MIRO: Microwave Instrument for the Rosetta Orbiter
MIRO will investigate the nature of the cometary nucleus, outgassing from the nucleus and development of the coma. MIRO is configured both as a continuum and a very high spectral resolution line receiver. Centre-band operating frequencies are near 188 GHz (1.6 mm) and 562 GHz (0.5 mm). Spatial resolution of the instrument at 562 GHz is approximately five metres at a distance of two kilometres from the nucleus; spectral resolution is sufficient to observe individual, thermally broadened, line shapes at all temperatures down to 10 K. Four key volatile species - H2O, CO, CH3OH, and NH3 and the isotopes H217O and H218O - are pre-programmed for observation. The primary retrieved products are the abundance, velocity, and temperature of each species, along with their spatial and temporal variability. This information will be used to infer the structure of the coma and its creation processes, including the nature of the nucleus/coma interface.
MIRO will sense the subsurface temperature of the comet nucleus to depths of several centimetres or more using the continuum channels at millimetre and submillimetre wavelengths. Model studies will relate these measurements to electrical and thermal properties of the nucleus and address issues connected to the sublimation of ices, ice and dust mantle thickness, and the formation of gas and dust jets. The global nature of these measurements will allow in situ lander data to be extrapolated globally, while the long duration of the mission will allow us to follow the time variability of surface temperatures and gas production. Models of the thermal emission from comets are very crude at this time since they are only loosely constrained by available data. MIRO will offer the first opportunity to gather subsurface temperature data that can be used to test thermal models. MIRO complements the IR mapping instrument on the orbiter, having a similar spatial resolution but greater depth penetration.
The five objectives of the MIRO instrument are to:
- Characterise the abundances of major volatile species and key isotope ratios in the nucleus ices.
The MIRO instrument will measure absolute abundances of key volatile species - H2O, CO, CH3OH, and NH3 - and quantify fundamental isotope ratios - 17O/16O and 18O/16O - in a region within several kilometres from the surface of the nucleus, nearly independent of orbiter to nucleus distance.
Water and carbon monoxide are chosen for observation because they are believed to be the primary ices driving cometary activity. Methanol is a common organic molecule, chosen because it is a convenient probe of gas excitation temperature by virtue of its many transitions. Knowledge of ammonia abundance has important implications for the excitation state of nitrogen in the solar nebula. By providing measurements of isotopic species abundances with extremely high mass discrimination, the MIRO experiment can use isotope ratios as a discriminator of cometary origins. The MIRO investigation will combine measurements of the variation of outgassing rates with heliocentric distance with models of gas volatilisation and transport in the nucleus to quantify the intrinsic abundances of volatiles within the nucleus.
- Study the processes controlling outgassing in the surface layer of the nucleus.
The MIRO experiment will measure surface outgassing rates for H2O, CO, and other volatile species, as well as nucleus subsurface temperatures to study key processes controlling the outgassing of the comet nucleus.
The MIRO investigation will use correlated measurements of outgassing rates and nucleus thermal properties to test models of gas formation, transport, and escape from the nucleus to advance our understanding of the important processes leading to nucleus devolatilisation.
- Study the processes controlling the development of the inner coma.
MIRO will measure density, temperature, and kinematic velocity in the transition region close to the surface of the nucleus.
Measurements of gas density, temperature, and flow field in the coma near the surface of the nucleus will be used to test models of the important radiative and dynamical processes in the inner coma, and thus improve our understanding of the causes of observed gas and dust structures. The high spectral resolution and sensitivity will provide a unique capability to observe Doppler-broadened spectral lines at very low temperatures.
- Globally characterise the nucleus subsurface to depths of a few centimetres or more.
The MIRO instrument will map the nucleus and determine the subsurface temperature distribution to depths of several centimetres or more. Morphological features on scales as small as 5 m will be identified and correlated with regions of outgassing.
The combination of global outgassing and temperature observations from MIRO and in situ measurements from the Rosetta lander will provide important insights into the origins of outgassing regions and of the thermal inertia of subsurface materials in the nucleus.
- Search for low levels of gas in the asteroid environment.
The MIRO instrument will search for low levels of gas in the vicinity of asteroids and measure subsurface temperature to provide information on the presence of water ice, and on near surface thermal characteristics and the presence or absence of a regolith.
The MIRO experiment will acquire both high resolution molecular line spectra in absorption and emission, and broadband continuum emission data from which gas abundances, velocities, temperatures, and nucleus surface and subsurface temperatures will be derived.
The MIRO instrument is composed of a millimetre wave mixer receiver operating with a centre-band frequency of 188 GHz, and a submillimetre heterodyne receiver operating with a centre-band frequency of 562 GHz. The two receivers are fixed tuned. The submillimetre wave receiver provides both broad band continuum and high resolution spectroscopic data, whereas the millimetre wave receiver provides continuum data only.
The submillimetre wave spectroscopic frequencies allow simultaneous observations of six molecules which are known constituents of comets. The submillimetre wave lines observed include the ground-state rotational transition of water 1(10)-1(01) at 557 GHz, the corresponding lines of two oxygen isotopes of water, and the 572 GHz ground state rotational line of ammonia J(1-0). Since these lines are ground-state transitions (between the lowest rotational levels of these molecules), they are expected to be the strongest in cometary conditions. The submillimetre spectrometer can also observe the CO J(5-4) line, and three methanol lines.
The millimetre and submillimetre wavelength continuum channels will sense the subsurface temperature of the nucleus to depths of several centimetres or more. Model studies will relate these measurements to electrical and thermal properties of the nucleus and address issues connected to the sublimation of ices, ice and dust mantle thickness, and the formation of gas and dust jets.