Venus’ mysterious haze is actually cosmic dust

金星神秘的雾霾实际上是宇宙尘埃

Venus is shrouded in a veil of mystery. The yellowish “lower haze” at the bottom of its atmosphere was discovered by the Venera and Pioneer Venus probes in the 1970s. These spacecraft were some of the first to successfully send images of the planet back to Earth, but the haze they saw went unexplained for decades. Now the mystery Venus was keeping has been unveiled. 金星被一层神秘的面纱所笼罩。20世纪70年代，苏联的“金星号”（Venera）和美国的“先驱者金星号”（Pioneer Venus）探测器在金星大气层底部发现了黄色的“下层雾霾”。这些航天器是首批成功将金星图像传回地球的探测器，但它们所观测到的雾霾几十年来一直无法解释。如今，金星隐藏的这个谜团终于被揭开了。

What nobody knew then was that the haze is an accumulation of particles from meteorites. “Shooting stars” that burn up in the atmosphere leave behind cosmic dust particles, and after sulfuric acid interacts with these particles, it leaves behind a haze. Planetary scientist Hiroki Karyu and his research team at Tohoku University in Sendai City, Japan, finally figured out how the haze formed using a microphysical model. 当时无人知晓的是，这种雾霾其实是陨石颗粒的堆积物。在金星大气层中燃烧的“流星”留下了宇宙尘埃颗粒，当硫酸与这些颗粒发生相互作用后，便形成了雾霾。日本仙台市东北大学的行星科学家狩野裕树（Hiroki Karyu）及其研究团队，最终利用微物理模型弄清了这种雾霾的形成过程。

This type of model involves microphysics—the micro-scale processes that create clouds and precipitation on Earth and other bodies. “The continuous influx of cosmic dust is sufficient to sustain this lower haze layer with the particle size distribution observed by the entry probes,” Karyu said in a study published in Nature Astronomy. “These haze particles of cosmic origin act as efficient condensation nuclei, promoting cloud formation in the main cloud deck even far from their initial source.” 这种模型涉及微物理学，即在地球和其他天体上产生云和降水的微观过程。狩野裕树在《自然-天文学》（Nature Astronomy）杂志发表的一项研究中表示：“持续涌入的宇宙尘埃足以维持这种下层雾霾，其颗粒大小分布与探测器观测到的结果一致。这些源自宇宙的雾霾颗粒充当了高效的凝结核，即使在远离初始来源的地方，也能促进主云层中云的形成。”

Hazed and confused

雾里看花

Previous speculation suggested the haze formed from volcanic ash. Venus is known as the most volcanically active planet in our Solar System, with signs of volcanism first observed by the Magellan spacecraft in the early 1990s. Recent studies discovered the planet is more volcanically active than previously thought, but the researchers found that ash belched out by Venusian volcanoes is not the source of haze particles. Surface dust was also ruled out. 此前的推测认为雾霾是由火山灰形成的。金星被认为是太阳系中火山活动最活跃的行星，20世纪90年代初，“麦哲伦号”（Magellan）探测器首次观测到了金星火山活动的迹象。最近的研究发现，金星的火山活动比之前认为的更为频繁，但研究人员发现，金星火山喷出的灰烬并非雾霾颗粒的来源。地表尘埃也被排除了。

Karyu found that even if there were much larger influxes of volcanic or surface dust on Venus, the particles would still not be able to interact with the atmosphere’s sulfur in the right way to form the haze. Instead of simply accumulating dust, Venus’ haze forms through a process that starts similarly to how clouds form on Earth. 狩野裕树发现，即使金星上有大量火山灰或地表尘埃涌入，这些颗粒也无法以正确的方式与大气中的硫发生相互作用从而形成雾霾。金星的雾霾并非简单的尘埃堆积，其形成过程与地球上云的形成方式有相似之处。

Clouds come into being via condensation nuclei, tiny aerosol particles floating around in the atmosphere. These can range from dust and soot to volcanic ash and even sea salt. Liquid water can also condense around ice. Water molecules are usually too sparse to bond to themselves in the air, but they stick to the surfaces of these hygroscopic particles, forming the nucleus of a cloud droplet. These droplets then accumulate into clouds. 云是通过凝结核形成的，即漂浮在大气中的微小气溶胶颗粒。这些颗粒包括尘埃、烟灰、火山灰甚至海盐。液态水也可以在冰周围凝结。水分子在空气中通常过于稀疏，无法自行结合，但它们会附着在这些吸湿性颗粒的表面，形成云滴的核。随后，这些云滴聚集在一起形成云。

Karyu and his team used a microphysical simulation to show how cosmic dust forms the lower haze. When space rocks are pulled in by the planet’s gravity, they shoot through the atmosphere and encounter gas particles that cause friction. The faster and further a meteorite plummets, the more friction it encounters, which causes it to burn up and leave a trail of particles in its wake. Like water particles condense around the dust on Earth, droplets of sulfuric acid condense on the surface of the meteorite particles left hovering over Venus. 狩野裕树及其团队利用微物理模拟展示了宇宙尘埃如何形成下层雾霾。当太空岩石被金星引力吸引时，它们会高速穿过大气层，与气体颗粒发生摩擦。陨石坠落得越快、越深，遇到的摩擦就越大，这导致它燃烧并留下一串颗粒轨迹。就像地球上的水分子围绕尘埃凝结一样，硫酸液滴也会在悬浮于金星上空的陨石颗粒表面凝结。

Heavy clouds and heavy metal

厚重的云层与重金属

The researchers found that meteorite particles increase cloud production on Venus since they provide more opportunities for condensation. But surprisingly, more particles do not mean more cloud droplets. Instead, several particles stick together and form larger clusters before sulfuric acid condenses around them. Clouds of these condensates continue to grow heavier until they sink into the scorching lower atmosphere, where the sulfuric acid evaporates at temperatures that can reach 100° C (212° F). The naked particles left behind form the haze. 研究人员发现，陨石颗粒增加了金星上的云层产量，因为它们提供了更多的凝结机会。但令人惊讶的是，颗粒越多并不意味着云滴越多。相反，多个颗粒会粘在一起形成更大的团簇，然后硫酸才会在它们周围凝结。这些凝结物形成的云层不断变重，直到沉入炽热的下层大气，在那里，硫酸在高达100°C（212°F）的温度下蒸发。留下的裸露颗粒便形成了雾霾。

Finding out what creates the lower haze on Venus unexpectedly solved another mystery. Something in Venus’ atmosphere had been absorbing ultraviolet rays, but until now, scientists had been unsure exactly what it was. Meteorites often contain metals such as magnesium, silicon, and iron, and iron compounds were previously speculated to be absorbers. The magnesium and silicon found in meteorites are terrible absorbers of UV radiation. But iron was more promising; Venera and the Vega probes had detected atmospheric iron, while the mass spectrometer aboard the Pioneer Venus Large Probe found the compound iron sulfate, but these discoveries eluded explanation for decades. 弄清金星下层雾霾的成因，意外地解决了另一个谜团。金星大气中一直有某种物质在吸收紫外线，但直到现在，科学家们仍不确定那到底是什么。陨石通常含有镁、硅和铁等金属，此前曾推测铁化合物是吸收剂。陨石中的镁和硅对紫外线的吸收效果很差，但铁的可能性更大；“金星号”和“维加号”（Vega）探测器曾探测到大气中的铁，而“先驱者金星号”大型探测器上的质谱仪发现了硫酸铁化合物，但这些发现几十年来一直无法得到解释。

Lingering suspicions that iron compounds were the mysterious absorber were confirmed when researchers found that iron sulfate matched the haze’s properties. It turns out that for haze particles at a certain height (40 to 50 km or about 25 to 31 miles above the surface), condensation is not possible because the high energy of the particles creates a barrier that does not allow droplets of sulfuric acid to stick to their surface. (This is called a nucleation barrier.) 当研究人员发现硫酸铁的特性与雾霾相符时，关于铁化合物是神秘吸收剂的长期怀疑得到了证实。事实证明，对于特定高度（地表以上40至50公里，约25至31英里）的雾霾颗粒而言，凝结是不可能的，因为颗粒的高能量形成了一道屏障，阻止了硫酸液滴附着在它们的表面。（这被称为成核屏障。）

Particles with a strong nucleation barrier are taken to the upper cloud layer by hotter air rising in the process of convection, where they cool enough to be incorporated into sulfuric acid particles. While previous observations had suggested this, Karyu’s team now has stronger evidence. 具有强成核屏障的颗粒会被对流过程中上升的热空气带到上层云层，在那里它们冷却到足以被纳入硫酸颗粒中。虽然之前的观测结果暗示了这一点，但狩野裕树的团队现在有了更有力的证据。

This discovery has implications for other planets. Upper hazes on gas giants have been studied ad nauseam, but microphysical processes beneath the main cloud layer remain an enigma. Venus may have shed light on at least some of the secrets of haze. On a planet like Jupiter, it might be that particles that cannot evaporate, like those from meteorites, settle on the outer layers of clouds. Further studies could give more insight into how cosmic dust promotes cloud and haze formation on planets like Jupiter, Saturn, and Neptune. 这一发现对其他行星也有启示意义。气态巨行星的上层雾霾已被研究得非常透彻，但主云层下方的微物理过程仍然是一个谜。金星的研究可能至少揭示了雾霾的部分秘密。在像木星这样的行星上，可能存在无法蒸发的颗粒（如来自陨石的颗粒）沉降在云层外层的情况。进一步的研究可以更深入地了解宇宙尘埃如何促进木星、土星和海王星等行星上云和雾霾的形成。

“As on Venus, observing the metal layers in the atmosphere of the outer planets would help [determine] the deposition rates of [metals] within their atmospheres and, by extension, the resulting haze abundances,” said Karyu. “正如在金星上一样，观测外行星大气中的金属层将有助于确定其大气中金属的沉积率，进而推断由此产生的雾霾丰度，”狩野裕树说道。