As quickly as hopes for ISON’s display rose after an outburst on November 13, they started falling again this morning based on reports from the IRAM millimeter telescope in Spain. The telescope, which studies radio wavelengths, reported that several key emission lines in ISON’s spectra have faded 20-fold since the 21st. These spectra are a fingerprint for gases being released from inside ISON’s interior.
The finding suggests, at the very least, that gas production inside ISON has shut down again. Michal Drahus (Caltech/NRAO), who is involved with IRAM, concluded on Yahoo’s comets-ml: “This may indicate that the nucleus is now at best marginally active or that… it no longer exists.”
This is just the latest of several worrying developments to occur in the last few days. The ESO’s TRAPPIST observatory indicated a 3-fold decline in water production inside the comet starting on November 21st, while the CARA Project measured a rapid increase in dust production beginning a few days after the outburst began. Both are indications that ISON’s outburst were the result of the nucleus breaking up.
Also worrying are measurements of the comet’s position by Terry Lovejoy, an Australian amateur. He found the comet was persistently lagging behind its expected position due to large non-gravitational forces. These forces are usually the result of strong outgassing from the comet’s nucleus, and may be a sign that the outgassing observed in ISON’s outburst was explosive, possibly resulting in the breakup of the nucleus.
Before ISON started its outburst, it was close to its survival limit, an empirical formula developed by comet expert John Bortle. This formula relates the nucleus’s brightness to the distance at which comets have been known to break up. When ISON’s outburst began, it was close to its survival limit, another possible sign that the outburst was the result of a breakup.
However, there are a couple of alternative explanations. ISON’s spin axis is not known to any degree of certainty, while the outburst blocked any view of possible active areas on the nucleus’s surface. Since summer, astronomers had gotten glimpses of two active regions on the comet’s nucleus. Features seen in the comet’s coma during the outburst suggested that activity from these regions had ramped up. It is possible that the veins of ice that were feeding these regions have finally been exhausted.
Another suggestion is that ISON’s spin axis was nearly parallel to its orbital direction, which means that only one side of the nucleus has been in sunlight since it was discovered. The outburst might have been started by a strong expulsion of gas that changed the comet’s spin axis. This change in spin axis would have turned the dark side of ISON’s nucleus sun-wards. The end result would be a sudden jump in gas production from ices that had burned off months ago on the permanently lit side. Supporting this explanation is that ISON’s brightness jumped twice – the first jump could be explained by the initial burst of activity comet’s rotation, while the second was fresh material being brought into play.
The spin axis suggestion could also account for the observed offset of the comet’s position. Nearly all of the activity on the comet would have taken place on the sun-facing side, meaning that the gas being expelled from the comet was being expelled sun-ward. This would have acted as a small, low-thrust rocket over months, changing the comet’s forward motion slightly but measurably.
Any number of explanations could be the correct one. What we do know is that the comet is doing something interesting and unexpected, another wild card to keep the experts stumped. ISON’s survival was never a sure thing, and it’s possible that the outburst was one last gasp for glory. We’ll have a better idea of what happened as it continues to approach the Sun.
Now that the comet is moving into the range of space-based solar observatories, the comet will be under near constant surveillance. While they won’t provide the same kind of data that Earth-based observatories did, they will give us a better understanding of the comet’s overall behavior from multiple angles. If ISON has indeed broken up, we will be able to tell by its behavior over the next three days.