 Like to introduce Dr. Larry Knitler, he's a Cosmochemist in the Department of Trustual Magnetism of the Carnegie Institution of Washington and the Deputy Principal Investigator on NASA's Messenger Mission to Mercury. He obtained a PhD in physics from Washington University in St. Louis in 1996. His analysis of measurements from the near-earth asteroid rendezvous mission helped provide the first chemical analysis of a minor planet, and he's led the analysis of X-ray fluorescence data from Messenger. In addition to remote-sensing geochemical measurements, his research focuses on the laboratory study of extraterrestrial materials, including meteorites and interplanetary dust particles, as well as samples returned by NASA's Genesis and Stardust missions to understand the formation of the solar system, the galaxy, and the universe. He received the Alfred O'Neill Prize of the Meteor ITICAL Society in 2001, and it was the name of the fellow of the same society in 2010. Asteroid 5992 Nettler is named in his honor. So I'd like to welcome, and I need to shift gears just a little bit, Dr. DeLarie Nettler. So I need to unmute you first. Okay, can you hear me? Yeah, I need to, can you unmute your video? I did it on this end, and... It says I cannot start my video because the host has stopped it. Ah, here we go. Okay, I think that might do it. Okay, I think that's gonna work. That did it. There we go. Great. Hi, thanks. Thanks a lot, Bruce. I mean, Brian, sorry. Now I've gotta find the share screen, share screen, and then actually start the video. Oh, sorry. There we are. Okay, I think you can see me and you can probably see my slide, so I will just start talking. Well, I thank Brian for inviting me to give this webinar. It's always fun to talk about one of my favorite planets and the exciting work we've done with the Messenger mission at Mercury. Mercury, of course, is one of the five planets that one can see with the naked eye, unless it's probably been known to humans for as long as there have been humans. But in fact, most people I know have never seen Mercury because it's hard to see. It's close to the sun, so it's only ever slightly above the horizon. It's not that bright or it can be very bright because it's close to the sun. But anyway, it's hard to see in many, many people. Probably more people on this webinar have seen it than typical people I talk to. But in any case, it's a planet that's certainly known to the ancients but not very well understood even to the present date, though we've certainly made a lot of progress on that and that's what this talk's gonna be about. But it was known to the ancients and there's some record of scientific investigation of Mercury going back to ancient times. Greek astronomer Eudoxus accurately measured its synodic period, a few hundred years BC. This clay tablet in the upper right is supposedly, I don't actually read cuneiform, but I'm told that it describes an observation of Mercury. But of course, once people started building telescopes in the 1600s, people like Galileo and Harriet started observing Mercury because they observed everything they could with their telescopes and started describing Mercury in more detail. In 1639, it was discovered to have phases, which is an important step in proving the Copernican theory that planets orbit the sun. And of course, by the late 1800s, telescopes started getting better and astronomers started looking at all the planets and making maps. And of course, a lot of this was more fanciful than was actually supported by the data at the time. For example, Percival Loa looked and he happened to find a bunch of canals on Mercury just like he saw on Mars. And I will give a spoiler alert that we have not confirmed the presence of canals on the surface of Mercury. But really, there wasn't that much you can do. It's difficult to study Mercury. And the reason is, of course, that it's close to the sun. So it's very hard to observe by telescope. The time when you can is limited, when it's above the horizon and amenable to study. Many telescopes can't be pointed that close to the sun. For example, the Hubble Space Telescope cannot point that close to the sun. So it's never observed to Mercury. And it's also very difficult to study by spacecraft. And this is because, although it's easy to get there and a spacecraft called Mariner 10 visited in 1974 and 1975, it's very difficult to get there and be going slow enough to be captured into orbit because you're falling into this big gravity well of the sun. And it's very hard to slow down. So before Messenger started exploring Mercury, the last time as spacecraft visited was in the mid-1970s and there were just three quick flybys of the planet then. And much of what we know about Mercury came from those flybys of Mariner 10. And among the very important observations of Mariner 10, well, first of all, it flew by the same side of the planet, all three flybys. So we only got high resolution imaging of half the planet. I mentioned about 45% of the surface. Discovered it looked an awful lot like the moon, dark gray covered with impact craters. But discovered it was very much not like the moon in many respects. Discovered that it has an intrinsic magnetic field which was a big surprise and therefore has a very dynamic magnetosphere due to the interaction of its field with the solar field and the solar wind. Also detected a very, very thin exosphere, a very thin atmosphere of hydrogen, helium, and oxygen. But of course, astronomy did a ground-based astronomy also advanced in the 20th century and some very important ground-based results were made. Starting in the 60s, for example, radio astronomy discovered that Mercury were used to characterize Mercury's orbit and discovered it's in a very interesting three-to-spin orbit resonance. So it has three of its own days, three rotations around its axis every two times it goes around the sun. So it's about 60 days for a day and about 90 days for a Mercury year. And ground-based astronomy starting in the 80s discovered more species in the exosphere of Mercury. For example, this plot on the side is a color-coded map of sodium emission from the early 2000s, showing this very weak exosphere of sodium atoms and also calcium in Mercury's exosphere. And another very important discovery I'll come to at the end was in the early 1990s, radar, radio astronomers pointed telescopes at polar craters, at impact craters at the north and south pole of Mercury and discovered that many of them have material in them that had reflective, radar-reflective properties essentially identical to water ice and postulated that these are deep impact craters that are in permanent shadow and therefore can have water ice stable at the surface or near surface over geological time scales and that's what we're looking at. So as I'll get to, that was one of the prime science goals of Messenger was to determine whether indeed there was ice in these polar craters. So before I get to Messenger, what we basically knew about Mercury before Messenger and why we're interested in Mercury is that it's really a planet of extremes. It's the closest planet to the