1 00:00:04,200 --> 00:00:07,120 This is Geology Bites with Oliver Strimpel. 2 00:00:08,720 --> 00:00:12,880 Why is there water on Earth? We asked this question because 3 00:00:12,880 --> 00:00:16,280 one might naively expect water to be too volatile to be 4 00:00:16,280 --> 00:00:21,120 retained by the hot early Earth. Rather, water should have been 5 00:00:21,120 --> 00:00:24,680 confined to the outer solar system, beyond the so-called 6 00:00:24,680 --> 00:00:28,720 snow line, where the distance from the sun is such that water 7 00:00:28,720 --> 00:00:31,840 can freeze. In the solar system. 8 00:00:31,920 --> 00:00:35,320 That line runs between the orbits of Mars and Jupiter. 9 00:00:36,880 --> 00:00:40,680 Lindy Elkins Tanton's research has focused on how to rest your 10 00:00:40,680 --> 00:00:44,960 planet's form and evolve. She has a particular interest in 11 00:00:44,960 --> 00:00:48,520 where the Earth's water came from and is a major proponent of 12 00:00:48,520 --> 00:00:51,480 one of the competing theories that attempt to answer this 13 00:00:51,480 --> 00:00:54,280 question. She's a professor in the School 14 00:00:54,280 --> 00:00:57,960 of Earth and Space Exploration at Arizona State University. 15 00:00:59,320 --> 00:01:02,120 Lindy Elkins Tansen Welcome to Geology Bytes. 16 00:01:02,320 --> 00:01:04,920 Thank you so much, Oliver. It's a pleasure to be here. 17 00:01:04,920 --> 00:01:08,400 Thanks for inviting me. Did I summarize the problem of 18 00:01:08,400 --> 00:01:10,360 the origin of the Earth's water correctly? 19 00:01:10,880 --> 00:01:14,800 Yeah, this is just how people think about it with the snow 20 00:01:14,800 --> 00:01:17,520 line. The old view of water in the 21 00:01:17,520 --> 00:01:20,800 planets, in the rocky planets and water in the Earth is that 22 00:01:20,920 --> 00:01:24,280 the planets formed from a planetary nebula that consisted 23 00:01:24,280 --> 00:01:28,080 of gas and dust orbiting around our young sun while the sun was 24 00:01:28,080 --> 00:01:31,520 forming. And high temperatures and the 25 00:01:31,520 --> 00:01:35,520 solar wind from the active young sun evaporated the volatiles 26 00:01:35,520 --> 00:01:38,160 from the inner parts of that planetary disk and blew them 27 00:01:38,160 --> 00:01:40,280 outwards past this so-called snow line. 28 00:01:40,800 --> 00:01:43,840 And so the inner terrestrial planets like the Earth formed 29 00:01:44,240 --> 00:01:48,760 dry with just rock and metal. But there's a new view. 30 00:01:49,280 --> 00:01:52,320 First of all, we've learned that that planetary nebula only 31 00:01:52,320 --> 00:01:54,640 lasted a few million years, and then it dispersed. 32 00:01:54,640 --> 00:01:57,240 We can see it in exoplanets, and we can measure it in certain 33 00:01:57,240 --> 00:01:58,560 ways in our own solar. System. 34 00:01:59,040 --> 00:02:02,280 So the planets had to have formed very rapidly because 35 00:02:02,280 --> 00:02:04,480 after that there's no more material available to form a 36 00:02:04,480 --> 00:02:07,080 planet. So even though volatiles were 37 00:02:07,080 --> 00:02:10,639 relatively depleted from the inner solar system by those very 38 00:02:10,639 --> 00:02:12,320 processes of solar, wind and high. 39 00:02:12,320 --> 00:02:15,680 Temperature minerals still. Formed with water trapped within 40 00:02:15,680 --> 00:02:18,840 them in crystal voids or as part of the crystal lattices 41 00:02:18,840 --> 00:02:23,520 themselves, and fast forming planetary cores of metal and 42 00:02:23,520 --> 00:02:26,480 rock in the outer solar system. Grew large enough to 43 00:02:26,480 --> 00:02:29,920 gravitationally gather their volatile envelopes before the 44 00:02:29,920 --> 00:02:33,600 planetary nebula dispersed, where the littler planets and 45 00:02:33,600 --> 00:02:36,640 the inner solar system never got big enough to gather that water. 