Ep. 627: Mercury Revisited

It’s been about a thousand years since we last looked at Mercury, so we figured it’s time for an update. What new things have we learned about Mercury, or even new questions? Fortunately, there’s a mission on the way to help get us some answers.

Download MP3 | Show Notes | Transcript

Show Notes

PATREON: Universe Today

Mercury (NASA)

PODCAST: Ep. 49: Mercury (Astronomy Cast)


Mariner 10 (NASA JPL)

IMAGE: Mercury’s Caloris Basin (NASA)

Why Have Parts of Mercury’s Landscape Deflated? (Sky & Telescope)

IMAGE: Ridges and Cliffs on Mercury’s Surface (NASA)

What is a fault and what are the different types? (USGS)

2P/Encke (NASA)

Mercury (The University of Utah)

Mercury’s magnetic field (Planetary Society)

BepiColombo (ESA)

Neptune-like (NASA Exoplanets)



The Moon and Mercury May Have Thick Ice Deposits (NASA Goddard)

Characteristics of Mercury (Universe Today)

Kim Stanley Robinson

Back to Top


Transcriptions provided by GMR Transcription Services

Fraser Cain:                Astronomy Cast, Episode 627, Mercury: Revisited. Welcome to Astronomy Cast, our 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’m doing well. We have what you would refer to as an adorable two inches of snow, and it brings me joy.

Fraser Cain:                That’s awesome. So, I wanted to begin this episode of Astronomy Cast with a shameless self-promotion for the Patreons that are associated with all of the projects that we do. So, Pamela is going to talk about the Patreon at the end of this episode for Astronomy Cast, and you should definitely be a part of that, as well as the one they do for CosmoQuest.

But if you’re a regular reader of Universe Today, you might not know that, if you join my Patreon for a one time – just one amount of money – any amount at all – I will remove all the ads from Universe Today for you, forever. So, you’ll never see another ad. Even if you cancel your Patreon immediately, you’ll still have no ads for the rest of your life. As long as that WordPress login exists. Plus, you get other cool stuff: advance access to all the videos and behind-the-scenes stuff that we’re doing and lots of other cool projects that we’re working on.

But, that’s one of the main ones if you come to our website – because I have a lot of people who become my patron, and they’re like “yeah, I found out about you because of Astronomy Cast,” so, hey – people of Astronomy Cast – I run Universe Today. If you go to that website and you don’t want the ads, you can remove them all for the rest of your life. All right, let’s get into the episode.

Dr. Pamela Gay:         All right.

Fraser Cain:                It’s been about 1,000 years since we last looked at Mercury, so we figured it’s time for an update. What new things have we learned about Mercury? Or even new questions? Fortunately, there’s a mission on the way to help us get some answers. So, last week we talked about the various terrestrial planets, and you wanted me to hold off talking about Mercury until this week because it is actually quite different from the rest of the terrestrial planets. Why? What’s going on?

Dr. Pamela Gay:         Well, it’s more like a moon in terms of it doesn’t have an atmosphere that leads to weathering. It had volcanism in the far distant past, maybe 4.6 billion years ago when it formed, but that volcanism didn’t stick around, and the mare seas that we see on it are, in many ways, like the mare seas that we see on the moon, where they were crated when massive impacts caused the liquid, internal bits to leak out onto the surface, and that liquid on the inside is due to tidal heating, which is more like we’d expect with a moon of Jupiter, and none of this is how a planet behaves.

And then on top of all of that, if you were able to just uncompress all the worlds and let them exist without being squished by their own self-gravity, it would be the densest object around due to its high metal content. And Earth is the only thing that – when you let things get squished down by gravity – is denser, and we know we lost all of our light stuff to the moon in a collision in the past. So, that just makes Mercury even more weird.

Fraser Cain:                Right? Right. That it is super close to the sun, baked by the solar radiation, the solar wind, lost a lot of its volatiles. There’s some really bizarre shapes on its surface, that I’m sure we’ll go into, that are fairly newly discovered. Yeah, it’s such a crazy world that – I mean, it’s smaller than some of the moons, right?