sun. It has the highest diurnal variation in temperature. It's the smallest planet. It's the densest planet and not very, very high density which was also determined from, mostly from Mariner 10 observations, indicated that it has a very high metal to silicate ratio compared to the other terrestrial planets. So the core on Mercury makes up a much larger fraction of the planet than the core of the Earth, for example. And this is shown in the cartoon on the bottom left. And it has this very interesting magnetic field. So it's viewed as an end member of planet formation. We think the planets in our solar system formed by a common process starting from a protoplanetary disk and it made a whole range of planets and Mercury's at one end of this. So understanding Mercury's properties is very important for understanding the range of things that happen in a protoplanetary disk and better understanding the origin of planets. Moreover, we now of course know a lot more about planets around other stars than we did when this mission was starting to be discussed. And what we of course know, both from radio doppler surveys of exoplanets and more recently from the Kepler mission is that there are lots and lots of planets around lots and lots of other stars. And what we also know is that many, many of those planets are much closer to their host stars than the planets in our solar system. And this is a plot, it's about a year old. I didn't update it for this talk, but it shows the known exoplanets as of about a year ago in terms of semi-major access and orbital eccentricity. And what we can see is that Mercury is in the field of extra solar planets. And in fact, there have been at least one extra solar planet for whom the density is so high it's been suggested it might be an analog of Mercury and that's an artist's rendition of such a planet on the bottom, right? But the point is that understanding how Mercury forms is not important just for understanding our solar system but for understanding planet formation in general. So that brings me to our little mission messenger which is the first spacecraft to actually orbit Mercury. This is the seventh mission in NASA's line of discovery missions which are PI driven missions of the exploration. They were started as faster, better, cheaper mission line. And as I said, they are proposed and competed and led by a principal investigator. The principal investigator I messaged her is Sean Solomon who was formerly the director of my department at the Carnegie Institution of Washington and he's now director of the Lamont Earth Observatory at Columbia University. But he's led this mission from the very beginning and very ably. And of course, flying a mission to Mercury is non-trivial if one would say. There are a number of significant technical challenges to flying a mission to Mercury. And the first is the thermal environment. It's much, much hotter. There's this whole R squared law with radiation and you get a factor of two closer to the sun. You have, and it's actually more than a factor of two. We have something like 10 or 11 times hitting Mercury that you have at the Earth. So we had to protect the spacecraft and this is done by putting the whole spacecraft behind a big sun shade that is made out of this interesting ceramic cloth. And it basically, you can shine the heat of 11 times the solar radiance at the Earth onto this thing on the one side and the spacecraft stays at room temperature on the other side. So this worked very, very well, but it required of course extremely well written autonomy software on the spacecraft to ensure that the shield was always between the spacecraft and the sun. And also, someone is on, making noise on the call, sorry. In any case, we also helped with the thermal environment by making the solar panels, or actually two thirds of them are mirrors to reflect the sunlight and only a third of the area is actually generating electricity or was. Another challenge is what I mentioned earlier which was the trajectory. It's very difficult to get into orbit around Mercury without flying by it because you're following into the big gravity well of the sun. And in the early 80s, some very smart people discovered trajectories that would work to get a spacecraft into orbit around Mercury and it requires sneaking up on it. And this is a very complicated plot that shows the whole history of the cruise. But essentially we launched in August of 2004 and then it took six and a half years to get to Mercury because we snuck up on it doing multiple orbits around the sun and multiple planetary flybys. We flew by the Earth once, we flew by Venus twice and we flew by Mercury three times before we went into orbit. So already before we went into orbit, we already doubled the amount of time a spacecraft had spent at Mercury after the Mariner 10 measurements. The point is that with all these flybys and stuff in March of 2011, we were able to sneak up on the planet at close enough velocity to the planet that we could fire our engine for some period of time to put on the brakes and be captured by the gravity field of Mercury and get into a stable orbit or stable-ish orbit, which I will get to. The mission was sold on the basis of a number of science questions. This is how missions are sold to NASA. You can't just say I wanna go there because it's cool, you have to say I wanna solve. Answer some well-defined science questions. And this is the list, I don't wanna get into them, but you will see as I go along that many of these, we have answered or at least provided new constraints on all of these. And they're things like, what is the geological history? Why is Mercury dense? What is this stuff at the poles? So it's a lot of ice and so on. And of course, to answer those questions, you need to make measurements. And so we also came up with a list of measurement objectives and use those mapped onto the science questions to determine a suite of instruments. And the spacecraft had a remarkable number of really good instruments, especially given the cost, that this was a faster, better, cheaper mission and cost-capped at not that high a cost and had to be built in fairly short period of time. The mission was selected in 1999 and had to fly in 2004. But they had a whole series of instruments. As you see here, laser altimeter for measuring topography, various instruments for chemistry, which I'll talk about, a couple of different cameras and mirror infrared all the way to UV spectrographs and a number of instruments for measuring the plasma environment around the planet doing space physics, as well as a magnetometer to really map out the planet's field. And I'll talk about results for most of these instruments. So for scale, this picture on the top left is the spacecraft during its vibration test in December of 2003. And this, those are human beings for scale. So you can see it's about the size of a mini Cooper, basically, and I'll fold it up here. Beautiful night launch in August of 2004, spectacular. And as I said, we did some planetary fly-by, so that's a picture of Earth and a picture of Venus taken by the main camera on Messenger. And we flew by Mercury three times. So if you remember, I said Mariner 10 imaged about 45% of the surface of Mercury. With our three fly-bys, just our three fly-bys, before we even got into orbit, we imaged another 45% or more of the surface. So even before we got into orbit, we then had at least an idea at some resolution of what almost the entire planet's surface looked like. And these are pictures taken on the three different fly-bys. And one other very interesting and important thing I want to mention is during our cruise, we also became the first spacecraft to use solar sailing as part of our mission plan and a very clever engineer, our main system engineer, Dan O'Shaughnessy at the Flight Physics Lab, realized early on that we were close to the Sun and there's a fair amount of radiation pressure. It's an old science fiction idea. And did the calculations and figured out that if we kept the attitude of the solar panels just right, the pressure from the Sun gave us a little extra boost and we were able to conserve fuel this way by taking advantage of this radiation pressure. And that gave us fuel, extra fuel for later on which turned out to be a really good thing. So on March 18th, 2011, just over five years ago, we entered orbit successfully around Mercury. And this shows our orbit, the basic orbits during the first year of our mission. And this is another very, very important point and this gets back to our technical challenges. The thermal environment at Mercury is very, very harsh. And as I said, we protect the spacecraft from the Sun by putting it behind the sunshade. But that doesn't protect the instruments in the spacecraft itself from the heat from the planet. Of course, we wanna look at the planet so we can't put a shade between the instruments in the planet. So instead we designed a thermal design that put us into a polar, highly, highly elliptical orbit. So we basically zoom in over the North Pole, very, very close through most of the mission between 200 and 500 kilometers minimum altitude over the North Pole. And then we would fly out to 13 or 12 or 15,000 kilometers and cool off. So the first year of the mission, we did this twice each day. And after that we did it three times each Earth Day. So it was first a 12 hour orbit and then an eight hour orbit. But the point is that means that we spent very little time close to the planet relatively speaking in a lot of time, far from the planet. But we flew by over and over and over again, orbit it. So we were able to collect a huge amount of data. And in fact, by the time we impacted into the planet just about a year ago, we completed 4,104 orbits around the planet. And because of this unusual geometry, it could be like over 4,000 flybys, but it's really orbits. And of course, the distance parts of our orbits allowed us to really track the magnetic field and the exosphere and the plasma environment in great detail that we couldn't do if we weren't only staying near the planet. So this also allowed us to scientifically investigate the mission, the planet in all of its manifestations. So as I mentioned, after the third flyby, we had reached 90% coverage in imaging. It actually took almost two years of orbit to finally get that last 10% and get to the final where we have images over 100% of the surface of the planet. And this is a beautiful picture. Just looking over the limit of Mercury. But this now is a global map of Mercury. So you can see what it looks like. These of course are not the real colors of Mercury. The previous picture was a very close approximation to true color. What this image is, is taking color data from multiple filters, multiple wavelengths and running a mathematical principle component analysis on this to dig out subtle differences in color and albedo and assign them to red, green and blue channels. And it makes us what we call an enhanced color image. And what we can see is that Mercury is quite variable on the surface in terms of its brightness and color. And we're exaggerating it here, but these differences between the dark blue material and the bright orange material are very, very important, very significant. And we know from other measurements now something about what they mean. Anyways, just a gorgeous movie. I wanted to show this before I get to the scientific results in detail. And this shows the same exact area in the same orientation of Mercury, with one picture taken by the Mariner 10 camera and one taken by the Messenger camera early in the mission. And this shows how important it is to not rely on flybys. Why orbital missions, institute exploration missions are very, very important because illumination matters for all, essentially all, well geometry matters for all observations and illumination for any observations involving light. It's very hard to believe unless you look really carefully that this is the same area and clearly it depends on the lighting conditions when you see as well. So the fact that we've orbited for four years and we're able to observe the same areas from with multiple lighting conditions, multiple angles means that we really have a very good idea of what the surface of Mercury looks like and its topography. So the number one science question for the mission was what is the, basically what is origin of Mercury? Why is it so dense? Why does it have this very large iron core? And to answer that question, we have to think about how the planets form and we do believe we have a picture of how planets form in our solar system and around other stars or at least a basic agreed upon framework which is that the planets share a common formation process of accretion. That you start with a protoplanetary disk of gas, high temperature that condenses out dust, the dust particles collide and form larger dust, they eventually form rocks, they eventually form planetesimals and eventually form planets. There are a lot of steps left out of here and a lot of steps we really do not understand but we do have plenty of evidence from the meteorite record and from other planetary data sets that this is what happens as well as astronomical observations of planet formation occurring in other disks, especially in recent years with new telescopes like ALMA. So why does Mercury, if it formed by the same processes that Venus and Earth and Mars did, why does it have such a large core? And before messenger, a number of models were put forward starting in the 70s when the large core was discovered and these include the idea it created a very high temperature that perhaps it was bathed in a very, very hot sun so it started out bigger with a larger rocky portion relative to the core and most of the rocky material evaporated by the sun, a related model is the idea that it was bigger to start with and something very large hit it and knocked off a good deal of the mantle and crust. Now analogous to how the moon is believed to have formed in our own system, only in this case it would have removed a lot of material. And there's also physical models that suggest that in this disk phase that there could be processes that separate metal from silicate and concentrate the metal in the inner part of the disk where Mercury formed. So these were ideas, but there was essentially no data to constrain them for decades after they were proposed until messenger came along. And what was recognized was one thing that was key was to be able to measure the composition of Mercury. And the reason for this is that the surface composition of the planet depends both on the whole geological history but also on the starting materials and these various models for its formation it very, very distinct predictions in many cases for the chemical composition of Mercury. So if we could determine the surface composition we can say something about the bulk composition and from that constrain these models. And in this regard messenger was designed with three separate instruments for measuring the surface composition and that use x-rays and gamma rays and neutrons to infer directly or indirectly what the surface is made out of. And I was focused mainly on the x-ray spectrometer and I'll get to that data in a second. But one of the most important very, very early results right after we got into orbit was the discovery that Mercury had a high abundance of potassium dothorium that has a fair amount of potassium on the surface. And this is a plot from several years ago from after we got into orbit showing the potassium dothorium ratio of the four terrestrial planets in the moon. And what's clear is that the moon is highly depleted in potassium. And potassium is a volatile element, moderately volatile element. It likes prefers to be in the gas-faced rocks at high temperatures. And the volatile depletion in the moon is believed to reflect thermal processing during this giant impact that formed the moon. And what we immediately see is that Mercury has the very similar potassium abundance compared to the other planets. And I should mention it's normalized dothorium here because potassium and thorium behave very similarly in geological processing. So when part of the mantle melts and forms a volcanic flow at the surface, the potassium-dothorium ratio of the mantle is preserved at the surface. So we think we're looking at the bulk ratio of this. Dothorium is very, very non-volatile of potassium-dothorium. So what we found early on is that Mercury is not volatile-depleted. And it's quite similar to the Earth and Mars in its potassium content. And we've further made measurements with the gamma rays to measure sodium and chlorine and discovered these other volatile elements also are in high abundance on the surface of Mercury. So Mercury is clearly not depleted in volatile elements which immediately rules out some of the pre-messenger models that suggested this thing formed at very, very high temperatures. We also used mainly the X-ray spectrometer but also the gamma-ray spectrometer for some measurements to measure the major elements, the major rock-forming elements on the surface of Mercury. And of particular importance, our magnesium, aluminum and calcium these are very diagnostic of different types of rocks and geological processes on the planets. And these are the results we've reported from the first few months of the orbit. And already we could see pretty much the full range of compositions on the surface of Mercury which are these red points. And what we can see here is on this plot of magnesium silicon versus aluminum to silicon, the moon is in green symbols, the earth is in yellow symbols. And most of the earth's surface are these oceanic and conchmill crust basaltic type rocks and granites and things that are down here. The moon has mori basalts that are here close to the earth and the lunar highlands that are very, very rich in aluminum compared to the earth. And Mercury doesn't look like those things. It looks higher in magnesium and lower in aluminum than most rocks on the earth and the moon. In fact, it looks very similar to some rare old rocks called Kamadiites on the surface of the earth. And similarly it's distinct in calcium. And so we already can tell Mercury is quite different in terms of its major element composition compared to the other terrestrial planets. Well, we orbited the planet for four whole years so we got a huge amount of data and we're able to turn these kind of plots into maps with all those data. Oh, I should mention that the moon, that's high aluminum is interpreted and has been since the Apollo days as indicating when the moon formed early on it was fully emulated and aluminum rich rocks made of largely of felt spar minerals were lighter than the magma itself in this magma ocean and floated to the surface and formed this crust. And that's why these rocks have aluminum. We see nothing like that on Mercury but I'll get back today I do both flotation crust in a couple of minutes. But like I said, we were also able to take the data and turn it into maps. And these are now spinning maps of magnesium silicon aluminum silicon based on four years of X-ray data. And you can see very interesting variations in some cases these correlate with the geology in some cases they don't we don't fully understand all these variations but there's a lot of people using these data now to model the geology and the petrologic processes that could explain these based on plausible compositions in the planet's mantle. Another very important discovery we made early on that they have fleshed out with the maps is that Mercury has a remarkably high amount of sulfur on its surface and a very low amount of iron despite having such a large amount of iron in its bulk composition. The sulfur is on the order of a couple of weight percent on average of the surface. And that's compared to on in the Earth's crust it's only a few hundred parts per million. So it's much, much higher on the surface of Mercury. And similarly, the iron is much, much lower on order of one to two weight percent on average much lower than on the Earth. These chemical properties tell us that Mercury formed with less oxygen relative to the overall mix of elements it's formed under less oxidizing more reduced chemical compositions than the other plants. So this is coupled from the idea of why it has a big core or at least we think it is, it may not be but it doesn't have to be related to the large core and what it tells us is that Mercury seems to have formed from different materials than the other plants. And finally, I want to point out highlight a very, very recent result. We just published this in the last few weeks in Nature Geoscience, a new discovery based on data acquired very late in the mission. And this was the discovery that Mercury also seems to have a lot of carbon on its surface. And the basic idea is that there, if you remember the globe I showed before had all this dark blue material. And it doesn't look blue to the eye but it's bluer than the surrounding stuff but it's the darkest material on Mercury. Mercury overall is very dark. It's darker than the moon, for example, even though it looks similar colored, it's darker. And what we've seen from very careful work over all these years is that the darkest materials are always associated with impact craters and are clearly material that's been excavated from several kilometers deep in the crust. Now at the very end of the mission we flew at very low altitudes and I'll show that in a little while. And during these low altitudes we were able to fly over some of these dark materials and measure the neutron signal coming from it. And it turns out the number of neutrons streaming out of the surface of the planet is sensitive to carbon. And it's hard to tell on this graph but these red points are just when we're right over this blue stuff. And essentially there's a little bit larger neutron signal over that than there is elsewhere. And of course we added up hundreds of these measurements and got a very significant signal. And the signal we found is that this is almost certainly a few weight percent carbon here and probably a weight percent or so carbon across the surface. It's probably graphite and we can base this on comparing the spectra of the planet with laboratory measurements. And this is postulated to be maybe instead of having an aluminum rich crust from an agma ocean phase like we see on the moon, perhaps Mercury formed a little lot of carbon in it. When it was an agma ocean it formed a graphite crust that floated to the surface. And it's subsequently been buried and mixed with other volcanic rocks but we're seeing remnants of it. So this is a very intriguing idea. Very different from what we've seen on any other planets. And this is gonna be a theme of the rest of what I say scientifically is what we find is Mercury is a very strange place and we find things there that are different than the other planets and we do not fully understand this place yet. But we have a huge and very rich data set to try to do so. So to summarize the chemistry, essentially Mercury's volatile rich chemically reduced this rules out some of the models but not all of the models for example, we don't know it could still be possible to strip a large portion of the rocky mantle by a giant impact because the models just aren't good enough to have made precise geochemical predictions. So we're waiting on the modelers to do that but they're working on it and there's of course renewed interest in doing this kind of modeling now that we have data that can strain things and hopefully gain understanding. But what we definitely see is that Mercury is made of a different mix of materials than the other terrestrial planets and this has to be taken into account in new theories that try to explain how the planets formed. So I wanna talk about now getting away from chemistry which was my main as well as mentioned talk about some of the other really interesting science results we got from this mission. And one of them was the discovery that the surface of Mercury seems to have been formed almost entirely by volcanism. That there's widespread volcanism occur very early in the planet's history. This was controversial based on the Mariner 10 data and we've proven it with beyond a doubt. In fact, this pink area here shows that there's a huge area of volcanic planes at the North Pole, Northern latitudes of Mercury that make up something like 6% of the surface area of Mercury. There's one giant expanse of flooded volcanic lavas. On the top right here, you can see this is a caldera remaining eventus and volcanic material. On this bottom right, you can see very clear morphological evidence for flowing lava for cutting channels and flooding basins and so on. So we see a lot of very strong evidence of volcanism. We also made a very interesting discovery. We discovered a new landform we haven't seen anywhere else in the solar system. When we started looking in many impact craters, we saw that many of them had these very bright materials and when we look closely, we see that there are these rimless depressions. They're just kind of pits at the bottom of these craters that don't look like craters. They don't look like anything else we've seen on rocky planets. They look like things seen on the polar ice caps of Mars where you have material evaporating away and we think that's what's happening here is that there's a volatile component at the surface that is wasting away into space and the stuff is slumping down. So this is very interesting. We're still trying to understand them better but it's one of these unexpected discoveries that we love as scientists, of course. Tectonically, Mercury is a very interesting place. More than 10 observations found that Mercury was covered with these things. The geologists call lobate scarps that are to a physicist like me, I would just call them very large cliffs and these can be like a mile high cliff and a thousand kilometers long, long linear features. And these are the results of the planet contracting. It's been cooling for four and a half billion years and as it has, the rocks have been shrinking, the planet's been shrinking and it causes giant faults in the crust and big blocks will fall relative to others and you have these very large cliffs and this is a lovely false color representation of one of these cross-cutting impact crater. This was a paper published two years ago by Paul Vernon coworkers who did a detailed analysis of thousands of these features on Mercury and were able to determine that Mercury has shrunk much more than had been previously thought and in much better agreement with what thermal models predicted. So this was a good result. People studied the geophysics of Mercury in great detail and are continuing to do so. For example, we can use radioscience which is really monitoring using the radio signal that we talk to the spacecraft with. Doing science with this by looking at very subtle Doppler shifts of the signal itself to map out the subtle pull of gravity on the spacecraft as it goes around the planet using that to measure the gravitational field of the planet combined with measuring topography with the laser altimeter where you shoot a laser pulse at the surface, measure how long it takes to get it reflected back and from that you can tell very precisely the distance. And you can do this to determine gravity maps and we see that there are indeed, this is an old one we have down here on the bottom right. We have updated once, but the basic features are there. We see a range, there's not a sphere. There are variations in the gravitational field. There's concentrations of mass here and there. And these can be used, the gravity field plus the topography can be used to constrain the interior structure of the planet and this has been done in a lot of work by Steve Hauck as a geophysicist at Case Western Reserve at University has done basically solved millions of internal structure models to find which ones were most consistent with the gravity data. And they found early on a very interesting structure that there's a very large solid and liquid iron core and very thin crust and mantle and in between there may be a solid layer of iron sulfide. This has not been suggested on other planets. We don't actually know that it's there, it's not required by the data, but it could be there from the data. So Mercury looks different than the other planets and this is making people rethink how