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Possible Detection of Hydrazine on Saturn’s Moon Rhea
M. Elowitz*, B. Sivaraman*, A. Hendrix, J.-I. Lo, S.-L. Chou, B.-M. Cheng, B. N. R. Sekhar, and N. J. Mason
We present the first analysis of far-ultraviolet reflectance spectra of regions on Rhea’s leading and trailing hemispheres collected by the Cassini Ultraviolet Imaging Spectrograph during targeted flybys. In particular, we aim to explain the unidentified broad absorption feature centred near 184 nm. We have used laboratory measurements of the UV spectroscopy of a set of candidate molecules and found a good fit to Rhea’s spectra with both hydrazine monohydrate and several chlorine-containing molecules. Given the radiation-dominated chemistry on the surface of icy satellites embedded within their planets’ magnetospheres, hydrazine monohydrate is argued to be the most plausible candidate for explaining the absorption feature at 184 nm. Hydrazine was also used as a propellant in Cassini’s thrusters, but the thrusters were not used during icy satellite flybys and thus the signal is believed to not arise from spacecraft fuel. We discuss how hydrazine monohydrate may be chemically produced on icy surfaces.

在卡西尼-惠更斯的土星系統探索任務中,卡西尼探測器幾次飛越了土星的第二大衛星土衛五-瑞亞(Rhea),並傳回許多珍貴的觀測資料,其中包含了卡西尼-紫外線成像光譜儀(UVIS)記錄的資料,這些資料讓我們得以對土衛五上的地質和地貌均有更進一步的了解。其中的遠紫外光(FUV)吸收光譜提供了土衛五表面冰晶成份的組成資訊。針對位於184 nm的未知微弱吸收,印度Physical Research Laboratory (PRL)的B. Sivaraman研究團隊利用TLS BL03光激放光實驗站嘗試模擬土衛五上的環境,並成功的取得了低溫下CHCl3、CCl4、H2O及N2H4等混合冰晶的遠紫外光吸收光譜。實驗數據更進一步與卡西尼觀測資料比對後推測在土衛五的表面冰晶中應含有N2H4(聯胺)的水合物,而此聯胺的形成推測可能來自於土衛五表面冰層所富含的水冰與氨冰受到輻射照射誘發化學反應所致,也可能是經由土衛六泰坦的大氣中逸散過來。