How the Moon Was Sculpted (Part One)

We all look up to the Moon as one of the greatest works of art in the night sky. It inspires thoughts that are greater than most of our day-to-day concerns. And the interest arises at all ages: from the 2 year old who squeals with glee at spotting the Moon to the grandparent whose sight of it may inspire somehow a hope for a better future. We have even been there, and walked on that alien land. So where did it come from?

The origin of the Moon is a topic of active debate. We used to be taught that the Moon came about as a result of an impact with a large asteroid. The story goes that an impactor striking Earth at a glancing blow can drag off enough of its outer mantle to explain its size and stable orbit. The story goes on to say that if this all happened very early on in Earth’s history, when it was still in a molten state, then Earth could recover from this blow to retain again its mostly spherical shape that is has today.

If true, then the composition of the Moon should be more or less equal that of the impactor. To test this hypothesis, lunar samples physically collected by humans and brought back to Earth were studied. When their compositions were measured and compared with that of Earth, it turns out that the Moon and Earth have similar compositions (apart from a lunar iron deficiency and a lack of lunar water). Uh oh - this would seem to rule out the major impactor story. So how did the Moon form?

Two new models to form the Moon are proposed already in 2017. In the first model, Earth sustained multiple hits from small impactors. In this scenario, each impact would result in Earth material being flung out into rings similar in concept to the rings we know and love around Saturn. Unlike on Saturn, the rings around Earth would have joined up to form larger and larger-sized masses until they accumulated into what we now call the Moon. This process would take about 100 million years, consistent with some measurements of the relative difference in age between the Moon and Earth (but not all).

In Part Two we will review the second formation model proposed to explain this great enduring sculpture.

Dr. Brenda Frye

About Dr. Brenda Frye

Brenda L. Frye is an observational cosmologist at the Department of Astronomy/Steward Observatory, University of Arizona. She earned her Ph. D. in Astrophysics from the University of California at Berkeley, assisted by a National Science Foundation Graduate Research Fellowship.

Her thesis work involved measuring the concentration of the total mass of visible plus dark matter in the fields of massive galaxy clusters, a program requiring the use of some of the largest telescopes in the world.

Moving a mile from her Ph. D. institution, she assumed a postdoctoral position with the Supernova Cosmology Project at Lawrence Berkeley National Laboratory under the direction of Professor Saul Permutter.

She then treked across the country to take a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship and a Princeton Council on Sciences and Technology Fellowship both at Princeton University.

Moving further east, she became a Lecturer in Physics at Dublin City University in Dublin, Ireland, where a number of European collaborations were formed.

From there she crossed back across the pond to the west coast of the U. S. to become a tenure-track Assistant Professor of Physics at the University of San Francisco.

Her travels have now landed her at her Alma Mater in Tucson, where she teaches and does research. The aims of her research continue to be to use gravitational telescopes in space as 'lenses' to study the properties of dark matter and those of distant galaxies back to when the universe was <900 million years old.

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