For #Apollo50th, 3 Things We Didn’t Know Before Landing on the Moon

Buzz Aldrin deploying scientific experiment tools during the Apollo 11 mission. Credit: NASA.
Thursday, July 18, 2019 


Saturday July 20 will be the 50th time we’ve orbited the Sun since humans set foot on the Moon for the first time. A groundbreaking achievement for aeronautics and space travel, NASA’s Apollo 11 mission contributed extremely valuable science for our understanding of Earth and what the Solar System might have looked like in its early, planet-formation phase.

In honor of Apollo 11’s 50th anniversary, Carnegie DTM Director Rick Carlson summarized some of the science made possible by lunar samples brought back to Earth. Here are three things we learned thanks to our trips to the Moon:

We changed our view of Earth (and other planets)

Most of Earth’s surface is quite young—at least geologically speaking, since the formation of our Solar System is measured to be 4.567 billion years old. Only a few parts per million by area of Earth’s surface consist of rocks older than 3.6 billion years.  “That means the first billion years of Earth’s geological history are missing,” says Carlson, who is an expert in the chemical and physical processes that formed the terrestrial planets. “It’s erased by the operation of plate tectonics that continually forms new crust at ocean ridges and returns old crust to Earth’s interior at subduction zones.”

Earth viewed from the Moon during the Apollo 15 mission. Credit: NASA.

That’s not the case for the Moon, where most of the surface is as old as 4.4 billion years. Unlike Earth’s, the lunar crust remembers the processes and conditions of Moon formation, and records the events occurring during the Solar System’s formation phase.

The Moon is thought to have formed from the debris of a collision between an object the size of Mars and the proto-Earth. Before Apollo, planetary scientists thought planets formed relatively cold and in much calmer environments as their gravity sucked in smaller objects to grow.  The first samples returned by Apollo 11 showed instead that the Moon likely formed largely, if not totally, molten. “The Apollo samples provided the evidence that planets form through extremely violent process that drive the redistribution of elements to form first-order features of a planet, such as the separation of metallic cores from silicate mantles,” Carlson says.

We got a better sense of how the Moon formed

Before scientists got their hands on lunar samples, models for how the Moon formed had a much colder Moon. These models also had the dark basins we can see with our naked eyes as remnants of evaporated ocean water. The very first samples returned from Apollo 11 helped planetary scientists conclude that instead, the Moon was extremely hot when it formed—so hot it likely had a magma ocean all around. These data suggest the Moon’s thick crust eventually grew when crystals forming in the cooling magma floated to the surface and solidified to form a crust.

We also now know the dark basins were not oceans, but craters resulting from ancient and huge meteorite impacts. “The Pre-Apollo Moon formation models were pretty much wrong,” Carlson says. Being so old, the Moon’s surface also holds an extremely important record of the Solar System’s history of meteorite bombardment. The information on the ages of the lunar crust, determined by measurements of the Apollo samples, helped planetary scientists calculate the ages of the surfaces of Mars and Mercury based on the density of their own craters. “The lunar cratering record also tells us that large meteorite impacts were so common in the early Solar System they may have sterilized any early life trying to form,” Carlson says.

Astronaut James Irwin sampling Moon rock during the Apollo 15 mission. Credit: NASA.

We reconsidered our views of water on rocky worlds

Water was initially thought to have escaped the proto-Earth and into space as a result of its collision with a Mars-sized object. But chemical analysis of lunar samples led by the late Carnegie geochemist Erik Hauri and Brown University’s Alberto Saal in 2008 suggested the Moon took some of our water after that collision, changing our planet forever. “Many volatile compounds like water could have been lost from that impact,” Carlson says. “An impact of this magnitude potentially changed the composition of the Earth.”

Finding water in lunar magmas also reignited a discussion on the fate of volatile elements at the time of a giant impact. Scientists restarted the conversation about the mechanism thought to be responsible for the presence of water on planets like Earth. Conventional theory says that icy bodies such as comets or asteroids might have added water to rocky planets—Earth included. “Results from lunar samples suggest this water might have been here all along,” Carlson says. 

 

Read more via Science: Analysis of lunar samples: Implications for planet formation and evolution

 

Roberto Molar Candanosa



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