planets evolve on the inside. This has a very different geological history than the Earth by necessity for being such a different geometry. So it's a very interesting place geophysically. Another thing, as I said, we wanted to map out the magnetic field and this turned out to be extremely interesting. This is one of my very favorite science results of the whole mission. Determine that the magnetic field of Mercury is a dipole like that of the Earth, like a giant bar magnet coming out of the North or out of the South going into the North, except that very, very precise mapping of the magnetic field found that it is in fact offset from the planets, from the planet, that the magnetic equator is about 500 kilometers North of the planetary equator. This is very, very strange. We think the field is being formed by a dynamo effect from the liquid outer core, from movement of molten iron leading to electrical currents that lead to the magnetic field. But it's not clear what could lead to an asymmetry like this. And there's some models and ideas about with different heat flux going across different parts of the core, but it's fully not understood and it's very, very strange. So it's one of these who ordered that and it's a very exciting, interesting result. A very, another result that when we started flying closer to the planet towards the end, we got closer and closer. And one of the things we were able to determine is that not only do we have this planetary field from the core, there's also remnant magnetism in the crust. When you fly over certain parts of the crust, you get a very small magnetic field signal from magnetized rocks near the surface. And I don't wanna explain this whole graph, but the point is that we only see this when we're within a few tens of kilometers of the surface. But this is happening in crust that we know from counting craters and calibration to the moon and so on and models, we know that this crust where these magnetic anomalies are was solidified something like four billion years ago, maybe 3.7, but not one billion years ago, which tells us that Mercury had a magnetic field even four billion years ago. We don't know if it was the same kind of magnetic field it has now, but this gives us a window into the past history of the planet. I definitely don't wanna talk about this except to say that it's a really, really cool cartoon. And the point is that because Mercury has this weak magnetic field and is so close to the sun, there's extremely interesting interactions of the solar magnetic field and the solar wind, which is a stream of charged particles coming out of the sun. The interactions of these fields leads to extremely complex and dynamic electromagnetic effects around Mercury. And we called the magnetosphere and you have giant magnetic storms happening on time scales of seconds. You have huge bursts of energetic electrons and all kinds of things analogous to what's going on on the earth when you have solar storms or high in the atmosphere or aurora borealis, but happening in very different ways. This is kind of a playground for space plasma physicists and they've only begun to scratch the surface of what's going on there, but we have a very rich data set. The exosphere, as I said, we were able to characterize that in considerable detail through the mission. And what we found is that sodium, calcium and magnesium are the most abundant elements in the exosphere. They're still very, very low abundance. They're coming from the surface due to some combination of being knocked off by micrometeorite impacts or solar winds knocking atoms off the surface. But we also see that they're put into this planet's exosphere by different processes that we see that sodium is pretty much just always there whereas calcium is always more abundant over the dawn terminator where the sun is coming up over the horizon. But both of them actually vary in how much is there by the season. And these are not well understood effects yet, but there's a lot of modeling going on. And again, now we have a very rich data set so the modeling, accurate modeling can now commence because there are data that can strain the models. And I finally want to close with information about these interesting polar deposits. So I mentioned earlier that the ground-based radar found evidence for these deposits. And this is a view of the North Pole of Mercury and the yellow shows where these deposits were inferred to be from ground-based radar astronomy. And we had a number of approaches to try and figure out if these were indeed water ice. And first we had to image the regions and make sure that the hypothesis that these, where these materials were in areas of permanent shadow where they never see the sun. As you mentioned, the tilt of Mercury's axis is almost non-existent. It's pretty much perfectly perpendicular to the ecliptic. So these deep hole craters near the poles really never do see sunlight. And we confirmed this both with topography and just imaging from different angles. We were also, because we could get such a good shape model and understand the elimination conditions to very, very accurate thermal modeling and determine that the temperatures where these regions are is where you have stability of water ice and in some cases hydrocarbons. And in fact, we were also able to use the laser altimeter to tell that some of these deposits are bright and some of them are dark. So to cut into the punchline, we think that some of them are pure water ice and some of them are pure water ice covered with organic material. And the real proof that these are, or the strongest evidence we have that this really is water ice came from neutrons. I mentioned neutrons earlier being used to find carbon. Earlier, we've used them to map hydrogen and basically because neutrons are coming out of the planet that they can be absorbed by materials at the surface because hydrogen's about the same mass as a neutron and hydrogen absorbs neutrons very, very well. And what we see here is the neutron signal in red as a function of latitude as we go from the equator or from 30 degrees north over to the close to the North Pole, we see that the number of neutrons coming out of the planet decreases. And the blue is a model of what you would predict the signal to be based purely on those deposits seen by the radar being made of water ice. And the match is very, very good. And this is in one energy of neutrons when we can measure that in a different energy regime too, and which also has good agreement, but some differences allow us to get some information that's the stratigraphy that shows us that indeed the water ice is buried in most cases and we think in most cases by some sort of organic material. So this tells us that, well, first of all, in one of the hottest places in the solar system, Mercury, there are regions that are so cold that there is water ice and organic matter. So we think there's water ice and organics probably everywhere in the solar system from Mercury to Pluto and to the Kuiper Belt and beyond. We know the Oort Cloud Comets have water and organics too. So the entire solar system has these