A computational model calculates the excitation of water rotational levels and emission-line spectra in a cometary coma with applications for the Microwave Instrument for Rosetta Orbiter (MIRO). MIRO is a millimeter-submillimeter spectrometer that will be used to study the nature of cometary nuclei, the physical processes of outgassing, and the formation of the head region of a comet (coma). The computational model is a means to interpret the data measured by MIRO.
The model is based on the accelerated Monte Carlo method, which performs a random angular, spatial, and frequency sampling of the radiation field to calculate the local average intensity of the field. With the model, the water rotational level populations in the cometary coma and the line profiles for the emission from the water molecules as a function of cometary parameters (such as outgassing rate, gas temperature, and gas and electron density) and observation parameters (such as distance to the comet and beam width) are calculated.
This work was done by Paul A. Von Allmen and Seungwon Lee of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Software category.
This software is available for commercial licensing. Please contact Daniel Broderick of the California Institute of Technology at
This Brief includes a Technical Support Package (TSP).
MIRO Computational Model
(reference NPO-46508) is currently available for download from the TSP library.
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Overview
The document outlines the development of a computational model designed to calculate the excitation of water rotational levels and the emission line spectra in the coma of comets, specifically for the Microwave Instrument for Rosetta Orbiter (MIRO). This model is crucial for interpreting data collected by MIRO, which is a millimeter-submillimeter spectrometer aimed at studying cometary nuclei, the physical processes of outgassing, and the formation of the coma.
The model incorporates the seven lowest rotational levels of ortho-water, which are primarily populated in the rotationally cold coma. It accounts for various factors influencing water molecule excitation, including collisions with other water molecules and electrons, as well as infrared pumping due to solar irradiation. The document includes several figures that illustrate the model's processes and results.
Figure 1 summarizes the physical model, while Figure 2 depicts the algorithmic process of the accelerated Monte Carlo method used to simulate a spherically symmetric gas density distribution. Figures 3 through 6 present the calculated water rotational level populations and optical line profiles for different transitions between these levels, based on varying outgassing rates and two isotopes of water (H₂¹⁶O and H₂¹⁸O).
The model's calculations show how water rotational level populations change with distance from the comet's center and how these populations vary with different water production rates. For instance, the document details the line spectra for transitions from levels 1₁₀ to 1₀₁, 2₁₂ to 1₀₁, and 3₀₃ to 2₁₂, highlighting the effects of outgassing rates of 10²⁸, 10²⁹, and 10³⁰ molecules per second for H₂¹⁶O and H₂¹⁸O.
Overall, the computational model serves as a vital tool for understanding the dynamics of water in cometary comas, providing insights into the physical processes at play and aiding in the interpretation of observational data from the MIRO instrument. This work represents a significant advancement in the study of cometary science and the broader implications for understanding the formation and evolution of comets in our solar system.
