The Moon keeps drifting away from Earth. Will it ever leave?

One caveat for the rate of the Moon's recession is that it hasn't always been the same as it is now. Our current estimate comes from the 1969 Apollo mission by NASA, which set up five reflective panels on the lunar surface. Scientists beam laser pulses at the reflectors from Earth and note down the round-trip time to come up with distance measurements to an accuracy of about 3 centimeters.

But, if you extend the rate of regression of 3.8 cm backward in time, you'd get the Moon and the Earth colliding 1.5 billion years ago, while we know that the Moon formed about 4.5 billion years ago. This shows that the speed at which the Moon is drifting away has varied in the past.

To get a better understanding of how far away the Moon was in our planet's past, geoscientists from Utrecht University and the University of Geneva looked at ancient layers of rocks in the gorges of the Karijini National Park in western Australia.

The paper recently published in the Proceedings of the National Academy of Sciences, shows that about 2.46 billion years ago, the Moon was about 60,000 kilometers closer to Earth than it is now. This means our days were shorter then, just around 17 hours instead of the 24 hours we've come to expect.

The look into the past by the scientists is based on studying erosion patterns in rock formations, which are related to variations in Earth's climate linked to so-called "Milankovitch cycles."

These orbital cycles, proposed by Serbian scientist Milutin Milankovitch a century ago, tie small, periodic changes in the shape of the Earth's orbit and the orientation of its axis to how sunlight received by the planet is distributed over time.

The changes can lead to extremely cold or warm periods, as well as wetter or dryer regional climate conditions. What's remarkable is that the rock record keeps track of these cycles in the way that sediments are laid down, and their frequency is connected to the distance between Earth and the Moon, as demonstrated by the scientists.

Interesting Engineering (IE) spoke with the co-author of the new study, Dr. Margriet Lantink of Utrecht University in the Netherlands, about the paper's methodology and the implications of their findings.

The following conversation has been lightly edited for clarity and flow.

Interesting Engineering: Are the Milankovitch cycles linked to the climate changes we've been observing?

Dr. Margriet Lantink: The Milankovitch cycles have nothing to do with the steep rise in atmospheric CO2 concentrations and associated climate warming that we observe since the Industrial Revolution. Milankovitch cycles play a role on timescales of tens of thousands to millions of years – the recent CO2 rise and climate warming are many orders of magnitudes faster and of larger amplitude than what has ever been observed in the geologic history, and we humans are responsible for it.

Of course, the shape of the Earth's orbit and the orientation of its spin axis are still undergoing slow periodic changes (arising from gravitational interactions with the other bodies in our solar system), and these will keep slowly changing the distribution of incoming solar radiation on the Milankovitch timescales. Earth is currently close to a 405,000 years eccentricity cycle minimum – so its orbit is relatively circular – and close to a precession maximum – meaning relatively low seasonality and low summer insolation on the Northern Hemisphere. The inclination angle of Earth's axis is intermediate.

In the below figure, you can see how over the past 800,000 years, glacial-interglacial cycles followed a ~100,000-year pattern that is attributed to the Milankovitch cycles. We are now in an interglacial period, and we should actually start to slowly move towards the next ice age. However, the recent human CO2 input is visible as an almost vertical, very high spike on the plot. As a result, we are very likely not going to enter this glacial period in the next ~50,000 years due to the unprecedented amounts of CO2 we have emitted into Earth's atmosphere.

IE: Can you predict the distance from the Earth to the Moon in the future, in another 2.46 billion years? 

Lantink: Our estimate of the Earth-Moon distance 2.46 billion years ago is based on empirical data that are stored in the geological record, but for the future, there is of course no geological record, and so we have to rely on models for the Earth-Moon tidal evolution, which is certainly not my area of expertise.

Earth's rotation rate and the orbital distance of the Moon evolve over time as a consequence of the dissipation of tidal energy (occurring mainly within Earth's ocean). This tidal dissipation (or frictional drag) depends on a lot of different parameters and complex processes that vary with time and on various timescales, and are, therefore, very difficult to precisely constrain in the past, such as changes in the configuration of the continents (due to plate tectonics). This is why there exists a lot of uncertainty about the tidal history of the Earth-Moon system and the associated evolution of the lunar orbit.

The fact that we have been able to acquire information about the Earth-Moon system 2.46 billion years ago doesn't mean that we now understand what happened between then and now: with our study, we have "just" provided an important snapshot of the situation 2.46 billion years ago. We now need more reliable geological data points that can be compared against robust theoretical models (such as the recent model of Farhat et al., 2022: The resonant tidal evolution of the Earth-Moon distance) to trace the evolution of the Moon through time, and try and understand what were/are the main parameters controlling its evolution.

For the future, it is possible to make predictions about the evolution of the Earth-Moon system and lunar orbit. But these predictions will be less accurate/contain a larger margin of uncertainty than predictions in the past because of the many unknowns in Earth's geophysical condition, such as the plate tectonic evolution.

IE: Is there a point when the Moon would be too far away and drift out of Earth's orbit?

Lantink: Let me first clarify that it's not the recession of the Moon that causes the Earth's spin rate to decrease – both things happen as a consequence of tidal dissipation. This tidal dissipation (frictional drag exerted on the Earth's tidal bulge) slows down the spin of the Earth, and through transfer of angular momentum, the Moon is accelerated into a higher orbit.

Whether it is possible for the Moon to drift out of Earth's orbit: in infinite time, yes, but we will reach the end of our solar system in a shorter time – there is simply not enough time for the Moon to move far away enough to escape the gravitational pull of the Earth. The effect of tidal dissipation slows down with time as the Moon is moving further away.

Study Abstract:

The long-term history of the Earth-Moon system as reconstructed from the geological record remains unclear when based on fossil growth bands and tidal laminations. A possibly more robust method is provided by the sedimentary record of Milankovitch cycles (climatic precession, obliquity, and orbital eccentricity), whose relative ratios in periodicity change over time as a function of a decreasing Earth spin rate and increasing lunar distance. However, for the critical older portion of Earth's history where information on Earth-Moon dynamics is sparse, suitable sedimentary successions in which these cycles are recorded remain largely unknown, leaving this method unexplored. Here we present results of cyclostratigraphic analysis and high-precision U–Pb zircon dating of the lower Paleoproterozoic Joffre Member of the Brockman Iron Formation, NW Australia, providing evidence for Milankovitch forcing of regular lithological alternations related to Earth's climatic precession and orbital eccentricity cycles. Combining visual and statistical tools to determine their hierarchical relation, we estimate an astronomical precession frequency of 108.6 ± 8.5 arcsec/y, corresponding to an Earth–Moon distance of 321,800 ± 6,500 km and a day length of 16.9 ± 0.2 h at 2.46 Ga. With this robust cyclostratigraphic approach, we extend the oldest reliable datum for the lunar recession history by more than 1 billion years and provide a critical reference point for future modeling and geological investigation of Precambrian Earth–Moon system evolution.

Read the full study here.