With the discovery of water ice in so many locations in the Solar System, scientists are hopeful in the search for life on other worlds. Guest Morgan Rehnberg returns to Astronomy Cast to explain the best places we should be looking for life.
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Speaker 1: This episode of Astronomy Cast is brought to you by Swinburne Astronomy On-line, the world’s longest on-line running astronomy degree program. Visit astronomyswin.edu.au for more information.
Fraser: Astronomy Cast Episode 375: The Search for Life in the Solar System.
Welcome to Astronomy Cast for weekly facts, fakes and journey through the cosmos where we help you understand, not only what we know, but also how we know what we know.
My name is Fraser Cain. I’m the publisher of the Universe Today and with me is Morgan Rehnberg, a graduate student in the Department of Astrophysical and Planetary Sciences at the University of Colorado at Boulder.
Hey Morgan, how are you doing?
Morgan: Hey Fraser. It’s good to be back.
Fraser: Good to be back here. We do seem to do this every year. So, it’s been around a year since you since you joined me on Astronomy Cast. We did a very special episode of Astronomy Cast on Saturn’s rings. This time around, we are going to be doing a different show, still about ice and water – about the search for life in the solar system or the search for water in the solar system, and therefore, life.
Morgan: Well, we are doing this episode as part of the CosmoQuest 2015 Hangout-a-thon. So, if you are listening to this show, we are still seeking donations for the Hangout-a-thon. We’re still going to be doing it for weeks and months after the show itself. So, even though it’s been a couple of weeks since the Hangout-a-thon happened, you can still participate. So, just go to CosmoQuest.org/hangout-a-thon and you can give some donations to help keep all of the educational and outreach activity that we do on the air. We really appreciate it.
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Fraser: With the discovery of water in so many locations in the solar system, scientists are hopeful for the search for life on other worlds. Morgan Wrenberg returns to Astronomy Cast to explain the best places we should be looking for life.
All right Morgan, your day job – graduate student – what do you do?
Morgan: Yeah, so I study the rings of Saturn as part of NASA’s Cassini Mission. Cassini has now been in orbit about Saturn for about 11 years. It’s hard to believe that we’ve been studying Saturn that long, but every time we look at something, we discover something new. So, we’ve got at least two more years – around two more years left in the Cassini Mission. We packed them full of exciting planned observations to help us understand the planet, the rings, but also the moons of Saturn. And as we’ll find out, the moons of Saturn are some of the most exciting places in the solar system.
Fraser: Very cool. So, specifically, you look for very small pieces of water ice, but we’re going to be talking about maybe some larger chunks of water here. So, to sort of set the stage here for what we are talking about today, wherever on Earth we find water, we find life. We find it in Antarctica. We find it in the hottest places. We find it in pools of super hot water. We find it in the cooling towers of nuclear reactors. So, why is water what astrobiologists really looking for when the look around in the solar system?
Morgan: So, right. There are not a lot of things that all life on Earth has in common. In fact, we can identify two things that all life on Earth has. One, is it is based on carbon. So, we call these things based on carbon organic compounds, and as far as we can tell, all the life that we’ve discovered here on Earth is based on carbon.
Fortunately, carbon is one of the most common elements in the universe and it is often combined with hydrogen, the most common element in the universe, and so we’re not lacking for carbon on Earth and we’re really not lacking for it out there in the solar system.
The other thing is water. And again water – we call it H2O – is made up of hydrogen and oxygen, two of the most plentiful elements here in the solar system. And it seems like every living thing on Earth needs water in some part of its life process.
Water is a remarkably fascinating molecule itself. It has properties that are very rare amongst other chemical compounds, and this makes it uniquely suited to help facilitate the processes that life needs. Because life here on Earth seems to require carbon and water, that’s what we’re stuck looking for out there in the solar system. It could well be that life could survive with other things, but we wouldn’t know what it would look like and it would be difficult to know how to search for it.
Fraser: Yeah, it feels like it’s the low hanging fruit, right? We don’t what life will require. But we do know – we don’t know what life might look like, as we don’t understand it, right? So, we can only look for life, as we understand it right now.
Now, I know NASA has put together some theories about maybe there could be life that uses ammonia as a place for the chemicals to mix together or maybe it might use liquid nitrogen. There are different kinds of things could act as a solvent, but we then wouldn’t really know the chemical processes, what kinds of things would be happening.
So, water we understand. We find it here. We might as well look for water in the solar system and life that is based on that, and maybe if we exhaust that we can go through other kinds of chemicals and other solvents.
So, what are the good candidates then for water that we may not find here on Earth?
Morgan: All right. So, if you go fifty years or so, it seemed like the solar system was a really dry place. We knew that there was water here on planet Earth and we knew that there was likely water trapped in the polar ice caps of Mars. But other than that, anywhere we seemed to point our telescopes seemed to be pretty devoid of water.
But as we’ve sent spacecraft to more and more places in the solar system, we’ve started to find water almost everywhere we look. We’ve found it in craters on the moon. We’ve found it in craters on Mercury. We’ve found it in asteroids. We’ve found evidence for water in meteorites that come back and land on the Earth. So, now that it seems like water is plentiful in the solar system as long as you know the right places to look.
But not all life – we think – well, not all water is equally capable of supporting life. The key is you need to have liquid water, and in order to have liquid you need pressure. So, you need an atmosphere or something solid to keep the water in liquid form. Otherwise, water that is liquid will just evaporate out into space and it’ll float away and be lost forever.
And so, although we see water on the moon and Mercury, those aren’t places we really want to look for life out there in the solar system because that is very, very cold – very icy water and not good for supporting life.
Fraser: Yeah, this is part of the problem with even Mars. The pressure on the surface of Mars is super low that you are never going to get pools, lakes, oceans of water on the surface of Mars. You are going to be looking for places. You are going to be looking for places under pressure – potentially deep underground.
Morgan: Right. So, Mars might have been a good place for life to exist billions of years ago, and Venus was a good place for life to have existed long ago before it got extremely warm, but today, neither of those places are likely locations – I think – for us to find life alive today. We might be able to find record of past life on Mars if we are particularly lucky, but the odds of finding actual living organisms on Mars today are very low.
It turns out that actually that Earth is the only planet in the solar system that seems like it’s likely to support life today. In order to find other places that we might be interested in, we’ve got to look to the moons. And the moons, especially of the outer planets, are some of the most fascinating places in the whole solar system.
And the one, people are most excited about today is the moon Europa. This is the second of the four Galilean moons of Jupiter. So, the Galilean moons are the four large moons. They are all about the size of Earth’s moon. They were first discovered by Galileo back in the 1600s when he first pointed his telescope at Jupiter, and they orbit extremely close to Jupiter. So, Jupiter exerts very strong tidal forces on them.
Of course, we’re very familiar with tides here on Earth. When the moon passes over an ocean, it lifts that water up and causes high tide, and when it goes to the other side of the earth, that water falls back down and creates low tide.
The same thing happens in the Jupiter system, but instead of having an object the size of the moon you have the largest planet in the solar system. And so, it can lift up not just water on the surface – because of course there isn’t water on the surface of these extremely cold bodies – it can lift up the surface itself. As the surface rises and falls, it rubs and scrapes against itself. This actually creates a lot of heat and this heat can actually melt the interior of these moons.
The closest moon to Jupiter, Io, is the most volcanically active place in the solar system because if all this molten rock forms magma and then bursts out of Io. And then if you go one moon further out, you get to Europa. So, the stresses there are not as strong because it is further away from Jupiter and it’s not enough to melt rock that’s probably at the core of Europa, but it is enough to melt the water.
We have pretty good evidence now from the Galileo spacecraft, which orbited Jupiter from 1995 to 2003, that there is an ocean of liquid water underneath the surface of Europa that is probably equal to or greater than all the oceans, rivers and lakes of water that are here on planet Earth. So, we’re talking about a truly vast amount of water, but it is different than here on Earth. It’s extremely salty. So, much saltier – we think probably – than even the Earth’s oceans, but more importantly, it’s trapped very far beneath the surface. We don’t have a very good idea how deep it is, but most people – most scientists would probably quote you something like 10 km ± 5 km. So, we’re talking about very deep, no matter how deep your number you chose to be, much deeper down than the average depth of most of Earth’s oceans are today.
Fraser: And so that depth means that any way we could try and get at it – to try and actually drill down – you know we would probably have a really hard time getting down through the ice here on Earth. So, to send a spacecraft to try and get down through that amount of ice and try and probe the water below is going to be pretty tough.