46 00:02:36,640 --> 00:02:40,040 So they look very different. But it turns out the materials 47 00:02:40,040 --> 00:02:43,760 that made those cores are not very different, because a lot of 48 00:02:43,760 --> 00:02:46,720 material migrated regularly both inward and outward while the 49 00:02:46,720 --> 00:02:50,160 planets were forming. And so that idea that the inner 50 00:02:50,160 --> 00:02:54,080 ones were just dry and the outer ones were wet is really almost 51 00:02:54,560 --> 00:02:58,360 the product of what the solar system looks like today and not 52 00:02:58,360 --> 00:03:02,400 the actual process by which things form. 2/3 of the Earth's 53 00:03:02,400 --> 00:03:06,280 surface is covered by oceans. It certainly looks as if there's 54 00:03:06,280 --> 00:03:10,360 a lot of water on Earth, but actually, what proportion of the 55 00:03:10,360 --> 00:03:13,400 Earth is water, and is it mainly in the oceans? 56 00:03:13,840 --> 00:03:17,760 We are such surface creatures. We really feel that all the 57 00:03:17,760 --> 00:03:19,480 water is here and we're standing in it. 58 00:03:19,760 --> 00:03:22,160 But the interior of the Earth still contains quite a lot of 59 00:03:22,160 --> 00:03:23,920 water. And again, it's mostly water 60 00:03:23,920 --> 00:03:28,680 that's either just an oxygen and a hydrogen and a hydroxyl ion 61 00:03:28,680 --> 00:03:31,320 instead of a full water molecule, and in some cases a 62 00:03:31,320 --> 00:03:34,960 full water molecule that are trapped in crystal voids or part 63 00:03:34,960 --> 00:03:37,040 of crystal lattices. So they almost seem like they're 64 00:03:37,040 --> 00:03:39,840 part of the solid Earth, but inside the Earth. 65 00:03:39,840 --> 00:03:43,640 The estimate range is from 2 to 10 times as much water as we 66 00:03:43,640 --> 00:03:46,200 have fear on the surface. So there's actually more water 67 00:03:46,200 --> 00:03:48,000 inside the Earth than there is outside. 68 00:03:48,600 --> 00:03:51,680 Okay, can you summarize the theories that have been put 69 00:03:51,680 --> 00:03:53,960 forward to explain the origin of Earth's water? 70 00:03:54,400 --> 00:03:55,440 There's a. Range. 71 00:03:55,440 --> 00:03:56,600 Of course, there's always a range. 72 00:03:56,600 --> 00:03:58,760 It's not just. Like one thing or exactly? 73 00:03:58,760 --> 00:04:01,320 Another thing, the way these theories go, but the simplest 74 00:04:01,320 --> 00:04:04,080 way to describe it is. One end of the idea. 75 00:04:04,360 --> 00:04:06,400 Is that Earth's water came from cometary? 76 00:04:06,400 --> 00:04:09,400 Impacts that the earth formed. Really dry, and then it was 77 00:04:09,400 --> 00:04:13,880 later wet impactors like comets that brought water to the Earth. 78 00:04:14,440 --> 00:04:19,240 And then the other end member concept is that although comets 79 00:04:19,240 --> 00:04:22,840 have definitely hit the Earth in the past, water was delivered to 80 00:04:22,840 --> 00:04:25,960 the Earth in the very rocky building blocks of the Earth 81 00:04:25,960 --> 00:04:27,920 itself. Found inside. 82 00:04:27,920 --> 00:04:32,440 Those rocky material crystals. And so the earth actually formed 83 00:04:32,440 --> 00:04:34,680 with water from its original rocky material. 84 00:04:34,960 --> 00:04:39,080 So what observations can we make to help distinguish among the 85 00:04:39,080 --> 00:04:43,640 different theories? One thing is detailed analysis 86 00:04:43,640 --> 00:04:48,680 of meteorites and asteroids and comets by audacious space 87 00:04:48,680 --> 00:04:51,920 missions that have measured particles of comets and also 88 00:04:52,080 --> 00:04:54,960 observations from the Earth. We now know a lot more about 89 00:04:54,960 --> 00:04:58,040 what comets are made of, even on the isotopic level. 