materials. We think on Mercury, it's probably a fairly recent impact of one or more comets has deposited this stuff and the molecules have found their way across the planet to the coldest spots and formed these ice deposits. And in fact, we can even do more. We can look at, we can take images, even though these things are in permanent darkness, which means we shouldn't even be able to see them, there's enough light scattered off of other parts of these craters onto them that if we stare with the camera really, really, really long, we can see, we can actually get images of what they look like. And this was worked by Nancy Chabot and coworkers of these. And I don't want to get into detail except to say that from the morphologies of these things, we can tell they're fairly young. So we think that comet impact hasn't been recently or more than one in fact. And I'm talking tens of thousands of years, not millions and millions of years. So I want to briefly mention, I said at the end of the mission, we got lower and lower in altitude. And this was the cause caused by gravity mostly that we, as we started to run out of fuel, we can no longer fire our engines periodically and the combined gravity of the sun and mercury gradually pulled us down. So this yellow, I mean, this green curve shows the very, very end of our mission, about a year ago, the last month and a half of our orbits. And it shows our minimum altitude in our orbit. And these vertical lines are when we fired our engines to get a little bit more time. So we're about to crash. We fire our engine to go up a little bit. So from 10 to 30 kilometers. But the important reason, the reason I wanted to show this one is you can see we had some time at the end with very, very low altitude, which we got some very high resolution data. But also very interesting, we actually ran out of fuel before this engine burn. But again, Dan O'Shaughnessy, our engineer who had figured out solar sailing, also figured out that we had some helium in the system, just ask us helium that we were using to push fuel around the system. And you blow it out the end of the spacecraft, it gives a thrust, just like, you know, letting go of a balloon after you blow it up and flies away as the air comes out. So we, this is the mission also is the first use of helium as a spacecraft propellant. And it allowed us to prolong the mission by a few weeks, which was really, really great. So this is the last image I just wanted to show. This is on the left is the first image we took in orbit after our orbit insertion and returned to Earth. And this is the, on the right is our very last image that was returned to Earth, taken on the next to last orbit. And just spanning, we've got thousands and thousands, hundreds of thousands of images in between. So I want to acknowledge this, of course, is the work of a huge team of very, very talented scientists and engineers and managers and mission operations people and it couldn't have been done without them as a fabulous team to work with. And as I hope I've shown you, it was a very successful mission and we've done a lot of interesting science. And that's just what I'll stop with here with my final word that messenger was successful. And even though it's small and gray and moon looking, it's a very, very weird and wonderful place. It's very different in a lot of ways from other planets and may, should provide valuable information both for understanding how planets form in our solar system and around other stars. And I will stop and take questions now, thank you. Okay, so can you hear me okay? I can. Okay, cool. We had a little mix up with our software. So now I'm the host somehow and I can see the Q&A window. And so I'll be giving questions together. I'm Dave, by the way. A lot of folks know me already, Dave Prosper. So we had just a couple of questions earlier. So I think you might have answered a lot of this already just to clarify about Mercury and the ice at the craters. So the ice at the craters is not there for geologically a very long time. It's there for just a few thousand years. Yeah, or maybe tens of thousands of years. Okay. And it's not actually like sheets of ice or glaciers just kind of like mixed in. Well, it might be like a small glacier. It's not a moving glacier, but it's probably a large expanse of solid water ice, but with maybe a tens of centimeter thick layer of carbonaceous material on the surface. Or maybe not. Some of them probably are pure water ice. And the radar people tell us it's actually very pure water ice, but I don't really know what that means in terms of very pure. But that's a good point. Cool. And it's supported there basically because it's just in the shadows. And there's not really a lot of heat. Yeah, it's stable there. So the water-moved molecules found their way there and then stuck to there and often has happened to them since. Awesome. And another thing a couple of people were talking about is a couple of orbital questions about mercury in general. One of them was just like traditionally for a long time there was, it was thought that mercury was in a complete tidal lock. And now it's like, it's like at that three two sort of spin going on. Yeah. So it's been known since the mid 1960s that it's in a three two spin orbit resonance. And I was born in 1969 and I learned as a kid in the 70s that it was tidally locked. And I'm called to find out that people still hear this. But there's a very interesting historical reason why people thought this. And that was because of essentially because of its spin orbit resonance and because of the vagaries of its orbit relative to the earths, when astronomers in Europe could see it when it was visible to Europe which is where all the astronomers were in the 17, 16, 17, 1800s they always saw the same side. And it was just, it wasn't that the planet wasn't rotating it was just that the only times that the astronomers could look at it it was showing the same side. So that was why it was believed to be tidally locked. It took modern radio astronomy where you could really track the movement to tell. But that was already in the 1960s. Okay. We had a couple of things about, what was it? So Jeff and Jeff, Hey Roland and John Berry had related questions. They're talking about why is, any ideas about why is Mercury's orbit so erratic? In a sense, is there like an erratic, like a kind of a craziness to the orbit or is it all just, is it just? To Mercury's orbit is very, very stable. It's resonance, it's more eccentric. Okay. Of the other planets. And I don't really know the, I mean I don't think anyone really knows the origin of Mercury's spin orbit resonance or why it has the orbit it has. Okay. But it's very stable. Long term and stable. Yes. Okay. Cool. And so for Jim Smalls, wondering for the upcoming transit on Mercury of the sun on May 9th, is there anything that people can look for in particular during the trans and anything interesting? Potentially. I don't know, but I don't think so, except that it's very interesting to watch a planet go across the face of the sun. But no, you won't be able to see anything, you know, other than a disk