In English

Constraints on the CO content of Callisto’s exosphere based on ALMA archival data

Negar Entekhabi
Göteborg : Chalmers tekniska högskola, 2018. 73 s.
[Examensarbete på avancerad nivå]

Callisto is the second largest moon of Jupiter and the third largest in the Solar System and in a wide enough orbit to avoid Jupiter’s harmful magnetosphere. It is tidally locked in its orbit so that the same side of Callisto always looks at Jupiter. Water (H2O) detections from the two leading and trailing hemispheres of Callisto are somewhat different which might be an indication of asymmetry in its atmosphere. A theoretical atmosphere density model of Callisto predicts that if the atmosphere of Callisto has been formed in oxidizing state, the top constituents of sublimation profile density are expected to be H2O, CO and CO2. Callisto’s tenuous CO2 atmosphere have been observed directly by the Galileo Near Infrared Mapping Spectrometer. Likewise, water absorption lines are detected from the leading hemisphere of Callisto. These findings and lack of information about CO, motivated us to search for CO lines in Callisto’s atmosphere. ESA’s (European Space Agency) JUICE spacecraft in 2030 will investigate Callisto’s exosphere in more detail. To prepare for that investigation it might be possible to use ground-based data from Atacama Large Millimeter Array (ALMA) in Chile to detect carbon monoxide (CO) lines. All available observations covering the COJ=2−1 transition frequency toward Callisto are extracted from ALMA archive. Then, to find the best fitted visibilities, the extracted data are analysed using a circular disc model for Callisto and spectra are extracted directly from the interferometric visibilities. Afterwards, averaging of the fitted spectra are performed for both leading and trailing hemispheres. Since the average spectra does not show any emission or absorption CO line, upper column density of CO by using rms noise level is calculated which gives  2 × 1015 cm−2. The result shows that, CO is likely not the most dominant species in Callisto’s atmosphere. For comparison, COJ=2−1 spectra towards Saturn’s moon Titan, are also extracted from the ALMA archive. The detected lines are strong and broad and CO column density is estimated to be at least five to six orders of magnitude higher in the atmosphere of Titan, than in that of Callisto.

Nyckelord: Callisto, Atmosphere, CO, ALMA

Publikationen registrerades 2018-11-21.

CPL ID: 256325

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