Long period comets originate in the Oort Cloud, a huge sphere comprising trillions of comets that surrounds the solar system, 20,000 to 100,000 Astronomical Units (AU) from the Sun. Long period comets will often return after thousands or even millions of years, or not at all. The Oort Cloud is estimated to contain 7,000,000,000,000 comets. There are 5 to 10 long period comets that approach the Sun each year. Some of these comets are behemoths, which are several hundred miles across.
What would happen if a massive Oort Cloud comet struck the Earth? The impactor would smash through the planetís crust and drive deep into the planetís interior where it would release most of its energy. A large comet, 200 miles in diameter, traveling at 100,000 mph with a density of 0.75 gm/cc would have impact kinetic energy equivalent to 3.1 x 1012 megatons of TNT. The kinetic energy from the Cretaceous/Tertiary (K/T) impact of 65 millions years ago, that caused the extinction of many lifeforms including dinosaurs, is estimated at 108 megatons of TNT. One of these large comets would release 31,000 times the energy of the impactor that caused the K/T extinction. The shock waves released inside the planet would tear Earthís crust apart. A large part of the 1.24 x 1025 BTU of impact energy would be released as thermal energy superheating Earthís magma.
The destruction of Earth's crust would release flood volcanic eruptions on a massive scale. The surface of the Earth would become a molten sea of lava. Volcanic eruptions would release vast quantities of gases into the atmosphere. Magma contains hydrocarbons that ignite and burn as they come into contact with oxygen in the atmosphere. This would draw down oxygen levels. These gases consist of steam, carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen and fluorine. The impact would produce a thick heavy atmosphere. Magma is very hot, with temperatures ranging from 1,650o to 2,200oF. Heat release from the magma would dramatically expand the atmospheric envelope to many times its original thickness. An outer layer of clouds composed of water vapor and sulfur dioxide would form around the entire planet. The outer layer would reflect most of the solar radiation back into space, leaving the planet in continual darkness. The inner thick layer of carbon dioxide would act as a heat blanket, sealing and trapping the volcanic heat at the surface of the planet. Carbon dioxide is very effective at blocking infrared radiation. The planet's surface temperature would hover around 2000oF.
After several million years, the planet would cool down and a crust would form. The crust creates an insulation barrier between the hot magma and the atmosphere. Surface temperatures would drop below 1,000oF. Clouds composed of sulfuric acid droplets would rain acid down onto the planet and over time bleed out into surface mineralization. Sulfur dioxide/sulfuric acid clouds would thin substantially. Sunlight, although dim, would make its way to the planet's surface. The atmospheric shield inhibiting photosynthesis would dissolve away. The intense period of flood volcanic eruptions on the planet would begin to come to an end.
The planet Venus is almost an identical twin with Earth. The two planets have approximately the same size, mass, density, volume, bulk composition, gravity and distance from the Sun. But thatís where the similarities end. Venus has no oceans. The average temperature at the surface of the planet is 865o F. The planet has a very heavy atmosphere, which is composed of primarily carbon dioxide (97%). Atmospheric pressure at the surface of the planet is 92 times that of Earth. Clouds of sulfur dioxide float high above the surface of the planet and rain down pure sulfuric acid. Because of its cloud cover, Venus is the brightest planet in the night's sky. It reflects approximately 80 percent of the sunlight incident on the planet.
Venus has more volcanoes than any other planet in the solar system. Over 1,600 large volcanoes have been identified and there are perhaps a million smaller ones. Volcanic intensity on the planet has slowed. There are very few impact craters (~ 900) on the surface, indicating the planet's crust is still quite young. Scientist estimated the crust was resurfaced 300-500 million years ago. The impact craters on Venus are quite different than those on the moon and on other planets in the solar system. Approximately 30% of the impact craters are partially embayed with lava. Lava floods the crater floor and in some cases breached the crater's rim and overflows. This observation would be expected if the planetís crust were quite thin.
Venus is presently in a stage of development analogous to the end of the Hadean Era of the Precambrian Eon on Earth. At this stage the earliest forms of life appeared. These hardy forms thrived in the superheated, very toxic environment. They fed off the sulfur and the carbon dioxide and transformed the planetary environment into a viable modern environment of moderate temperatures, oxygen and water.
Venus is different from Earth in other ways. Venus has only minor axis rotation. The planet makes one complete spin approximately every 243 days. Venus has an extremely weak magnetosphere. The magnetic field of Venus is 100,000 times weaker than Earth's field. The magnetosphere provides the planet protection from solar winds, plasma of hot ionized gas that flows out from the sun, and from cosmic radiation. The lack of a planetary magnetic field allows charged particles to collide with its atmosphere. These collisions strip the outer atmosphere of the lighter atoms, hydrogen and helium. Collisions of charged particles would also destroy the ozone layer that protects the planet from ultraviolet radiation. Radiation would split apart water molecules into hydrogen and oxygen. The hydrogen would then be blown away into deep space by the super-fast hydrodynamic escape process and lost forever. As a result, Venus has no oceans and minimal atmospheric water vapor. Water makes up only 0.003 percent of the present atmosphere. The water has essentially been bled off the planet. The lack of available moisture represents one of the key factors making it difficult for life to establish a foothold on the planet.
What would happen if a massive Oort Cloud comet were to strike the Earth? The answer is Venus. It's not everyday that we have the opportunity to witness a planet being reborn. In the case of Venus, the big question is "Will it be a stillbirth?" Or maybe a better question is "Should we help in the delivery?"