Dr. Pamela Gay:         Oh yeah. Yeah…You have – Titan’s bigger. Ganymede’s bigger. And it’s just weird. It’s just weird.

Fraser Cain:                So, I guess one thing that I think is really important is that, when last we talked, it was – again, I said 1,000 years ago because I was too lazy to actually look up the date that we talked about it last – but it had to have been before 2011 when NASA’s MESSENGER spacecraft arrived at Mercury and did a lot of its work. So, what did we learn about Mercury from MESSENGER?

Dr. Pamela Gay:         Well, it’s a whole lot more cratered than we imagined, and those craters aren’t even majority direct impact, some researchers think. Because Mercury is such a solid object that, when you hit it with a big rock, it tosses out chunks that go form additional secondary craters.

Fraser Cain:                Wow.

Dr. Pamela Gay:         And trying to untangle those primary craters from those secondary craters is a challenge. The other thing that we learned is, it wasn’t the first spacecraft to go there. It had previously been visited by Mariner 10. But the details with which we saw it allowed us to understand that there’s things like the massive Caloris Basinon one side and the weird terrain – they actually refer to it as the weird terrain – it’s a lot like Pluto’s –

Fraser Cain:                The chaotic terrain?

Dr. Pamela Gay:         Yeah, on the other side … and this partnering of massive crater and weird surface features is thought to either be due to the shock waves rebounding around the world and tossing up the terrain at the antipode, or some papers even think that it’s from everything getting flung up and landing at the antipode and, of course, somewhere in between is possible. And so, you have this world that has really been knocked around a lot. It’s had a hard life, and all of that is apparent in the never-getting-weathered, never-getting-filled-in-with-lava surface that we get to explore.

Fraser Cain:                And this – you know I mentioned this chaotic terrain – I mean, one of the things that’s really – because Mercury is so small, it has a relatively low mass compared to Earth, Venus, etc. It has cooled down quite a bit despite being next to the sun. And what effect has that cooling down had to the planet?

Dr. Pamela Gay:         Well – this is one of those things that – we look at the moon and Mercury and, again, they’re so similar. They both have what’s called lobate scarps.And what has happened is – and it’s the most ridiculous term – I don’t know why I find it so ridiculous – but even you snorted at the name.

Fraser Cain:                Yeah, it’s awesome. Sorry, that was a snort of appreciation.

Dr. Pamela Gay:         It’s also just a cute name, let’s admit that. So, you start out with larger, hot object of molten goodness. The outer surface cools first, even under the influence of the sun, and then as the inner parts cool, they shrink. And you end up with this outer shell that is now bigger than the inner world.

Fraser Cain:                Right.

Dr. Pamela Gay:         And it goes shrink. And as it compresses and crumples down, it creates these faulting areas where the land can slide up on top of each other. We have lobate scarps here on Earth, as well, but the ones on the moon are the ones that are indistinguishable in images from the ones on Mercury, and that’s just kinda cool.

Fraser Cain:                I’m sort of imagining this planet crumpling in on itself – crunching smaller and smaller – and these big cracks opening up across its surface and jumbled up terrain as it’s trying to process this. Not to mention getting hammered by various meteorites and all the comets that are drawn in to the sun’s gravity. I think you’re right – rough life.

Dr. Pamela Gay:         Yeah. Compressionally, it has thrust faults, lobate scarps, wrinkle ridges – so it starts out all wrinkled up and then you just keep throwing things at it. Even Comet Encke has thrown bits and pieces at its surface, and I just love that we can blame one specific comet for at least some of the damage that has occurred.

Fraser Cain:                We should talk about the atmosphere of Mercury.

Dr. Pamela Gay:         Yes.

Fraser Cain:                Which is a phrase that I’m sure people never thought they would say.

