木卫二是一颗冰冻的岩石卫星，它的表面温度在赤道地区平均约为-163℃，并且拥有15~25千米厚的冰层。乍一看，它似乎不太可能存在生命。然而，厚厚的冰壳隐藏着冰下海洋。我们之所以知道这个海洋存在，是因为我们探测到了它的磁场(当然，在我们到达那里之前，我们不能100%确定)。
最终，科学界的共识是，由于木卫二本身与附近其他天体之间的引力相互作用，木卫二拥有液态海洋，海洋下是岩石地幔。简而言之，木星(以及它的几个大卫星，如木卫一、木卫三和木卫四)通过引力不断拉扯着木卫二，导致木卫二伸缩变形。如果没有这些引力相互作用，以及它们产生的热量，木卫二将是一个固态的冰卫星。
如果木卫二取代月亮成为我们的卫星，我们的隔壁会有一个雄伟的水世界吗?
这是不言而喻的，但是思维实验需要估计和猜测。值得注意的是，其中一些过程可能发生在较短的时间尺度上(几天到几个月)，比如稀薄大气的形成，也可能发生在较长的时间尺度上(数万年到数千万年)，比如全月球范围的冰川融化。时间的长短取决于许多因素，以下是基于我们已有数据的假设。
木卫二目前位于太阳系的宜居带之外。“宜居带”是液态水可以存在的区域(这个区域的大小和远近是由恒星的温度决定的)。所以，如果木卫二被移动到地球旁边，它会突然发现自己处于宜居带，然后开始融化。
由于木卫二没有大气层(它的大气层非常稀薄，接近真空)，冰会直接从固体升华为气体。很快，木卫二的大气层就会变得越来越厚。最终，在没有任何磁场保护的情况下，强烈的电离辐射(这也是导致冰融化的原因)会分解大气中的水，留下氢气和氧气。由于氢气的分子量很小，木卫二的引力无法留住它，因此氢气会缓慢地散逸到太空中。
电离辐射对木卫二还有另一个影响——它们产生了一个临时的保护磁场!
我们目前在金星和火星上观察到的情况就是如此。太阳风与金星的大气层相互作用，将其电离并剥离，同时也在此过程中产生了一个临时磁场。这个临时磁场偏转了部分太阳风，从而减缓了剥离过程。
一段时间后，电离辐射和磁场强度之间达到了一个微妙的平衡。木卫二月亮稳定下来了，拥有了一个液态水表面和浓厚的含氧大气层。
那么我们会拥有一个适合生命存在的水世界?嗯，不完全正确。这样的平衡只能维持一小段时间。
地球的液态铁镍核心的流动给我们带来了地磁场。地磁场是地球的第一道保护屏障，它偏转了来自太阳的高速带电粒子流（被称为太阳风）。太阳风是如此强大，以至于它可以轻易地将气体从行星的大气中分离出来，直到几乎没有气体残留为止。
火星曾经很像地球，有着液态水的海洋和厚厚的大气层，其表面大气压曾经相当于地球大气压的30%左右。但是火星太小了，它的核心很快冷却了下来。于是在数十亿年前，它的磁场消失了。火星的大气层没有磁场的保护，被太阳风逐渐吹走，液态水开始大量蒸发，直到它又被太阳风带走。慢慢地，火星表面上的水消失殆尽，大气层也无影无踪，如今火星表面大气压只相当于地球表面大气压的0.8%。所以，没有磁场，也就没有大气，没有大气，液态水就会消失，最终只会留下一个干燥寒冷的世界。
木卫二处在木星强大的磁场内，诱发了一个感应磁场，这是因为它在冰壳下存在着导电的含盐海洋层。但是木卫二没有像地球或木星那样的固有磁场，因此它注定要失败。它的大气层会慢慢地被剥离，连同上面的水也会逐渐蒸发。
那么我们会得到什么?嗯，像这样的东西……
看起来熟悉吗?好像是一个小一点的月球。木卫二现在是一块围绕地球运行的贫瘠的岩石卫星;它已经变成了一个比我们现在的月球更小的卫星，因为它所有的水都消失在太空中了。这个最初巨大的宇宙冰块已经融化并逐渐被太阳风剥离了。
总结
木卫二之所以是现在这个样子，正是因为它位于宜居带之外。当行星在太阳系中形成时，在大约5天文单位处存在一个霜线。在这个距离上，物体离太阳足够远，水将会开始形成冰晶。像木卫二那么大的冰卫星不可能在地球附近形成，如果它被移到地球附近，它也不可能一直是冰卫星。
同样的原理也适用于土卫六（泰坦）这样的卫星。它能够存在其甲烷湖泊，是因为它处于太阳系的寒冷区域。如果它更靠近太阳，它将会过于温暖，无法自然形成烃类化合物。此外，如果它靠近太阳，它的大气层将会被剥离掉大部分。
总而言之，在宜居带的地球大小的行星周围形成可居住的卫星是不太可能的。卫星太小不能形成很厚的大气层；缺乏磁场保护会使它的表面逐渐被太阳风剥离。一个可居住的行星应当更大一些，它和地球将会是两个在宜居带内绕太阳运行的行星，而不是一个行星和一个卫星。
Europa is a frozen moon, so at first glace, it doesn't seem like a great candidate for life. However, the thick icy crust hides a sub-glacial ocean. We know about this oceanic wonderland because, among other things, we detected its magnetic field (however, of course, we won't be 100% sure until we get there).
