By Seungwon Lee, MIRO Co-I, Jet Propulsion Laboratory
From ground-based observations and previous spacecraft flyby data, we know that comets are made out of ice and dust. What we do not know well is what types and proportions of ice and dust comets are made of and how they are distributed in the nucleus. The Rosetta mission gets us one step closer in our scientific quest to understand comet composition and structure. The Microwave Instrument on the Rosetta Orbiter (MIRO) has a spectrometer tuned to frequencies that are sensitive to key cometary gases. The most abundant gas emitted by comets is water vapor, and MIRO is very sensitive to it. Using MIRO’s measurements, we can probe the abundance of water vapor in the vicinity of the comet nucleus, which we call the “inner coma”.
By watching how the inner coma changes over the course of a day or a season, we can learn about how comets produce the cometary gases. We see that the inner coma persistently has a much larger density in the sunlit side than the dark side over the course of a day and also a much larger density toward the summer hemisphere than the winter hemisphere in a seasonal cycle. This strong correlation with the sun illumination condition (day versus night and summer versus winter) indicates that the cometary gas is produced by the sublimation of the ices, which is very sensitive to the amount of heat given by sunlight.
The inner coma observations can also tell us about the process by which gas escapes from the nucleus. For example, does it bounce around within a porous dusty layer on the comet before emerging into the vacuum, does it blow off a dusty crust from the built-up gas pressure, or does it come shooting out through cracks in the crust? While it emits gas and dust continuously, the comet C/G 67P has also several times exhibited sudden outbursts (i.e. higher gas and dust activities), each of which lasts for just a couple of hours. We are trying to understand the mechanism of the outbursts as well as the continuous outgassing.
Furthermore, the spatial structure of the inner coma gives us a hint about the distribution of ice in the comet’s nucleus. For example, is the ice uniformly distributed or localized in some regions? How deep below the surface is the ice located? We use a physics-based model to relate the inner coma structure to the ice distribution. In the model, we calculate the flow of water vapor from the nucleus surface into space by solving a set of equations that describe the rules that such a flow needs to obey according to the laws of physics. This allows us to predict the number density of water molecules in the coma as a function of the amount and distribution of ice assumed to be present on the nucleus surface. By matching the observed densities with those from the coma model, we can better understand how much ice there is and how it is distributed in the nucleus.
Figure 1 illustrates the inner coma structure observed with MIRO in July 2015 when Comet 67P was at 1.3AU from the Sun and its southern hemisphere was in summer. The colored circle in the figure indicates the MIRO signal strength, which is related to the gas density at that position. The red color means a high gas density and blue means a low density. The blue solid line indicates the north pole of Comet 67P, while the dashed blue line indicates the south pole. The sun was located in the positive X direction (to the right). The MIRO observation clearly shows that the south pole direction (the blue dashed line) has a higher gas density and the sunlit/day side (the right side) has a higher gas density. Qualitatively, this pattern is consistent with our expectation since the main mechanism of outgassing is from the ice sublimation and the efficiency of the sublimation is determined by the solar illumination condition.
By adjusting the ice distribution in the model, we can attempt to trace the inner coma structure back to the ice distribution in the nucleus. We designed two models: one with the ice distributed uniformly over the entire nucleus and the other one with the ice distributed only near the south pole. You can see that in Fig. 2, with ice everywhere, the entire right side (the day side) of the inner coma has a lot of water vapor in it. In Fig. 3, with ice only at the south pole, most of the water vapor is to the lower right. By comparing the spatial structure of the models and observations, we find that the model with ice only near the south pole matches the observation best. We are now conducting more detailed studies to refine the ice distribution and to quantify the amount of ice.