Earth-Moon: A Watery "Double-Planet"
New work on lunar samples reveal a shared source for water in the deep interior of both Earth and Moon
Paul D. Spudis
A watery double-planet: Luna and Terra
Science Magazine recently published a paper that reports that minute quantities of water contained in lunar volcanic glass appear to be identical in isotopic composition to terrestrial water. According to subsequent press reports, this finding revolutionizes our understanding of the origin of Earth and Moon. But does it?
Water is a simple molecule, made up of two hydrogen atoms and one oxygen atom. However, these atoms are not all made the same – they always contain the same number of protons and electrons but the number of neutrons they contain varies. In particular, some naturally occurring hydrogen contains an extra neutron and hence has twice the mass of normal hydrogen. This "heavy hydrogen" (called deuterium, for its atomic weight of two) is much less abundant than its lighter version. Planetary scientists use the amounts of deuterium, relative to normal hydrogen, as a measure of the provenance of the material, i.e., where it formed relative to the Sun.
Ultimately, substances that have identical deuterium/hydrogen ratios are presumed to have come from the same source. We have reason to believe this ratio increases systematically outward from the Sun, depending upon where in the early "solar nebula" the material condensed and its subsequent geological processing. Oxygen (the other element in water) also has an isotopic variation; normal oxygen has 16 protons in its nucleus, but the other isotopes of oxygen can have an additional neutron or two. As with hydrogen, the variation in the ratios of normal to "heavy" oxygen is thought to be indicative of where the material comes from.
Of course nothing is ever quite so simple and straightforward. Subsequent processing, such as interaction with cosmic rays, can sometimes alter the composition of samples but if these effects can be accounted for and eliminated, isotopic composition can be used as a tool to map the ultimate sources of Solar System debris. This has been done with many different elements and compounds, but oxygen and hydrogen are very volatile and thus, sensitive indicators of the thermal environment in which they formed.
When the isotopic composition of an element like oxygen is plotted for the various groups of Solar System materials – meteorites, lunar, martian and terrestrial samples – they all form distinct groups, indicating that the source reservoirs of these materials formed in different locations of the nebula. The most primitive type of meteorite – carbonaceous chondrite – appears to have formed at the farthest distance from the Sun. These rocks are thought to have originated within once icy bodies, the cores of objects known as comets. Comets form in the outer Solar System where low temperature substances are abundant and are occasionally perturbed by gravity to enter the inner Solar System, i.e., inside the orbit of Jupiter. Once there, they are heated by the Sun and their most volatile components are sublimed away; after multiple passes through the inner planet zone, only a small fraction of this primitive material remains.
The new findings indicate that the isotopic composition of the hydrogen in water in the mantle (deep interior) of the Moon is nearly identical to that in the water of Earth's mantle, and both appear to have come from carbonaceous chondrite (most primitive) meteorites. When compared to a variety of data from other Solar System objects (including the giant planets, icy outer planet satellites and meteorite groups) the Earth-Moon system is compositionally distinct and identical, indicating that, whatever our origins, the description of Earth and Moon as a double-planet is even more appropriate than we had thought.
What does this mean for lunar origin and what does it say about the water at the Moon's poles? The bulk composition of the Moon has long been recognized as a key constraint on models of lunar origin. A basic question is whether the Moon is made of the same material as the Earth or not. The new results indicate that it is and as such, is another contributory piece of evidence that the materials of the Earth and Moon were brewed in the same pot. Interestingly, this pot of material is distinct from virtually every other Solar System object (as near as we can tell based on limited information from the other planets). Whatever process formed the Moon, it involved objects that were created more or less in this neighborhood of the Solar System. The new results also suggest that both Earth and Moon had a significant component of water early in its history. Earlier studies had suggested that the terrestrial hydrosphere was a late addition, a veneer of cometary debris from deep space that was added to the Earth late in its history. We now know that this water was incorporated into the Earth very early, possibly from the beginning of accretion. The Moon shares this trait – and the same source of water.
So is the giant impact model of lunar origin still viable? The existence of water in the lunar interior is not a prediction of the giant impact model but as has happened previously, the model will probably be modified to accommodate the new findings. We have a tendency to imagine (and desire) simple systems in chemical and thermal equilibrium, in which materials and energy behave in a straightforward, predictable manner. But this event (if it occurred) was a singular one, possibly involving complex, chaotic behavior. Thus, some of the difficulties created by the new data will probably be explained away. A hypothesis elastic enough to be stretched to fit any new discordant observation isn't particularly useful and certainly isn't scientific.
How does this affect our thinking about the water ice trapped at the Moon's poles? As we continue to find that the interior of the early Moon was more water rich than previously thought, we must add lunar water to the long list of possible sources for polar-trapped water. (As a reminder, the previous idea was that polar water was derived from external sources – the Sun via the solar wind hydrogen, water-bearing meteorites and comets). Could at least some of the water at the poles be of lunar origin? One problem that we still don't understand is the geological age of the polar cold traps – they exist because the spin axis of the Moon is normal to the ecliptic plane. How long has the Moon been in this orientation? We suspect that the Moon has been stable for at least the last 2 billion years but water is being found in volcanic glass over 3 billion years old and thus, released before the current polar cold traps existed. So at least for now, it seems that the Moon's own water is an unlikely contributor to the ice at its poles. But that story could change too.
The Moon's surprisingly complex and interesting history continues to confound the experts. We may have already "been there" but we still don't fully understand the Moon's story and true potential.
Paul D. Spudis | READ MORE
Paul D. Spudis (1952-2018) was a senior staff scientist at the Lunar and Planetary Institute in Houston, Texas, and a prolific author on the subject of the moon. His books include The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources.