對土衛二恩塞勒達斯羽狀噴流之化學組成的 ALMA光譜研究

Abstract

土衛二恩塞勒達斯是一個引人注目的冰冷世界，因為在其南極地區發現的水 蒸氣羽狀噴流令人信服地表明土衛二的冰殼下可能存在一個全球性的地下海洋。 再者，類似於地球上發現的海底熱泉也可能存在於土衛二的海洋和岩石核心之間。 因此，透過對土衛二外氣層和周圍 E 環的高角分辨率和高靈敏度分子譜線觀測 可以幫助我們更好地了解地下海洋的化學成分，從而為土衛二的適居性提供重要 資訊。我們於 2018 年 5 月和 6 月使用 ALMA 對土衛二進行了觀測。不幸的是，在 觀測過程中，為土衛二南極噴流來源的虎紋裂縫因背對我們使我們無法直接觀測 到。雖然如此在我們的 ALMA 觀測中依然發現六種分子，包括一氧化碳 (CO)、 氰化氫 (HCN)、二氧化氮 (NO2)、二氧化硫 (SO2)、甲醇 (CH3OH) 和水 (H2O)。 部分 E 環的連續發射光譜也被觀測到。而在土衛二以北我們也發現甲醇的訊號; 此外，當應用 uv-taper 的加權方法作圖時，甲醇的訊號會出現以類似環狀的結構 圍繞土衛二。於此次的觀測結果裡我們也發現，當恩塞勒達斯的頭半球 (Leading hemisphere) 朝向我們時，沿著視線方向看過去的分子相對於土衛二的運動速度 為紅移;而當為尾半球 (Trailing hemisphere) 時，分子的相對運動速度就會為藍移。除了發現分子的存在之外，我們也利用虎紋裂縫的表面溫度來計算分子的柱密度 (Tex = 180 K 用於計算水分子以外的分子的柱密度;Tex = 140 K 用於計算水分子的柱密度)。在計算的過程中，我們也假設這些觀測到的分子處於光學深度薄和局部熱力平衡的狀態。a) CO, 2.1 × 10^14 cm^-2; b) HCN (GND) and HCN (v2 = 1), 1.0 × 10^12 cm^-2 and 1.2 × 10^14 cm^-2, respectively; c) NO2, 2.8 × 10^15 cm^-2; d) SO2, 6.3 × 10^13 cm^-2; e) CH3OH (土衛二以北的訊號), 9.9 × 10^14 cm^-2; f) H2O1, 6.1 × 10^14 cm^-2。我們的 ALMA 結果很大程度上解決了關於 INMS 所偵測到的含有 28-Da 質量 粒子是主要為 CO 還是 N2 的長期爭論。再者，存在於海洋中的 HCN 和 CO 都是 天文生物學中重要的研究對象。最後，甲醇的發現2也對土衛二地下海洋的可居住 性研究具有重大且深遠的意義。Enceladus is a fascinating icy world because a global subsurface ocean may exist beneath its ice shell as suggested convincingly by the water vapor venting plume discovered in Enceladus’ south polar region. Hydrothermal systems similar to those found on Earth could also exist at the interface between Enceladus’ ocean and rocky core. Ground-based high angular resolution and high sensitivity molecular observations of Enceladus’ exosphere and the surrounding E ring thus may help us better understand the chemical composition of the subsurface ocean and hence provide vital information on determining whether Enceladus is habitable or not.We therefore conducted ground-based ALMA observations of Enceladus in 2018 May and June. Unfortunately, Tiger stripes were hidden from our direct view on the far side during our observing run. In our ALMA observations, we detected six molecular species, including CO, HCN, NO2, SO2, CH3OH and also H2O. Continuum emission from E-ring was also partially detected. Methanol emission was detected north of Enceladus; in addition, an extended ring-like structure unveiled by CH3OH emission around Enceladus becomes apparent when uv-taper was applied. We note that molecular lines detected toward the leading hemisphere of Enceladus all exhibit a positive, i.e. redshifted, velocity with respect to Enceladus’ rest frame. By contrast, molecular lines detected toward the trailing side of Enceladus all display a negative, i.e. blueshifted, velocity with respect to Enceladus.Adopting the surface temperatures of tiger stripes measured (Tex = 180 K for all molecular transitions except water; Tex = 140 K was employed for the H2O line), and assuming the observed molecular lines are optically thin and in local thermodynamics equilibrium (LTE), we derived column densities of detected molecular species: a) CO, 2.1 × 10^14 cm^-2; b) HCN (GND) and HCN (v2 = 1), 1.0 × 10^12 cm^-2 and 1.2 × 10^14 cm^-2, respectively; c) NO2, 2.8 × 10^15 cm^-2; d) SO2, 6.3 × 10^13 cm^-2; e) CH3OH (土衛二以北的訊號), 9.9 × 10^14 cm^-2; f) H2O1, 6.1 × 10^14 cm^-2.Our ALMA results also largely settle the long-lasting debate on the 28-Da mass particle revealed by INMS whether it is primarily CO or N2. Both HCN and CO are important ingredients in ocean astrobiologically. Our unforeseeable detection of methanol2, another essential component for life, has a vast implication in the study of habitability in Enceladus’ subsurface ocean.