托福阅读材料推荐:距离土星第七远的土星卫星

　　Enceladus could also teach us about genesis in our solar system in other critical ways. “You’re not just searching for life, you’re searching for an understanding of the nature of that life, and how it compares to life on Earth,” says Chris McKay, a planetary scientist at NASA Ames Research Center. For instance, if we discover that creatures on Enceladus are nothing like those on Earth—if their biochemistry is completely different—then it would likely mean that the two forms of life arose separately and independently, and thus, that aliens might be likely to exist other places as well. “If life started at least twice in our solar system, then you know the universe is full of life,” McKay says. Or, if we find out that Enceladus organisms and Earth organisms are made in identical ways, it may indicate that life originated someplace else, and was carried to both worlds. If Enceladus is barren, however, it could support the theory that life needs an environment on dry land to get started, not an ocean. Regardless of what a mission to Enceladus might discover, the answer will tell us something fascinating.

　　SKIMMING GEYSERS

　　Enceladus has over 90 geysers that spew plumes of salty water vapor, organic compounds and ice particles from the underground ocean into the air. These present a great opportunity for a visiting spacecraft, which would not have to land to search for life (which is much more difficult and expensive) but could simply fly through the geysers to capture samples. “The plume is coming right out of the ocean,” McKay explains, “So why would we want to land? We can get the freshest stuff, coming right from the source.”

　　Yet even if life exists on Enceladus, it may or may not show up in plume samples. If the pelagic ocean on Earth (that is, the open water away from the shore or seafloor) is an analogue for Saturn’s icy ocean moon, then the outlook is depressing—the pelagic zone has an extremely low density of life even on our planet. “If we had this in Enceladus’s ocean, it would be very hard to even pick up an organism,” Porco says. Scientists would need to sample a ridiculously large amount of water in order to capture any organisms.

　　Thankfully, a few months ago a microbiologist told Porco about decades-old scientific research that makes her optimistic about finding life in the plumes. At the Berkeley meeting, she described this research on a process called “bubble scrubbing” that occurs in Earth’s oceans—and it could make quite a difference in Enceladus’s geysers. It turns out that wherever bubbles rise through water, they scrub the water column so that organisms and organic materials become concentrated at the surface. And when the bubbles burst (like in ocean spray or in Enceladus’s jets), they eject those microbes in the spray. So if life exists on Enceladus, its plumes may contain a much greater concentration of organisms than the rest of its ocean—all thanks to bubbles. “Even if the ocean on Enceladus starts out being as microbially poor as the pelagic ocean on Earth, which is the worst case, we still have a chance of seeing lots of organisms in the plumes,” Porco says. Still, this scenario immediately presents another issue: A spacecraft must find a way to capture a sample without smashing the delicate organisms to bits as it makes a high-speed pass through the jets.