PH104 L9

Observations and the Solar System : Lecture 9

Comets, Meteorites and Impacts

We will finish off the Solar system with a discussion of the "extras" or junk which is flying around.

Asteriods and Meteorites

Both Asteriods and Meteorites are rocky objects. There is no clear distinction -usually a large rock is called an asteriod and a small rock a meteorite (particularly if it hits earth : nice example )

There is an area in the solar system where asteroids are rather common. Between Mars and Jupiter there is a belt of asteroids. This Asteroid belt is about 1.5 A.U. wide and lies between 2.0 and 3.5 A.U. from the sun. Some of the asteroids are rather large, for example Pallas-600km across, Cerres 900km. There are many such asteroids - about 100,000 can be seen using photographic plates. Many of the orbits are eccentric. For example icarus's orbit brings it well within that of Mars.

These asteriods are responsible for most of the craters and impact sites we see around the inner planets. Sometimes these hit earth. We looked at slides of Barringer crater in Arizona USA where 50,000 yrs ago a 50m across meteorite landed and created a crater 1.2km across. ( website ) An impact with a large meteorite or comet is one possible explaination for the demise of the dinosaurs. ( possible site underneath Yucatan )

Comets

A comet is - and looks - very different from a meteorite. It is, simply, a large piece of ice which is orbiting the sun in a eccentric orbit. It can be well described as a "dirty snowball". Typically Comets are in very large very eccentric orbits. A "typical" comet might be 10km across For most of its orbit (remember Kepler!) it is far from the sun and very difficult to see, however as it nears the sun the water vaporises this makes the comet much larger and spectacular. Around the 10km nucleous a coma of perhaps 10⁶ km will develope around the nucleous, It also has a tail which might be 10⁸ km long.

Comets also might impact on Planets - Comet Shoemaker-Levy impacted upon Jupiter in 1994. (Since Jupiter is by far the largest planet its gravitional effect is much more than other planets) Images of this include images of the disintegratinging comet and images of the ``bruised'' jupiter - one , two and three

Comets might also pose a significant threat to Earth. The tunguska event in 1908 was probably a comet impacting in Siberia ( photo of fallen trees )

On 12 November 2014, ESA's Rosetta mission soft-landed its Philae probe on comet 67P/Churyumov-Gerasimenko. Data is still being analyed to test various hypothesis such as that the chemical building blocks of life originated in Comets.