Did Cosmic Collisions in Earth’s Early History Help with Development of Life on Earth?

Compared to Mars and meteorites from ancient asteroids, Earth contains a much lower quantity of certain elements, such as chlorine. If Earth had the same levels of these elements as Mars, we would have a poison planet by human or biological life standards.
Two scientists, Dr. Zachary Sharp of the University of New Mexico in Albuquerque and Dr. David Draper of the NASA Johnson Space Center in Houston estimated that the concentration of chlorine on Earth is 17 parts per million. And that’s a very puzzling number. Scientists have found that the meteorites that formed at the birth of the solar system have much higher levels of chlorine. Based on those studies, Sharp and Draper estimated that the early Earth had 10 times the amount of chlorine than it has today.
They investigated the hypothesis that the chlorine dissolved into Earth’s metallic core during its early development. Their results suggested that the core is not, in fact, where chlorine went. In high pressure experiments, Sharp and Draper added iron metal, rocks representing the mantle, and a chlorine compound to a capsule heated to 1900° Celsius under pressures about 150,000 times higher than Earth’s surface atmospheric pressure. They measured chlorine’s “partitioning” behavior, meaning they determined whether chlorine became incorporated into the metal or into the rocky material, and found that almost none of it goes into the metal. From an article in Science News, “Early Earth’s chlorine blown away by giant impacts,” “Chlorine has long puzzled scientists because modern levels are so low.” “The result: Chlorine still didn’t dissolve in iron. That means chlorine probably isn’t hiding out in the core, Sharp says. So he and Draper looked elsewhere for a solution. After ruling out the possibility that Earth never accumulated chlorine in the first place, the pair concluded that the incipient Earth rammed into giant planetary bodies more than 4 billion years ago and the repeated impacts blew the element away.” One such giant impact has been postulated for two decades as the likely process resulting in the formation of Earth’s Moon.
From a New York Times Article, “Chlorine, Swimming Pool Helper, Has a Checkered Past”, “Dr. Sharp believes that most of Earth’s chlorine was wiped out by the huge meteorites that slammed into the planet during its first few hundred million years. Those impacts vaporized some of the oceans, sending chlorine out into space.
If those impacts hadn’t gotten rid of all that chlorine, Earth would be radically different. The oceans would have been loaded with as much salt as the Dead Sea. Such salty water wouldn’t be able to dissolve oxygen. Much less vapor would rise from it, leading to much less rain. The continents would have become parched, preventing nutrients like phosphorus from flowing from the land to the sea. “It would be a very nasty place,” said Dr. Sharp.”
“If the process we envisage for explaining Earth’s chlorine abundance also took place on Mars, then this situation would be even worse on that planet,” said Draper. “Because the martian chlorine content is much higher than Earth’s, implying its initial chlorine content was correspondingly higher, it would have had even more chlorine-rich oceans, if indeed it had oceans early in its history. These problems would then have been far worse than on Earth. Such conditions would make it very difficult, if not impossible, for life as we know it to arise on the red planet.”

Citations
Z.D. Sharp and D.S. Draper. The chlorine abundance of Earth: implications for a habitable planet. Earth and Planetary Science Letters. Published online April 16, 2013. doi:10.1016/j.epsl.2013.03.005. [Go to]
E. Wayman. Early Earth’s chlorine blown away by giant impacts.
Science News online, published April 23, 2013 [Go to]
C. Zimmer. Chlorine, Swimming Pool Helper, Has a Checkered Past. The New York Times online, published May 23, 2013 [Go to]