Morgan: Right. So, we have to try and understand the oceans of Europa to begin with indirectly and there are a couple of interesting ways that we can do that.
For one, there’s the idea that there might be giant plumes of water, like geysers of water shooting out of the surface of Europa. There’s been some circumstantial evidence from Hubble to suggest that this might be the case, but the reason that people really think this could happen at Europa is because we see it one other place in the solar system. That’s a moon of Saturn called Enceladus. Enceladus also has a localized ocean of liquid water beneath its south polar region.
That water somehow builds up in pressure and bursts forth through cracks in the surface of Enceladus to then shoot about 200 kg of water out of the surface of Enceladus every second. That’s enough to fill up a few Olympic-sized swimming pools every day – so not a trivial amount of water that is escaping. It’s enough water to create the entire E ring of Saturn, which is the largest ring of Saturn and one of the largest rings in the solar system.
So, we know that underground oceans can connect to the surface through geysers, and that’s great because that allows us to study them without actually having to go underneath the surface and find out what’s there. We can just wait for things to squirt out and fly our spacecraft by and either take pictures or collect some of those particles or make other kinds of observations.
Europa also has these large cracks on the surface that could be related to the stresses of this tidal interaction with Jupiter. It’s possible that at times, these cracks are deep enough that water from underneath the surface, from within the ocean of Europa, can actually leak up to the surface. This would allow us to land on the surface and study that top level of ice that could be formed based on that water that was somewhat recently in the ocean of Europa and get a better sense.
And these cracks play another important role because it is not just carbon and water that all life on Earth needs. It also needs energy. Humans get energy by eating food. Plants get energy by doing photosynthesis to turn the light of the sun into energy that they can then use to grow, that we can then use to pick them and eat them. You could also get energy from hydrothermal vents. This happens at the bottom of the ocean. And really, you could get energy from the combination of certain chemicals, for example, like dropping sodium in water releases a lot of energy in that reaction. These are sources of energy then that animals or these creatures could use in different places in the solar system.
And these cracks are a great way to get that because remember that these volcanoes on Io are shooting up tons of ash, and as this ash deposits onto the surface of Europa – and we’re pretty sure that it does – these cracks offer a way for that ash to sink down into the ocean. This could provide as a sort of organic material that could then form the basis for the actual structures of life, but also infuses the oceans with some unique chemicals. These chemicals could react with salts and the rock at the bottom of the ocean to provide some of the energy needed for microbial life to exist within these oceans.
Fraser: So, could the ash that we see on the surface of Europa – could that help explain some of the coloration that we see on Europa?
Morgan: Yeah. It is likely that what astronomers often term as ‘brown gunk’ is at least partially material that has fallen from Io – from the volcanoes on Io – onto Europa.
The surface of Europa is extremely young. It is one of the youngest surfaces in the solar system, just millions of years old as opposed to say the moon’s surface, which is billions of years old. That means that the surface must be getting recycled relatively frequently and when that happens anything that is on the surface is going to get sucked down into the moon. That would allow it to recombine with material that is already in the oceans to enrich those oceans with more complex chemicals – more complex molecules – that could either form the basis for life or the food source for life or some combination of those.
Fraser: That’s really cool. I hadn’t heard about that research. That’s really kind of amazing. It’s often that I ask the questions and I kind of already know the answer but I actually had no idea that that was one the explanations. It’s great to have a planetary scientist here on Astronomy Cast every now and then.
So, you’ve got this situation where the cracks on Enceladus are potentially materials falling down. It’s almost like they are sub ducting like the plates on Earth. You’ve got this ice potentially crossing on top of each other, cracking open and things going back down and a way for the material to cycle back down into the water. So, you mentioned Europa as one of the classic places that we want to look, as it’s potentially got this energy source. It’s got this water – although it’s pretty salty – but how is Enceladus as a place for life?
Morgan: Enceladus has water going for it, and it has heat – a lot of heat in fact in the South Pole region of Enceladus – more heat than we can explain through any source that we understand. So, if you consider radioactive heating, like the breakdown of elements like Uranium releases heat naturally in the crust of the Earth – also in pretty much every body in the solar system.
