WEEK 10: MARCH 1-7

TOPIC: FRED WHIPPLE'S COMET MODEL

     

Once it had become clear that comets are bona fide members of the solar system just as planets and asteroids are, the question then becomes just what their physical nature might be. While they may appear to be fairly large in our nighttime sky, the fact that background stars shine through their tails and their comae undimmed indicates rather strongly that these are not solid but instead are mere clouds of material. Furthermore, while a comet that passes near a planet such as Jupiter will have its orbit strongly affected, the orbit of the planet itself is not affected by any measurable extent, indicating that a comet’s mass is negligible compared to that of a planet. Astronomers of the 19th and early 20th Centuries who studied comets that were passing close to Earth were unable to detect any kind of solid object in or near the center of their comae, suggesting that any such object had to be small, no more than a couple of hundred km across.

Once spectroscopic observations of comets began to be made during the 1860s these revealed the presence of several gaseous substances, including those containing carbon atoms. Meanwhile, studies of cometary tails had revealed that one of their primary constituents is dust, a conclusion that was reinforced once the relationship between comets and meteor showers was established during the latter decades of the 19th Century.

As is true with any scientific model, any explanation of a comet’s physical nature had to take all these various facts into account. One of the early widely accepted explanations of a comet was the so-called “flying sandbank” model developed in the 1880s by British astronomer Richard Proctor and championed most strongly in the early- to mid-20th Century by British astronomer Raymond Lyttleton. Within this model, comets are composed of a swarm of small solid dusty “objects,” the largest perhaps being a few meters in diameter, which would contain the various gases absorbed within them. As this “sandbank” approaches the sun the heat from sunlight would cause the objects within it to release these gases, producing the cometary effects we see such as the coma and the tail. Eventually, after the comet has made numerous returns to perihelion, all the gas would be released, and the comet would be nothing except a swarm of dust particles and a few larger objects, perhaps to form part of a meteor shower should Earth ever intercept it in the future.

This “flying sandbank” model, despite being the dominant theory for the structure of cometary nuclei for much of the first half of the 20th Century, suffered from several serious difficulties. Clearly, a comet would lose a significant amount of gas with each perihelion passage, and consequently its “lifetime” before running out of gas would be relatively short. Several comets, however – including 2P/Encke, which returns every 3.3 years – were observed at return after return after return with their supplies of gas seeming to undergo little change. To get around this, the idea was introduced that the “swarm” would accumulate additional gas as it traveled through the solar system, however it soon became clear that comets lost far too much gas when near the sun to ever be replenished by what they might sweep up during the rest of their travels.

What would prove to be the “flying sandbank’s” downfall, however, was a phenomenon first noticed in the 1820s by the German mathematician Johann Encke while studying observations of the comet that is named after him. Encke noted that, even after he allowed for all possible gravitational perturbations by the various planets, the comet was returning to perihelion a few hours earlier than predicted at each return. To account for this, Encke proposed a “resisting medium” in the solar system that was slowing the comet down and pushing it into a smaller orbit.

Throughout the remainder of the 19th Century a few other comets were seen to exhibit this phenomenon, which is now known under the name of “non-gravitational forces.” Despite the fact that there were problems with the idea, Encke’s idea of a “resisting medium” continued to hold for the most part . . . until the 1930s when two short-period comets were found to be accelerated into larger orbits – the exact opposite of what a “resisting medium” would do. While the “flying sandbank” model could be consistent with the effects of a “resisting medium,” it fell apart completely when confronted with these observed accelerations. The time had come for a new model.

In a series of papers, the first of which was published in March 1950, Harvard University astronomer Fred Whipple – who had spent the previous two decades studying phenomena associated with meteors and who managed to discover six comets in the process – proposed what he initially called his “icy conglomerate” model for a comet’s nucleus. In this model – which has come to be more popularly called the “dirty snowball” – a comet’s nucleus is a solid object made up of ices of various volatile gases which in turn have large amounts of dust grains embedded within them. Under the heat of sunlight these ices “sublimate” – i.e., turn directly from a solid into a gas, as will happen in the near-vacuum environment of space – and in this process the gas along with the embedded dust grains is ejected off the nucleus into the coma. The sunlight also breaks apart the parent molecules of these sublimated ices – water ice as well as other ices such as carbon monoxide and carbon dioxide – into the daughter molecules that were being spectroscopically detected at the time.