Chapter 12***

the mass extinction

Fateful day some 65 million years ago--asteroid or comet slammed into Mexico with the force of a hundred million hydrogen bombs, hitting at a slight angle causing a huge tsunami, ignited fires. Dust and smoke remained in the atmosphere for weeks or months, blocking sunlight and causing temps to fall as if Earth were experiencing a global and extremely harsh winter. Stoped photosynthesis, released large amounts of carbon dioxide into the atmosphere which strengthened the greenhouse effect. Caused chemical reactions in the atmosphere that produced large quantities of harmful compounds, such as nitrous oxides. Acid rain.

Is the impact threat a real danger or media hype?

Geological data shows the impacts large enough to cause mass extinctions happen many tens of millions of years apart on average (unlikely in our future). Smaller impacts can be expected more frequently. In 1908, a tremendous explosion occurred over Tungaka. Another way to gauge the threat is to look at objects in space that might strike Earth. The threat comes from comets is lower, but we are unlikely to see a comet plunging in from the solar solar system until it is well on its way, giving us no more than a few years to prepare. These impacts are unlikely to strike densely populated areas, because most of Earth's surface is ocean, and even on land humans concentrated in relatively small urban areas. Some asteroid bring valuable resources tantalizingly close to EArth (iron rich). It may also be possible to gather fuel and water from asteroids for use in missions to the outer solar system.

evidence for the impact

Key evidence comes from further analysis of the sediment layer. Besides being unusual rich in iridium, this layer contains four other unusual features: 1)high abundances of several other metals, including osmium, gold, and platinum 2) grains of shocked quartz, quartz crystals with a distinctive structure that indicates they experienced the high pressure conditions of an impact; 3) the spherical rock droplets of a type known to form when drops of molten rock cool and solidify in the air 4) soot (at some sites) that appears to have been produced by widespread forest fires

mass extinction

The death of the dinosaurs was only a small part of the biological devastation that seems to have occurred 65 million years ago. The fossil record suggests that up to 99% of all living organisms died around that time and that up to 75% of all existing species were driven to extinction. This makes the event a clear example of mass extinction--the rapid extinction of a large fraction of all living species.

survivors of extinction?

The most astonishing fact is not that 75% of all species died but that 25% survived. Among the survivors were a few small mammals. These mammals may have survived because they lived in underground burrows and managed to store enough food to outlast the global winter. With the dinosaurs gone, mammals became the new kings of the planet. Over the next 6.5 million years, the mammals rapidly evolved into an assortment of much larger mammals--ultimately including us.

Did an impact kill the dinosaurs?

There's no doubt that major impacts have occurred on Earth in the past. Geologists have identified more than 150 impact craters on our planet. An impact could cause widespread physical damage. Large impacts may have altered the entire course of evolution. Alvarez team found a thin layer of dark sediments deposited about 65 million years ago--about the time the dinosaurs went extinct--was usually rich in the element iridium. The Alvarez team suggested a stunning hypothesis: the extinction of the dinosaurs was caused by the impact of an asteroid or comet.

How do the jovian planets affect impact rates and life on Earth?

Ultimately, every asteroid or comet that has impacted Earth since the end of the heavy bombardment was in some sense sent our way by the influence of Jupiter or one of the other jovian planets. There is a deep connection between the jovian planets and the survival of life on Earth. The role of Jupiter has led some scientists to wonder whether we could exist if our solar system had been laid out differential. Could it be that civilizations can arise in solar systems that happen to have a Jupiter like planet in a Jupiter like orbit?

Have we ever witnessed a major impact?

We have never witnessed a major impact on a terrestrial world, but have seen one on Jupiter: the 1994 impact of a comet named shoemaker-levy 9. Each of the individual nuclei crashed into Jupiter with an energy equivalent to that of a million hydrogen bombs. Comet nuclei barely a kilmeter across left scars large enough to swallow Earth.

controversies and other mass extinctions?

measuring precise extinction rates becomes more difficult as we look to older fossils, but there appear to have been at least 4 other mass extinctions during the past 500 million years.