Enceladus also experiences tidal heating from Saturn. It’s much farther from Saturn than Europa is form Jupiter, so this heating is much weaker, but if you combine these, we don’t get near as much heat as we observe in coming out of Enceladus. So, there is some incredible source of heat providing this localized region of Enceladus with a lot of energy.
So, we have energy and we have liquid water because of that. The rub with Enceladus is it is difficult to know where you get the organic compounds that you need to sustain life because there is not like an Io around Saturn that is spewing out all of these kinds of compounds. When we look at the water coming out of the ocean – the plumes of Enceladus – they are almost perfectly pure water. We can see some salts and things within them that sort of act as an antifreeze to keep the ocean from freezing even given this tremendous heat, but we don’t see a lot of more complicated compounds.
Some of that is because they are heavier and they don’t fly out as far from the surface when these geysers erupt, but also, it is likely that there is just a lot less organic type material at Enceladus than you would find at say Europa. So, I think that makes Enceladus a second tier candidate in comparison to Europa in the possibility in having this ocean.
Fraser: Yeah, that little saltiness – there are places on Earth that have higher levels of salinity and they have found life. There are very salty pools that they have found life in on Earth. So, it’s not a deal killer.
Morgan: No, it’s not.
Fraser: It turns out that Europa is definitely one of the best candidates, but the other Jovian moons seem to have underwater, under – I guess – under ice crusts and internal oceans as well, right? I mean Ganymede? Callisto? There’s something going on there in all of the them.
Morgan: Right, we think that Ganymede and we suspect Callisto have oceans of liquid water underneath their surface. Because they are further out than Europa, they don’t receive nearly as much tidal heaving from Jupiter as does Europa. So, probably, these aren’t either as warm or as large and they could be more irregular in how capable they would be of supporting life. And so basically, anything that Callisto or Ganymede has, Europa has better.
So, if you are going to play the odds game and say, “What’s the most likely place in the Jovian system to look for life?” it’s definitely Europa. It’s the closest to Jupiter of the three. It’s the closest to Io of the three. It’s surface shows the most remodeling and the youngest surface, meaning the most materials being incorporated into it – underneath into the interior of the moon. So, if you are going to go to one of those three, you’re going to want to go to Europa.
And that is exactly what NASA is planning to do in the coming 10-15 years.
Fraser: One question is how does this Panspermia idea – that life could be moving within worlds of the solar system – how does that play into it?
Morgan: That’s really important because there are two questions when you think about life in the solar system. There is the question, “Could this place support life?” And I think the answer is yes, Europa could support life. It’s likely, yes, that Enceladus could support life.
In fact experiments that we’ve conducted on the International Space Station suggest that just normal ‘plain-Jane’ Earth bacteria can survive well more than a year when exposed to the vacuum of space and then reconstitute themselves back into their original living organisms.
But in these locations, the conditions are still extremely harsh and the question is, “Is it possible for life to have arisen in these locations?” Sure, Europa could support life, but could life be formed in conditions as harsh as Europa? Because even though we see life all over the Earth – whether we’re talking about hot volcanic environments or super salty underground lakes in Antarctica – it’s not likely that the life we see there formed there. Life on Earth probably formed in some of the most ideal conditions on the planet, whatever those happened to be and we don’t really understand exactly what is necessary to form life.
But whatever it was, it formed in the best possible conditions on Earth, and then over millions and billions of years spread to all of the nooks and crannies of the planet. So, Europa and Enceladus, they probably never had these nice balmy conditions that would make it most probably that you could form life. Therefore, it’s a question of if life has to arrive on that body; probably it is unlikely to life on Europa or life on Enceladus.
But the idea of Panspermia, which is that life can travel from world to world within the solar system and within the galaxy, you can side step that issue. And there are two ways to think about Panspermia.
The idea of interstellar Panspermia, which is that life formed somewhere in the galaxy, and then that world or a chunk from that world was ejected out of its solar system and ended up some how lumbering into our solar system early in the formation of the solar system and it basically, infected our solar system with life from another star. That is a speculative way of thinking about Panspermia because there is evidence that dust particles and things move between stars, but it is not the most likely situation in the world.
On the other hand, there is the idea of interplanetary Panspermia and this is the idea that life somehow manages to luck into arising on some planet in our solar system. Most people would probably say that planet was probably Earth, but it could have been Mars or Venus earlier in their lifetimes.
