Is life on Earth a gift from the comets?
By Quamrul Haider
Nothing in the sky is quite as beautiful as comets sweeping through the inner solar system with the stately grace of a majestic ship at sea. But our fascination with comets goes beyond their beauty. In antiquity, when astrological omens were taken very seriously, great import was attached to the occasion when comets made infrequent and unpredictable appearances from the gelid, dark, outermost fringes of the solar system.They were considered to be harbinger of death and doom.
While it may seem straight out of a science fiction novel, comets are also linked with the appearance of life on Earth, the only planet in the far reaches of one of billions of spiral galaxies, the Milky Way, that we know can support life. The link is based on the argument that when comets left their homethe Oort Cloud and the Kuiper Belt and pummeled the newly formed Earth during the Heavy Bombardment Period about 3.8 to 4 billion years ago, they deposited a rich range of organic material that are chemical precursors of life.
Attributing the origin of life to comets is a radical departure from the traditional views based on purely scientific to the religious doctrine, to others that border on science fiction. Before the publication of Charles Darwins On the Origin of Species in November 1859, the widely held view of life was that it could arise spontaneously from nonliving matter. Darwins work, showing how species develop gradually as a result of environmental pressures, made such an idea seem improbable. However, he made no attempt to explain how life came into being on a once lifeless Earth.
An alternative to the spontaneous formation of life was proposed in 1907 by the Swedish chemist and Nobel laureate, Svante Arrhenius, who suggested that life on Earth was introduced billions of years ago from space, originally in the form of microscopic spores that float through the cosmos, landing here and there to act as seeds for new biological systems. This idea, called the Panspermia hypothesis, can be traced back to Anaxagoras, the Greek philosopher who lived in the 5th century BC. He claimed that the Universe is made of an infinite number of spermata, or seeds, which gave rise to life forms on reaching the Earth.
There are ample evidences to suggest that the Panspermia hypothesis,vis-a-vis cometary origin of life, may not be a far-fetched idea. Analysis of Hale-Bopp, the comet that lit up the night sky for the better part of 1995-1997, revealed that the chemicals that evaporated from its nucleus a ball of ice a few kilometres in diameter were not just tons of water, but many organic materials that are the basic ingredients of life. These materials were also found in Halleys Comet by Giotto, a robotic spacecraft of the European Space Agency, as it flew by and studied its composition in March 1986.
These two comets together with the Hyakutake Comet that made its closest approach to Earth in March 1996 led researchers to conclude that life-forming material may not have been produced on Earth. Instead, they were brought here by comets when they bombarded the nascent Earth. Indeed, the earliest evidence for life on Earth suggests it was present some 3.83 billion years ago, overlapping with the bombardment period.
Other space missions also confirmed the possibility that life on our planet was kick-started by comets. The 2004 Stardust mission found amino acid when it collected particles from Comet Wild 2. When NASAs Deep Impact spacecraft crashed into Comet Tempel 1 in 2005, it discovered a mixture of organic particles inside the comet.
Further credence to the cometary theory of lifes origin was lent in 2015, when Japanese researchers claimed that their experiments simulating the impact of high-speed mock comets frozen soup of amino acids, ice and rock on artificial planets caused amino acids to change into peptides, the first step towards more complex molecules that are necessary for life. In an article published in the journal Science in April 2016, French scientists have for the first time shown that ribose, a sugar that is one of the building blocks of genetic material in living organisms, may have formed in cometary ices. Clearly, these studies open the door not only to the possibility that life-forming molecules were brought to Earth aboard comets, but they could have implications for life on other worlds too.
An ambitious new NASA research project aims to bring together an interdisciplinary team of scientists from around the world to study how organic molecules are created in interstellar clouds and delivered to planets as they form. In particular, they will study the molecular make-up of comets to better understand what chemical reactions formed them. This may provide clues to the evolution of carbon-based chemistry on Earth in its early history.
So, is life on Earth really a gift from the comets? Surely, analysis of cometary debris shows that comets were heavenly vans that transported tons of carbon-rich materials from interstellar space that may have played a pivotal role in the beginning of life on Earth. This does not preclude the hypothesis that we are simply highly evolved primates monkeys progressively turning into upright humans.
The writer is a Professor of Physics
at Fordham University, New York.
Photos: Google Image
