Observations of Gas Activity Evolution

The evolution of production rates along a cometary orbit depends on the available solar energy, the volatility of the species and the structure, and outgassing processes in the nucleus as outlined above. Key to constrain the nucleus composition and structure are observations of the cometary activity at large heliocentric distances and over a wide range of rh to cover the long-term evolution. In future, in situ measurements from landers will also become available. Unfortunately, observations of gases are often possible only in the water-driven sublimation regime inside rh = 3 AU, because at large rh the sublimation rates are low (Fig. 4), and the excitation of line emissions is weak. Only exceptional bright long-period comets allow us to detect gas emissions also in the CO-driven regime at large heliocentric distances. Here we discuss observations of the gas evolution in comets in view of the different model concepts for sublimation.

The unusually bright long-period comet Hale-Bopp provided us with the widest coverage of gas activity observations so far (Fig. 8). We therefore look at its activity evolution in more detail. Beyond rh = 5 AU activity of CO has been detected in comet Hale-Bopp in the radio range [24, 25] as well as emissions of CN and HCN in the optical [177,187] and radio [24]. The other minor volatile species were detected in the water-driven sublimation regime at less than 3-5 AU heliocentric distance. All production rates increase toward perihelion and decrease on the outbound path, following the variation in solar

Jupiter Family Comets

Heliocentric

Fig. 8. Gas production rates over heliocentric distance of comet Hale-Bopp [25]

Heliocentric

Fig. 8. Gas production rates over heliocentric distance of comet Hale-Bopp [25]

energy input. Water sublimation is first detected around 5 AU, and at rh = 3.5-4 AU, it started to dominate over CO activity, as expected for a water ice dominated comet.

H2CO, CS, and HNC show a steeper increase of their production rates toward perihelion than other species. This is most likely linked to their formation as a daughter product in the coma, rather than from pure nucleus sublimation (see Sect. 5).

When comparing pre- and post-perihelion observations, some systematic differences in the evolution of gas activity can be seen, like a sudden increase in activity for most species within 1.5 AU pre-perihelion and a stagnation in CO production rate near 2-3 AU pre-perihelion (Fig. 8). However, the overall activity evolution is very similar for the monitored parent species on the inbound and outbound path.

Do the measurements constrain the nucleus interior? To see how we can use observations like in Fig. 8 to study the sublimation processes, we discuss the observations in view of the two extremes of a pure amorphous and pure crystalline nucleus. First, we treat comet Hale-Bopp as an unprocessed nucleus consisting of a homogeneous mixture of amorphous ice. As the comet approaches the Sun, we expect crystallization of its surface layers that should lead to differences of the production rates between pre- and post-perihelion. This is not observed. The difference is illustrated when comparing model predictions based on amorphous water ice with some CO trapped and some CO in a separate phase (Fig. 9) to the observed activity evolution (e.g., [73,175]). We note that in the models CO is trapped by amorphous water ice only by a few percent. Most of CO is assumed to condense in a separate phase, able to outgas from the interior of the nucleus, similar to the scenario for crystalline water ice outlined above. The models predict the increase in CO production rate on the pre-perihelion path, although the simulations come to different results on where the crystallization of the amorphous ice sets in and on the detailed subsequent activity evolution. However, these models do not agree with the post-perihelion evolution of CO. Updated models, taking into account the improved knowledge on comet Hale-Bopp (Fig. 10) give significantly improved results. Nevertheless, the CO production rate post-perihelion is still high. This effect is attributed to the thermal inertia of the nucleus causing seasonal effects. For more realistic models, we need to know parameters, such as the nucleus size and the rotation axis, which are usually not available for a modeled comet. Nevertheless, there remain differences between models

Was this article helpful?

0 0
Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

Get My Free Ebook


Post a comment