A new research from physicists at the University of Alberta provides the first model of the rotational dynamics of our Moon, taking into account its solid inner core. His model helps explain why, seen from Earth, the Moon seems to swing on its axis.
The answer, said physicist Mathieu Dumberry, lies in the complex geometry of the Moon's orbit, locked in what is known as a Cassini state.
"The Moon revolves around the Earth, but its orbit is tilted about five degrees from the normal to the ecliptic plane, the plane on which the Earth revolves around the Sun. But just as the axis of rotation of the Earth is inclined at 23.5 degrees in space, the axis of rotation of the Moon is also tilted by about 1.5 degrees, "explained Dumberry, associate professor in the Department of Physics. "Along an orbit, it points in the same direction in space – which is in the same plane as normal for the moon's orbit. This defines a Cassini state. "
This type of lunar orbit was first observed by Giovanni Cassini more than four centuries ago. Since then, the complex mathematical and physical elements of the Cassini state have been examined by scientists around the world. But what makes this model unique is responsible for a solid inner core in the center of the Moon.
The heart of matter
"Basically, we take all forces into account and try to predict the angle of the Moon's inner core," Dumberry explained. "The angle of inclination can be predicted, but we need to know precisely the deep inner structure of the Moon. However, we know that it is not aligned with the mantle or with the core of the fluid. We determine that the inner core is inclined to 17 degrees of the mantle in one direction or 33 degrees in the other. "
And if scientists can identify the angle of the inner core, they will be able to develop a more accurate picture of the interior of the Moon.
"This is the first model of the rotational dynamics of the Moon that fully takes into account the presence of a solid inner core," said Christopher Stys, a graduate student who conducted this research under the supervision of Dumberry. "Understanding the composition of the Moon's interior can provide insight into the events that led to the formation of the Moon and the beginning of Earth's history."