OLDEST EARTH ROCK FOUND ON MOON?
An earth-rock took a four billion year trip to the moon and back.
The amazing Odyssey of a small chunk of Earth rock which was carried on a four billion-year round-trip from the Earth to the Moon and back is described in an article published by NASA, in Earth and Planetary Science Letters, January 23, 2019.
The article reveals the astounding science behind the near mythical journey of a tiny piece of ancient rock formed deep in the Earth’’s crust. It follows this rock’s ejection into space by a meteor impact and its fall to the lunar surface and burial there for unknown eons. The buried earth rock was subsequently “unearthed” by another meteor impact —this time on the moon (!)—which stranded it on moon’s surface. It remained undisturbed there for unknown eons until in January of 1971 it was miraculously found and collected by our own USA lunar astronauts Alan Shepard and Edgar Mitchell of the Apollo 14 team. These men collected it and —-brought it back to Earth from where it had originated some 4 billion years earlier. !
This is a story to pique the imaginations—and curiosity of our youth—and I share a summary of it with you—my readers.
What caused the Apollo team to pick up this particular hunk of rock debris among the jumble of broken rocks (called lunar regolith) scattered around on the lunar surface? How were scientists able to decipher the history of this piece of earth rock? How do we know it is 4 billion years old? The story should not only awake our interest in science (and prod our imaginations) but make us all proud of the achievements of our scientist’s, the courage of our astronauts and the ability of the human race to understand the magnificent complexity our world and to execute such wonders of discovery— using the methods of science, and the knowledge gleaned by generations of geologists, physicists, astronomers and mathematicians.
Let’s begin with the collection of the specimen by astronauts—Commander Alan Shepard and Edgar Mitchell near Cone Crater in the Fra Mauro region of the moon on January 31 1971. What was it that caused Shepard to pick up this insignificant-looking rock (smaller than a football) from many others strewn about on the lunar surface?
I suspect what caught Shepard’s eye was a small “clast” or embedded rock fragment —an inclusion— of light-colored rock surrounded by almost entirely dark gray rock material in an an irregular shaped cobble (about the size of a small football). Almost all the rocks on the moon are of a gray-to dark gray color. This rock an was different! Shepard picked it up and placed it in the special collection bag for its return trip back to Earth. it remained unanalyzed for many years until until sometime in 2018 when it was unpacked, studied and reported on in this recent publication (above) authored by David Kring who finally decoded the history of the sample at the Lunar and Planetary Institute in Houston Texas. it was there these many years after it was collected that its full history was finally revealed.
(PS I was a graduate student 1971, studying “soft-rock” geology at a NY university. Some of those samples the Apollo team brought back were delivered for preliminary analysis to my university Department where I did get a chance to see lunar sand-samples collected by Shepard and others. In fact I loaned a piece of my microscopic equipment ( a picking tray) to one of the physical geologists studying the mineralogy of these lunar samples. I eventually got it back and have it to this day! )
The clast of light-colored rock in the Shepard sample was composed of felsic minerals (i.e. granite). These minerals (granite) make up the bulk of continental rocks on Earth. Granite or felsic rocks are the result of chemical and physical differentiation of the Earth’s crust and mantle. Earth’s complex geological processes—weathering, erosion, transportation, and plate tectonics operating in an oxygenated, water-rich earthly environment are responsible for the creation of these lighter colored and lower specific gravity (felsic) rocks. Felsic rocks are almost non-existant on the moon—and certainly its surface. Upon careful analysis, back on Earth—the small rock-clast (about an inch by two inches long ) was found to be composed of minerals common in granite (i.e. K-feldspar, quartz and zircon). The first two minerals are among the most common in the Earth’s continental crust, while the latter mineral— zircon, is a common accessory mineral in granite and felsic rocks. Their presence indicated that the clast was almost certainly derived from the Earth!
So how and when did it get onto the moon?
The answer was helped along by the presence of the mineral zircon in the sample. Zircon crystals (they may occur as gemstones) often have small amounts of the naturally radioactive element Uranium as an inclusion in its crystal structure. Uranium is radioactive. That is uranium atoms decay naturally into the element Lead at a known rate (you have heard of the “uranium-lead clock”). So carefully calculating the ratio of uranium to lead within the zircon crystals permitted scientists to date the age of the rock which contained the zircon. The analysis concluded that the zircon crystal (and the rock) was formed more than 4 billion years ago. That means that the felsic rock clast crystalized from a melt very early in Earth’s history. (The oldest earth rocks are found in Australia where they have been dated to about 4.4 billion years) That date puts the origin of the Shepard-rock very near the time the Earth itself was formed—estimated to be about 4.5 billion years ago. So this little clast of earth rock was very likely crystalized during the very early history of our Earth, or only about half a billlion years (500 million years) after Earth had formed from its primordial dust cloud. The fact that even at that early date geological processes were similar to the present—erosion, weathering and rock differaention were in progress.
Astronomers have suggested that at that time in Earth history (@4 billion years ago) the moon’s orbit ranged much closer to Earth. Also we know that Earth itself, in those early eons experienced more frequent bombardment by meteors and comets. When meteors strike a planet the force of impact is so great the collision sprays plumes of “ejecta” —fragments of earth rock-dust-molten materials, etc. —upward in all directions. In this shower of debris some of the smaller fragments—may reach a speed sufficient to launch them into space ( i.e. 11 km/sec is Earth escape velocity). These rock fragments shoot our beyond the Earth atmosphere and enter near-by space. The moon being near by would have been a likely place for these fragments of earth-rocks (“earth meteors”) to fall into.
The fact that the Shepard felsic-earth-rock clast was surrounded by and cemented into a larger piece of lunar rock suggests that it had probably been buried among lunar rock fragments at some depth below the surface for a long period.
On the moon there is no atmosphere and no wind, no flowing water and therefore there are few agencies to move rock debris other than infrequent and limited volcanism, lunar gravity or meteor impact. Scientists theorize that a meteor impact—another one—this time impacting the lunar surface near where the “Shepard clast” was buried—dislodged the rock class and it’s enclosing “country rock” from its place of burial and ejected this onto the moon’s surface. The Fra Mauro area was chosen by Apollo 14 planners, for the very reason that it sits near the rim of a meteor crater, strewn with ejecta from an ancient impact.
It was there that Commander Shepard landed Apollo 14 close to the Fra Mauro highlands and Cone Crater and where the now famous sample was collected. The moon rock—in the hands of the Apollo crew—made its return trip to Earth in the Apollo Lunar Module and then in the Command Service Module.
What a fantastic 4 billion year-old journey!!.
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