Earth's birth date turned back: Formed earlier than believed (Forwarded)

Office of News & Public Affairs
Harvard University
Cambridge, Massachusetts

July 17, 2003

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

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.

[NOTE: Images supporting this article are available at
http://www.news.harvard.edu/gazette/...arthbirth.html ]