Okay. So continuing on this theme of extant habitability, worlds where we think life could be alive today, where liquid water environments might exist that could support life. We all know and appreciate that water is not enough. So what, what is the kind of framework that we use for habitability? Broadly speaking, we can break that down into three keystones. We have to have water, we have to have the essential elements, and we have to have some form of energy. Europa, in this context, and, and to some extent, Enceladus and some of the other moons satisfy various components of these keystones through various mechanisms. We talked earlier about why we think Europa does have this liquid water ocean, and we discussed why it's a global ocean. One of the things that I didn't mention is that, it's an ocean that's there today, and we think it's been there for much of the history or solar system. So it's it's been around for a long time, which is great from the standpoint of, of habitability. We don't know how long the origin of life took, so the longer an environment can be around to some extent, the happier it will be when it comes to searching for life then. As far as the elements before life we all know and, and love the CHNOPS, the carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur. But along with those elements we think that life needs a total of about 54 of the naturally occurring elements. And on a world like Europa or Enceladus for that matter, we think that the initial chondritic or sort of space rock composition, helps provide that menu of, of essential building blocks. And then even after formation you've got exogenous delivery, you've got various things coming in from from the Jovian environment and beyond that would also deliver some of the raw materials. Then we get to that keystone of energy. And here's where an environment like Europa's subsurface ocean might have some problems. When you think about the base of the energetic food chain on Earth, we of course typically think of photosynthesis. Using energy from the sun to form the base of the food chain, and then the animals eat the plants, etc. Well what about in an ocean that's covered by kilometers of ice? And an ocean where photosynthesis is a very small niche, at best. Well, what I'd like to do now is talk a bit about this energy keystone for habitability on a world like Europa. And we'll be talking a little bit about tidal dissipation, but we'll really be focusing on the radiolytic chemistry on the surface. But, before diving into that let's just put the energy dynamic in the broader context of what it means for life. Now, this sounds unromantic, but for the most part, biology is a layer on top of geology. And the role of life is to alleviate chemical disequilibrium in an environment. A good analogy is a battery in a flashlight. If you buy batteries at the convenience store, and just leave them out on your counter, eventually the energy that's in those batteries will be dissipated because the electrons will kind of scurry along the outside, etc., and after a certain number of years those batteries die. If, however, you now take that battery and put it in a flashlight and complete the circuit. You will dissipate that stored energy a lot quicker. But in so doing you will get work done. And ultimately what biology does is it completes the biochemical, the geochemical circuit in the environment. Take for instance rust. Rust occurs naturally, but there are certain microbes that can help accelerate the oxidation of iron. And so if you had a rock out in the environment without microbes it would rust at rate x. Then with microbes, it would rust at a rate 2x x and those rate kinect, kinetics are, are environment and, and microbe dependent. Key thing is that life completes the circuit whether it's humans combining oxygen and food, or microbes combining some sort of oxidant and reductant. And the generic terms here, oxidant and reductant, refer to your electronic chapter, the oxidant and your electron donor, the reductant. Typically the reductant is your food and your oxidant is some sort of gas or other things that you're combining to to create the geochemical reaction that you need to survive. Earth microbes use a whole host of different oxidants and reductants. And broadly speaking when you think about reductants you can think of say, fluids coming out of a hydrothermal vent. And so, on a world like Europa, you could well have a very vigorous water-rock interaction on the sea floor that sources reductants to Europa's ocean. But if you do not have an oxidant to couple with that reductant then the ecosystem there will be energetically constrained, energetically limited. So what do the energy dynamics on Europa look like? And, and what sources for oxidants may exist? This is a rough diagram of what we think the energy dynamic on Europa might look like. We of course have solar and this is the, this is calculated for a full day and distributed over latitudes. The title energy in the, in the mantle and the, the ice shell is somewhere between, say, a few to possibly as much as 190 or so milliwatts per square meter. Radiogenic decay is in the range of a few to maybe as much as 7 or 8 or, or 10 or so milliwatts per square meter. Interestingly you actually have about 20 to 40 or so milliwatts per square meter. Reflection from Jupiter because that's a, a big old source out there. But, what I