Next week, it will be a half-century since humans first landed on our Moon. Nobody has visited there since the last of the Apollo Moon missions, in 1972. So what’s the talk in space circles? That we should send humans to the Moon again.
There are reasons for this. For example, the Moon is a likely stepping stone for manned voyages further into space, like to Mars. Going there will help us rediscover what it takes to send humans to another world. After all, and despite the several Moon landings in the early 1970s, by now the largest fraction of humanity’s manned experience in space is from orbiting the Earth. We’ve done that in the Space Shuttle and the International Space Station, and learned plenty from those expeditions. But what do we really know about exploring other objects out in space?
So there’s sense in considering going to the Moon again.
But there are also, in space circles, suggestions that we should look beyond the Moon. That we should consider a moon, yes, but not necessarily the Moon. One of the other moons out there in our solar system, that is, is actually receiving some serious attention. The National Aeronautics and Space Administration (Nasa) has for a while had plans to send an unmanned spacecraft to a distant object we call Europa.
Europa is one of nearly 80—eighty!—moons that rotate around the largest planet in the solar system, Jupiter. We know that, for various reasons, it is impossible to land a craft on Jupiter — though we did send one named Galileo crashing onto its surface in 2003—but Europa is another kettle of organisms altogether, and I use that phrase deliberately. Nasa’s plan is to have the craft—Europa Clipper, it’s called—only fly past Europa and photograph it. But this particular moon is intriguing enough that there’s already at least a proposal for a follow-up mission that will not just fly past, but actually, land there.
Keep in mind that all this is not happening any time soon, partly because it will all take a great deal of planning and effort, and partly because any craft will need several years to reach that distant corner of our solar neighbourhood. Clipper is supposed to launch in 2023, a date that has already been pushed back a year. If it does take off that year, the earliest it can reach Jupiter is 2026. Meanwhile, Nasa has proposed launching Europa Lander in 2025, to land on Europa in 2031. For now, though, Lander is still just a speculative proposal.
Nevertheless, why the interest in Europa? One clue is in the name that’s generally used for the Europa Lander project: it’s an “astrobiology mission". Put another way, the Lander is supposed to search for signs of life (called “biosignatures") on Europa. True, we have no actual evidence that there is life on Europa. But from what we know of its composition and physical characteristics, scientists think it is about the most likely place in the Solar System, apart from Earth of course, for life to exist.
This speculation about Europa dates from at least 1979, when the Voyager spacecraft flew past it and sent back photographs. A few years earlier, Pioneer 10 had sent back low-resolution photographs that weren’t very useful. But the much clearer Voyager images showed plenty of intriguing detail. For one thing, Europa’s surface seems extraordinarily smooth. For another, it is crisscrossed by many intersecting lines, like a gigantic cobweb laid over it. It’s as if, said one scientist, the lines were “painted on with a felt marker". What could cause such smoothness and those lines? The explanation that best fits these images is that Europa’s surface is a relatively thin crust of ice, and the ice floats on top of a deep ocean. The crisscrossing lines, then, are long fissures in the ice. Now of course ice and oceans mean liquids, and if it’s liquid it could be water, and water immediately suggests the possibility of life. But what’s fascinating about Europa is that there’s more to it that suggests life as well.
Or actually, what’s really fascinating is the reasoning that leads to this suggestion.
Unlike some other moons out there that have similar ice-covered oceans, the bottom of Europa’s deep ocean, scientists think, is rocky. This is important, because if it were ice instead, like they think is the case on those other moons, it would be hard for the building blocks of life as we know it to come together in the ocean. But a rocky bottom probably has holes and cracks, just like the rocky bottoms of oceans here on Earth do. And if so, these vents could be like underwater springs or small volcanoes, through which minerals from deeper inside Europa come spewing out into the ocean.
This is what prompted Michael Russell and two Nasa colleagues to conduct a study of Europa and write a paper titled The Possible Emergence of Life and Differentiation of a Shallow Biosphere on Irradiated Icy Worlds: The Example of Europa (Astrobiology, 1 December 2017). They knew that the reaction between hydrothermal springs and saline seawater was critical to the appearance of the earliest forms of life here on Earth. Could something similar have happened on Europa?
Maybe so, but the problem is that this process of forming life on Earth needed energy from the Sun, and that’s in pretty minimal supply on distant Europa. Russell suggests a different mechanism. Europa’s surface may not get much sunlight, but it gets a heavy dose of radiation from the nearby behemoth we know as Jupiter. Let’s say gases and minerals from deep inside Europa are churned upward through the ocean by the heat from the hydrothermal springs. They reach the underside of the crust of the ice. But there are cracks too, remember?
Scientists have actually observed plumes on Europa, erupting through the cracks in the ice. Such plume activity means there’s an exchange happening between the ocean and the surface which is being bombarded non-stop by electrons from Jupiter. And if that’s so, that radiation can play the life-giving role that solar energy did on Earth.
That is, there might just be some kind of life gathered around these cracks and vents, perhaps spreading through Europa’s oceans. Here on Earth, we’ve observed communities of simple life forms coming together where water and ice meet, like under Antarctica’s ice cover. Again, could something similar have happened on Europa?
Well, the abstract of Russell’s paper puts it in these words: “Ionic gradients imposed across these inorganic barriers…could drive the hydrogenation of carbon dioxide and the oxidation of methane through thermodynamically favourable metabolic pathways leading to early life-forms." Dense academic language, you think? In an interview, Russell used simpler terms: “All the ingredients and free energy required for life are all focused in one place." But however it’s described, this is why scientists are looking at Europa with hope and excitement. This is why they are starting to think about what they would send to Europa aboard Lander, to search for those biosignatures.
For one example: once Lander lands on Europa, is there a way to cut through the ice crust to the ocean below, to be able to conduct tests? This is no simple task. For even though I called it relatively thin above, the crust is thought to be about 30km thick. We’ve never drilled that deep on Earth, not even close, so we need to figure out how to do it. Besides, how will we equip such a drill with the power it will need? No convenient electrical outlets on Europa, remember.
There are a few ongoing efforts to solve this puzzle, and two even come with charming acronyms. Powered by plutonium, SLUSH (Search for Life Using a Submersible Heated Drill) will drill into the crust and, at the same time, partially melt the material it encounters. Its designers think this will be more efficient than just drilling. Still, they expect it might take as long as three years before SLUSH strikes the ocean. The team at ARCHIMEDES (A Really Cool High-Impact Method for Exploring Down into the Europan Subsurface) think they can proceed entirely without a drill, melting the ice using a laser. One advantage here is that the laser probe can carry with it the instruments that will search for biomarkers.
Both these projects have already produced prototypes that are being tested. It will be a decade and more before either of them, or something else altogether, lands on an icy and distant rock. But once that happens, this moon that whirls about our largest planet might just offer us lessons and perspective about our place in this universe, and a whole new understanding of the meaning of life.
So yes, we should send men back to our Moon. But hark! Europa calls.
Once a computer scientist, Dilip D’Souza lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun