Ammonium salts on comet 67P The distribution of carbon and nitrogen in the Solar System is thought to reflect the stability of carbon- and nitrogen-bearing molecules when exposed to the heat of the forming Sun. Comets have a low nitrogen-to-carbon ratio, which is contrary to expectations because they originate in the outer Solar System where nitrogen species should be common. Poch et al. used laboratory experiments to simulate cometary surfaces and compared the resulting spectra with comet 67P/Churyumov-Gerasimenko. They assigned a previously unidentified infrared absorption band to nitrogen-containing ammonium salts. The salts could contain enough nitrogen to bring the comet's nitrogen-to-carbon ratio in line with the Sun's. Science, this issue p. eaaw7462 Laboratory experiments show that comet 67P contains ammonium salts, which may dominate its nitrogen content. INTRODUCTION Comets and asteroids preserve information on the earliest stages of Solar System formation and on the composition of its building blocks. The nature of their solid material can be investigated by analyzing the sunlight scattered by their surfaces. The nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P) was mapped by the Visible and InfraRed Thermal Imaging Spectrometer, Mapping Channel (VIRTIS-M) on the Rosetta spacecraft from 2014 to 2015. The nucleus appeared almost spectrally uniform from 0.4 to 4 μm, characterized by a low reflectance of few percent, a reddish color, and an unidentified broad absorption feature around 3.2 μm, which was ubiquitous throughout the surface. The darkness and the color of comet 67P could be due to a mixture of refractory organic molecules and opaque minerals. Although water ice may contribute to the 3.2-μm absorption, it cannot explain the entire feature. RATIONALE Semivolatile compounds of low molecular weight, such as carboxylic (−COOH)–bearing molecules or ammonium (NH4+) ions, have been proposed as potential carriers of the 3.2-μm absorption feature. To test these hypotheses, we performed laboratory experiments to measure the reflectance spectra of these compounds mixed in a porous matrix of submicrometric opaque mineral grains, under simulated comet-like conditions (170 to 200 K, <10−5 mbar). RESULTS The 3.2-μm absorption feature is consistent with ammonium salts mixed with the dark cometary surface material. We attribute additional absorption features to carbonaceous compounds and traces of water ice. Several ammonium salts can match the absorption feature equally well: ammonium formate, ammonium sulfate, or ammonium citrate. A mixture of different ammonium salts could be present. Ammonium salts at the surface of comet 67P could have been synthesized through acid-base reactions of ammonia (NH3) with the corresponding acid molecules in solid ices. That reaction may have occurred in the interstellar medium, in the protoplanetary disk, or during the sublimation of the ices in the cometary nucleus. The depth of the band suggests that the cometary surface contains an upper limit of ~40 weight % (wt %) of ammonium salts, but the exact concentration remains unknown. If the amount of ammonium salts is higher than ~5 wt %, they constitute the dominant reservoir of nitrogen in the comet, containing more nitrogen than the refractory organic matter and the volatile species, such as NH3 and N2. Consequently, the abundance of nitrogen in this comet is closer to that of the Sun than previously thought. CONCLUSION Ammonium salts may dominate the reservoir of nitrogen in comets. Their presence in cometary dust may explain increases of gas-phase NH3 and HCN observed in some comets when close to the Sun, which could be caused by the thermal dissociation of ammonium salts. Several asteroids in the Main Belt, Jupiter’s Trojan asteroids, and its small moon Himalia have similar spectra to that of comet 67P, with a broad spectral absorption feature at 3.1 to 3.2 μm, which we suggest could also be due to ammonium salts. The dwarf planet Ceres has ammoniated phyllosilicates on its surface, which may have formed from ammonium ions inherited from outer Solar System objects with compositions similar to that of comet 67P. The presence of these salts on comet 67P, and possibly on other primitive Solar System bodies, suggests a compositional link between asteroids, comets, and the proto-solar nebula. Comparison of ammonium formate spectrum with the average spectrum of comet 67P. The average reflectance spectrum of comet 67P (black line) and the spectrum of a mixture of ammonium formate (NH4+ HCOO–) with opaque grains measured in the laboratory under comet-like conditions (blue line). Also shown are views of the 4-km-diameter comet nucleus (Credit: ESA/Rosetta/NAVCAM–CC BY-SA IGO 3.0; http://creativecommons.org/licenses/by-sa/3.0/igo) and the 48-mm-diameter laboratory sample. The measured nitrogen-to-carbon ratio in comets is lower than for the Sun, a discrepancy which could be alleviated if there is an unknown reservoir of nitrogen in comets. The nucleus of comet 67P/Churyumov-Gerasimenko exhibits an unidentified broad spectral reflectance feature around 3.2 micrometers, which is ubiquitous across its surface. On the basis of laboratory experiments, we attribute this absorption band to ammonium salts mixed with dust on the surface. The depth of the band indicates that semivolatile ammonium salts are a substantial reservoir of nitrogen in the comet, potentially dominating over refractory organic matter and more volatile species. Similar absorption features appear in the spectra of some asteroids, implying a compositional link between asteroids, comets, and the parent interstellar cloud.

