A comet is one of several types of solar bodies present in the night sky. They orbit the sun and when close enough to the sun, the often display a coma, which is a temporary atmosphere surrounding the comet, and a tail. The presence of these two features separates comets from asteroids. The composition of comets also differs from asteroids as they are thought to originate from different areas of our solar system. While the composition of many comets has been determined through the use of light spectroscopy of the tails, two recent space missions have helped expand upon the known compositions of comets in the solar system.
As part of looking at the night sky, telescopes have been used to examine the light spectrum of the tails of comets passing by the earth and determine the composition of materials coming off of many different comets. These did not involve any special spacecraft missions but were just examinations done on passing comets. The detected substances include: Argon, hydrogen, carbon, water, carbon dioxide, nitrogen, and oxygen. Comets are also known to contain methane and ethane gas as well as the amino acid glycine.
The Deep Impact Mission involved the launching of the Deep Impact spacecraft whose mission was to further research into comets. On July 4, 2005, the spacecraft launched an 800 pound projectile at the comet Tompel 1. The purpose of this action was to hit the comet and release a large amount of particles and debris off the surface layer of the comet the comet. The projectile was successful and many observing telescopes took pictures of the event. These pictures were in the infrared spectrum and from the colors detected, the composition of surface of the comet was revealed.
The comet’s surface layer was comprised of silicates, which comprise the majority of the earth’s crust, crystalline silicates, organic material that they were unable to further evaluate, sodium, a large layer of very fine dirt. There was no ice found but it is suspected that it had either dried out or was covered by the fine dirt layer and was deeper below the surface than what was ejected by the impact. Probably the most interesting discovery were the minerals that were detected. These minerals, clays and calcium carbonates, are only formed in liquid water. This means that at some point, liquid water was present on the comet to form these minerals.
As part of a secondary mission, the Deep Impact spacecraft also measured the gas emissions from the comet Hartley 2. This happened on November 4, 2010 and the most abundant gas that was measured coming off the comet was found to be carbon dioxide.
A later space mission, known as the Stardust mission, passed by the comet Wild 2 and was able to collect samples from its tail. This occurred on July 2004 and the spacecraft returned to earth about two years later. The findings from the particles indicated that the material in the tail contained water molecules, abundant organics, and many chemical elements such as calcium, magnesium, titanium, aluminum, and sulfur. These elements rarely found in space but commonly found on the Earth’s surface.
Though similar to other solar bodies in the solar system, comets are unique and have different compositions and characteristics than other solar bodies. Recent space missions by unmanned spacecraft have helped in improve knowledge on comet composition. Examining the materials of the top surface layers as well as sampling the particles in the tail of a comet have helped scientist discover and learn more about comet composition.