The Japaneese IR spacetelescope IKARI have analyzed comets for water-, CO- and CO2 ice, and thus been able to give an answer to the question of the original CO/CO2 balance of the Solar-system

Comets are often described as “dirty snowballs”. A comet’s main body (cometary nucleus) is composed of ice and dust (e.g. grains of sand, small rocks). As a comet from a distant place in the universe approaches the Sun, its ice sublimates and changes to gas. This gas and dust form a “coma” that thinly spreads around the cometary nucleus and forms a long tail. In this way, the familiar shape of the “broom star” is produced. There are still many questions about the ratio of ice and dust in comets. One theory in the latest studies suggests that comets contain much more dust than previously thought, making them more like “icy dirtball” than “dirty snowball.” Using such humorous terms, researchers conduct serious discussions.

It is thought that ice, the main constituent of cometary nucleus, is comprised of about 80% water and the remaining 20% carbon dioxide (i.e., dry ice), carbon monoxide, etc. In addition, it is thought that it contains trace matters: hydrocarbons such as methane and ethane, ethanol (alcohol!), and ammonia. “Snowball (or dirtball)” made of ice and dry ice including other trace materials are flying across the solar system.

AKARI has observed comet Lulin several times at various timings. This article reports the observation results of March 30 and 31, 2009, after the comet passed through the perihelion (the nearest point to the Sun). The InfraRed Camera (IRC) onboard AKARI is able to conduct both imaging and spectroscopic observation. On March 30, we conducted near-infrared spectroscopic observation in the 2 to 5μm wavelength. On the next day, March 31, we performed three-color imaging observation in 2, 3, and 4μm.

A false-color picture obtained at 2, 3, and 4μm wavelengts of comet Lulin. The field of view (FOV) is 10arcmin x 10arcmin. The coma  extends to the full FOV.

It was mentioned above that comet ice mostly consists of water, carbon dioxide, and carbon monoxide. With near-infrared or radio wave, we can observe molecules of water and carbon monoxide from the ground. We have been actively researching to estimate their abundances. However, it is impossible to observe carbon dioxide, the source of dry ice, from the ground. Carbon dioxide radiates in around 4.26μm and 15μm induced by its molecular vibration. To observe such radiation, we need to use a rocket or satellite to avoid the earth’s atmosphere. The near-infrared wavelength region (i.e. 2 to 5μm) in AKARI’s observation covers the 4.26μm radiation by molecular vibration of carbon dioxide. AKARI can also cover both the 2.66μm radiation of water molecules and 4.67μm of carbon-monoxide molecules. Thus, AKARI is the ideal satellite for observation of molecules contained in the cometary nucleus.

There have been only several observational cases of the carbon-dioxide molecules of comets across the world in the past. The cases are: two comets by nearby and in-situ observations by explorers (i.e. comet Halley by the Vega explorer of the former Soviet Union and comet Tempel 1 at the time of its collision with the impactor of NASA’s Deep Impact mission); and comet Hale-Bopp and comet Hartley 2 by Europe’s infrared astronomical satellite ISO. In addition, there was only one case with comet Hale-Bopp by ISO where the three main molecules of water, carbon dioxide and carbon monoxide were “simultaneously” detected “with the same instrument.” If we could observe many comets from various heliocentric distances, we could determine more accurately the ratio of these molecules. AKARI’s observation is revolutionary. It allows us in a short time to increase observational data of carbon dioxide contained in cometary nuclei more than fivefold.

Then, what we can discover from such observation? First of all, let’s consider where and how cometary nuclei were created. There are many types of comets: some reappear in cycles of several years to tens of years; some come from the outer rim of the solar system and disappear beyond the system without returning. Tracing their origin, it is thought that the home of most comets is a relatively limited region a little outside the rim of the early solar nebula, specifically the region from Jupiter to beyond Neptune. A number of planetesimals from several km to several tens of km in size were born from gas and dust in the disk of the primitive solar nebula 4.5 billion years ago. These planetesimals merged and grew to protoplanets, and grew further to become today’s planets. This is a typical theory of the solar system formation. Planetesimals born in the cold area outside of Jupiter contained much ice. It is thought that many planetesimals collided with each other or were absorbed into protoplanets or planets. Meanwhile, there must have been many planetesimals that did not collide but whose orbits were changed by planetary gravity. Finally, they were blown out to the outer rim of the solar system. They became the origins of comets.

According to this history, comets are considered as fossils of the solar system, and contain various data of the time the system was formed 4.5 billion years ago. The composition of the gas released from these fossils is an important clue to discover the true face of the solar system in the age of the early solar nebula. Cometary nuclei were created in the solar system 4.5 billion years ago and, further, in a deep disk made of gas and dust. We have no means of knowing such place directly. All the information, including the ratio of ice and dust, of water and carbon monoxide, and of water and carbon dioxide, is a clue to know the deepest area of the primitive solar nebula of eons ago.
 

This is an example of the spectrum of comet Lulin observed by AKARI. Strong radiation is clearly visible around 2.6 to 2.7μm of water and around 4.2 to 4.3μm of carbon dioxide. Meanwhile, it is noticeable that carbon monoxide radiation, which is expected to appear around 4.7μm, is weak. By applying a model assuming the material distribution of comet’s coma, we forecast a ratio of molecular abundances in comet Lulin’s coma. The result is that, assuming the number of water molecules is 100%, the relative number of carbon dioxide is about 4 to 5% and that of carbon monoxide is less than 2%. These values of carbon dioxide and carbon monoxide are rather low compared to the observation results of comets in the past. Carbon dioxide (dry ice) transforms to gas at a lower temperature than water. Similarly, carbon monoxide changes to gas in lower temperature than carbon dioxide. Considering this fact, we infer that there is a high possibility that Lulin’s cometary nucleus was created in a relatively hot place within the primitive solar nebula, in other words, closer to the Sun.

This type of observation and research focusing on carbon dioxide in comets is still in its developmental phase. At this stage, we can identify the ratio of molecule abundances of several comets, but cannot forecast where their cometary nuclei were formed in the past solar system. Nevertheless, AKARI is actively observing comets (e.g., Fig. 3). We expect that observation cases will increase and data accumulate to the degree that we can discuss them statistically. The data will bring important clues to elucidate what evolution occurred in the deeper regions of the dust disk in the primitive solar nebula 4.5 billion years ago, and what materials composed the cometary nuclei (planetesimals) or protoplanets. The combination of comets and AKARI is a kind of time machine to explore the history of the solar system.

Source: JAXA