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| Titel |
Submillimeter Spectroscopic Observations of Asteroid (21) Lutetia with MIRO Instrument on the ESA Rosetta Spacecraft |
| VerfasserIn |
Seungwon Lee, Samuel Gulkis, Mark Hofstadter, Paul von Allmen, Jacques Crovisier, Nicolas Biver, Dominique Bockelee-Morvan |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049951
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| Zusammenfassung |
| The European Space Agency (ESA) Rosetta spacecraft flew by asteroid (21) Lutetia on July
10, 2010. The spacecraft carried a millimeter/submillimeter instrument named MIRO. MIRO
made continuum measurements which are presented in a companion paper by Gulkis et al.
Here we present spectroscopic measurements made near 560 GHz (0.5 mm) to search for an
exosphere of Lutetia. The MIRO instrument is sensitive to four volatile species – CO,
CH3OH, NH3, and H2O, – including three oxygen-related isotopologues, H216O, H217O, and
H218O. While over 1000 spectra were taken throughout the encounter, only seven
spectra near the closest approach can be expected to contain the targeted lines. This is
because the large flyby speed and consequently large Doppler Effect shifts the lines
out of MIRO’s bandpass during most of the approach and departure. The seven
spectra are used to estimate the upper limit of the column density of the four volatile
species and their corresponding outgassing rates from Lutetia’s nucleus. In order to
increase the signal to noise ratio, the spectra are first individually Doppler-shift
corrected, then averaged, and finally smoothed for Doppler shift broadening during the
spectrum integration time of 5 seconds. The resulting spectrum did not exhibit a
signal in either absorption or emission. Therefore, the standard deviation of the
resulting smoothed spectrum is used to estimate the upper limit of the optical depth,
which is related to the column density and the outgassing rate. The calculated upper
limits are model dependent, requiring assumptions on the excitation temperature of
molecules, their velocities, and their spatial distribution. If sublimation of ices is the
gas-generating process, and gases are emitted in a spherically-symmetric shell with velocity
0.25 km/sec and an excitation temperature of 180 K, we estimate upper limits for
the outgassing rates of H2O, CO, CH3OH, and NH3 to be ~1023, ~1026, ~1024,
and ~1023 molecules per second, respectively. If sputtering-based outgassing is
the dominant gas production process, the molecules would be expected to have
much higher excitation temperatures, perhaps 1000 K, making them easier to detect.
In that case, the upper limits for each species are an order of magnitude smaller. |
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