The Earth Story

The Last Great Impact and the Birth of Our Moon

We recently published a post about how important the Moon is to our existence (http://on.fb.me/1vSgIix), but have you ever considered how our moon first formed? While there are several hypotheses, the ‘Giant Impact Hypothesis’ is currently the most favoured.

It is proposed that a protoplanet the size of Mars (often referred to as Theia) struck the newly formed Earth 4.5 billion years ago. The debris from this event began to coalesce and out of the destruction rose our Earth and a brand new Moon. However, it was George Darwin in 1898 who first proposed that the Moon and Earth may have originated as the same body. He proposed the Moon was formed from centrifugal forces spinning off molten material from the Earth.

Using Newtonian mathematics he concluded that the Moon had orbited much closer to the Earth in the past and was actually drifting away with an ever increasing orbit. This idea was bolstered when both American and Soviet experiments confirmed that the Moon was in fact moving away.

Despite his best efforts George Darwin could not create a mathematically sound reconstruction of how the Moon first separated from the Earth’s surface. It was not until 1946 that Reginald Aldworth Daley of Harvard University proposed an alternative theory to that of centrifugal forces. Could an impact have separated the Earth and Moon?

Little attention was paid to Daley’s ideas and it wasn’t until 1974 at a conference on satellites that the idea was brought back to light. In 1975 Dr William Hartmann and Dr Donald Davis published a book called Icarus, in which they suggested that a satellite sized body could have collided with Earth ejecting enough material to form the Moon.

The key piece of evidence supporting the giant impact hypothesis is the geochemical analysis of lunar rock collected during the Apollo 11, 12 and 16 missions. If the Moon formed purely from the Earth then Earth rocks and lunar rocks would be expected to show the same geochemical abundances.

If the Moon had formed elsewhere in the solar system to become later entrained within the Earth’s gravitational field, then there would be no similarities in rock samples.

The results indicated that the samples were different enough not to have formed from the same material, but showed similarities that disputed the idea of the moon having formed in a separate part of the solar system.

Lunar rocks show relative depletions in volatile molecules such as water, which have low boiling points. In fact there are more water in bones than on the moon. It is suggested that the heat from a giant impact would have evaporated any volatile present and, as the Moon formed from the debris, that it inherited the deficiency in these molecules.

However, some have suggested that the similarities between Lunar and Earth rocks works to discredit the Giant Impact Hypothesis. It is suggests that the Moon should be 70% Theia in composition which should mean that there is greater variation in the geochemical values between Earth and Lunar rocks.

Recent research may have come across a solution to this problem. Scientists suggest that the Moon may comprise only 40% Theia and that Theia itself may have been similar to meteorites called enstatite chondrites, which have very similar compositions to the Earth. This would explain the similarity in Earth and Lunar samples in the case of a giant impact.

So the next time you look up into the night sky and see the moon shining majestically just remember the extraordinary manner of its birth. It is an amazing example of the beauty that can be born out of disaster and that even when you suspect the worst, not all is lost.

Further Reading:

http://bit.ly/1nEbeCq http://bit.ly/1zVsR6P

A Cool Infographic: http://bit.ly/19PGFef

Refrences:

http://bit.ly/1pViQXr http://bit.ly/1D0z1Y3

Image Credit:

Art – Dana Berry Source – Robin Canup, SWRI

Geologic Map of Asteroid Vesta In 2011, NASA’s ion-engine powered Dawn spacecraft entered orbit around the asteroid 4 Vesta, a fascinating body in the solar system and the likely source for one of the more abundant groups of meteorites discovered on Earth.  The spacecraft orbited the asteroid for almost a year, taking images and spectral measurements of the surface that allowed scientists to interpret the type of rocks exposed at the surface. Using those measurements and the basic principles of geology they were able to produce this lovely map – the first detailed geologic map of a differentiated asteroid. A geologic map is constructed by grouping units together based on common properties. On Vesta, the geology is controlled by a combination of impact cratering and avalanches driven by gravity. A single impact excavates material from the crust, throwing it onto the surface all at once and creating a single geologic unit processed by later avalanches. When another crater forms, it throws out material that covers the debris from the first impact, creating a second geologic unit and a setup that geologists call crosscutting relationships. If the debris from the one impact is buried by debris from another, then geologists can put an order to those impacts and create a stratigraphy. Over the last 2.5 years, the Dawn mission team used these principles to identify units across Vesta’s surface and put them in a stratigraphic context, allowing the completion of a full geologic map of the asteroid’s surface, possibly with tiny gaps in the northern hemisphere where some areas stayed in shadow while the spacecraft was present. Vesta’s surface is dominated by 2 giant impact craters near its south pole: Venenia and Rheasilvia (named for the “Vestal Virgins”). The larger of the 2, Rheasilvia, has a diameter almost as wide as the asteroid itself. These impact craters define some of the major boundaries in Vesta’s stratigraphic timeline: the oldest craters are pre-Venenian, the Venenian impact is older than Rheasilvia, and a smaller crater known as Marcia is younger than both.  Since we haven’t sampled the rocks in place it is difficult to know when those craters formed exactly, but estimates can be made using assumptions of how often craters form on Vesta. For the Rheasilvia impact, estimates range from 1 billion years ago to about 3.5 billion years ago – the best we can do without directly sampling the rocks.  As time goes on, Geology is gradually staking its claim to the study of many bodies throughout the solar system. We have geologic maps of other planets and moons, now we’re making geologic maps and geologic timelines of asteroids. -JBB Image credit: NASA/JPL/UCLA http://dawn.jpl.nasa.gov/feature_stories/maps_reveal_surface_features_Vesta.asp