Deep Impacts From time to time, the orbits of some small bodies in the solar system are slightly modified, and a comet or an asteroid happens to hit the Sun or a planet. In 1994, the fragments of a comet named Shoemaker-Levy 9 impacted the planet Jupiter, throwing enormous amounts of dust into its atmosphere. The giant planets (mainly Jupiter but also Saturn, Uranus and Neptune) behave like goal keepers: their huge gravitational fields attract small bodies coming from the outer Solar System and prevent most impacts on the inner planets (Mercury, Venus, Earth and Mars). Impacts and mass extinctions It has been estimated that a significant impact, produced by a body in the 10-km size range, could take place on Earth about once every 100 million years. Indeed, the disappearance of the famous dinosaurs, and of numerous other forms of life about 65 millions years ago, was most likely trigged by a cosmic impact (on what is now the northern part of the Yucatan peninsula in Mexico), which injected large amounts of dust into the atmosphere and induced major changes in the ecosystems. The early bombardment The impact rate was much higher in the early Solar System. Numerous small bodies of ice and dust, orbiting the Sun in the region where the giant planets had just been formed, were subjected to the attraction of the massive Sun and of nearby giant planets. Many of them were hurled into the inner Solar System, and many of them hit our planet. Of course, there was no human being to watch the fierce impacts which took place at that remote time, between 4 and 3.8 billion years ago. However, we can still see huge impact basins on the Moon and Mercury, which are the scars of some of these impacts. We can even compute that the Moon itself has probably been formed out of the debris of a giant impact on the young Earth. The harbingers of Life? Now, some of these small bodies were certainly comparable to the cometary nuclei that are studied to-day by cometary space missions. There are good reasons to believe that some of these porous chunks of ice (mainly water, also some carbon dioxide) and dust (silicates with carbonaceous compounds) broke off in the primitive atmosphere, releasing water vapour and fluffy particles. The water condensed while the fluffy particles slowly fell to Earth. Possibly, the ponds of water enriched in amino-acids that then appeared on the surface of our planet were the factories where the most pristine forms of Life appeared... It is most likely that impacts of small bodies have played a major role in the appearance and evolution of Life on Earth. Could the same process have allowed the appearance of Life on Mars? And could similar processes have allowed the appearance of Life on exo-planets around other stars, all around the Universe? Links to NEO pages Atlin Realty Asteroid Comet Impact Hazards Asteroid Radar Research AMICO 2000 Home Page Asteroid Comet Impact Hazards: The Torino Scale Comet Shoemaker-Levy Home Page (JPL) Database of Physical and Dynamical Properties of NEOs Near-Earth Asteroid Tracking International Meteor Organization Jarmo Moilanen´s WWW-site - Impact crater list The LINEAR Project Home Page LONEOS Description Near Earth Object Program Near Earth Object Dynamics Site Regional Geophysics Section Spaceguard Survey:Table of Contents The Spaceguard Foundation Home Page The Spacewatch Project Unusual Minor Planets

One of the Shoemaker-Levy 9 impact craters on Jupiter obtained by the HST. This spectacle occured in 1994. The "Noerdlinger Ries" crater in Germany, the remnant of a meteorite impact 15 million years ago.