As Rosetta’s miniature UV Imaging Spectrograph, ALICE will help us determine where comet C-G came from, what it is made of, and how its nucleus, coma, and tails interact. (Unlike the names of other U.S. instruments, “ ALICE” is not an acronym; it is simply a name that the instrument’s principal investigator, Alan Stern, likes).
ALICE is a key tool for discovering a comet’s thermal history - how hot it has gotten in the past. That thermal history can tell us something about where the comet was formed and traveled in its orbits. If we know that a comet never got above a certain temperature, we can infer that it never got closer than a certain distance from the sun (or, for that matter, any other star).
ALICE detects thermal markers in the far-UV range of light. These markers are a special group of volatile gases, called noble gases (which include helium, neon, argon, and krypton). Sometimes they are called inert gases because they do not interact with other chemicals to form new ones. They remain as they were when they first appeared on the comet. (Alan Stern and his colleagues discovered the first noble gas ever found on a comet (argon) in spectrographic studies of the comet Hale-Bopp. We know the exact temperature at which noble gases sublime (change directly from a frozen to gaseous state). Therefore, we can determine how hot or cold the comet has been in its past.
For example, if ALICE detects the noble gas, neon, we know that the comet has not heated to more than 16K degrees (-257 C or -430 F), which is only 16 degrees above absolute zero. If the neon appears to be part of the comet's original material (for example, it comes shooting up from deep inside the nucleus), the comet must have formed in a region of space colder than 16K. This gives us a strong clue for solving the mystery of where comets come from. ALICE is Rosetta's only remote-sensing instrument that can measure noble gases.
Besides the inert gases, ALICE is also a tool for detecting other components in the comet that are critical to the development of molecules that nurture life on Earth. The far-UV, which ALICE excels at observing, is ideal for seeing water, carbon monoxide, and carbon dioxide, three key molecules formed in the coma by the comet's nucleus. ALICE will also measure the abundance of carbon, hydrogen, oxygen, nitrogen, and sulfur atoms in comet C-G’s coma. In other studies of comets, scientists have seen significant amounts of the first four atoms. Along with sulfur, these atoms are important to lipids, carbohydrates, and proteins - complex organic molecules important for life on Earth. We don’t know if any of these molecules have ever developed on comets, and, if they have, we don’t know if comets delivered them to Earth or other worlds. The possibility is intriguing. ALICE is the only remote-sensing instrument on Rosetta that can detect these key atoms if they exist in the comet's atmosphere.