Comets 'not cause of extinctions'

Comet strikes are an unlikely cause of past mass extinctions on Earth, according to computer simulations.

Scientists used the simulations to model the paths of long-period comets, to determine the likelihood of these "dirty snowballs" striking our planet.

The University of Washington, Seattle, research appears in Science journal.

How many mass extinctions in Earth's history were caused by these icy bodies crashing into our planet has been a subject of considerable debate.

Many scientists agree that an asteroid strike 65 million years ago wiped out the dinosaurs. But there is uncertainty about how many other such events were triggered by asteroid or comet colliding with Earth.

Comets are composed of dust, rock, water ice and frozen gases. When their orbits bring them into close proximity with the Sun, parts of them warm up, causing material to sublimate (turn directly from a solid to a gas state) and form a "fuzzy" envelope around the comet nucleus.

Asteroids are distinguished from comets precisely because they lack this envelope, or "coma".

via BBC NEWS | Science & Environment | Comets 'not cause of extinctions'.