A study led by 抖阴旅行射听Boulder researchers provides new insight into the moon鈥檚 excessive equatorial bulge, a feature that solidified in place over four billion years ago as the moon gradually distanced itself from the Earth.
The research sets parameters on how quickly the moon could have receded from the Earth and suggests that the nascent planet鈥檚 hydrosphere was either non-existent or still frozen at the time, indirectly supporting the theory of a fainter, weaker sun that at the time radiated around 30 percent less energy than it does today.
The moon currently recedes from the Earth at a rate of about 4 cm per year according to lunar laser ranging observations from the Apollo missions. The recession is believed to result from gravitational or tidal interaction between the Earth and its moon. The same process also causes Earth鈥檚 rotation to slow down and the length of day to increase. However, the recession rate for the early moon is largely unconstrained. 听
鈥淭he moon鈥檚 fossil bulge may contain secrets of Earth鈥檚 early evolution that were not recorded anywhere else,鈥� said Shijie Zhong, a professor in 抖阴旅行射 Boulder鈥檚 Department of Physics and the co-lead author of the new research. 鈥淥ur 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.鈥�
The moon's rotation makes it flattened at its poles and bulge at its equator. 抖阴旅行射 two hundred years ago, French mathematician and physicist Pierre-Simon Laplace determined that the moon鈥檚 equatorial bulge was too large (by twenty times) for its one-revolution-per-month rotational rate. Scientists have theorized that the moon, born hot, rotated fast after its formation听and possessed an equatorial bulge much greater in size than it does today. The bulge tended to shrink in adjustment to reduced rotational force as听the moon moved farther from the Earth and reduced its rotational rate, until the moon cooled and stiffened enough to have solidified a permanent bulge in its crust, creating the feature known as the fossil bulge.
The timing and necessary conditions of this fossil bulge formation, however, have remained largely unknown given that no physical models have ever been formulated for this process. Using a first-of-its-kind dynamic model, Zhong and his colleagues determined that the process was not sudden but rather quite slow, lasting several hundred million years as the moon moved away from the Earth during the Hadean time or听around听4 billion years ago. But for that to have been the case, Earth鈥檚 energy dissipation in response to tidal forces鈥攚hich for the present-day Earth and its recent past is largely controlled by the oceans鈥攚ould have to have been greatly reduced at the time.
鈥淓arth鈥檚 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,鈥� Zhong said, adding that a weaker, fainter young sun could have made such conditions possible in theory.
The 鈥渟nowball Earth鈥� hypothesis has been suggested previously for the Neoproterozoic around听600 million years ago听based on geological record. Similar ideas were also hinted for the early Earth on the basis of the fainter young sun, but direct observational evidence in the geological record is currently lacking, making it the subject of debate among scientists.
The researchers plan to continue optimizing their model and will attempt to fill in other knowledge gaps about the moon and Earth鈥檚 early days between 3.8 and 4.5 billion years ago.
The study was in Geophysical Research Letters, a journal of the .听Co-authors of the research include Chuan Qin (formerly of 抖阴旅行射 Boulder and now with Harvard University) and Roger Phillips of Washington University in St. Louis (formerly at the Southwest Research Institute in Boulder, Colorado). NASA and the National Science Foundation provided funding for the research.