90 00:04:58,320 --> 00:05:01,480 So what we can do is we can compare the isotopes of certain 91 00:05:01,480 --> 00:05:04,680 elements as they're found on the Earth versus the ones that are 92 00:05:04,680 --> 00:05:09,520 found in comets or in meteorites that are parts of asteroids or 93 00:05:09,520 --> 00:05:14,240 in asteroids themselves. And when you look at deuterium 94 00:05:14,240 --> 00:05:18,800 to hydrogen, those two hydrogen isotopes, or nitrogen 14 to 95 00:05:18,800 --> 00:05:23,600 nitrogen 15, those nitrogen isotopes and also noble gas 96 00:05:23,600 --> 00:05:28,240 compositions, it's been shown that cometary water contributed 97 00:05:28,440 --> 00:05:32,720 1% or less of the Earth's water. Some people say perhaps up to 98 00:05:32,720 --> 00:05:36,160 15%, but really a very low amount. 99 00:05:36,160 --> 00:05:38,360 So water really can't have come from comets. 100 00:05:38,760 --> 00:05:42,640 It had to have come from the water that's bound up in rocky 101 00:05:43,040 --> 00:05:45,080 asteroidal material like meteorites. 102 00:05:45,840 --> 00:05:49,480 When you talked about the isotopic measurements, you 103 00:05:49,480 --> 00:05:52,240 talked about nitrogen. Does that actually bear directly 104 00:05:52,400 --> 00:05:56,440 on where the hydrogen and oxygen in the water came from as well? 105 00:05:56,640 --> 00:05:59,040 Yeah. We think of comets mainly as ice 106 00:05:59,040 --> 00:06:02,480 balls, balls of ice, and we think of them as water ice, but 107 00:06:02,480 --> 00:06:05,640 they're about half ice. And that ice is not just water 108 00:06:05,640 --> 00:06:08,440 ice, but it's also things like ammonia ice that contains a lot 109 00:06:08,440 --> 00:06:10,960 of nitrogen. And the other half of comet 110 00:06:10,960 --> 00:06:14,680 materials, broadly speaking, are organic materials. 111 00:06:15,240 --> 00:06:19,360 And so things that are made of carbon and nitrogen and hydrogen 112 00:06:19,360 --> 00:06:23,000 and then other elements as well. And so when you add the water 113 00:06:23,000 --> 00:06:26,240 from comets, you actually are forced to also add those other 114 00:06:26,240 --> 00:06:28,680 ices and the organic material as well. 115 00:06:29,120 --> 00:06:33,480 And so you can compare those, the whole volatile inventory of 116 00:06:33,480 --> 00:06:35,360 comets to what we find on our Earth. 117 00:06:36,040 --> 00:06:41,080 So if we can say that the other volatiles have the isotopic 118 00:06:41,120 --> 00:06:45,280 signature of comets, then we can infer that the water must have 119 00:06:45,280 --> 00:06:47,360 come along for the ride. Yeah, that's right. 120 00:06:47,360 --> 00:06:51,240 Or conversely, if none of the other measurements fit, then 121 00:06:51,240 --> 00:06:53,080 cometary water is not the main source. 122 00:06:53,640 --> 00:06:56,920 Perhaps you've already indicated your preference, but which 123 00:06:56,920 --> 00:07:01,600 theory do you favour and why? I came at this not so much to 124 00:07:01,600 --> 00:07:05,360 solve the problem of Earth's water, but instead looking at 125 00:07:05,360 --> 00:07:09,360 the processes by which rocky planets form from giant impacts 126 00:07:09,400 --> 00:07:11,680 of material in early solar systems. 127 00:07:11,680 --> 00:07:15,080 And those giant impacts, as they creep into the planet, deliver 128 00:07:15,080 --> 00:07:19,640 enough kinetic energy that they melt the target body as it's 129 00:07:19,640 --> 00:07:21,360 growing. And the target body goes through 130 00:07:21,360 --> 00:07:26,920 this phase called a magma ocean. And so my interest was how can 131 00:07:26,920 --> 00:07:30,240 we use our quite extensive knowledge of how magma 132 00:07:30,560 --> 00:07:35,920 solidifies and how volatiles interact with magma to predict 133 00:07:35,960 --> 00:07:39,240 and understand how rocky planets form from that starting point of 134 00:07:39,240 --> 00:07:42,560 a magma ocean? And it had been thought the 135 00:07:42,560 --> 00:07:45,600 magma oceans were necessarily fully dry, that the water was 136 00:07:45,600 --> 00:07:49,360 driven off by heat. And again, it's