木卫二存在海洋——日渐明朗（英文原文） |
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当导线放在随时间变化的磁场中时，感应电流就会产生。这些电流会依次产生能被探测的磁场，这就是机场的金属探测器以及使“伽利略”木星探测器能“看”到木星卫星中带电导体的物理学原理。在本期杂志的1340页，克沃尔森（Kivelson）以及其他人公布了木卫二的表面附近存在导电层的压倒性证据。最可能的解释是木卫二在其冰壳之下有一个含盐的、全球性的海洋。 木卫二的大小与月球相似而且在成分上被认为是最像地球的，但在其表面有一层厚100公里左右或是液态或是固态的水。由于木为二环绕木星的奇异轨道引发的潮汐力使表面非常冷的冰层普遍发生破碎和变形。在冰层表面下10公里处可能存在水，使那些对地球以外适合生命存在的环境感兴趣的人兴奋不已，同时也确定了木卫二在NASA外太阳系计划中扮演主要角 色。 理论上的、地质学的以及光谱分析的论据都被用来证明木卫二冰壳下海洋的存在，但是没有一个能让人心悦诚服（见表）。作为比较，磁场的证据却惊人得强有力。为理解这一点，设想在靠近木卫二表面处存在良导体层会发生的现象。在这种“高导电性”的情况下（达到这一导电性的磁场弱于对金属的要求），一个简单的感应模型预言由感应电流产生的随时间变化的磁场几乎可以精确的抵消垂直于木卫二表面随时间变化的外部磁场。后一磁场主要来自于木星磁轴的倾斜，这使得朝向木卫二的磁场以大约20度的振幅震荡，其周期为10小时。木卫二感应磁场的强度、磁极的方向取决于木卫二在木星旋转的磁场中的瞬时位置。 观测的数据和模型匹配得很好。这就是更惊人的设想，而且这个模型没有需要调整的参数。另外，最近的数据显示当探测器遇到180度经线时，木卫二由电流产生的磁场会转向180度，这正如模型所预计的那样。这一现象和固定的磁场有很大的差别。这一模型没有对数据进行精确的描绘：木卫二深深地沉浸在木星的磁层及其包含的等离子体中，而且可以弄清（尽管很复杂）磁场扰乱了因木卫二的环境而产生的交互作用。 木卫二存在海洋的证据 技术： 1、潮汐变形及加热效应的理论研究 2、对其表面变形的观测，尤其是“混乱”地区、圆形山脊，可能在低粘性的表面上流动 3、近红外光谱预示有盐在表面沉淀 4、磁场证据证明存在感应效应 5、利用高度测量和重力测定潮汐的变化 推测： 1、假设海洋一旦形成就会持续存在 2、暗示薄冰和高运动性的冰与其下存在海洋相符 3、盐会在盐水的“喷发”中升华 4、一个接近表面的、全球性的导体层的最好解释是一个含盐的海洋 5、是否存在海洋可以参考冰的厚度 挑战： 1、对冰的流动能力几乎一无所知，尤其是对潮汐的频率，因此预报尚无法确定 2、这也可解释为薄、冷、脆的冰“浮”在厚、暖、软的易变形的冰上 3、就算有水包含其中，它也没必要形成海洋——它可能在融化的冰中 4、还有其他可能的导电层？ 5、木卫二的轨道 为了更好地近似，感应现象取决于导电层的厚度和导电性。而且这一导电层也必须是一个完整的球壳；观测数据无法用比全球覆盖少得多的高导电区域拼凑而成的模型来解释。一个成分与地球海水相似、厚度超过10公里的全球性水层可以解释这些数据。在木卫二的海洋中占支配地位的离子可能与地球的不同，但它们必须符合导电的要求。一个厚得多的水冰层，即使掺杂有大量的盐水冰也无法解释这一现象，因为与在液态水中比起来其离子相对稳定。任何似是而非的流动海洋（相对冰层）是无关紧要，因为其速度远低于木卫二表面的转速。一个部分融化的冰层可能符合导电的要求，但其本身难以令人信服，因为融化的液体在大范围上是互相连接的，这将导致由于冰和水的密度不同水从冰层中过滤分离出。克沃尔森等人证明木卫二的外部电离层同样无法提供符合要求得导电层。感应区的强度与其到导电层的距离的平方成反比，任何深层的导电层（例如金属核或岩浆核）所形成的磁场远比观测到的弱。 一些奇异的可能性也不应被排斥（例如石墨或其他高导电性的物质，或者在冰中混杂着大量的碳但其在颗粒范围内互相连接），但是一个水层是最似是而非的解释。一个引人注目的证明海洋存在的证据可能来自被提议的木卫二的人造卫星测得的重力和高度的数据。预计中的每天潮汐的振幅在木卫二有全球性海洋的情况下将大于没有的情况。更复杂的是，介于这两者之间的设想可能是可行的（例如冰在有些地方与下面的岩石接地）。但是人造卫星的数据可能会解开木卫二是否存在海洋之谜。只要定义足够广泛，海洋可能并不稀罕，但木卫二的情况可能很特殊，因为潮汐的加热可能使液态水接近表面，包括偶然的喷发或流动。在火星之后，它仍是太阳系中最吸引人的搜寻可能存在生命证据的地方。 |
译自 《科学》2000-8-25 |
Europa's
Ocean--the Case Strengthens |
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When a conductor is placed in a time-varying magnetic field, electrical currents are induced. These currents in turn create magnetic fields that can be detected. It is these physical principles that underlie the operation of metal detectors at airports and that give the magnetometer on the Galileo spacecraft the ability to "see" electrical conductors inside the moons of Jupiter. On page 1340 of this issue, Kivelson et al. present overwhelming evidence for a conducting layer near the surface of one of these moons, Europa. The most likely explanation is that Europa has a salty, global water ocean beneath its ice shell. Europa is similar in size to Earth's moon and is thought to be mostly Earthlike in composition, but with a layer of some 100 km or so of water on top, which may be either liquid or solid. The very cold surface ice is extensively cracked and deformed, a testament to the flexing by tides as Europa follows its forced eccentric orbit about Jupiter. The possibility of water beneath this ice, perhaps as little as 10 km below the surface, has excited those interested in extraterrestrial environments for life and established a major role for Europa in NASA's plans for outer solar system missions. Theoretical, geological, and spectroscopic arguments have all been used to support the presence of an ocean beneath Europa's icy shell, but none of these arguments are compelling (see the table). In contrast, the magnetic field evidence is remarkably strong. To appreciate this, consider the behavior expected for a sufficiently conducting shell close to Europa's surface. In this "high conductivity" limit (reached for a conductivity many orders of magnitude lower than those for a metal), a simple induction model predicts that the time-dependent field created by the induced currents almost exactly cancels the component of the time-dependent external field perpendicular to Europa's surface. The largest contribution to the latter comes from Jupiter's dipole tilt, which causes the field direction at Europa's location to oscillate with an amplitude of roughly 20° and a period of about 10 hours. The predicted strength and direction of Europa's induced magnetic dipole depend on the instantaneous position of Europa in the reference frame defined by Jupiter's rotating magnetic field . The observational data match the model well. This is all the more remarkable considering that the model has no adjustable parameters. Moreover, these most recent data show that the field arising from currents or magnetization within Europa changes direction by 180° for spacecraft encounters 180° apart in longitude, as the induction model requires. This behavior is very different from that expected for a fixed field [such as the core dynamo that appears to dominate at Ganymede]. The model does not represent the data perfectly: Europa is deeply immersed in the jovian magnetosphere and in the plasma contained within that field, and understandable (though quite complex) field disturbances arise from this interaction of Europa with this environment. EVIDENCE FOR EUROPA'S OCEAN Technique: 1.Theoretical study of tidal deformation and heating 2.Observations of surface deformation, especially "chaotic" regions, rafting, cycloidal ridges,possible low-viscosity surface flows 3.Near-infrared spectroscopy suggesting salt deposits on surface 4.Magnetic field evidence for an induction response 5.Altimetry and gravity field with sufficient resolution to determine tidal variation Implications 1.Predicts that an ocean will persist once formed 2.Suggests thin ice and highly mobilized ice, consistent with an underlying ocean 3.Salt could arise from sublimation of a salty water "eruption" 4.Requires a near surface,global conducting layer,most readily explained by a salty ocean 5.Clear determination of whether there is an ocean; information on ice thickness Challenge 1.Rheology of ice is poorly known, especially at tidal frequencies, so predictions are uncertain 2.Might be explained by thin,cold, brittle ice "floating" on thick,warm, soft, easily deformed ice 3.Even if water is implicated,it need not come from an ocean--there may be melting within the ice 4.Is there any other possible conducting layer? 5.Requires Europa orbiter To a good approximation, the induction response depends on the product of conducting layer thickness and conductivity. However, the layer must be a nearly complete spherical shell; the data cannot be explained by a patchwork of highly conducting regions with much less than global coverage. A global layer of water with a composition similar to Earth seawater and a thickness greater than about 10 km could explain the data. The dominant source of ions in Europa's ocean may be different from those in Earth's oceans , but they should satisfy the conductivity requirement. A much thicker layer of water ice, even if it is heavily contaminated with frozen brine, cannot explain the data because the ions are relatively immobile compared with those in liquid water. Any plausible ocean flow (fluid currents relative to the ice shell) is unimportant because it will have a much lower velocity than the rotational motion of Europa's surface. A partially melted ice layer could match the required conductivity but is physically implausible because the melt would have to be interconnected over large distances, which would result in the melt percolating through and separating from the ice driven by the density difference of ice and water. Kivelson et al. argue that Europa's tenous, external ionosphere also cannot provide the required conducting layer. The induced field declines as the inverse cube of the radius from the surface of the conducting layer, and any deep-seated conducting layer (such as a metallic core or a magma ocean in the rocky core) would therefore lead to a much lower field than is observed. Some more exotic possibilities cannot be excluded (such as graphite or some other relatively high conductivity material, plausibly carbon-rich, intermingled within the ice but interconnected at the grain size scale), but a water layer is the most plausible explanation. A compelling demonstration of its existence or absence may be reached from gravity and altimetry data in the proposed Europa orbiter. The predicted diurnal tidal amplitude is over an order of magnitude larger for a Europa with a global ocean than for a Europa without one. More complex, intermediate scenarios can be envisaged (such as ice "grounding" on the underlying rocky topography in some places and not others). But the orbiter results will likely settle the fascinating question of whether Europa has an ocean. Defined broadly enough, oceans may not be that rare, but Europa's case may be special because the tidal heating may allow liquid water to get closer to the surface, possibly including occasional eruptions or flows. After Mars, it remains the most attractive extraterrestrial environment within our solar system in which to seek evidence of past or present life. *The author is in the Division of Geology and Planetary Science, California Institute of Technology |
Copied From[Science Aug. 25th 2000] |