Then, an impact on the surface of Earth or one of these other bodies that jars loose pieces of rock that have this life attached to it and then flies on to other worlds in the solar system and infect them with Earth life. This is a lot less speculative. It is still far from an accepted idea but we know material moves regularly between the planets in our solar system.
Fraser: And this life is rugged enough to handle each one of the stages. That you can go from an asteroid striking Earth or striking Mars, for example, and throwing up all that material – life can survive that process. Life can survive time inside a meteorite in space, and life can even survive the re-entry process of coming back through to a planet.
Morgan: That’s right. Of about 60,000 meteorites we’ve found on Earth, more than a 100 we know came from Mars. Models that simulate this suggest that the transfer of materials from Mars to Earth happens quickly, not millions of years, but thousands of years or tens of thousands of years for these sorts of things. That seems like a long time, but in the span of a solar system, that is the blink of an eye.
And while we don’t have confirmed samples from Mercury or from Venus, it’s not out of the question that there could be meteorites of those worlds here on Earth. And it is more likely that those worlds have Earth and Mars meteorites that were blasted off of those planets, fell in towards the sun, and happened to land on Venus or on Mercury.
It’s a little trickier to get those things out into the outer solar system, but it’s not impossible. This would enable life that arose in ‘comfortable environments’ on Earth or on a warm, wet Mars to travel to one of these places capable of supporting life, but not capable of forming it.
We will have an idea if this happened because if we manage to find life on Europa, or Enceladus, or Titan or any of these other places, and it looks anything like Earth life, that will be strong support for the idea of Panspermia. Because the idea of life arising in two different places and looking similar, while certainly possible are not particularly high, and this gives us a clue as to whether or not Panspermia actually happened if we’re able to find life somewhere out there in the solar system.
Fraser: So, now one of the things – we’ve talked about Europa, we’ve talked about Enceladus – but in fact, the new research says that it feels like there is probably liquid water in a lot of even the Kuiper belt objects because potentially have some radioactive heating inside of them. So, you could have liquid water on Pluto or Sedna?
Morgan: It’s possible. Radioactive heating is more effective on larger bodies, and it’s more effective early in the history of the solar system than later on because you start with a finite amount of uranium and plutonium and these other radioactive elements and there is no way to replenish them.
You start off with a lot and you start using them up, and over time you use them up and use them up, and over time the heat you generate is less and less. So, with a larger body you start with more and you can more effectively heat the interior of yourself. I think we would be even less likely to find life in one of these far-off, isolated bodies than in the inner solar system where we have all these additional sources of heat.
We have tidal heating. We have temporary heating from large impacts. That could have been important at the very beginning of the solar system. And we have light. It’s a form of energy that heats the surface, but as we know here on Earth, it’s a source of energy that life has harnessed extremely effectively. Light is something that is almost non-existent when you get out past the orbit of Neptune. Jupiter receives about 4% as much light as does planet Earth and Saturn receives a 100 times less light than does the earth, and every time you go out a planet, it drops off precipitously.
So, light on Earth is the basic energy medium that we collect most of our energy here from on the surface, and it does warm the surface of our planet. It warms the surface of Venus, Mars, and Mercury, and that just doesn’t exist in the outer solar system.
So, while it is possible the some Kiuper belt objects supported oceans early in the solar system, and some that had exceptionally large concentrations of radioactive elements or were exceptionally large might still support those oceans today. The isolation that they experience, I think, is a major impediment. Sort of the same way that Enceladus is you don’t have a source of organic material, and even if you started off with some, you are going to expend these in the chemical reactions needed to sustain life. These bodies tend to be relatively small, and so they just don’t start off with a whole lot of material. And unfortunately, even if life is out there, they are too far away for us to get to.
New Horizons plans to fly by a Kiuper belt object something like three to four years after it passes Pluto, and we know it took nine years to get from Earth to Pluto. So, we’re not in a position to study a selection of these objects that is large enough to have a good chance of finding life.
We’d be much better off trying to drill through the surface of Europa or peer into the oceans of Enceladus or land on Titan than we would go out to the Kiuper belt.
Fraser: Do you think with all these kinds of clues – the plumes that are coming out of Enceladus, the plumes that are potentially coming out of maybe even Ceres but also Europa potentially, you’ve got this opportunity where maybe the chemicals for the concentrence of life are maybe being ejected out into space that you could maybe fly a spacecraft through it or near and take a ‘whiff’ as you pass through it. I mean is there potential there?