because I think 137 00:07:49,360 --> 00:07:53,200 that some of our scientific intuition comes from our simple 138 00:07:53,200 --> 00:07:58,440 experiences at one atmosphere, pressure and Earth gravity and 139 00:07:58,440 --> 00:08:01,520 the kind of conditions we normally live in where if you, 140 00:08:01,560 --> 00:08:04,840 for example, put your clothes in a hot dryer, it drives off the 141 00:08:04,840 --> 00:08:08,640 water. And that's true, but also the 142 00:08:08,640 --> 00:08:11,440 thing that we forget is that water is actually incredibly 143 00:08:11,440 --> 00:08:13,840 sticky and you never drive it all off. 144 00:08:14,160 --> 00:08:18,120 And so is the amount of water that is retained enough to do 145 00:08:18,120 --> 00:08:19,640 what we need to make a rocky planet. 146 00:08:19,800 --> 00:08:23,640 And so my work showed that if you build a rocky planet with 147 00:08:23,800 --> 00:08:27,760 rocky material that contains just 100 parts per million of 148 00:08:27,760 --> 00:08:32,039 water bound up in crystals, just the way we find it in meteorites 149 00:08:32,039 --> 00:08:35,200 and in asteroids, many of them are in fact wetter than 100 150 00:08:35,200 --> 00:08:38,200 parts per million. But if you process that through 151 00:08:38,200 --> 00:08:41,880 a magma ocean, a lot of water is released into an early steam 152 00:08:41,880 --> 00:08:45,480 atmosphere that's free to later cool and collapse into an ocean, 153 00:08:45,880 --> 00:08:48,960 but then a lot is actually retained inside in the magma 154 00:08:48,960 --> 00:08:51,080 because the magma won't give up all the water. 155 00:08:51,520 --> 00:08:54,280 And. So it turns out that just 156 00:08:54,280 --> 00:08:57,440 starting with just 100 parts per million of water produces. 157 00:08:57,560 --> 00:08:59,960 A nice wet. Planet one with water inside, 158 00:08:59,960 --> 00:09:03,160 similar to what the Earth has today and with a deep water 159 00:09:03,160 --> 00:09:05,520 ocean. In fact, the problem becomes 160 00:09:05,520 --> 00:09:08,400 getting rid of the surface water, not making more. 161 00:09:08,840 --> 00:09:13,000 And So what that work indicated that really confirmed my 162 00:09:13,000 --> 00:09:15,320 opinions about this whole field is that. 163 00:09:15,560 --> 00:09:19,480 Any rocky planet. Born anywhere in our universe at 164 00:09:19,480 --> 00:09:22,960 a material that has just 100 parts per million of water will 165 00:09:22,960 --> 00:09:25,520 be born wet. And therefore if it's in a 166 00:09:25,520 --> 00:09:28,160 habitable zone, it will be born habitable. 167 00:09:28,560 --> 00:09:33,080 And so when you think about it that way, requiring comets to 168 00:09:33,080 --> 00:09:37,000 strike the surface later, which is a stochastic process that you 169 00:09:37,000 --> 00:09:42,080 can't count on, sounds a little bit like special pleading when 170 00:09:42,080 --> 00:09:44,680 you know that the planets going to be wet from the beginning. 171 00:09:44,960 --> 00:09:47,920 The problem is not delivering water to a planet, the problem 172 00:09:47,920 --> 00:09:50,000 is getting rid of it. These planets are born with so 173 00:09:50,000 --> 00:09:52,640 much water. Did the water in the oceans then 174 00:09:52,760 --> 00:09:56,040 have to be extracted from the magma ocean, or do we think it 175 00:09:56,240 --> 00:09:58,160 just came out of the steam atmosphere? 176 00:09:58,520 --> 00:10:01,600 That steam atmosphere is a bit vulnerable. 177 00:10:01,720 --> 00:10:04,080 A hot steam atmosphere is expansive. 178 00:10:04,080 --> 00:10:07,520 It rises up from the planet's surface farther than a cold 179 00:10:07,520 --> 00:10:09,240 atmosphere like what we have today. 