轨道器、着陆器和漫游车已前往火星，但没有一架返回。日本航空探索局（JAXA）希望改变这一现状。10月20日，其火星系外探测（MMX）任务从日本南部的种子岛升空。该探测器将在未来一年前往火星，计划在火星的两颗卫星中着陆较大的那颗——法乌斯（Phobos），采集至少10克岩石和尘土样本，并将收集物于2031年带回地球。行星科学家希望这项任务能解开一个长期存在的谜题：火星的卫星是如何形成的——同时也将进一步揭示火星本身的诸多信息。
ORBITERS, LANDERS and rovers have travelled to Mars, but none has returned. The Japan Aerospace Exploration Agency (JAXA) intends for that to change. On October 20th its Martian Moons eXploration (MMX) mission lifted off from Tanegashima, an island in southern Japan. The probe will spend the next year travelling to Mars, where it aims to land on Phobos, the larger of the planet’s two moons, collect at least ten grams of rock and dust, and return the haul to Earth in 2031. Planetary scientists hope the mission will resolve a long-standing puzzle: how Mars’s moons were formed—and, in the process, reveal much about the planet itself.
法乌斯和火星的另一颗卫星——狄摩斯（Deimos）体积小，分别约22公里和12公里，外形像土豆块。这可能暗示它们是被火星引力捕获的小行星，但被捕获的小行星通常具有倾斜、椭圆的轨道，而这两颗卫星几乎呈圆形轨道，且沿赤道运行。这提示另一种可能性：一次巨大撞击抛射出的碎片进入轨道，随后聚合成卫星。
Phobos and Mars’s other moon, Deimos, are small, respectively about 22km and 12km across, and look like lumpy potatoes. That might suggest they are asteroids that were captured by Mars’s gravitational field after straying too close. Captured asteroids, however, usually have tilted, elliptical orbits, whereas both moons follow nearly circular paths around the planet’s equator. That hints at another possible origin: a giant collision that threw debris into orbit that later gathered into moons.JAXA has spent over a decade developing MMX, working with the space agencies of America, Europe and Australia.
JAXA已投入超过十年开发MMX，并与美国、欧洲和澳大利亚的太空机构合作。该任务基于日本在太空采样方面的记录：其“隼鸟”任务（Hayabusa）从接近地球的小行星“伊托卡瓦”（Itokawa）采集了少量颗粒，并于2010年带回地球；“隼鸟2”于2020年从另一颗近地小行星“龙宫”（Ryugu）带回了五克材料——超过任务目标，且样本富含含水分子和有机物，为探讨小行星如何携带生命之初的物质至地球提供线索。
The mission builds on Japan’s record of returning samples from space. Its Hayabusa mission collected small amounts of grains from Itokawa, an asteroid which comes close to Earth, and brought them home in 2010. Its successor, Hayabusa2, brought back five grams of material—well above the mission’s target—from Ryugu, another near-Earth asteroid, in 2020. The samples proved rich in water-bearing molecules and organic compounds, offering clues to how asteroids may have carried the ingredients for life to Earth.MMX’s mission is more demanding.
MMX的任务更具挑战性。法乌斯表面的重力约为地球的千分之一，但仍是龙宫（Ryugu）的40倍。
Phobos’s surface gravity is roughly one-2,000th of Earth’s, but it is 40 times greater than Ryugu’s.
亚利桑那大学的行星科学家埃里克·阿斯普豪格（Erik Asphaug）解释说，这足以对脆弱的航天器构成撞击风险。“隼鸟号”任务也采用了“触地即走”的方法，即航天器短暂地与小行星接触。相比之下，“火卫一探测器”（MMX）将降落在火卫一上，并从其表面以下提取样本。
That is strong enough to pose a crash risk for a delicate spacecraft, explains Erik Asphaug, a planetary scientist at the University of Arizona. The Hayabusa missions also followed a “touch-and-go” method in which the spacecraft briefly kissed the asteroids. In contrast, MMX will land on Phobos and extract samples from beneath its surface.
而且，“火卫一探测器”的制造者们希望在其环绕火星运行的三年里做更多的事情。由美国国家航空航天局（NASA）开发的一台名为MEGANE（发音类似于日语中的“眼镜”一词）的仪器将测量宇宙射线撞击火卫一时刻射出的伽马射线和中子，从而揭示其化学成分。一个欧洲的小型巡视器将探索其表面。在离开火卫一之后，“火卫一探测器”还将对火卫二进行飞越，以对其进行近距离观测。
And MMX’s makers want to do a lot more during its three years orbiting Mars. An instrument called MEGANE (pronounced “meh-gah-neh”, like the Japanese word for spectacles), developed by NASA, will measure gamma rays and neutrons given off when cosmic rays hit Phobos, thus revealing its chemistry. A small European rover will explore its surface. After departing Phobos, MMX will conduct fly-bys of Deimos to observe it, too, up close.
如果这些样本能够安全送回地球，科学家们认为它们可能会厘清这两种起源说究竟哪一种是真实的。如果火卫一是被俘获的小行星，其岩石可能会类似于太阳系其他地方的陨石。如果它是由一次巨大的撞击后形成的，它将包含由剧烈加热产生的熔融碎片。
If the samples make it safely back to Earth, scientists reckon they might settle which of the two origin stories is true. If Phobos is a captured asteroid, its rocks may resemble meteorites from elsewhere in the solar system. If it instead formed after a giant impact, it will contain melt fragments generated by intense heating.
无论火卫一的历史如何，它紧邻火星，意味着它应该也收集了被小行星撞击从这颗行星上炸飞的物质。“火卫一探测器”收集的样本中，火星碎片应当占到大约万分之一。科学家们已经拥有火星陨石，但这些大多是经受住了穿梭太空和地球大气层考验的坚硬火山岩。伦敦自然历史博物馆的行星科学家萨拉·罗素（Sara Russell）将协助分析“火卫一探测器”的样本，她希望该任务能够回收更软的沉积物。“那是我们可能会看到生物印记的物质，”她说道，这为火星过去是否曾宜居提供了线索。
Whatever Phobos’s history, its proximity to Mars means it should also have collected material blasted off the planet by asteroid impacts. Martian debris should make up about 0.1% of the samples MMX collects. Scientists already have Martian meteorites, but these are mostly hard volcanic rocks that survived the journey through space and Earth’s atmosphere. Sara Russell, a planetary scientist at the Natural History Museum in London who will help analyse MMX’s samples, hopes the mission could recover softer sedimentary material. “That’s the material where we might see biosignatures,” she says, offering clues to whether Mars was once habitable.
人们有理由对此产生好奇。古老的河床和湖泊沉积物表明，火星曾拥有液态水和更厚的大气层。与这颗行星本身不同，它的卫星几乎没有经历过地质变化。
There is reason to wonder. Ancient riverbeds and lake deposits suggest Mars once had liquid water and a thicker atmosphere. Unlike the planet itself, its moons have undergone little geological change.
JAXA的天体物理学家伊丽莎白·塔斯克说，因此它们可能像时间胶囊一样保存了那个遥远过去的碎片。这两颗小卫星因此可能有助于回答一个更大的问题：火星上是否曾存在生命。
They may therefore have preserved fragments of that distant past like a time capsule, says Elizabeth Tasker, an astrophysicist at JAXA. The two small moons could thus help answer a much bigger question: whether life existed on the red planet.