want to focus your attention to is the 125 milliwatts per square meter on average of charged particle irradiation. These are energetic electrons, ions and protons, that are within the Jovian magnetosphere that bombard Europa's surface. Now Europa does not have an atmosphere, so this is direct from the sort of vacuum of space onto the solid ice of Europa. To put this in perspective that kind of that kind of a radiation environment is comparable to a strong energetic solar storm that creates very strong aurora in the upper atmosphere of the earth. Here's what those charged particles would look like if we could actually see them in cartoon form. At the left here is Jupiter with it's magnetic field in green. Europa in blue, and then the electrons and ions and protons circle around with their gyro-radii, which are determined by their velocity and mass and charge and, and host of other dynamics. And, these gyro radii, then help determine where on the surface of Europa these charged particles are going to impact. The key dynamic here is simply that Europa's surface gets bombarded by all of these charged particles. And that creates radiation chemistry or radiolytic processing or radiolysis on the surface of Europa. And so if we zoom in on Europa's upper say millimeter of ice. Notice the change in scale here from 1 millimeter to 10 kilometers or so. We zoom in on that upper millimeter. We've got all these charge particles coming in and splitting apart water and whatever else is there. Some things can recombine, other things escape. And the overall effect of this radiation chemistry and the interaction with the water ice, is that oxidants such as hydrogen peroxide, H2O2, Oxygen, Sulfate, CO2, SO2 are created in Europa's surface ice. And then if there's some fracture in the ice that allows that surface material to be delivered. You now got a means for transferring these radiolytically produced oxidants to the ocean below perhaps providing that terminal of the, of the biochemical energy equation. And we know that these oxidants exist. The Galileo spacecraft detected hydrogen peroxide on the surface of Europa. Ground-based spectroscopy has observed oxygen. The Galileo spacecraft also observed the sulfates and the CO2 and the SO2. We can use laboratory simulations for instance up in our lab at JPL where we have, what we like to call our Europa in a can. To replicate that radiation chemistry, and make sure that we closely, that we can understand in detail exactly what's happening. And that's what I'm showing you here, is the 3.5 micron hydrogen peroxide feature, created in the lab, by irradiating water ice, and mapping that onto the red dots here, which are the data from the Galileo spacecraft. So in the lab, we can recreate what we think is happening on the surface of Europa and map that onto our observation's both from spacecraft and ground based telescope observations. And what that leads to is that the surface of Europa contains about 0.13% by number abundance, relative to water molecules of hydrogen peroxide. It's roughly equivalent to taking a, a bottle of peroxide that you would get at a pharmacy, and diluting it with about three gallons of water. That's what the surface of Europa would have in terms of hydrogen peroxide. Then, if that's delivered to the ocean below, that peroxide decays to things like oxygen, and microbes are off to the races. And its really not that hard to get Europa's ocean to have a molarity of oxygen and hydrogen peroxide in its ocean, beyond the oceanic minimum zones found here on Earth. And I won't go into detail on this plot, but, what this is showing here is a relationship between the delivery period of those surface oxidants and the molarity of Europa's ocean. And what you find is that as long as the surface ice of Europa is mixing into the ocean on a time scale of say tens of millions of years, which is consistent with our geological assessment of Europa's surface age, then Europa's ocean can become rich with oxidants. Even for those somewhat conservative estimates of that delivery and, and abundance measurements. And so, if in fact these radiolitically produced surface oxidants do get delivered to the ocean below, there are all sorts of biogeochemical pathways that microbes on Earth could utilize to survive in Europa's oceans. And shown here is a network of just a few of those possibilities. Shown in grey are the things like sulfate, and peroxide, and oxygen, and CO2, that are known to exist on Europa's surface. And then coming up from the seafloor in this diagram are things like sulfide and, and reduced iron that can be coupled with those oxidants through metabolisms. And here's a list of some of those known metabolic processes, that microbes on Earth utilize given those raw materials of the oxidants and, and reductants. So in conclusion, from the habitability standpoint and thinking about that keystone of energy to help power life on Europa. The maintenance of a, an ocean that is out of chemical equilibrium and, and which could sustain life within the ocean, could come down to the following dynamic. You've got tidal energy allowing liquid water to mix with this rocky sea floor, helping to supply reductants. And then, the liquid water ocean mixing with the ice shell to help deliver those radiolitically produced oxidants to the ocean below. And then it's off to the races for the microbes and whatever else could be there.
