Europe's Jupiter Icy Moons Explorer is unlikely to find life. Here's why.
欧洲的木星冰卫星探测器不太可能发现生命。原因如下。
Even after Europe's Jupiter Icy Moon Explorer (JUICE) concludes its mission more than ten years from now, we are unlikely to know for sure whether life might be thriving in the vicinity of the solar system's largest planet.
即使欧洲的木星冰卫星探测器（JUICE）在十多年后结束了任务，我们也不太可能确切知道这颗太阳系最大的行星附近是否有生命存在。
Liquid water, a source of energy and nutrients are the three things scientists believe are necessary for life to emerge anywhere in the universe. Some of Jupiter's moons are more likely than others to provide all three of these elements. The European Space Agency's JUICE mission, set to launch this week, aims to help scientists better understand which of these moons possess the right stuff and could possibly support life, and which couldn't.
液态水是能量和营养的来源，是科学家认为生命在宇宙任何地方出现所必需的三样东西。木星的一些卫星比其他卫星更有可能出现这三种元素。欧洲航天局（European Space Agency）定于本周发射的果汁探测器任务旨在帮助科学家更好地了解这些卫星中哪些拥有合适的物质来支持生命，哪些卫星不能。
JUICE, however, is not going to detect life, nor its direct signatures, scientists say. And by the time the mission ends, most likely by crashing into Jupiter's largest moon Ganymede, we will still be decades away from knowing for sure whether life, even if only microbial, thrives on any of Jupiter's four main moons. So what exactly is JUICE going to tell us?
然而，科学家说，果汁探测器无法探测生命，也无法探测生命特征。任务结束时，很可能会撞上木星最大的卫星木卫三，我们还需要几十年才能确定，即使是微生物，是否有生命体在木星的四个主要卫星中的任何一个上繁衍生息。那么果汁探测器到底要告诉我们什么呢？
Related: ESA's Jupiter mission JUICE is not 'strong enough' to orbit potentially life-harboring Europa. Here's why
相关：欧空局的木星任务果汁探测器不够“强大”，无法围绕可能存在生命的木卫二运行。原因如下
Many of the moons orbiting the giant planets of the solar system, such as Jupiter, Saturn and possibly Uranus, are quite different from Earth's moon. Data from probes that flew past them suggest that these moons may be harboring abundant oceans of water. Due to the extremely cold temperatures in these distant parts of the solar system, these oceans are covered by shells of ice dozens of miles thick, so peeking inside of those water bodies is not an easy task.
许多围绕太阳系大行星运行的卫星，如木星、土星，可能还有天王星，与地球的卫星截然不同。从飞越它们的探测器获得的数据表明，这些卫星可能拥有大片海洋。由于太阳系中这些遥远的地方温度极低，海洋被几十英里厚的冰壳所覆盖，所以窥探这些水体的内部并不容易。
On some of these moons, such as Saturn's moon Enceladus, scientists have detected evidence of water geysers that spray upwards for miles into space through cracks in the ice. Measurements by the Hubble Space Telescope suggest that Jupiter's smallest moon Europa might also produce such plumes. These plumes indicate that some source of heat must be at work inside of those moons, increasing the possibility that conditions favorable for life might exist on these worlds.
在其中一些卫星上，如土星的卫星土卫二，科学家们已经探测到水间歇泉的证据，这些水间歇泉通过冰的裂缝向上喷射数英里到太空中。哈勃太空望远镜的测量表明，木星最小的卫星木卫二也可能会产生这样的羽状物。这些羽状物表明，卫星内部一定有某种热源在起作用，增加了这些星球上存在有利于生命的条件的可能性。
Jupiter's largest moon Ganymede, the main target of the JUICE mission, is also believed to possess an ocean, and so might the most distant, crater-riddled Callisto.
木星最大的卫星木卫三是果汁探测器任务的主要目标，它可能拥有海洋；最遥远的，陨石坑遍布的木卫四也可能拥有海洋。
JUICE will be studying these two moons from the altitude of several hundred miles, mostly measuring the physical properties that can help scientists confirm the existence of these oceans, determine their depth and gain some insights into their chemical composition. The probe will also make two flybys of Europa, the second closest of the four main moons to Jupiter. Despite the valuable measurements JUICE will take of these moons, scientists don't think that any signs of life could be detected from so high above.
果汁探测器将从数百英里的高度研究这两颗卫星，主要是测量物理属性，这些属性可以帮助科学家确认海洋是否存在，确定深度，获得对它们化学成分的一些见解。探测器还将两次飞越木卫二，木卫二是四颗主卫星中离木星第二近的一颗。尽管果汁将对这些卫星进行一些有价值的测量，但科学家们不认为从这么高的地方可以探测到任何生命迹象。
"If life exists on these moons, we expect it to be in the water, and that's very hard to access," Adam Masters, senior lecturer in space and atmospheric physics at Imperial College London and a member of the team that built one of the scientific instruments for JUICE, told Space.com. "We don't expect life on the surface of these moons and it's not feasible yet to go down [underneath the ice crust] to where there might be life."
“如果这些卫星上存在生命，我们预计会在水中，很难接近，”伦敦帝国理工学院太空和大气物理学高级讲师、为果汁探测器建造科学仪器的团队成员亚当·马斯特斯告诉空间官网，“我们不认为这些卫星表面会有生命存在，而且到(冰壳下)可能有生命的地方去也并不可行，”
Of the two missions set to study Jupiter's moons in the early 2030s, JUICE has a disadvantage when it comes to gathering evidence of extraterrestrial life. That's simply because JUICE's main target is Mercury-sized Ganymede, the solar system's largest moon and the third most distant from Jupiter out of the planet's four main natural satellites. NASA's Europa Clipper mission, which will launch next year but will reach the Jupiter system one year ahead of JUICE, has a better chance of producing ground-breaking insights in that respect, as its focus is the smaller Europa. This moon, closer to Jupiter than Ganymede, is much more likely to possess all three of the main prerequisites for the emergence and existence of life, scientists think.
在2030年初计划研究木星卫星的两项任务中，果汁在收集外星生命证据方面处于劣势。这仅仅是因为果汁的主要目标是水星大小的木卫三，这是太阳系最大的卫星，也是地球四颗主要天然卫星中离木星第三远的卫星。美国宇航局的木卫二快船任务将于明年发射，但将比果汁提前一年到达木星系统，在这方面更有可能产生突破性的见解，因为它的重点是较小的木卫二。科学家认为，这颗卫星比木卫三更靠近木星，更有可能拥有生命出现和存在的所有三个主要先决条件。
"Evidence from previous missions as well as from theory and modeling suggests that on Europa, under the ice shell, if you keep going down into the water, you will eventually reach a rocky bottom," said Masters. "This rock is the source of nutrients. And because we think there might be water plumes on Europa, just like on Enceladus, we think there must be some source of energy. Ganymede, however, is a different category."
马斯特斯说：“从以前的任务以及理论和模型中得到的证据表明，在木卫二的冰壳下面，如果你继续向下进入水中，最终会到达岩石底部。岩石是营养物质的来源。因为我们认为木卫二上可能有水柱，就像土卫二上一样，我们认为那里一定有某种能量来源。然而，木卫三不同。”
Ganymede, over 3,200 miles wide (5,200 kilometers), has an ocean that is likely much deeper than that of the smaller Europa. In fact, scientists think that Ganymede's ocean might hold 25 times as much water as Earth's ocean. But the immense depth of this body of water may be one of the reasons why life may not exist on this moon.
木卫三宽超过3200英里(5200公里)，其海洋可能比较小的木卫二要深得多。事实上，科学家们认为木卫三海洋的水量可能是地球海洋的25倍。但这片水域非常深，可能是这颗卫星上不存在生命的原因之一。
"What we think happens inside Ganymede if you go deep enough is that the water becomes ice again," said Masters. "So Ganymede's ocean does not only have an ice shell above but also below and that means that the water is likely not in touch with the rocky core."
马斯特斯说："我们认为，如果你走得足够深，在木卫三内部水又会变成冰，因此，木卫三的海洋不仅有上面的冰壳，而且还有下面的冰壳，这意味着水很可能没有接触到岩石核心。"
No rocky bottom means no source of nutrients. However, Masters adds, all may not be lost for Ganymede as cracks may exist in the bottom ice layer, created perhaps by some form of thermal activity within the planet-sized moon's core. The size of Ganymede, coupled with its greater distance from Jupiter (665,000 miles or 1,070,000 km compared to Europa's 417,000 miles or 671,000 km), means the intensity of the tide-producing gravitational forces that Ganymede is subject to is much weaker than those that Europa experiences, which in turn might result in less thermal activity inside the moon and therefore a lower likelihood of life.
没有岩石底部意味着没有营养来源。然而，马斯特斯补充说，木卫三并不是什么都没有，因为底部冰层中可能存在裂缝，这可能是由行星大小的月球核心内某种形式的热活动造成的。木卫三的大小，加上它距离木星更远（665，000英里或1，070，000公里，而木卫二的距离为417，000英里或671，000公里），意味着木卫三受到的潮汐引力强度比木卫二经历的要弱得多，反过来可能导致月球内部的热活动更少，因此生命存在的可能性更低。
Scientists are so confident that there is no chance for JUICE to encounter anything alive around or on Ganymede's surface that they plan to dispose of the spacecraft by crashing it into the moon at the end of the mission. This maneuver is intended to prevent JUICE from turning into an out-of-control piece of space junk that could potentially collide with the more promising Europa in the future and contaminate it with Earthly germs.
科学家们非常自信，果汁探测器不会在木卫三表面周围遇到任何生物，他们计划在任务结束时将航天器撞向月球处理掉。这一策略旨在防止果汁探测器变成失控的太空垃圾，可能会和未来更有希望的木卫二相撞，并被地球上的细菌污染。
"There would need to be a real shake-up of knowledge about Ganymede to put it into a different category," said Masters. "If we for instance find that there isn't such a thick ice shell or if we find that there is material on the surface coming from below and that Ganymede is more like Enceladus and Europa, that might change things."
马斯特斯说：“我们需要对木卫三的知识进行调整，才能把它重新归类。例如，如果我们发现没有这么厚的冰壳，或者我们发现有更下层的物质，而且木卫三更像土卫二和木卫二，情况可能会有所不同。”
Even though JUICE will not touch down on Ganymede and attempt to reach down below the moon's crust of ice (which may be up to 90 miles, or 150 km, thick), its package of ten scientific instruments will allow scientists to glean a surprising amount of information about the moon's interior ocean.
尽管果汁探测器不会降落在木卫三上，也不会到达月球冰壳（可能高达90英里或150公里厚）以下，但它有着10个科学仪器包，科学家能够收集到关于月球内部海洋的大量信息。
One of the key questions scientists want the mission to answer is what the chemical composition of the water below Ganymede's icy crust might be, as that could hint at the moon's ability to sustain life.
科学家们希望这次任务回答的一个关键问题是木卫三冰壳下面的水的化学成分可能是什么，因为这可能暗示了月球维持生命的能力。
Remote analysis of the composition of the ice crust surface might provide some hints, as might remote measurements of the magnetic field around the moon and its variations. The magnetic field of Jupiter is more than 10,000 times stronger than that of Earth. In the 1990s, NASA's Galileo mission detected odd disturbances in this magnetic field that scientists eventually traced to the effects of oceans inside Jupiter's moons.
对冰壳表面成分的远程分析、对月球周围磁场及其变化的远程测量也可能会提供一些线索。木星的磁场比地球强10,000多倍。在20世纪90年代，美国宇航局的伽利略任务探测到这个磁场中奇怪的扰动，科学家们最终追踪到了木星卫星内部的海洋。
Magnetometers on JUICE are more sensitive than those on Galileo, and Masters hopes they will provide much more detailed views of the water body under Ganymede's icy shell. These sensitive instruments could even reveal, even through the dozens of miles of ice, how much salt is dissolved in the subsurface ocean's waters.
果汁探测器上的磁力计比伽利略上的磁力计更灵敏，马斯特斯希望它们能够提供关于木卫三冰壳下水体的更详细的视图。这些敏感的仪器甚至可以揭示出，即使是透过几十英里的冰层，地下海洋的水域中溶解了多少盐。
"If there is an ocean that has salt in it, it makes it electrically conducting," said Masters. "Then we expect to detect a sort of pulsing magnetic signal of a very low frequency. It's a very weak signal that is quite hard to resolve. But that's where JUICE fits in. If it's very, very salty, it's going to be more electrically conductive and the signal will be stronger. If it's got very little salt in it, it will be a weaker signal."
马斯特斯说：“如果海洋中含有盐，就会使海洋导电。我们希望检测到一种非常低频的脉冲磁信号。这种信号非常微弱，很难分辨。但这正是果汁的用武之地。如果海洋非常非常咸，导电性会更强，信号也会更强。如果含盐量少，信号会更弱。”
The chemical composition of the hidden ocean may help scientists decide whether searching for life or its direct signatures with a future mission would be a worthwhile endeavor.
研究可能存在海洋中的化学成分可以帮助科学家决定在未来的任务中探寻生命或生命特征是否值得。
Data gathered by both JUICE and Europa Clipper will help scientists determine how to best look for direct traces of life in the Jupiter system in the future. Masters, however, cautions that a mission that might provide the ultimate answer about life on Jupiter's moons may still be a "generation away."
果汁探测器和木星二快船收集的数据将帮助科学家确定未来如何更好地寻找木星系统中的直接生命痕迹。然而，马斯特斯警告说，为木星卫星上的生命提供最终答案的任务可能仍然需要“一代人的时间”。
"JUICE will set the scene for future missions that will look for biomarkers on these moons," said Masters. "We don't even know how these biomarkers should look like and there will be a lot of work done by our colleagues in life sciences and chemistry, who will try to understand the implications of what JUICE is going to find and how that perhaps fits in with what we know about life in extreme environments on Earth, such as the bottom of Earth's oceans."
马斯特斯说：“果汁探测器将为未来在这些卫星上寻找生物标志物的任务奠定基础。我们甚至不知道这些生物标志物应该是什么样子，在生命科学和化学领域的同事会不懈努力，尝试搞明白果汁将会发现什么，以及果汁探测器如何与我们所了解的地球上极端环境下的生命相吻合，比如地球海洋的底部。”
Even without the discovery of life, the JUICE mission is expected to produce a slew of fascinating scientific discoveries. In addition to the Galileo mission, NASA's Juno spacecraft has studied the Jupiter system, but that spacecraft's main focus has been on the planet itself, rather than its moons. In the 1970s, the Pioneer 10 and Pioneer 11 and the Voyager 1 and Voyager 2 mission made brief visits to Jupiter, as did the Saturn explorer Cassini and the New Horizon probe bound for Pluto and beyond.
即使没有发现生命，果汁探测器任务也会产生一系列令人着迷的科学发现。除了伽利略任务，美国宇航局的朱诺飞船也研究了木星系统，但该飞船的主要焦点是行星本身，而不是它的卫星。在20世纪70年代，先锋10号和先锋11号以及旅行者1号和旅行者2号任务对木星进行了短暂的访问，土星探测器卡西尼号和新地平线探测器也前往了冥王星和更远的地方。
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