Dr. Pamela Gay:         I kinda feel like atmosphere is too strong a phrase. Exosphere – I think that’s it – again, the same word that we use for the moon. So, Mercury has a magnetic field. It’s significantly weaker than the Earth’s magnetic field, but it is sufficient to generally protect Mercury from the solar wind.

This magnetic field comes from – Mercury has a highly elongated orbit and because of this combination of the squished orbit – the most squished in the solar system for a planet – as it goes around the sun, it isn’t tidally locked to the sun the way the moon is to the Earth, so that the exact same side is always facing the sun. That would have happened if it had a circular orbit. But because it has this squished orbit, you end up with these weird moment arms inside of it, so that for every two times it goes around the sun, it ends up experiencing three days.

And this constant squishing and unsquishing with its elliptical orbit leads to it continuing to have a molten core – lots of metals – and it still has a magnetic field. That magnetic field means that various materials coming off of its surface get to stay put for a little while, but they do end up streaming behind it so that, in certain solar images, you see this tail of particles constantly streaming off of Mercury.

So, there’s this one-two situation of magnetic field allows things to hang out for awhile; you have cosmic rays coming in from the sun that are knocking ions off the surface of Mercury, and all of this ends up eventually leaking ever so slowly out behind it to form a tail.

Fraser Cain:                It – I was just thinking about a planet having a tail – which is such a great idea. So, how did MESSENGER die?

Dr. Pamela Gay:         We crashed it into Mercury…? Because of course we did, it’s what we do with things – we crash them.

Fraser Cain:                Right. Did you see – I tweeted an animation of something where someone was crashing – they were talking about crashing Cassini into Jupiter, and in the end, they came together for a hug?

Dr. Pamela Gay:         Saturn.

Fraser Cain:                Saturn, that’s right. Sorry, yeah. It was a cartoon they crashed –  

Dr. Pamela Gay:         Wait – well, we plunged Galileo into Jupiter –

Fraser Cain:                Yeah, they crashed – that’s right – yeah, they crashed – they crashed Cassini into Saturn, and the animation was the spacecraft just stopped right at the last second, and Cassini and Saturn had a hug because all good missions end in a hug. Which I thought was very cute. But MESSENGER had its lithobraking with Mercury?

Dr. Pamela Gay:         Yeah, that was a thing. It got a few good years in there. It returned a whole lot of really solid images, and some of its instruments even allowed us to get hints at its surface, which is really the only way we’re ever going to totally figure out how this weird world formed so close to the sun. And there’s a lot of different theories, and it’s only BepiColombo that, hopefully, is eventually going to help us untangle exactly which one of these theories is correct.

Fraser Cain:                Now before we actually started this show – I think before we even started recording – you were just going on about these questions that you had about Mercury. So, why don’t we kind of interleave the questions that you have about it with the reality that a spacecraft is going back to Mercury to try and answer some of these questions.

Dr. Pamela Gay:         So, my biggest question is, “Why,” – and someone can probably explain this to me, I just haven’t found the answer yet because people don’t publish enough on why things aren’t the case – so, we know from exoplanet studies that there are a lot more mini-Neptunes than just about anything else out there.

We know that there are a lot of worlds that migrate in towards their sun and suffer greatly, eventually losing their atmosphere. And looking at Mercury, we see a world that is much denser, uncompressed than it should be. And so, what I would love someone to explain to me is, “How do we know that it’s not a mini-Neptune that had its atmosphere removed in the early days of the solar system?”

Fraser Cain:                Well, we see that.

Dr. Pamela Gay:         Yeah, in other solar systems.

Fraser Cain:                I mean, we see exoplanets now with – they’re clearly being dismantled by the – the gas parts are being dismantled by the star.

Dr. Pamela Gay:         Yeah.

Fraser Cain:                So, is that a possibility?

Dr. Pamela Gay:         Well, this is what I don’t know because trying to google, “Why is something not the case?” has failed me. So, I’m hoping someone has thought on this – has published on their other theories and will someday hear this and explain to me the other theories that people have published.

Fraser Cain:                Right. Well, let’s talk about BepiColombo, though. So, this is – so, who is sending this mission?

