文章基本信息

标题：The Origin of Titan’s External Oxygen: Further Constraints from ALMA Upper Limits on CS and CH2NH
本地全文：下载
作者：N. A. Teanby ; M. A. Cordiner ; C. A. Nixon 等
期刊名称：The Astronomical journal
印刷版ISSN：0004-6256
电子版ISSN：1538-3881
出版年度：2018
卷号：155
期号：6
页码：1-8
DOI：10.3847/1538-3881/aac172
语种：English
出版社：American Institute of Physics
摘要：Titan’s atmospheric inventory of oxygen compounds (H2O, CO2, CO) are thought to result from photochemistry acting on externally supplied oxygen species (O+, OH, H2O). These species potentially originate from two main sources: (1) cryogenic plumes from the active moon Enceladus and (2) micrometeoroid ablation. Enceladus is already suspected to be the major O+ source, which is required for CO creation. However, photochemical models also require H2O and OH influx to reproduce observed quantities of CO2 and H2O. Here, we exploit sulphur as a tracer to investigate the oxygen source because it has very different relative abundances in micrometeorites (S/O∼10−2) and Enceladus’ plumes (S/O∼10−5). Photochemical models predict most sulphur is converted to CS in the upper atmosphere, so we use Atacama Large Millimeter/submillimeter Array (ALMA) observations at ∼340GHz to search for CS emission. We determined stringent CS 3σ stratospheric upper limits of 0.0074ppb (uniform above 100 km) and 0.0256ppb (uniform above 200 km). These upper limits are not quite stringent enough to distinguish between Enceladus and micrometeorite sources at the 3σ level and a contribution from micrometeorites cannot be ruled out, especially if external flux is toward the lower end of current estimates. Only the high-flux micrometeorite source model of Hickson et al. can be rejected at 3σ. We determined a 3σ stratospheric upper limit for CH2NH of 0.35ppb, which suggests cosmic rays may have a smaller influence in the lower stratosphere than predicted by some photochemical models. Disk-averaged C3H4 and C2H5CN profiles were determined and are consistent with previous ALMA and Cassini/CIRS measurements.
关键词：planets and satellites: atmospheres;planets and satellites: individual (Titan);radiative transfer