180 00:10:09,600 --> 00:10:13,640 And the early young sun was very active and so a lot of that 181 00:10:13,640 --> 00:10:17,760 steam could have been lost to solar wind, but not all of it 182 00:10:17,760 --> 00:10:19,400 before it. Cooled, in fact. 183 00:10:19,400 --> 00:10:22,560 This is one of the ideas of the difference between Venus and the 184 00:10:22,600 --> 00:10:25,320 Earth. Just that distance from the sun 185 00:10:25,320 --> 00:10:28,440 could have meant that Venus's atmosphere was stripped, but the 186 00:10:28,520 --> 00:10:31,920 the Earth retained more of its atmosphere to allow it to 187 00:10:31,920 --> 00:10:35,720 collapse into an ocean. And so I think that the physics 188 00:10:35,720 --> 00:10:39,080 and chemistry would indicate that some steam atmospheres may 189 00:10:39,080 --> 00:10:41,760 be stripped, and then the planet needs to start over with 190 00:10:41,760 --> 00:10:45,480 volcanism, releasing water from the inside, and then others will 191 00:10:45,520 --> 00:10:48,240 collapse and make something of an ocean before they're 192 00:10:48,240 --> 00:10:50,640 stripped. Is that the main mechanism then 193 00:10:50,640 --> 00:10:53,480 for getting the water from the inside to the outside then via 194 00:10:53,480 --> 00:10:55,280 volcanism? That's right. 195 00:10:55,320 --> 00:10:58,200 That's part of why people care so much about plate tectonics. 196 00:10:58,200 --> 00:11:01,440 When we think about habitability, that the way that 197 00:11:01,440 --> 00:11:05,760 we understand that carbon is controlled on our planet and 198 00:11:05,760 --> 00:11:07,920 water is refreshed into the atmosphere. 199 00:11:08,400 --> 00:11:12,920 If the problem then becomes how to get rid of the excess water, 200 00:11:12,920 --> 00:11:16,080 so to speak, what do we think happened to make it disappear? 201 00:11:17,040 --> 00:11:21,040 I think that the primary mechanism would be stripping of 202 00:11:21,040 --> 00:11:24,240 that inflated hot steam atmosphere by the young sun. 203 00:11:24,560 --> 00:11:28,320 But another way to remove water from the surface is through 204 00:11:28,640 --> 00:11:31,320 ongoing impacts. For example, on the Earth, our 205 00:11:31,320 --> 00:11:34,080 last really giant impact had to have been the one that formed 206 00:11:34,080 --> 00:11:36,360 our moon. But impacts? 207 00:11:36,360 --> 00:11:39,720 Didn't just end then. And in fact, the moon forming 208 00:11:39,720 --> 00:11:45,040 impact or impacts of that scale would create a trail of debris 209 00:11:45,040 --> 00:11:47,520 in the Earth's orbit, which the Earth and the Moon would 210 00:11:47,520 --> 00:11:50,000 necessarily sweep up again over time. 211 00:11:50,400 --> 00:11:54,400 And so those impacts, plus other objects flung in from the outer 212 00:11:54,400 --> 00:11:57,600 solar system or out from the inner solar system during those 213 00:11:58,040 --> 00:12:01,440 early chaotic times before the planets completely cleared their 214 00:12:01,440 --> 00:12:04,920 orbits, all of those impacts would have removed some amount 215 00:12:04,920 --> 00:12:07,320 of atmosphere and water from the surface. 216 00:12:07,720 --> 00:12:11,200 Although it's been shown also that that removal is not as 217 00:12:11,200 --> 00:12:14,840 efficient as our surface bound brains might think. 218 00:12:15,040 --> 00:12:16,720 They remove some, but not everything. 219 00:12:17,600 --> 00:12:21,560 As you already mentioned, water plays a critical role in plate 220 00:12:21,560 --> 00:12:26,120 tectonics as the presence of even minute quantities of water 221 00:12:26,120 --> 00:12:29,360 in a rock has a very big effect on its strength and melting 222 00:12:29,360 --> 00:12:32,280 temperature. This in turn effects then what 223 00:12:32,280 --> 00:12:35,480 happens to plate boundaries, especially at subduction zones 224 00:12:35,480 --> 00:12:38,040 where water is released by dehydration reactions in the 225 00:12:38,040 --> 00:12:40,680 down growing plate. And as you should have 226 00:12:40,680 --> 00:12:44,040 mentioned, that's the source of the volcanism that we now see 227 00:12:44,040 --> 00:12:47,120 above, subduction zones in volcanic arcs. 228 00:12:47,640 --> 00:12:51,440 Can we envisage how the Earth might have evolved if it had 229 00:12:51,560 --> 00:12:55,240 much less water? And you mentioned Venus, Is that 230 00:12:55,240 --> 00:12:58,160 a case in point? That's certainly one of the 231 00:12:58,160 --> 00:13:01,320 predominant views in the scientific community, that Venus 232 00:13:01,320 --> 00:13:04,800 is just dry on the inside. Also, as you point out, even a 233 00:13:04,800 --> 00:13:08,880 very small amount of water, again in those crystals within 234 00:13:08,880 --> 00:13:12,760 the crystals not wandering around as a free fluid, reduces 235 00:13:12,760 --> 00:13:15,040 the viscosity tremendously. And that the thing to think 236 00:13:15,040 --> 00:13:19,720 about here is that the interior of the Earth is moving 237 00:13:20,120 --> 00:13:24,560 centimeters to meters per year, so very, very slowly, and it 238 00:13:24,560 --> 00:13:27,880 moves through the deformation of solid crystals. 239 00:13:28,320 --> 00:13:30,920 And those solid crystals are the same as you might pick up in a 240 00:13:30,920 --> 00:13:33,400 piece of rock on the surface, but under heat and pressure they 241 00:13:33,400 --> 00:13:36,400 can flow. And you can imagine this makes 242 00:13:36,400 --> 00:13:39,040 some kind of intuitive sense. That if. 243 00:13:39,080 --> 00:13:42,800 In their little crystal voids are bits of water, or in their 244 00:13:42,800 --> 00:13:45,880 crystal lattices are water. Those are generally weaker 245 00:13:45,880 --> 00:13:49,280 crystals than ones without, and so they flow more easily. 246 00:13:49,680 --> 00:13:53,160 So just little bits of water inside the solid crystals. 247 00:13:53,160 --> 00:13:56,120 The mineral crystals actually allows the inside of the Earth 248 00:13:56,120 --> 00:13:59,880 to flow more easily, particularly right up underneath 249 00:13:59,880 --> 00:14:02,440 the tectonic plates, and that's where there's an area of 250 00:14:02,440 --> 00:14:04,920 particularly low viscosity that allows that. 251 00:14:04,920 --> 00:14:07,880 Plate tectonic movement, which we don't. 252 00:14:07,880 --> 00:14:09,760 Think is. Happening on Venus, although 253 00:14:09,760 --> 00:14:11,200 that's debated. It could be. 254 00:14:11,480 --> 00:14:14,600 Still happening on Venus. But the idea is that the 255 00:14:14,600 --> 00:14:18,880 interior of Venus in fact is drier and therefore much stiffer 256 00:14:19,240 --> 00:14:23,360 and much less willing to flow. But an interesting question is. 257 00:14:23,960 --> 00:14:27,120 If there was plate tectonics on Venus, how would we really know? 258 00:14:27,400 --> 00:14:30,240 Because the way that we absolutely confirmed it on the 259 00:14:30,240 --> 00:14:34,360 Earth was with seismic networks that were able to locate 260 00:14:34,360 --> 00:14:38,840 earthquakes that conform to the shape of a down going slab under 261 00:14:38,840 --> 00:14:40,760 these volcanic arcs that you refer to. 262 00:14:41,200 --> 00:14:43,360 We have no seismic network on Venus. 263 00:14:43,640 --> 00:14:47,600 We could hope, maybe someday. So just to get back to why Venus 264 00:14:47,600 --> 00:14:51,440 is dry and the Earth is not, is it you think fundamentally 265 00:14:51,560 --> 00:14:54,760 because it's slightly closer to the Sun and therefore it 266 00:14:54,760 --> 00:14:56,640 couldn't retain water in an atmosphere? 267 00:14:56,720 --> 00:15:00,760 That is a good solid hypothesis for why that is. 268 00:15:00,800 --> 00:15:03,680 And I think that's probably where I would go first to look 269 00:15:03,680 --> 00:15:07,480 for an answer. But there's still so much that's 270 00:15:07,480 --> 00:15:10,720 not known about Venus, so much more that we need to learn. 271 00:15:10,960 --> 00:15:14,680 And so I wouldn't venture to say that I'd place money on that 272 00:15:14,680 --> 00:15:16,720 bet, but it's absolutely plausible. 273 00:15:17,160 --> 00:15:20,120 I said in the introduction that the snow line runs between Mars 274 00:15:20,120 --> 00:15:22,680 and Jupiter. So is Mars wet? 275 00:15:23,080 --> 00:15:26,680 Mars is a bit wet at the present time and has a bunch of water 276 00:15:26,680 --> 00:15:29,440 ice on its surface that migrates back and forth. 277 00:15:29,800 --> 00:15:33,320 But there's evidence that it was much wetter in the past and so 278 00:15:33,360 --> 00:15:35,360 there's the thought that. Then in the. 279 00:15:35,360 --> 00:15:40,920 Distant past Mars was as wet and habitable as the Earth, and that 280 00:15:40,960 --> 00:15:45,080 it lost its water over time and became much much drier, which is 281 00:15:45,080 --> 00:15:48,120 where it is is today. The thought for that water loss 282 00:15:48,120 --> 00:15:51,920 is mainly just that Mars is much smaller than the Earth and water 283 00:15:51,920 --> 00:15:55,440 is not as well gravitationally bound to it, and especially if 284 00:15:55,440 --> 00:15:57,160 Mars lost. Its magnetic. 285 00:15:57,160 --> 00:15:59,480 Field early on, which it has. None now. 286 00:16:00,000 --> 00:16:03,240 It's not as protected on Mars as it is on the Earth, and so that 287 00:16:03,240 --> 00:16:06,080 water will be lost over time to the solar wind. 288 00:16:06,440 --> 00:16:10,000 That's one set of ideas. That hangs together well and 289 00:16:10,000 --> 00:16:11,600 might. Explain what we see on Mars 290 00:16:11,600 --> 00:16:15,240 today. As we search for exoplanets, 291 00:16:15,520 --> 00:16:18,160 there's a lot of discussion about the habitable zone, 292 00:16:18,720 --> 00:16:22,720 defined as the band within which liquid water can exist, because 293 00:16:23,280 --> 00:16:25,280 we believe water is essential for life. 294 00:16:25,960 --> 00:16:30,520 But could it be that a typical planet in the habitable zone 295 00:16:30,520 --> 00:16:34,720 might just be dry, even if any water would have been in liquid 296 00:16:34,720 --> 00:16:37,160 form if it was there? Would you think the sort of 297 00:16:37,160 --> 00:16:41,920 mechanisms that you describe for Earth would prevail in any 298 00:16:42,000 --> 00:16:46,440 plausible exoplanetary system? Well, I hate to generalize to 299 00:16:46,440 --> 00:16:48,520 the universe because we know so little about it. 300 00:16:48,880 --> 00:16:52,560 I can only say that if a planet like the Earth was formed around 301 00:16:52,560 --> 00:16:55,960 another star from materials similar to what we have in our 302 00:16:55,960 --> 00:16:57,920 solar system, then it would also be wet. 303 00:16:58,320 --> 00:17:02,880 But the concept of a habitable zone is a very, very simplistic 304 00:17:03,120 --> 00:17:06,720 concept because everything about the habitability of a planet 305 00:17:06,720 --> 00:17:10,920 depends upon its chemistry. In fact, more even than its 306 00:17:10,920 --> 00:17:13,599 distance from its star or how large it is. 307 00:17:14,000 --> 00:17:16,680 In a solar system where for some reason there's a higher 308 00:17:16,680 --> 00:17:19,880 percentage of radiogenic elements than interior, heating 309 00:17:19,880 --> 00:17:23,640 up the planet can keep its surface wet longer and you don't 310 00:17:23,640 --> 00:17:25,920 need to rely on that star distance. 311 00:17:26,240 --> 00:17:29,360 The composition of its atmosphere will entirely 312 00:17:29,360 --> 00:17:32,280 determine the current surface temperatures of that planet. 313 00:17:32,560 --> 00:17:34,440 And if you look at the difference between Venus's 314 00:17:34,440 --> 00:17:37,640 atmosphere and the Earth's atmosphere and how different our 315 00:17:37,640 --> 00:17:41,080 surface conditions are, knowing we could not discriminate 316 00:17:41,080 --> 00:17:44,280 between Venus and the Earth, when looking through telescopes, 317 00:17:44,760 --> 00:17:46,680 you see the composition is everything. 