Dr. Pamela Gay:         This is primarily a European Space Agency and JAXA working together. It’s actually two spacecraft that are flying together and will become two spacecraft once they get to Mercury.

Fraser Cain:                Right. And it’s a long journey. We’ve talked about this in the past – that Mercury is the hardest planet to reach in the solar system. Because you’ve got to get rid of all the Earth’s orbital velocity to get there. So, it’s taking a very tortured journey.

Dr. Pamela Gay:         This spacecraft was named in 1999. I – it’s been years since it launched – I don’t even remember its launch anymore – and it finally gets there in 2025.

Fraser Cain:                It launched four-ish years ago in October 2018.

Dr. Pamela Gay:         Yeah, it launched years ago. And so that’s a seven-year journey, basically, to get to a world that, in the grand scheme of things, isn’t that far away. But the problem is you have to match its orbit just right or you fall into the sun and, while you could do your standard Helmholtz orbital transfer like we do to Mars, the problem is that the velocity close into the sun on a Helmholtzorbit that has the Earth at the farthest point from the sun, and Mercury is the nearest point to the sun – you’d be going entirely the wrong velocity to be able to go into orbit without lots and lots of fuel being required. So, you have to figure out how to match speeds another way.

Fraser Cain:                Right. And the way you do that is you – I guess you give – because we talked about gravity assist in the past and how you are stealing orbital momentum from the giant planets as you go past them to speed yourself up, and you’re doing the opposite – you’re giving them a little bit of orbital momentum to change your trajectory to get closer and closer in to the sun, which is kinda crazy.

Dr. Pamela Gay:         So yeah, getting there’s hard. And the thing is that, by the time any mission gets to Mercury, we are going to have questions that we can’t answer with the instrumentation that’s on board because science has advanced, instruments have advanced, and it’s highly frustrating. But hopefully BepiColombo, with its ability to measure the chemistry of the surface and so many other things about the internal structure and the magnetic field – hopefully it’s gonna help us understand exactly how this weird world ended up forming.

And right now, one of the primary ideas is it had a massive collision, not unlike the Earth had, that knocked all the lightweight material off of it, and that it actually started out more than two times the mass of Earth.

But the other theory – there’s a variety of theories – the other theory to look at is, during the solar nebula, anything lighter than what Mercury is currently made of simply was blasted away by the solar nebula. And the way we can start to separate these ideas is by looking at the surface composition. And with MESSENGER we were finding higher potassium and sulfur levels, which seem to make sense for the giant impact hypothesis? So, yeah – imagine losing that much mass in a collision and still being left behind.

Fraser Cain:                One of the great interests of exploring the moon is the possibility – and I guess now almost the certainty that there are deposits of water at the south pole and the north pole of the moon. Is there something similar on Mercury? Are there permanently shadowed craters on Mercury?

Dr. Pamela Gay:         Yeah. And making it even more interesting: Mercury is tilted less than a degree off of the orbit around the sun. And so, the sun never gets more than a few arcminutes above the horizon. The horizon. So, you don’t need very much of a crater to have a permanently shadowed region. And so, there’s water that has been found through radar returns – at least we believe it’s water – found through radar returns within those permanently shadowed regions.

And the temperatures are completely reasonable for machinery to exist at the poles, and some science-fiction writers – like Kim Stanley Robinson comes to mind – have even imagined, because its rotation rate is so slow, that you could have essentially domed cities on massive tracks that simply moved just out of the sunlight as the world rotated to constantly stay at just the right temperature so they were never too hot or too cold and…

Fraser Cain:                That sounds crazy.

Dr. Pamela Gay:         Yeah. It’s a tantalizing world for future technologies we do not have.

