破解冥王星密码需要怎样的努力？

搞科研很难，搞尖端科研更难，这需要坚持和无限的耐心。逾15年前，我们开始筹划飞往冥王星的项目。当时我们就知道，和大家说的一样，这将是一场远足。但我们也知道，它有可能带来巨大的收获。70年来我们都只能遥望冥王星，而“新视野号”将首次近距离观察那个世界。这样的挑战你很难抗拒，而我选择了迎难而上。

最初的喧嚣

首次关于飞抵冥王星的认真探讨出现在20世纪80年代末。当时，人们用望远镜观察比冥王星更远的星体（被称为恒星掩星）时发现冥王星存在甲烷大气层，而且可能正在快速向宇宙空间流失。鉴于冥王星个头很小，看到其引力竟然足以“拴住”大气层很让人意外。

就在那段时间，几位行星科学家开始认真地研究冥王星。在整个20世纪90年代，谁是冥王星科学知识的最热心的贡献者显而易见，我们这些人也自然而然地在科学会议上聚在一起，讨论探访这颗第九大行星的问题。美国国家航空航天局（NASA）曾构想过一些探冥项目，但由于各种各样的原因均未成行，其中最主要的因素是经费不足。2000年NASA表示有可能启动这样的项目时，大家都已成竹在胸，对于如何通过飞近探测冥王星来优化我们的科学认知有了一些相当清晰的计划。

Science is hard and good science is harder — it takes persistence and tons of patience. When we began planning a mission to Pluto over 15 years ago, we knew it was going to be, as they say, a long haul. But we also knew it had a huge potential payoff. New Horizons would be the first closeup look at a world that we’d known about only distantly for 70 years. That kind of challenge is hard to pass by. And I didn’t.

Earliest rumblings

The first serious discussion of a mission to Pluto occurred in the late 1980s. That’s when telescope observations of distant stars going behind Pluto (known as stellar occultations) showed that it had a methane atmosphere, probably rapidly evaporating to space. Considering Pluto’s small size, it was a surprise to see that its gravity was strong enough to hold on to an atmosphere at all.

Several planetary scientists began working on Pluto in earnest around that time. Throughout the 1990s, it was pretty obvious who the enthusiastic contributors to Pluto science were, and we naturally congregated at scientific meetings to speculate about a mission to the ninth planet. Several attempts at a Pluto mission were formulated by NASA, but for various reasons none came to fruition, usually because of insufficient funding. By the time NASA announced the opportunity for a Pluto mission in 2000, everyone was ready with some pretty clear plans for how to optimize the science that could be done via a flyby.

冥王星的神秘大气层

1984年，我在加州理工学院读研究生时对冥王星产生了兴趣。当时人们对冥王星大气层所知甚少，无法用计算机建模来预测。但后来当那些恒星掩星揭示了冥王星出人意料的大气层后，我就迷上了这方面的研究。我关注的重点是其大气中的化学成分。“新视野号”项目首席研究员艾伦·斯特恩开始召集研究者时，邀请我加入了大气科学团队。其他小组的研究课题包括表面地质、内部结构以及冥王星五卫星体系的形成等等。

我的主要兴趣是了解冥王星大气层的化学组成和稳定性。在太阳系45亿年的历史中，冥王星怎么来约束自己的大气层？它的引力很弱，这些气体早该在宇宙空间流失殆尽。但情况显然并非如此。

我感兴趣的另一个问题是冥王星大气中的甲烷是否形成了复杂的碳氢化合物，以及后者是否凝聚成气溶胶并沉降，最终留在了冥王星的表面。经历了太阳系的漫长历史，这些碳氢化合物在冥王星表面的厚度应该有几十米。

而实际上，“新视野号”没有在冥王星上发现可能由这些光化学物质（冥王星大气中的甲烷吸收阳光中的紫外线后产生）形成的大片非常暗的区域。在太阳系外围，通过化学反应形成的类似碳氢化合物比比皆是。这些略带红色的物质统称为索林斯（tholins）。我们计划进一步观测冥王星大气层，以便弄清楚可形成气溶胶的乙烷、乙炔和乙烯等碳氢化合物气体的分布情况。

Pluto’s mysterious atmosphere

I became interested in Pluto as a graduate student at Caltech in 1984. So little was known about the atmosphere at the time that you could get a computer model to predict just about anything. But after those stellar occultations revealed its unexpected atmosphere, I was hooked on Pluto atmosphere research. I focused on the planet’s chemistry. When Alan Stern, the New Horizons principal investigator, started pulling together researchers to work on the mission, he asked me to join the atmosphere science team. Other mission members are focused on surface geology, interior structure, and formation of the Pluto-Charon system of five moons, among other things.

My main interest is understanding the chemistry and stability of Pluto’s atmosphere. How could Pluto hold onto its extended layer of gases over the 4.5 billion year age of the solar system? It should have long ago escaped to space because of tiny Pluto’s low gravity. But apparently it hasn’t.

Also, I’m interested to see whether the methane in its atmosphere produced complex hydrocarbons that would condense onto aerosols, settle downward, and be deposited on Pluto’s surface. Over the age of the solar system, these molecules should accumulate so that now we’d expect to see tens of meters of the stuff on Pluto’s surface.

And in fact, New Horizons does see large very dark regions on Pluto that might be made up of these photochemical products, generated when the methane in Pluto’s atmosphere absorbs ultraviolet sunlight. Similar types of chemically processed hydrocarbons are seen elsewhere in the outer solar system. They are generally known as “tholins” — a generic term to describe this reddish material. We have further observations planned that will probe Pluto’s atmosphere and map the distributions of hydrocarbon gases such as ethane, acetylene and ethylene that condense to form the aerosols.