Morgan: In a way there is. One of the more interesting places that has a lot of organic compounds are comets, and comets are endemic to the outer solar system. This could suggest to us that Kiuper belt objects have more organic material than we’ve previously considered. We’ve made some efforts in the last decade to try and understand comets as a source for these organic compounds.
Some people think that comets hitting Earth delivered organic compounds to Earth. This would work. We know that comets hit all the inner solar system objects early on in the history of our solar system.
So, we’ve been trying to study comets in more detail. We flew through the tail of a comet about 10 years ago with the Stardust Mission, and just, of course, in the last year, ESA’s Rosetta spacecraft and Philae Lander have approached very close to a comet and have been studying it in unprecedented detail.
We know from these observations that the tails of comets – the surfaces of comets do have organic compounds on them, and it could be that comets themselves could be possible locations for harboring life over long periods of time. They are not warm enough to sustain on-going liquid water, but if they came from an object that ever had life and ever had liquid, they are kind of buses bringing this life around the solar system and randomly hitting into things and depositing its passengers along with it.
Fraser: So, do you think space whales on Europa?
Morgan: Space whales would be nice, but I think there is nowhere in the solar system that we should expect to find microbial life. The amount of energy it takes to sustain macroscopic life here on Earth is tremendous. If you look at the food chain for any animal, you need an incredible amount of organisms below it.
Think about a cow, for example. Cows have to eat a tremendous amount of grass. The Earth is basically covered in grass and trees and leaves and things like that just to support a small – relatively, in comparison – population of animals.
So, at best, I think we’ll find microbial life that is well suited to living off the land in these extremely harsh environments. That’s the life we see living in these super salty lakes and these caves far from the reach of sunlight and places we see lots of pressure, like living underneath the surface of the Earth. All of these are microbial life. That’s what we should be looking for and expect to find, at least in our own solar system.
Fraser: If people are watching all the missions that are happening, what are some upcoming missions that are going to help us push these questions a little forward?
Morgan: Absolutely. On-going missions right now that will helps us understand these questions are Rosetta studying comet 67P and Cassini, which will continue to observe Enceladus and other moons of Saturn which could have similar features as Enceladus. But as we look forward, probably the most exciting mission coming up is the Europa Clipper, a mission whose name will probably change before it actually launches. The idea here is to fly a large, flagship sort of spacecraft to Jupiter, go into orbit about Jupiter and then make a series of 40-50 close fly-bys of Europa to study the surface, to try and sample any plumes that might be there, to shoot radar down through the surface and try to get a range on the depth of that ocean and possibly even to deploy a Lander to the surface.
Fraser: Right with support from the Europeans.
Morgan: Right with support from ESA if that works out. Then, on Mars, what we’re working up to is a sample-return mission. So, Curiosity has been searching Mars for understanding the large distribution of chemicals on the surface and what kinds of rocks are made up of what compounds. The next spacecraft that should advance this is a spacecraft that should launch in the early 2020s called Mac C, and what Mac C is going to do is collect samples of the surface of Mars, package them up in tightly sealed containers and it’s going to deploy those in a specific location on Mars.
Then, a future mission beyond Mac C is going to land, collect those samples, blast them off and return them to the Earth. This will allow us to study rocks from Mars that didn’t have to travel through thousands or millions of years of space. They didn’t have to burn up through the Earth’s atmosphere. They didn’t have to smack into the ocean or land on land.
They come pristine from Mars, and although the odds of finding fossilized microbes in them are probably relatively low, having those in the lab where we have much, much more sensitive instruments than we do on rovers and Landers on Mars, we’re going to be able to get a much better idea of the chemical composition of the Martian surface. That will ultimately lead us to a better understanding of its history, and perhaps if Mars was very Earth-like billions of years in its past.
Fraser: Well, Morgan thank you for joining me on this very special episode of Astronomy Cast. I think we got – really this question is just beginning – and I think the best is yet to come. We’ve got some pretty exciting explorations coming up.
So, will you join me again in a year from now?
Morgan: I will indeed. If we’re lucky, we’ll have even more exciting news to talk about from some of these searches.
Fraser: Absolutely. Well, thank you very much Morgan.
Morgan: Thank you.
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