Fraser Cain:                Right. But the same ideas that are being proposed about going and setting up some kind of operation at the moon’s south pole – you could do something almost the identical situation at Mercury. It might even be a little easier because Mercury – it doesn’t have as much – what is it? Eccentricity? Which is the one that rises and goes above and below the planet ecliptic? I forget. Anyway…

Dr. Pamela Gay:         It’s tilted more to the plane of the ecliptic, actually, than any of the other worlds. So, it’s tilted about eight degrees to the ecliptic. With the moon, it has a weird tilt because it’s going around the ecliptic in the sky, and then the Earth is tilted, so there’s just a lot of weirdness going on.

Fraser Cain:                Yeah, a whole bunch of added trigonometry going on there. So, I guess BepiColombo arrives Mercury 2025, as you said.

Dr. Pamela Gay:         Yes.

Fraser Cain:                Two spacecraft. It has a Japanese spacecraft and the main BepiColombo built by ESA. They’re gonna separate and then explore Mercury together.

Dr. Pamela Gay:         Yes. And it’s really an interesting new future that we’re looking at where we have multiple missions simultaneously going to different worlds. We’re looking at a future where we’re going to have three main, and then a bunch of smaller satellites, all going to Venus, where we have these two different missions going to Mercury; we keep throwing new things at Mars every proper alignment of the orbits.

And this combination offers both the ability for the two missions to confirm each other’s results – something that’s been extremely important at Mars, where we’ve needed to confirm things like the methane results that were highly controversial. So, we get the ability to confirm and we also – well, if one of them fails, hopefully the other one will still keep going – so there’s this redundancy in our ability to get data. It’s an exciting day anytime we can get just one spacecraft to a world, but to have this new era of multiple spacecrafts is – well – here’s hoping that we have funding to support enough scientists to get everything we can out of all that data.

Fraser Cain:                Yeah, it does feel like we’re just getting a more-and-more established presence out there in space when you add up all the spacecraft and all the different targets that they’re at right now. It’s pretty great. All right, well, I’m sure we’ll come back around in 2025 and talk about the mission itself. Until then, of course, I think we’re just gonna wait with beta breath.

Dr. Pamela Gay:         It remains dead to us.

Fraser Cain:                Right. All right, well thank you, Pamela.

Dr. Pamela Gay:         Thank you. And thank you to all of our patrons out there who are, well, allowing us to keep everything going to pay Rich, Ally, Nancy, to keep us on the straight and narrow, to have Beth out there promoting everything we do.

You’re doing awesome things, and we just wanna thank some of you by name, and this week we’re gonna thank Zero Chill, William E Kraus, Frank Stuart, Daniel Loosli, Jim Schooler, Benjamin Carryer, John Drake, Ganesh Swaminathan, Bob Zatzke, Thomas Sepstrup, Burry Gowen, Jordan Young, Stephen Veit, Kevin Lyle, Jeanette Wink, Mountain Goat, Gerhard Schwarzer, Andrew Poelstra, Brian Cagle, Venkatesh Chary, Aurora Lipper, David Truog, David, TheGiantNothing – I love that username – Will Hamilton, Ronald McCoy, Jean-François Rajotte, cacoseraph, William E Kraus, Bill Hamilton, Robert Palsma, Laura Kittleson, Joshua Pierson, Les Howard, Jack Mudge, Joe Hollstein, Gordon Dewis, Helge Bjørkhaug, Frank Tippin, Alexis, Adam Annis-Brown, Neuterdude, William Baker, WandererM101, William Andrews, Andy Cowley, Jeff Collins, Harald Bardenhagen, David Gates, Scott Bieber, Matthew Horstman, and Nicky Lynch.

And as always, if you would like me to pronounce your name correctly, please just spell it phonetically in Patreon. Stephen, I saw that you changed your username to ‘Stephen Veit it rhymes with right’ and that brought me so much joy. So much joy.

Fraser Cain:                And you know what? I forgot to mention – so go to patreon.com/universetoday. All right, thanks everyone. We’ll see you next week.

Dr. Pamela Gay:         Thank you.

Female Speaker:         Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a creative comment’s 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. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep this show going, please consider joining our community at patreon.com/astronomycast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.

Back to Top

Follow along and learn more: