Scientists have studied stars many light-years away, exoplanets, black holes, neutron stars, and even the invisible dark matter that permeates every galaxy. Given that, it hardly seems (at first) that the Moon could still surprise us. After all, the study of the Moon is as old as astronomy itself, and it's the only astronomical object a human being has ever set foot on. But a new study suggests that the Moon has a previously undiscovered low-viscosity region, residing just above the core. The region is partially molten, which fits with earlier models that suggest some melting may exist on the core-mantle boundary.
The region, referred to in the study as the “low-viscosity zone," could better explain measurements of tidal dissipation on the Moon. While scientists have previously calculated the effects of Earth's tidal forces on the Moon, none of those calculations have been able to account for certain observations. Specifically, there is a relationship between the Moon's tidal period and its ability to absorb seismic waves, which are converted to heat deep in the Moon's interior. That relationship was unexplained until now.
The authors of the study, however, were able to closely match those observations with their simulation when a low-viscosity zone was included in their models.
The tides on Earth are the most obvious effect of the Moon’s gravitational influence, but the Earth has a reciprocal tidal influence on the Moon. As these tidal forces from the Earth put pressure on the Moon, it creates seismic waves. Those waves ultimately dissipate, being converted to heat deep inside the Moon in a process called tidal heating. The low-viscosity zone plays a role in that process, helping the waves to dissipate. It's a measurable influence.
Using those measurements, researchers were able to calculate some specific characteristics of the low-viscosity zone. It has a viscosity of 2x1016 Pascal-seconds, which is extremely low compared to previous estimates of conditions at the bottom of the lunar mantle.
The zone begins about 500 meters above the lunar center, and it acts as a blanket to slow down the cooling of the core, influencing the thermal evolution of the Moon.
The new model is not perfect, however, and the paper’s authors acknowledge that it doesn’t exactly match all observations. “The asthenospheric viscosity and the lithospheric thickness are probably too soft and too thin, respectively, in our reference model,” they write. This doesn’t mean their results aren’t informative, but it means that a more precise model may still be required in order to understand the Moon’s internal structure in more detail.
Understanding the relationship between dissipation and tidal cycles in planetary bodies is important to various aspects of planetary science, the authors note in the paper. Among other things, it would give clues as to the evolution of the body in question—both its thermal and its orbital history. And it can help us understand the moons of other plants, such as those orbiting Jupiter and Saturn.
The Moon’s history is of particular interest, tied in as it is with our own past, and it will continue to be the subject of study for the foreseeable future. Who knows? Maybe it still has a few more surprises up its sleeve, waiting to be discovered.
Scientists have studied stars many light-years away, exoplanets, black holes, neutron stars, and even the invisible dark matter that permeates every galaxy. Given that, it hardly seems (at first) that the Moon could still surprise us. After all, the study of the Moon is as old as astronomy itself, and it's the only astronomical object a human being has ever set foot on. But a new study suggests that the Moon has a previously undiscovered low-viscosity region, residing just above the core. The region is partially molten, which fits with earlier models that suggest some melting may exist on the core-mantle boundary.
The region, referred to in the study as the “low-viscosity zone," could better explain measurements of tidal dissipation on the Moon. While scientists have previously calculated the effects of Earth's tidal forces on the Moon, none of those calculations have been able to account for certain observations. Specifically, there is a relationship between the Moon's tidal period and its ability to absorb seismic waves, which are converted to heat deep in the Moon's interior. That relationship was unexplained until now.
The authors of the study, however, were able to closely match those observations with their simulation when a low-viscosity zone was included in their models.
The tides on Earth are the most obvious effect of the Moon’s gravitational influence, but the Earth has a reciprocal tidal influence on the Moon. As these tidal forces from the Earth put pressure on the Moon, it creates seismic waves. Those waves ultimately dissipate, being converted to heat deep inside the Moon in a process called tidal heating. The low-viscosity zone plays a role in that process, helping the waves to dissipate. It's a measurable influence.
Using those measurements, researchers were able to calculate some specific characteristics of the low-viscosity zone. It has a viscosity of 2x1016 Pascal-seconds, which is extremely low compared to previous estimates of conditions at the bottom of the lunar mantle.
The zone begins about 500 meters above the lunar center, and it acts as a blanket to slow down the cooling of the core, influencing the thermal evolution of the Moon.
The new model is not perfect, however, and the paper’s authors acknowledge that it doesn’t exactly match all observations. “The asthenospheric viscosity and the lithospheric thickness are probably too soft and too thin, respectively, in our reference model,” they write. This doesn’t mean their results aren’t informative, but it means that a more precise model may still be required in order to understand the Moon’s internal structure in more detail.
Understanding the relationship between dissipation and tidal cycles in planetary bodies is important to various aspects of planetary science, the authors note in the paper. Among other things, it would give clues as to the evolution of the body in question—both its thermal and its orbital history. And it can help us understand the moons of other plants, such as those orbiting Jupiter and Saturn.
The Moon’s history is of particular interest, tied in as it is with our own past, and it will continue to be the subject of study for the foreseeable future. Who knows? Maybe it still has a few more surprises up its sleeve, waiting to be discovered.
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