中文翻译：

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铵盐是彗核上和一些小行星上的氮储存库

彗星 67P 上的铵盐 太阳系中碳和氮的分布被认为反映了含碳和氮的分子在暴露于形成太阳的热量时的稳定性。彗星的氮碳比很低，这与预期相反，因为它们起源于外太阳系，在那里氮物种应该很常见。波奇等人。使用实验室实验来模拟彗星表面，并将所得光谱与彗星 67P/Churyumov-Gerasimenko 进行比较。他们为含氮铵盐分配了一个以前未知的红外吸收带。这些盐中可能含有足够的氮，使彗星的氮碳比与太阳的氮碳比保持一致。科学，这个问题 p。eaaw7462实验室实验表明彗星67P含有铵盐，这可能主导其氮含量。介绍 彗星和小行星保存了有关太阳系形成最早阶段及其组成部分的信息。可以通过分析其表面散射的阳光来研究其固体材料的性质。2014 年至 2015 年，Rosetta 航天器上的可见光和红外热成像光谱仪映射通道 (VIRTIS-M) 对 67P/Churyumov-Gerasimenko（以下简称 67P）彗核进行了测绘。从 0.4 到 4，该核在光谱上几乎是均匀的μm，其特点是反射率低，呈红色，在 3.2 μm 附近有一个不明的广泛吸收特征，在整个表面无处不在。彗星 67P 的黑暗和颜色可能是由于难熔有机分子和不透明矿物的混合物。虽然水冰可能有助于 3.2-μm 的吸收，但它不能解释整个特征。基本原理 低分子量的半挥发性化合物，例如含羧基 (-COOH) 的分子或铵 (NH4+) 离子，已被提议作为 3.2-μm 吸收特征的潜在载体。为了检验这些假设，我们进行了实验室实验，在模拟的类彗星条件（170 至 200 K，<10-5 毫巴）下，测量混合在亚微米不透明矿物颗粒多孔基质中的这些化合物的反射光谱。结果 3.2-μm 吸收特征与铵盐与暗彗星表面材料混合一致。我们将额外的吸收特征归因于含碳化合物和痕量水冰。几种铵盐可以很好地匹配吸收特性：甲酸铵、硫酸铵或柠檬酸铵。可以存在不同铵盐的混合物。彗星 67P 表面的铵盐可以通过氨 (NH3) 与固体冰中相应的酸分子的酸碱反应合成。这种反应可能发生在星际介质、原行星盘中或彗核中的冰升华过程中。带的深度表明彗星表面含有约 40 重量% (wt%) 的铵盐上限，但确切的浓度仍然未知。如果铵盐的量高于~5wt%，它们构成了彗星中主要的氮库，比难熔有机物和挥发性物质（如 NH3 和 N2）含有更多的氮。因此，这颗彗星中的氮含量比以前认为的更接近太阳。结论 铵盐可能在彗星中的氮库中占主导地位。它们在彗星尘埃中的存在可以解释当靠近太阳时在一些彗星中观察到的气相 NH3 和 HCN 增加，这可能是由铵盐的热解离引起的。主带中的几颗小行星、木星的特洛伊小行星及其小卫星 Himalia 的光谱与彗星 67P 的光谱相似，在 3.1 至 3.2 μm 处具有广泛的光谱吸收特征，我们认为这也可能是由铵盐引起的。矮行星谷神星在其表面有氨化的页硅酸盐，这可能是由从太阳系外天体继承的铵离子形成的，其成分类似于彗星 67P。这些盐分存在于彗星 67P 上，也可能存在于其他原始太阳系天体上，这表明小行星、彗星和原太阳星云之间存在成分联系。甲酸铵光谱与彗星67P平均光谱的比较。彗星 67P 的平均反射光谱（黑线）和甲酸铵（NH4+ HCOO–）与不透明颗粒的混合物在实验室中在类彗星条件下测量的光谱（蓝线）。还显示了 4 公里直径彗核的视图（来源：ESA/Rosetta/NAVCAM–CC BY-SA IGO 3.0；http://creativecommons.org/licenses/by-sa/3。0/igo) 和 48 毫米直径的实验室样品。彗星中测得的氮碳比低于太阳，如果彗星中存在未知的氮库，这种差异可能会得到缓解。彗星 67P/Churyumov-Gerasimenko 的原子核显示出大约 3.2 微米的未知宽光谱反射特征，该特征在其表面无处不在。在实验室实验的基础上，我们将此吸收带归因于与表面灰尘混合的铵盐。该带的深度表明，半挥发性铵盐是彗星中氮的重要储存库，可能在难熔有机物和更易挥发的物种中占主导地位。一些小行星的光谱中出现了类似的吸收特征，这意味着小行星、彗星、