[meteorite-list] Earth's Birth Date Turned Back

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:21:06 2004
Message-ID: <200307172133.OAA28748_at_zagami.jpl.nasa.gov>

http://www.news.harvard.edu/gazette/2003/07.17/01-earthbirth.html

Earth's birth date turned back: Formed earlier than believed
By William J. Cromie
Harvard University Gazette
July 17, 2003

[Image]
Stein Jacobsen shows off a chondritic meteorite named Allende
for the place it fell in Mexico in 1969. (Staff photos by Jon Chase)

Our planet is 50 to 90 million years older than previously thought,
according to new evidence found in meteorites.

Mixtures of radioactive elements, which tick away like clocks, show that
most of Earth had formed only 10 million years after the sun was born as a
star, which took place about 4,567 million years ago. Previous measurements
indicated an Earth birth of 60 million to 100 million years after the sun's
nuclear fires began to burn.

Mars, about half the size of Earth, may have formed 5 million years or less
after our star's birth, or, like Earth, much faster than anyone believed.

While Earth was still young, a stray boulder the size of Mars, or about
4,000 miles across, struck it and knocked off enough pieces to form the
moon. "That occurred 30 million years after the sun formed, and it completed
the building of Earth from gas and dust particles left over from the
formation of the sun," says Stein Jacobsen, professor of geochemistry at
Harvard University, who made the measurements.

But Earth still was not covered by a hard crust. Jacobsen and Charles
Harper, a former research associate in his laboratory, previously determined
that this was not completed until 100 million years after the sun began to
shine.

The oldest rocks found on our planet date back to about 4,000 million (4
billion) years ago. Therefore, some 600 million years of history were lost
in the melting and reworking of rocks as the young planet cooled down. There
would be no way to make up for that loss without the radioactive clocks in
meteorites that geologists are sure formed at the same times as Earth and
the other planets. "One class of meteorites, known as chondrites, are the
most primitive material in the solar system," Jacobsen points out. "They
have never been melted like the rocks of the planets, so retain the earliest
record of our solar system."

Rock clock ticks

Radioactive elements in rocks decay in a predictable way, like the ticking
of a well-made clock that can run for millions of years. The decay marks a
change in character of the elements; one type of uranium, for example,
decays into lead. Jacobsen and his colleagues used a radioactive type of
hafnium, a rare heavy metal, which decays into tungsten, a more familiar
gray-white metal. The ratio of this type of tungsten to a stable variety of
the same metal reveals how much hafnium decayed away, or how long the clock
has ticked.

"After 50 million years, the hafnium-tungsten timepiece is a dead clock
because all the radioactive hafnium has decayed away," Jacobsen explains.
"But for the first 50 million years of solar system history, it is ideal for
tracking a planet's growth."

Hafnium was not even known until the early 1920s. It was named after the
Latin word for Copenhagen (Hafniae), where it was discovered. Measuring the
ratio of hafnium-derived tungsten to stable tungsten requires special
instruments, and is so difficult that no one succeeded until Jacobsen and
Harper did it in an iron meteorite in the mid-1990s.

Making the same measurement on chondrite meteorites, however, involves a
higher level of difficulty. Scientists at the University of Michigan tried
but did not find any differences in tungsten ratios between chondrites and
the Earth, so they concluded that our planet must have formed between 60 to
100 million years after the sun.

Jacobsen worked on improving the technique, and, with the help of research
associate Qingzhu Yin, found that the amount of tungsten produced by
radioactive hafnium in Earth's rocks is higher than that in the chondrites.
These more accurate measurements showed that Earth built itself up from
solar leftovers in only 10 million years, or quicker than anyone believed
before.

"Within 100,000 years of the formation of the sun, the first embryos of the
planets Mercury, Venus, Earth, and Mars had formed," Jacobsen reported in
the June 6 issue of Science. "Some grew more rapidly than others, and within
10 million years, about 65 percent of Earth had formed."

The Big Whack

No meteorites have been found from furnace-hot Mercury, the closet planet to
the sun, or from cloud-shrouded Venus, the next one out. Then comes Earth,
some 93 million miles from the sun, then Mars, another 50 million to 248
million miles away. (The distance varies with the orbits of the red planet
and Earth.)

Pieces of Mars have been knocked off the Martian surface by meteorites from
farther away, and were found in earthly places like Antarctica. From
measurements made of these rocks and geological data sent back by unmanned
spacecraft that landed on Mars, Jacobsen estimates that our neighbor could
have been built in 5 million years or less.

The hafnium clock was still ticking when a huge rock orbiting near Earth,
perhaps a sister planet, took a gravitational turn for the worse. It slammed
into Earth, scattering rocks beyond the pull of our gravity to attract them
back. Their own gravities, however, pulled them together into a satellite
2,160 miles wide, circling 240,000 miles from us.

Astronauts brought back hundreds of pounds of these rocks picked up during
six moon landings. Examinations of the samples fit well with the so-called
Big Whack theory of the moon's origin, and Jacobsen's work provides a lunar
birth date of 4,537 million years.

What on Earth happened after the hafnium clock stopped 4,517 million years
ago? Before Jacobsen learned to read the clock so precisely, he and Harper
used other kinds of radioactive rock clocks to determine when Earth became
solid enough to stand on. They came up with a date of 4,467 years, or 100
million years after the interstellar gas and dust accreted into a fiery sun.

Where did this gas and dust come from? Most astronomers believe it was
debris from more primitive stars that blew up when they ran out of nuclear
fuel. Such supernovas, as they are called, have been lighting up the
universe for 10-12 billion years. Gravity gathers these construction
materials into massive black clouds. Spectacular images of the insides of
such clouds, taken by the Hubble Space Telescope, show young stars being
born inside them.

Billions of years from now the sun will finish burning its gas, and implode
to pieces. The leftovers will join those from shorter-lived stars and solar
systems. There will be radioactive hafnium in the mix and the clocks will
start over again, timing the formation of new planets and moons, some of
which may develop life intelligent enough to tell time by these exotic
means.
Received on Thu 17 Jul 2003 05:33:20 PM PDT