Ep. 626: Terrestrial Planets — Mercury, Venus, Earth, and Mars

We continue our refreshed tour of the Solar System, checking in on the inner terrestrial planets: Mercury, Venus, Earth and Mars. What have we learned about their formation, evolution and what they might tell us about other planets in the Universe?

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Show Notes


Weekly Space Hangout

EMAIL: wshcrew@wshcrew.space

Mercury (NASA)

Venus (NASA)

Mars (NASA)

Microbes A-Z: Your Questions Answered (AMNH)

NASA climate modeling suggests Venus may have been habitable (NASA)

What is the carbon cycle? (NOAA)

Plate Tectonics (USGS)

The Coronae of Venus: Evidence for Ongoing Volcanic Activity? (LPI)

Alien Life Could Theoretically Survive Within Venus’s Clouds, Scientists Say (Science Alert)

Are mysterious dark patches in Venusian clouds microbial life? (New Atlas)




Venus Climate Orbiter AKATSUKI (JAXA)

Venus Express (ESA)

Magellan (NASA JPL)

Escape Velocity (Hyperphysics)

Solar Wind (NOAA)

Water on Mars May Be Trapped in the Planet’s Crust, Not Lost to Space (Scientific American)

Dusty Snow on Mars Could be Melting Just Below the Surface (Universe Today)

Salty Liquids on Mars — Present, but not habitable? (LPI)

NASA Confirms Evidence That Liquid Water Flows on Today’s Mars (NASA)

Scientists find water in Mars’ Grand Canyon (EarthSky)

Earth isn’t ‘super’ because the sun had rings before planets (Rice University)

Supernovae and life on Earth appears closely connected (EurekAlert)

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Transcriptions provided by GMR Transcription Services

Fraser Cain:                  Astronomy Cast Episode 626: The Terrestrial Planets – Mercury, Venus, Earth, and Mars. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos where we help you understand, not only what we know, but how we know what we know.

                                    I’m Fraser Cain, publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a Senior Scientist for the Planetary Science Institute and the director of CosmoQuest.

                                    Hey, Pamela, how you doing?

Dr. Pamela Gay:         I am cold. I am cold. In my studio, it is 18 degrees Celsius. It’s in the 60s inside the house because outside we’re in the negative numbers and our house needs new windows.

Fraser:                         We should at least acknowledge – I know you don’t wanna talk about it, but there exists a telescope called “James Webb” and things seem to be happening to this telescope. That’s all, I think, Pamela is willing to talk about this week. Maybe in future weeks, we will go into greater detail, but until it is fully deployed, it is a telescope that shall not be named.

So. I’ve already enraged her, bringing this up. Sort of brought forward deep-seated emotional scars and now she’s gonna need, probably, a nice stiff drink afterwards to get over the anxiety. But we’re just another week or two until we’re fully deployed and then we move into the testing out phase – anyway, I’m not gonna talk about it anymore.

The thing that I do want to talk about is that we are looking to bring on more of the science communicators for the Weekly Space Hangout crew. Weekly Space Hangout. And, of course, this is the show we do every Wednesday. It’s like a roundtable of variety, of space-science communicators; some are practicing astronomers, some are Ph.D. graduates, some are space journalists, like me, and we rotate through this group of people, and we got a bunch of slots open up.

And the person this is perfect for is someone who wants more practice, or wants any kind of practice, in doing live science communication. If you just wanna build your chops in communicating either your ideas or just science ideas in general, to a wide audience in a fairly – you know, I feel like it’s a fairly forgiving and non-hostile environment and it’s really a place for you to get training.

And so, if you feel like there’s just not a lot of places for you to get practice and this is a field that you want to do or skills that you want to add, then we would love to have you join us at the Weekly Space Hangout and be one of our journalists every week.

So, send an email to Nancy Graziano, wshcrew1@gmail.com, or you can just go to the website and there’ll be information there. And let her know that you’re interested and then tell her your info, and what you’re looking to get out of it, and we would love to have you join us.

So, that’s it. Let’s move on to the show.

We continue our refreshed tour of the solar system, checking in on the inner-terrestrial planets: Mercury, Venus, Earth, and Mars. What have we learned about the formation, evolution, and what they might tell us about planets across the universe?

