Was Johnny Appleseed a Comet?
Four billion years ago Earth was bombarded by a hail of comets and asteroids. The shattering collisions rendered our planet uninhabitable during a period scientists call the Late Heavy Bombardment (LHB).
It surely sounds like the LHB was an awful time for the beleaguered young planet — but perhaps the pelting was a good thing after all, say researchers. Kamikaze comets could have delivered important organic molecules to Earth — sowing the seeds for life.
Genesis by comets is a controversial idea, but it’s just received an important boost. A NASA-supported experiment reveals that complex molecules hitchhiking aboard a comet could have survived an impact with Earth.
"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," says Jennifer Blank, a geochemist at the University of California, Berkeley. Blank and colleagues simulated a comet collision by shooting a soda-can sized bullet into a metal target containing a teardrop of water mixed with amino acids – the building blocks of proteins.
|In this schematic diagram of Blank et al.’s apparatus the red arrow indicates the projectile, which is fired from the breach toward the stationary target (inset). The 3 smaller red triangles indicate transducer pins that measure the velocity of the projectile as it passes. "Upon impact, the sample container and a similarly-sized metal plug fly backwards into the recovery area, where they’re trapped as gently as possible in layers of felt," adds Blank.|
Credit Jennifer Blank, UC Berkeley
Not only did a good fraction of the amino acids survive, but many polymerized into chains of two, three and four amino acids, so-called peptides. Peptides with longer chains are called polypeptides, while even longer ones are called proteins.
"The neat thing is that we got every possible combination of dipeptide, many tripeptides and some tetrapeptides," said Blank. "We saw variations in the ratios of peptides produced depending on the conditions of temperature, pressure and duration of the impact. This is the beginning of a new field of science."
Freezing the target to mimic an icy comet increased the survival rate of amino acids, she added.
Blank’s ballistic test was designed to simulate the sort of impact that would have been frequent in Earth’s earliest history when rocky, icy debris in our solar system combined to form the planets. 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 16 miles per second (25 km/sec).
The severity of the laboratory impact was akin to that of an oblique collision between the rocky surface of Earth and a comet coming in at an angle of less than 25 degrees from the horizon.
|In a related project conducted by members of NASA’s Astrobiology Institute, scientists have created primitive organic cell-like structures. They did it in their laboratory by duplicating the harsh conditions of cold interstellar space! Did comets carry such protocells to Earth?|
Benton Clark, chief scientist of Flight Systems at Lockheed Martin Astronautics, proposed in 1988 that if comets are slowed sufficiently — by, e.g., drag from Earth’s atmosphere, which would be greatest at low impact angles — some water and organic compounds might survive the collision.
"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."
Though comet hunter Eugene Shoemaker 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, according to Blank’s estimates.
One well-known model for the beginnings of life on Earth posits that terrestrial life sprang from complex molecules such as amino acids and sugars produced by electrical discharges in a primeval atmosphere replete with gases such as methane, hydrogen, ammonia and water. The famous Miller-Urey experiment in 1953, conducted by Stanley Miller and Harold Urey of the University of Chicago, demonstrated that a lightening-like discharge in a test tube filled with these molecules could produce amino acids.
|The Miller-Urey experiment generated electric sparks — meant to model lightning — in a mixture of gases thought to resemble Earth’s early atmosphere.|
Other scientists believe that the building blocks of life on Earth arrived from space. Astronomers have detected many kinds of organic molecules in space, floating in clouds of gas or bound up in dust particles. They range from the simplest – water, ammonia, methane, hydrogen cyanide and alcohols, including ethyl alcohol – to more complex molecules.
Interestingly, of the more than 70 amino acids found in meteorites, only eight of them overlap with the group of 20 which occur commonly as structural components of proteins found in humans and all other life on Earth.
To test whether water and organic compounds could survive the high pressures and high temperatures of a collision, Blank and her colleagues worked for three years to design a steel capsule that would not rupture when hit with a mile-per-second (1.6 kilometer-per-second) bullet fired from an 80-mm bore cannon at the University of Chicago and later at Los Alamos National Laboratory. The target she and her team developed – a two-centimeter diameter stainless steel disk about a half-centimeter thick – was able to withstand about 200,000 times atmospheric pressure without bursting.
They filled the small cavity with water saturated with five amino acids: three from the list of 20 that comprise all proteins in humans (phenylalanine, proline and lysine) and two varieties detected in the Murchison meteorite (aminobutyric acid and nor-valine). That meteorite plummeted to the ground in 1969 in Australia and is thought to be from the core of a comet.
The survival of a large fraction of the amino acids and their polymerization during the collision makes the idea of an extraterrestrial origin of organic compounds a strong contender against Miller-Urey style theories, Blank said.
"About one comet per year arriving in a low-angle impact would bring in the equivalent of all the organics produced in a year in an oxidizing atmosphere by the Miller-Urey electric discharge mechanism," Blank estimated. "An advantage is you get all of it together in a puddle of water rather than diluted in the oceans."
The next hitchhikers she plans to subject to a shock test are bacterial spores, which some have proposed arrived on Earth via comets to jump-start evolution.
Related Web Pages
Life from a Dirty Snowball – article about the possibility that complex organic molecules form in comets
Life From Space — Jennifer Blank talks about comets seeding life on Earth
Organic molecules fall to Earth in meteors — article about the discovery that organic molecules on meteors might be able to survive the heat of entry into Earth’s atmosphere
How Does Life Begin and Develop? — A discussion of theories about the origin of life
Scientists find clues that life began in deep space — an article about research that produced "proto-cells" — membranous bubbles with organic chemicals inside — in the simulated environment of deep space
Comets — more information
The Miller-Urey experiment — a brief summary of the experiment that produced amino acids by sending sparks through a simulated early-Earth atmosphere
American Chemical Society annual meeting — home page for the meeting where Jennifer Blank presented her group’s findings.