Astronomers uncover the secret of the Moon’s ‘fossil bulge’

New models show that the Moon was once hot and rotating fast, which caused its equatorial bulge whilst retreating from Earth


The Moon’s has a large equatorial bulge with flattened surfaces at the poles. Image credit: NASA

How the Solar System formed, and evolved into what we see today, is a topic that is ever changing after the release of new information. The latest research in this case is a study conducted by the University of Colorado (CU) at Boulder, United States, which focused on the Moon’s equatorial bulge. The new results suggest that the Moon solidified after its formation over four billion years ago, before gradually moving away from Earth that, at the time, had all its surface water frozen.

CU Bolder researchers created models that set parameters on how quickly the Moon receded from Earth, in order to get see which results matched up with what we see today. The results best suggest that at the time of our only natural satellite’s solidification, Earth’s hydrosphere was either non-existent or still frozen, also referred to as the ‘snowball Earth’ theory. Coincidently, the results also support a different theory. It suggests that at the time of this runaway satellite, the Sun was weaker, fainter and radiated around 30 percent less energy than it does today.

What we can be certain about, based on direct observation, is that the Moon is moving away from Earth at a rate of around four centimetres (one and a half inches) per year, which is the rate your fingernails grow. This was discovered thanks to the lunar laser ranging observations during the Apollo missions. Astronomers have explained this to be the consequence of tidal interactions between the Moon and Earth. Both bodies exert a gravitational effect on each other, with the Moon’s gravitational effect causing the tides – hence the term ‘tidal’ interaction. This tidal interaction gradually slows the orbit of Earth while causing the Moon to slowly back away.

“The moon’s fossil bulge may contain secrets of Earth’s early evolution that were not recorded anywhere else,” says Shijie Zhong, a professor in CU Boulder’s Department of Physics. “Our model captures two time-dependent processes and this is the first time that anyone has been able to put timescale constraints on early lunar recession.”

The Solar System over four billion years ago was mayhem, as it contained copious amount of protoplanets and leftover debris. Image credit: NASA/JPL-Caltech

Roughly 200 years ago, a French mathematician and physicist by the name of Pierre-Simon Laplace discovered that the Moon’s equatorial bulge was too big – by the order of twenty times – for its one revolution per month rotational rate. Scientists have since theorised that the Moon was created after a Mars-sized protoplanet slammed into the Earth during the Hadean period, four and 4.6 billion years ago, and the consequential, scorching nascent body was then caught in our planet’s gravity. The Moon continued to rotate excessively after its formation, and this is what led to an increased equatorial bulge and flat poles before it finally solidified, creating the feature known as the fossil bulge.

It is only until now that the timing and conditions of the fossil bulge formation have been constrained. The physical, foremost, dynamic model of the lunar evolution determined that the process was not as hastened as first thought, and instead the process was rather slow. This event lasted several hundred million years, as our lunar companion distanced itself from Earth during the Hadean period. Although, for this to have occurred, the tidal interaction between the two bodies would have had to been incredibly reduced compared to now. This is why researchers suggest that the Earth was essentially a snowball in its frozen existence – if water even existed on the surface at the time.

“Earth’s hydrosphere, if it even existed at the Hadean time, may have been frozen all the way down, which would have all but eliminated tidal dissipation or friction,” says Zhong. This adds further evidence to the theory that our young Sun was much weaker and fainter, as it would’ve had to been, in order to make such conditions possible. The future plans, for Zhong and his collaborators, are to continue optimising the model and attempt to further our knowledge of the Moon and Earth’s evolution between 3.8 and 4.5 billion years ago.

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