February 24, 2005
The Moon: Earth’s Childhood Attic
The moon is sometimes referred to as Earth's childhood attic, a rich repository of what the early terrestrial geology might have promised prior to the advent of life. Europe's Chief Scientist, Bernard Foing, looks at what the moon can tell us about our past.
Astrobiology Magazine -- The European Space Agency's SMART missions - Small Missions for Advanced Research and Technology - are designed to test new spacecraft technology while visiting various places in the solar system. SMART-1 is now at the moon, mapping the surface mineralogy. Future missions can use the technology being tested by SMART-1 to go to Mars, Venus, Mercury, comets, and the sun.
SMART-1 flew to the moon using a new type of ion propulsion system. These engines work by expelling a continuous beam of charged particles --ions-- out the back, which then pushes the spacecraft forward. Because SMART-1 is also testing miniaturized instruments, it is tiny by spacecraft standards, weighing only 367 kilograms (809 pounds). It could fit into a cube just one meter (3.3 feet) across, although the solar panel wings extend out about 14 meters (46 feet).
In this article, Bernard Foing, Chief Scientist at ESA and Project Scientist for SMART-1, describes how lunar discoveries could improve our understanding of Earth's history.
The SMART-1 survey of mineral resources will be the first geologic map of the moon. We are using the infrared spectrometer on SMART-1 to take spectra of the lunar soil, which tells us the distribution of minerals on the surface. Some minerals were created by volcanism, others by impacts, so they've seen different types of pressures and temperatures. This shows up as different crystalline structures. By measuring the spectral fingerprints of these crystals, we can reconstruct the history of the different minerals.
Another scientific objective is to study how the moon responds to solar X-rays. When solar X-rays strike the moon, different elements such as magnesium, silicium, aluminum and iron emit X-rays at different wavelengths. By measuring those emissions, we are going to show where these elements are present on the surface. This will add to our understanding of the moon's composition. Is the moon made of green cheese, or something else?
When we know the composition of the moon, we can better understand the historical relationship between the moon and the Earth. We believe now - at least this is the prevailing scenario - that the moon was created 4.5 billion years ago when there were still a lot of planetary embryos (called planetesimals) floating around in the solar system. When the Earth was very young, only 10 to 50 million years old and still a global ocean of magma, a body about the size of Mars impacted the Earth.
This impact blasted a mixture of materials, from both the impactor and from Earth, into orbit around the primitive Earth. This material then re-condensed to form the moon. By measuring the chemical makeup of the moon now, and knowing what we think was the makeup of the Earth back then, we will be able to better understand how much of the early Earth is incorporated into the moon, and what happened during this great impact event.
Future missions might be able to collect samples from the lunar surface. Some of these samples could be meteorites - asteroids or perhaps rocks from Venus, Mars and the Earth. In particular, there might be meteorites from one planet that has completely disappeared now -- the early Earth.
Because of tectonic evolution and erosion reshaping the continental crust, the early Earth is gone. But 3.8 billion years ago, when there was a period of heavy asteroid bombardment, some pieces of this early Earth could have been projected onto the moon. Those rocks would now be buried under some protective layers.
The moon is the childhood attic of the Earth. It is where we put the toys in the closet, and we didn't play with them for 4 billion years. But now we can go back and look at them - we can go back to the moon and get some of these samples of the early Earth.
Maybe there would be still some organics embedded into the samples there, and maybe even some trace of an organism that predates the oldest organism we've found so far in the Earth's fossil record. But collecting those samples requires a very well organized strategy, and that is something we could put as an objective for future robotic or maybe even human missions. It may be that humans are needed to spot the best places where there will be some layers from this Earth ejecta on the moon from 3.8 billion years ago.
We could also look for rocks from early Venus, since we believe early Venus was also reshaped in a 50 to 100 million-year time scale. But for samples of early Mars, I believe it would better to go to Mars itself. Because Mars is a geologically less active world than the Earth, there are still some very old terrains there that we could access.
Future missions that will rely on technologies we are now testing with SMART-1 include the BepiColombo mission to Mercury and the Solar Orbiter, which will study the sun close-up. We are also planning astrophysics and fundamental physics missions, such as LISA, a gravitational wave detector. LISA will rely on a micropropulsion thruster to maintain, within nanometric accuracy, the positions of three spacecraft separated by a million kilometers.
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