All that popping and fizzing that give comets their distinctive fuzzy appearance and flowing tails isn't just from water blasting off their bodies, but ancient dry ice, astronomers say.
Preliminary analysis of information collected by a NASA science probe that passed by Comet Hartley 2 last week show unmistakable correlations between jets of dust around the comet's body and carbon dioxide, also known as dry ice.
"They are going hand-in-hand. Where there is CO2, there is dust, therefore CO2 is the material that is sublimating and taking dust grains with them," astronomer Lori Feaga, at the University of Maryland, told Discovery News.
Sublimation occurs when a solid transforms directly into a gas without pausing in a liquid state.
Scientists don't know how common dry ice-fueled jets are on comets, but they saw signs of a similar phenomenon when the probe visited its first comet, Temple 1, in 2005, as part of the Deep Impact mission.
After releasing an 820-pound metal slug into the comet to kick up streams of material for analysis, the spacecraft was renamed and given a new comet to visit. It passed within 435 miles of Comet Hartley 1 on Nov. 4.
Scientists have only just begun parsing through the thousands of pictures and the chemical analyses made by the spacecraft, but the carbon dioxide measurements from Hartley 2 literally jumped right out.
"We knew this was a very active comet. We just didn't know why," lead researcher Michael A'Hearn told Discovery News.
He suspects Hartley 2 has a richer supply of ancient dry ice, which can transform into gas with about half as much heat as it takes for water, than a comet like Temple 1.
The finding raises new questions about conditions in the early solar system, when the comets and planets were formed. Comets are believed to be icy remnants of the solar system's original building blocks.
Rather than a smooth mix of materials to work with, it appears the material was rather chunky, like chocolate chips in cookie batter.
"With Hartley 2, there seems to be a lot of mixing," Faega said. "It's a very compelling story and we think it ties in with what's happening on many comets."
Measurements of a comet's carbon dioxide cannot be made by telescopes on Earth because the gas is blocked by the planet's atmosphere.