318 00:17:47,080 --> 00:17:51,680 And I would just add that even in planets that are not in the 319 00:17:51,680 --> 00:17:56,320 classical habitable zone, there could be the liquid water just 320 00:17:56,320 --> 00:18:00,000 under the surface of those planets, Either brines or waters 321 00:18:00,000 --> 00:18:02,200 that are heated from radiogenic heating inside. 322 00:18:02,400 --> 00:18:05,280 Or the moons of planets that are not at all in the habitable 323 00:18:05,280 --> 00:18:09,120 zone, but tidal heating from their main planet keeps them 324 00:18:09,120 --> 00:18:14,760 warm enough to habitable. Have we in fact seen evidence of 325 00:18:14,760 --> 00:18:17,560 water on an exoplanet? Yes, traces of. 326 00:18:17,560 --> 00:18:20,640 Water and exoplanetary. Atmospheres have been found. 327 00:18:21,120 --> 00:18:23,160 So we know. There's water out there, There's 328 00:18:23,160 --> 00:18:25,200 water out there and then a. Whole. 329 00:18:25,200 --> 00:18:28,200 Chain of questions need to be asked about what that water 330 00:18:28,480 --> 00:18:32,880 might mean and does it indicate any kind of habitable situation 331 00:18:32,880 --> 00:18:35,440 that we could understand with our comprehension of life. 332 00:18:35,920 --> 00:18:39,840 Unfortunately, it seems that the most likely kind of life 333 00:18:40,440 --> 00:18:44,440 elsewhere in the universe is going to be microbial life 334 00:18:44,440 --> 00:18:48,120 living under the surface of a planet, which is unfortunately 335 00:18:48,120 --> 00:18:50,600 going to be virtually undetectable using today's 336 00:18:50,600 --> 00:18:54,280 technologies. If you were to be in charge of 337 00:18:54,560 --> 00:18:58,840 formulating missions to try and figure out if and where there 338 00:18:58,840 --> 00:19:01,080 might be life, how would you go about it? 339 00:19:01,360 --> 00:19:05,120 Well, my favorite thing to do would be to go to Mars and drill 340 00:19:05,120 --> 00:19:07,640 into the groundwater system and measure what's there. 341 00:19:08,000 --> 00:19:10,720 That's where I would go first. But that's already technology 342 00:19:10,720 --> 00:19:13,800 that we don't have. What about under the surface of 343 00:19:14,000 --> 00:19:17,560 Europa or Enceladus? Or under those icy exteriors. 344 00:19:17,720 --> 00:19:19,880 Sure. Famously great places that 345 00:19:19,880 --> 00:19:22,080 people want to go look for life? Absolutely. 346 00:19:22,840 --> 00:19:24,280 What are you working on at the moment? 347 00:19:24,880 --> 00:19:28,600 I am absolutely focused on the Psyche mission, which is a 348 00:19:28,600 --> 00:19:32,440 mission to a metal asteroid, not a site for searching for life 349 00:19:32,440 --> 00:19:35,000 and no liquid water. I've been working on that since 350 00:19:35,000 --> 00:19:38,760 2011. And we won our NASA competition 351 00:19:38,760 --> 00:19:43,520 in 2017 and we launched in 2023, and we'll get to the asteroid in 352 00:19:43,520 --> 00:19:47,320 2029. And so all of my time is spent 353 00:19:47,400 --> 00:19:51,480 leading this very large team of people on this project, making 354 00:19:51,480 --> 00:19:55,080 sure that our spacecraft stays happy and healthy on its its way 355 00:19:55,080 --> 00:19:57,160 to the asteroid so that we can get our science. 356 00:19:58,200 --> 00:20:00,400 Lindy Elkinstanton, thank you very much. 357 00:20:00,640 --> 00:20:03,120 Thank you very much indeed for this time, Oliver. 358 00:20:03,120 --> 00:20:05,800 I appreciate it. To see pictures and 359 00:20:05,800 --> 00:20:11,200 illustrations that support this podcast, go to geologybytes.com, 360 00:20:11,320 --> 00:20:14,080 where you'll also find a subject matter index of all the 361 00:20:14,080 --> 00:20:16,440 episodes. There you can also give me 362 00:20:16,440 --> 00:20:20,400 feedback which I welcome, as well as sign up to get my emails 363 00:20:20,400 --> 00:20:21,600 about new episodes.