Lesson9a - Formation Comets and their effect

Comet Hayukatake

Comet Halley up close

Comet Hartley2 from Deep Impact Space Probe

Comet Compositions Water Ice (H 2 O) Dry Ice (CO 2 ) Dust and Rock In the vacuum of space (no air pressure) what is the state of H 2 O and CO 2 ?

. 1.Gas. 2.Gas or a liquid, depending on the temperature 3.Gas or Solid, depending of the temperature 4.Gas, Liquid or Solid depending on the temperature 5.Solid.

Comets are on highly elliptical orbits A comet spends most of its time far from the Sun where temperatures are very low. When it passes close to the Sun, the comet heats up and the volatile gasses (H 2 O and CO 2 ) begin to evaporate. There is no liquid state. As the gasses explode outward they drag dust along with them. Comets first develop a Coma and then a tail.

Gas tail Dust tail

When the comet leaves the Sun, will it look like this…. 1)

Or this? 2)

. 1.#1 2.#2

It’s this one… Solar Wind and Radiation drives the tail. 2)

Comet’s tail always points away from the Sun In the vacuum of space the only thing pushing on the tail is the solar wind and the light coming from the Sun. The tail has to always point away from the Sun, regardless of how the comet moving. Charged particles in the solar wind exert a force on the gas molecules When dust particles absorb sunlight they receive a push as well.

Gas tail Dust tail

Elemental abundances in the Universe

Other than hydrogen and some helium, all other elements are made inside stars. When stars die they expel these elements back out into space where new objects incorporate them. Carbon and Oxygen are about 10 times more abundant than any other element, except for hydrogen and helium. Both of these elements are made in stars where 3 Helium nuclei are fused into one Carbon, and an additional Helium makes Oxygen.

What we find in the Solar System is lots water, carbon dioxide, and methane. H 2 O, CO 2, CH 4 But not in the inner Solar System. It looks like Earth has lots of water because of the oceans, but this is a tiny fraction of the Earth’s mass. What is the difference with Mercury, Venus, Earth and Mars compared to the rest of the Solar System?

. 1.The inner planets are geologically active 2.The inner planets are rocky planets 3.The inner planets all have atmospheres

The Frost line for water

Close to Sun, H 2 O, CO 2, and CH 4 are all gasses. Beyond the orbit of Mars these are solids.

What does the Sun do to comets that evaporate their water and dry ice?

The solar wind drives these gasses away from the Sun, causing a tail to form from the comet.

The Solar Wind drives the gasses out to the outer parts of the Solar System. Just like it does for the tail of a comet. In the outer solar system the volatiles are in the form of ice. Ices can stick together and make larger bodies. Jupiter originally was a mixture of rocky material and ice. It grew large enough to be able to hold onto hydrogen and helium. It then became a gas giant. Outer solar system moons and comets are covered in ice.

In the inner solar system it was cool enough for iron and silicate compounds to form and make rocky material. But not cool enough for ice to accumulate. The inner planets were devoid of these volatile compounds.

In this scenario of solar system formation, Earth should have virtually no water. Where do you think most of the water on Earth came from?

. 1.Water was trapped in the rocks and later released. 2.Water came from volcanic activity 3.Water came from collisions with comets.

Today it is thought that virtually all the water on Earth came from collisions with icy bodies. Many of these bodies probably came to Earth from the migration of Neptune and Uranus caused by the 1:2 resonance of Jupiter and Saturn. Even more interesting, comets have been found to contain amino acids. The building blocks of proteins and DNA. Comets might have seeded the Earth with the compounds of life.

Importance of comets Life needs water. Comet collisions provide this. Comets have amino acids, life needs this. It seems to be the case that without the giant planets in the solar system, we (humans) would not be here today. The planets shot almost all of the small icy bodies out into the Oort cloud.