Ultimately, the scientific consensus is that Europa has a liquid ocean due to gravitational interactions between itself and the other bodies in its neighborhood. In short, Jupiter (and a few of its large moons, such as Io, Callisto, and Ganymede) pull and tug on Europa, causing the moon to stretch and contract. Without these gravitational interactions, and the heating that they produce, Europa would be a solid ice moon.
But what if our moon was replaced with Europa? Would we have a majestic water world just next door?
This should go without saying, but thought experiments require estimates and guesstimates. It is important to note that some of these processes can happen on short time scales (days to months), like the formation of a thin atmosphere, or long time scales (tens of thousands to tens of millions of years), like moon-wide glacial melting. The amount of time depends on a number of factors, and the following is a hypothesis based on our best current data.
Now, all that said, Europa is currently located beyond the habitable zone in our solar system. As a refresher, the "habitable zone" is the region where liquid water can exist (this zone is defined according to the temperature of the planet's star). So, if Europa were moved and placed next to Earth, the moon would suddenly find itself within the habitable zone, and it would start to melt.
Due to the lacking atmosphere around the moon (it has one, but it is tenuous and very thin), the ice transitions straight from a solid to a gas. Soon, Europa would begin to have a thicker and more substantial atmosphere, which would slowly grow thicker and thicker. Ultimately, without any protection of a magnetic field, strong ionizing radiation (what's causing the ice to melt) would break apart the water in the atmosphere, leaving behind hydrogen and oxygen gasses.
And the ionizing photons have another impact on Europa—they create a temporary protective magnetic field!
This is what we currently see around Venus and Mars (to a lesser degree). The solar wind interacts with Venus' atmosphere, ionizing it, stripping it away, but also creating a temporary magnetic field in the process. This temporary magnetic field deflects some of the solar wind, thus slowing down the stripping process.
For a time, a fine balance would be found between the amount of ionizing radiation and the strength of the magnetic field. So things would look promising...for a time.
Glorious water world? Well, not quite.
At first glance, all of this sounds like a fantastic thing, it appears as if Europa will completely melt, leaving behind an oxygen rich water world. But this wouldn't last. Without a geomagnetic field like that of the Earth or Jupiter, it is doomed. It's atmosphere would slowly be stripped away.
So what would we have? Well, something like this...
Look familiar? Europa is now a small and barren rock orbiting around the Earth; it has become a smaller moon than our current one, as all of its water has been lost to space. What started off as a giant cosmic ice block has melted and gradually been stripped away by the solar wind (alas).
What have we learned?
What makes Europa such a nice target to explore is precisely its location beyond the habitable zone. When the planets were forming in our solar system, there was the frost line at about 5 AU, this is the point at which objects are far enough from the sun that ice crystals could begin forming. An icy moon the size of Europa could not form around the Earth, and it couldn't continue being an icy moon if it was moved here. It's too warm and the moon is too small.
This same principle applies for a moon like Titan. It is able to exist with its methane lakes because it's in a cold region of the solar system. If it was closer, it would be too warm for the hydrocarbons to form naturally. Moreover, if it were closer to the sun, it would have had most of its atmosphere stripped away.
So to sum, it's very unlikely that a habitable moon can form around an Earth-sized planet located in the habitable zone. The moon would have to be substantially larger, making it more like two planets as opposed to a planet/moon.