To Understand the Moon Do We Need to Study Venus?

nature |  Lunar-origin studies are in flux. No current impact model stands out as more compelling than the rest. Progress in several areas is needed to rule out some theories, support others or direct us to new ones.

First, a better understanding of what happened between the formation of the disk and the accumulation of the Moon from the disk is essential, because this phase established the Moon's properties. Did mixing homogenize the composition of the disk and the planet before the Moon formed? Were volatile elements lost from the disk, and, if so, did the pattern of loss vary with the disk's temperature? Canonical impacts produce a mostly liquid disk whereas in the high-angular-momentum impacts, the disks are initially largely vapour. Such disk-evolution models are technically challenging and will require a multidisciplinary approach incorporating both dynamics and chemistry.

Second, the likelihood that a resonance altered the Earth–Moon angular momentum needs to be assessed for a variety of physical states of the early Earth and Moon and using state-of-the-art models for the tidal interactions between them.

Finally, further isotopic comparisons of lunar and terrestrial materials would be extremely valuable. They should include highly refractory elements, such as calcium, to test the equilibration model. Finding that an element that could not have mixed in a vapour phase in 100 years is the same in the Moon and Earth but different in Mars would argue against equilibration; finding Earth–Moon isotopic differences in such a highly refractory element would support it.

Oxygen provides arguably the most important isotopic constraint on lunar formation. The distinct oxygen isotopic compositions of the Earth–Moon system, Mars and most meteorites reflect different initial compositional reservoirs in the inner Solar System. This simplifies the interpretation of oxygen compositions compared with elements such as silicon, whose isotopic abundances are affected by later planet-forming processes (such as crustal extraction). Increasing the precision of oxygen isotope measurements could potentially rule out some impact scenarios.

It remains troubling that all of the current impact models invoke a process after the impact to effectively erase a primary outcome of the event — either by changing the disk's composition through mixing for the canonical impact, or by changing Earth's spin rate for the high-angular-momentum narratives.

Sequences of events do occur in nature, and yet we strive to avoid such complexity in our models. We seek the simplest possible solution, as a matter of scientific aesthetics and because simple solutions are often more probable. As the number of steps increases, the likelihood of a particular sequence decreases. Current impact models are more complex and seem less probable than the original giant-impact concept.

A clue may lie in Venus. The assumption that the Moon-forming impactor had a composition very different from that of Earth is largely based on what we know about Mars. We do not know the isotopic composition of Venus, the planet most similar to Earth in both mass and distance from the Sun. If Venus's composition proves similar to that of Earth and the Moon, Mars would then seem to be an outlier, and an impactor composition akin to Earth's would be more probable, removing many objections to the canonical impact.

Determining the isotopic composition of Venus's key elements will probably require a mission to the planet. Such a tantalizing prospect reminds us how much there is still to learn in our Solar System backyard.