All right, where do you want to go with this? How should we start? I mean, it’s been – I don’t know, 10 years, 12 years, since we talked about the inner solar system? So, it may make sense to have another crack at it.

Dr. Gay:                      And what I love is, in the Trello notes for this show, I had omitted Mercury, to put Mercury in the next episode when we’re talking about other things that are super close to the sun. And –

Fraser:                         So, it’s not a terrestrial planet?

Dr. Gay:                      Well, it’s…a big old rock. And when you look at the histories of Mars, of Venus, of Earth, all three of these worlds, for their first three billion years or so, were experiencing the great heavy bombardment, and had liquid oceans, and have had tectonic episodes, and continue to have geophysical activity that alters the face of the planet. And little Mercury is literally off-screen in the image…that will not let me drag it. It’s not even on this image, in fact. I thought it was off-screen. And the thing is, because it’s small, because it’s unusually dense, because it has an ion tail, the geophysics of Mercury are radically different than what we see with these three larger worlds.

Fraser:                         Okay. We’ll cover Mercury more next week, but let’s focus then on the larger and farther planets, Venus, Earth, and Mars. If you were to sort of characterize them in their togetherness, what would you define them? How would you define them?

Dr. Gay:                      These are worlds that have had liquid oceans at some point in their history, have potentially had life at some point in their history, and continue to have their environment altered through meteorological processes of varying degrees of deadliness.

Fraser:                         So, let’s talk about the water phase. When – I mean, we’re familiar with the water here on Earth – when did this exist on Venus?

Dr. Gay:                      So, Venus and Mars, both, for roughly the first three billion years of their lives were able to support liquid oceans and their low-lying areas. And what is amazing, as we think about this, is life here on Earth really began to evolve during those first three billion years. We didn’t so much have anything that was more than like, single-celled and squishy, but we had life in those first three billion years. And if was only as they neared, well, two billion years ago that they became utterly inhospitable, there’s the potential that those early oceans contained microbes. And this gets to the whole idea though of why did they both die.

And the diverging history of these three worlds gives us a new view that we didn’t even imagine last time we talked about these worlds; on just how the Goldilocks Problem isn’t a problem for today, it’s a problem for how things evolved over time.                        

Fraser:                         Okay, so why did Venus die?

Dr. Gay:                      Venus, as near as we can tell, had a massive runaway greenhouse effect, and depending on what papers you read, it was either several hundred million years ago to billions of years ago. And that runaway greenhouse effect that was driven by a change in chemistry, potentially by sudden volcanism across the entire world or by ongoing volcanism that destroyed the planet a little more slowly, however you destroyed the environment, it was a greenhouse environment that thickened the environment, thickened that atmosphere. And as it got thicker, it trapped the heat in; sunlight comes in, infrared radiation comes off the soil, and then just bounces around.

Fraser:                         Do we think the volcanism happened first or the runaway greenhouse happened first?

Dr. Gay:                      The papers that I trust most have the two things being inter-related where for unexplainable reasons, Venus sat there building up heat, building up heat, and then just, either flipped its surface in mass volcanism across the entire world or had a massive epoch of volcanism that has continued across the planet, resurfacing the entire world over the past hundreds of millions of years.

Fraser:                         I mean, here on Earth we have the carbon cycle. We have the plate tectonics that shift the surface of the earth around. And as carbon dioxide is generated on the surface, it gets sequestered; it goes underneath the plates and disappears out of the atmosphere and out of the surface environment. And we don’t see that on Venus. We don’t see any plate tectonics on Venus. So, I guess, same question, right? Did Venus’s heat shut down its plate tectonics or did its lack of plate tectonics drive its heat?

Dr. Gay:                      And this is something we don’t know.

Fraser:                         Yeah.

Dr. Gay:                      And one of the problems is, we haven’t been able to get anything to live on its surface, robot-wise, long enough to go looking for earthquakes. So, while there are hints in research of areas called coronae, crowns, that look like recent volcanic activity, we can’t measure the earthquakes – venusquakes, I guess – that would either be caused by ongoing volcanism or would allow us to trace out the existence of plates. We can’t do either. So, we assume Venus doesn’t have plates. We don’t know if it has active volcanism.

What we do know is some horrible point in its past built up too much heat in its atmosphere, evaporated all of the water, which became a new greenhouse gas, caused deadly acidic chemistry to take place, and potentially created a place where life can exist only in the clouds. And that idea just refuses to go away. There could still be life on Venus in the clouds. There are a variety of researchers that think the dark patches that get seen in its clouds periodically may actually be the aerosolized version of an algae bloom. But we don’t know. But we’re going to send three spacecraft there and, hopefully, find out.

Fraser:                         Yeah, and I guess that was my point, or that I was about to make, was that we have three separate spacecraft that are going. That Venus is chronically, tragically, underexplored. Mars is crawling with robots, but Venus, we haven’t had a serious mission to Venus. There’s the Akatsuki, the Venus Express, and then like, Magellan in ’98? It’s been a long time and we haven’t had a really serious, and attempts to land on the surface was, like, in the mid-80s? It’s time to go back to Venus, for real. Bring the modern equipment and get to the bottom of these mysteries.

 Dr. Gay:                     And it’s just that much harder to go someplace that’s warm. And the thing that brings this home to me is, when I’m out gardening in the summer, if I set my iPhone down in the sun, it ceases to function in 20 minutes flat or less. But, in the winter, if I do the exact same thing, my phone’s like, “I’m cold; you’re gonna run out of battery fast, but I’m not going to give you any big warning messages and die until the battery goes.”

Fraser:                         Right, right.

Dr. Gay:                      And the difference between having to figure out how to shed heat, which is hard, versus trying to figure out how to prevent your batteries from dying in the cold. Generating heat is easy. Makes a, well, a world of difference between Mars and Venus.

Fraser:                         Alright. I wouldn’t say we know why Venus died. We have a series of inter-related complexities that may have contributed to the death of Venus. Answer: waiting for spacecraft to arrive. What killed Mars?

Dr. Gay:                      Well, with Mars, it lacked the ability to hold on to its atmosphere. So, Venus died because its atmosphere got thicker and thicker and thicker and thicker, and chemistry, and death. On Mars, early on, its atmosphere was somewhat maintained by the fact that it had a repository of materials – ices, oceans, all of that – that formed an atmosphere, could sublimate, evaporate into being an atmosphere, but over time, the sun and lack of gravity, just – and lack of a magnetic field that made the sun’s effects greater – it just slowly pushed it away. And it continues to do so.

If hydrogen or helium is released into our atmosphere, it’s gone very quickly. If you release pretty much anything into Mars’s atmosphere, it’s gone pretty quickly. Here on Earth, the reason we lose hydrogen and helium so fast is it’ll get hit by an oxygen molecule, nitrogen molecule, and that transfer of momentum sends it away at escape velocities. Without a strong magnetic field, pretty much anything you put into Mars’ atmosphere, it’s going to get eventually affected by the solar wind and blown away at escape velocities.

Fraser:                         Although, it is kind of interesting that the story of Mars’ water, I mean, that was definitely the understanding was that all of the water on Mars had evaporated and gotten broken up and pushed out, you know, by this solar wind. But it does appear now that a lot of Mars’ water went into the planet; got absorbed by the rock underneath and that it might have larger sources of water under the surface than we originally thought.

Dr. Gay:                      And this is where the keyword is: the surface water –

Fraser:                         Right.

Dr. Gay:                      – Capable of sublimating into the atmosphere. That’s what’s went away. There is tantalizing evidence of dust-covered buried glaciers, of subsurface brines.

And every month, it seems like there’s a new press release on new evidence of water at more equatorial radii than ever before, and yeah. It’s a world that just might have ice we can access. And potentially even vast amounts at that massive trench, shown in the image here, along the bottom of that trench, where the atmosphere is the thickest it is anywhere on Mars, so here you have the largest canyon in the solar system. With not enough air to breathe, but more than elsewhere, and potentially water and protection from a whole lot of radiation all down at the bottom of that canyon.

Fraser:                         Yeah, we talked about this last, or two weeks ago, with the –

Dr. Gay:                      Yeah.

Fraser:                         – 2021 show that I thought this was one of the biggest stories of the year. And hopefully, we’ll see a lot more exploration information about this. That the water on Mars could be…tantalizingly close, easy to access, and could be a total game-changer both in the search for life, but also in just the support of exploration of the planet.

Dr. Gay:                      To Earth?

Fraser:                         Yeah, so I wanna talk about Earth, but I also just want to talk about, like – I mean, we asked what killed those two planets. What killed Earth? Us. Alright, moving on. But I want to know, I guess, I want to put this in context for how we feel about terrestrial planets in general as we think about this in the context of extrasolar planets. What is this starting to tell us? What are we starting to think of what we’re going to find as we look around the rest of the galaxy? Both what we know about Earth and what we know about Venus and Mars.

Dr. Gay:                      I think instead of viewing planets as following some Goldilocks scenario of this one’s too hot, this one’s too cold, this one’s just right, it is much more like an avocado, where it’s too hard, too hard, too hard, perfect… mush. And so, you have this moment, and that moment is different depending on which avocado or planet you look at, and it’s how long a world is habitable that is really the question. That we can have all sorts of different scenarios where right now, something evolving fast enough could exist…could exist. But, if you wait, even our world is going to become no longer habitable.

                                    And so, the question becomes how big is the window? And it’s the worlds with the biggest window, the biggest period of being not too hard and not too mushy, that’s the exciting time to find them and we’re still figuring out what is that window for different kinds of worlds?

Fraser:                         What does it look like? I mean, where would we find the best chance of picking up an avocado in the store, in an exoplanetary sense, and it being ready to spread on your avocado toast?

Dr. Gay:                      Well, there’s some really cool research that just came out in the past…I want to say in the past week, that was looking at a new way of simulating solar systems where they added into their simulations the difference in pressure you have in the disc around a star at the different places where materials change face. So, you’re gonna get this change in pressure at that point where water suddenly goes from being water vapor to being water ice. You’re gonna get a change in pressure at that point where silica suddenly is able to either vaporize or solidify.

And a star, like our sun, creates planets at the places our solar system created planets given the fullness of time and that temperature distribution through the early disc. And so, if that person’s right – and never believe a planet formation model, but I like this one because it actually reproduced our solar system. If this is right, then to get the distribution planets we see, you have to have the temperature profile that we have.  

So, the question becomes – and I’m guessing these simulations still need to be run – what other temperatures end up with the pressure boundaries just right, that in the fullness of time with everything evolving, moving around over time, do you end up with planets at that – I’m not gonna say magical, I’m not gonna say golden –

Fraser:                         Goldilocks?

Dr. Gay:                      – I’m gonna say…what was that?

Fraser:                         Habitable? Goldilocks?

Dr. Gay:                      Yeah, it’s…

Fraser:                         Just right?

Dr. Gay:                      Well, yeah. “In that sweet spot,” that’s the best word I can come up with. That ends up with planets in that sweet spot where they are the right size to be able to generate a magnetic field and not cool off too fast; where they don’t get hit so much that they end up with the rotation getting slowed down and ending their magnetic field early, which probably happened to Venus. They have that sweet spot where they’re able to hold onto their magnetic field, have enough heat, and are able to have, and hold onto, an atmosphere long enough to evolve life.

Fraser:                         But this sounds like you’re throwing in a lot more variables. Is this constraining the size of a habitable zone for an extrasolar planetary system? Like, narrowing the size of it to where you have those pressure gradients? Or is it just that it also depends on the chemistry of the planets, the size, the magnetosphere, etc. What would be habitable to Mars-sized planets might be different than what’s habitable to Earth- or Venus-sized planets.

Dr. Gay:                      And then, we also get down to the life as we know it versus life that’s living beneath a shell of ice. So, when we look at strictly the Goldilocks scenario of Mars, Earth, Venus, there we’re starting to say: okay, you’re able to get planets cropping up at these places that have these experiences if you have a sun-like star with this temperature profile. We would have to rerun all the scenarios to see, what do you get with bigger and larger stars, and can you get a similar outcome with different distances?

                                    And then yeah, chemistry is part of that because depending on the chemistry, things glom together in different ways, and you may or may not have enough stuff to form big, rocky worlds. So, I guess what I’m getting at is, one really awesome paper published one really awesome way to actually explain how we got our solar system and we’re a long way from figuring out other solar systems. More computer time, please.

Fraser:                         Yeah, yeah. But it’s always coming back to that question, right? Are we normal?  Because we’re wonderful, and I’m talking about that in the solar system sense, the fact that we have a planet, a blue planet, with life covering it in this corner of the Milky Way is phenomenal. And we see horrible hellscape Venus to the left of us and awful cold, frigid, dusty Mars to the right of us, and yet we’re doing great.

