star formation areas inside molecular clouds in more detail.
In the meantime, while raising a glass of water here on Earth, we can celebrate the fact that we’re living in an absolutely magnificent Universe as revealed with every new study.