Re: Why are the same substances present on more than one planet?

Dear Miss Arbogast's Science Class
Temperature and gravity in the early forming solar system did indeed cause differentiation between different elements and molecules in what we call the pre-solar nebula (sometimes also known as an accretion disc). However, because this accretion disc was generally hot and turbulent, there was a lot of mixing going on. As the earliest planetesimals were forming this mixing means that separation between different chemical types would not have been anything like as complete as you suggest in your question. Naturally, nearest to the sun as the proto-sun was accreting (before it ignited nuclear reactions to shine as a star), it was hottest and so we find minerals and elements there that "can stand the heat"---what are called refractory elements and minerals. There are compositional differences between the planets which reflect this, for instance, Mercury is thought to have proportionally more iron and a larger core than the planets further out whilst the outer planets (which I think you were thinking about in your question) do indeed have substantial atmospheres of gases like hydrogen, methane and ammonia. That is largely because when the core of the sun became large and hot enough to ignite thermonuclear reactions, the sun went through a violent, short-lived phase known as a T-Tauri phase (named after a star in the constellation of Taurus that seems to be going through a similar phase now). This violent reaction literally blew away everything light (like hydrogen) near the sun and so the inner planets have relatively little hydrogen or other very volatile elements.

Interestingly you mention ammonia and methane in the Earth’s atmosphere. This leads to another fascinating question you might like to think about because, unlike what you imply in your question, none of the methane and ammonia in the Earth’s atmosphere today was formed at the time of the formation of the Earth. If you think about it, the Earth’s atmosphere has a unique and chemically bizarre composition. Methane and ammonia are only trace constituents but there should not be even that much present, they should have reacted completely with the oxygen in the atmosphere. Indeed, all that oxygen in the atmosphere is chemically ridiculous---it should react with the rocks (oxidising them) and disappear completely within a fairly short geological time.

The answer to this is life on Earth. Without life there would be no oxygen, and all the methane comes mainly from cattle! Ammonia on Earth too is formed mainly by biology. All these chemicals are being constantly renewed in the atmosphere by life. A good project would be to look at the Earth’s atmosphere and ask how it compares with the atmosphere of Mars and Venus and why it is so incredibly different. Bizarre compositions like this form a good way of searching for life on other planets---try searching the NASA web site for projects designed to search for life on other planets!