And I think the question still, we know of thousands of planets, the question just really hangs over us is that: were there a thousand variables that all had to come together to make Earth the kind of planet that it is, or did it just take one or two fairly common situations to happen? And as we explore and as the tools come online for us to explore these other planets, will we realize that Earth is extra special, as opposed to just regular special.

Dr. Gay:                      And this is where I think back to when I was learning to solve the quadratic equation and there were annoyingly two possible answers. And yes, there are probably far more than thousands of variables that go into it, that all have to line up just right to get our world. But then, there may be a different solution. There may be infinite different solutions that can get to other forms of habitable.

Fraser:                         Yeah. That it could be like a steady state that the universe reaches, like the variables could all play together and get you to this standard point all the time.

Dr. Gay:                      There’s another paper you probably saw that came out this week that talked about how the history of supernova explosions near our solar system affected our world’s ability to have life because the ionizing radiation of the supernovae generated greater cloud cover and affected the temperature history of our world.

Fraser:                         Yeah. So then could you not have life without a whole bunch of supernovae going off at the right times nearby? Like, that just gets extreme at this point. Yeah.

Dr. Gay:                      Yeah. So, we’re just starting to know how many variables we have to figure out and we haven’t figured out all the variables, and we certainly haven’t figured out the values of all the variables. We just know we have life here, for now. Mars and Venus could have had life, may still have life that as we do not know it, and that’s amazing.

Fraser:                         If you switch, what do you think, if you switched Mars and Venus in their orbits, do you think that would make a difference for either one of them? Would Venus be more habitable? And Mars, I think, would be ruined either way.

Dr. Gay:                      Yeah, Mars is ruined, either way, that lack of a magnetic field. Venus… if it hadn’t had the bejesus knocked out of it by whatever changed its rotation to be counter to the direction you would expect so that its days aren’t longer than its years… I feel like playing with its rotation right would get you a long ways as well. So, the question becomes, how much do you have to change the day/night cycle, and where do you need to put it to rescue it?

Fraser:                         Right.

Dr. Gay:                      Magnetic fields.

Fraser:                         Yeah, yeah. Absolutely fascinating. Alright. Well, thank you, Pamela.

Dr. Gay:                      Thank you.

Fraser:                         Do you have some names for us?

Dr. Gay:                      I do. So, as always, our show is brought to you by you. We are immensely grateful to all of you who are part of our Patreon community and to those of you who have become one-time donors through PayPal. Now, those of you who made the one-time donations, I love you, you are amazing, but the Patreons’ names I get to read right now. So, I’m gonna go ahead and read some of our Patreons.

Andrew Stephenson, Benjamin Davies, Glenn McDavid, Steven Coffey, Elad Avron, The Mysterious Mark, Jen Greenwald, Cemanski, Kseniya Panfilenko, Joe Wilkinson, planetar, Sean Freeman (Blixa the cat), Peter, Roland Warmerdam, Dean, john öiseth, The Air Major, Saebre Lark, Brian Kilby, Arcticfox, Claudia Mastroianni, Aron Tannenbaum, Bart Flaherty, Corinne Dmitruk, Naila, Tim Gerrish, Lew Zealand, Jordan Turner, Rayvening, Allen M Price, Mark Van Kooy, Leigh Harborne, Mark Phillips, Kathleen Mattson, Bob the cat, chris wheelwright, Jason Kardokus, Olivia Bryanne Zank, Ron Thorrsen, PAPA1062, Robert Hundl, Kim Barron, Vitaly, Paul Esposito, Arthur Latz-Hall, Scott Briggs, Ruben McCarthy, Uhmu, Geoff MacDonald, Wayne Johnson, and Iggy Hammick.

And if I’ve mispronounced your name, I’m very sorry.

Fraser:                         It’s inevitable.

Dr. Gay:                      If you could like, change your Patreon username to be phonetically spelled, then I’ll do better, I promise. Thank you.

Fraser:                         Alright. Thanks, Pamela.

Dr. Gay:                      Buh-bye.

Female Voice:            Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a Creative Commons attribution license. So, love it, share it, and remix it, but please, credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, astronomycast.com.

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