[CTRL] Stardust We Are

[William Shannon]

http://news.bbc.co.uk/hi/english/sci/tech/newsid_1262000/1262216.stm



Thursday, 5 April, 2001, 15:08 GMT 16:08
UK

Comets could have seeded life on Earth

By BBC News Online science editor Dr David Whitehouse By simulating a
high-velocity comet collision with the Earth, a team of scientists has shown
that organic molecules hitch-hiking aboard a comet could have survived an
impact and seeded life on Earth.

The results add weight to the theory that the raw materials for life came
from space.

"Our results suggest that the notion of organic compounds coming from outer
space can't be ruled out because of the severity of the impact event," said
Jennifer Blank of the University of California, Berkeley.

Blank and her colleagues presented their findings at the national meeting of
the American Chemical Society in San Diego.

New science

The researchers shot a can-sized bullet on to a coin-sized metal target
containing a droplet of water mixed with amino acids, the building blocks of
proteins.

To date, more than seventy varieties of amino acids have been found in
meteorites and some in interstellar dust and gas clouds.

It was observed that not only did a good fraction of the amino acids survive
the collision, many had been polymerised into chains of two, three and four
amino acids, so-called peptides, the first stage of building proteins.

What is more, freezing the target to mimic an icy comet actually increased
the survival rate of the amino acids.

Oblique impact

The test was designed to simulate the type of impact that would have been
frequent during Earth's early history, some four billion years ago, when
rocky, icy debris in our Solar System accumulated to form planets.

During this violent time, much of the debris would have resembled comets -
dirty snowballs thought to be mostly slushy water surrounding a rocky core -
slamming into Earth at velocities greater than 25 km per second (16 miles per
second).

The severity of the laboratory test was equivalent to an oblique collision
with the rocky surface of the Earth - a comet coming in at an angle of less
than 25 degrees from the horizon, rather than head on (perpendicular to the
Earth's surface).

"At very low angles, we think that some water-ice from the comet would
remain intact as a liquid puddle concentrated with organic molecules, ideal
for the development of life," Blank said.

"This impact scenario provides the three ingredients believed necessary for
life: liquid water, organic material and energy," she added.

Though it has been estimated that in Earth's early history only a few percent
of comets or asteroids arrived at low enough angles, the bombardment would
have been heavy enough to deliver a significant amount of intact organic
material and water, Blank said.

The next hitch-hikers she plans to subject to a shock test are bacterial
spores, which some have proposed arrived on Earth via comets to jump-start
evolution.