go across the sun. Yeah. Very small disk. Yeah. They like, there's no moons. There's no, there's no atmosphere to speak of just this weekend. So severe. Messenger itself is now a puddle of molten metal on the surface. So. Okay. So as far as you know, there's no any research going on with the transit other than maybe possibly solar observatories taking a peak. Yeah, I don't know. I know there's gonna, I know there's a lot of interest in watching it because it's an actual occurrence, but I don't know of any scientific applications. There might be. I just don't know. And we had an anonymous question asking about what is reduction versus oxidation in regards to planetary chemistry? Well, oxidation or reduction in chemistry is technically the movement of electrons, but the simplest way to think about it in terms of planetary chemistry is the relative amount of oxygen relative to the other elements in the system. And if you have, if you think about it in terms of iron, if you have a lot of oxygen, oxygen sucks up a lot of the iron and you have, you can have both metal. If you have a lot of oxygen, if it's really, really oxidizing, all the iron becomes oxidized and it'll be like rust or the equivalent in a silicate. If it's very, very reduced, like on mercury, you have very, essentially all of the iron isn't in the form of metal. And it's because there's less oxygen to grab on to the iron. And so we see it in the iron, but it impacts how all the elements behave in rocks and it's a very complicated geochemical thing. But that's essentially, we think what it means for mercury is that mercury, for whatever reasons, the building blocks of mercury have less oxygen than the other terrestrial planets. And this could have to do with ice because ice has a lot of oxygen and it could be that close to the sun, there was less ice around in the starting materials. Just literally blasted away. Yeah, or was vaporized before the material came together to form the planet. There's a question about the contraction of mercury speaking of. Any idea about, like someone was saying, an anonymous viewer asked, why would they would think that it would be collecting some material from maybe getting blasted from the sun or from comets impacting? So how come it seems to just be, is it losing mass? It's not losing mass. It's that as the rocks cool, they shrink. And this is, most solids actually do this as they get colder, they get smaller. Now the one solid that we think of that we're really familiar with that does the opposite is water ice. You freeze water and it actually expands but that's very unusual. Most solids get smaller. So what's happening is the planet is cooling and the rocks themselves, the atoms are getting closer to each other. And it's shrinking. And the questioner is absolutely right that there is material being added to mercury and probably at a very high rate because it's vacuuming up stuff coming into the sun. But even so it's nowhere near enough to counteract this shrinking. But I should also mention this shrinking is it's shrunk maybe by 10 kilometers in its diameter over four million years and the diameter is three kilometers or no, 5,000 kilometers. So we're talking about a very, it's still a very small amount but it's more than the amount being added. I should mention on the earth where we're adding something like 20 to 40,000 tons of material from outer space every year. And that sounds like a lot until you add it up and realize it's a very thin layer when you add it over the whole surface. Okay. Thank you. So it's not like a honey I shrunk the kids, it's more like a- No, thank you. The question from Matthew Ota about the radar images that show the crater, I guess the crater is a circular as if the radar beam was perpendicular to the poles. So you're just wondering how it was done before the messenger mission showing those like kind of figuring out the geometry. Was it complex image processing from the earth? No, I- Well, first of all, I'm not deeply familiar with these observations but I would say that it was just done, I think they were done with the AeroSebo radio telescope in Puerto Rico, which you know is a giant dish that points up. And- In one spot. And I think, yeah, and I don't know how well it points or whatever but I think- But the point is that I think even though the sun can't get in, our orbit is not perfectly aligned with Mercury. So I think you go above and below the plane of Mercury a little so we can actually look down on the poles from the earth and that's how it works. So. Awesome. Cool. And it's a quick question, another anonymous one. For what accounts for the slow rotation of Mercury and Venus compared to the earth? Is it because they're so much closer to the sun or like so tidal forces or is that still? I don't really know. It's clearly, I mean Mercury's rotation is slow because it got locked into this resonance, which we're not, we're not in a resonance like it is what's in this three two resonance. So it really does three times around its orbit but I don't know the history I don't think anyone does but there's theories as to how it could have and it could have been knocked into this by an impact so it could have been spinning faster and something at it and knocked it just the right way and there's theories for this but I don't really know the details and I know I don't know anything about Venus's rotation state but I think what we know is that every planet had a unique history and that giant impacts played a very major role in the final formation of the planets which means that each one could be completely different. So it's not just an issue of being close. We think the Earth probably spanned much, much, much faster early on and then got slowed down. Cool, that is all the time we have for our questions right now. So that's all for tonight and you'll be able to find this telecon along with or this webinar along with many others on the night sky network under the outreach resources section just search for webinar. Tonight's webinar will be posted on the night sky network YouTube page and the dedicated research page will be up by the end of this week. And now for our raffle. It's gonna count and determine this month's winner in the ASP's total sky watchers manual. Now numbers entered before I say go won't count and so last month we had a few people kind of jumped the count and so we wanna make sure that this is fair for everyone and so please wait until I say go and then Dave's gonna count and then we'll figure out who wins the book. So tonight's criteria is the eighth person to enter the number eight after the signal will win. And so are we all ready? So you have to wait for the signal. So here's the signal, go. Listen to the chat. Okay, well we got a lot we're trying to find now. People are good one, two. Okay, we've got more than eight people. Let's see, one, two, three, four, five, six, seven, eight. Why it's Jim Small is the eighth person. Congratulations Jim, thank you so much. Copy of our lovely book. And Jim, if you make sure that we have your contact information I think that we do since you're a regular and so we probably have your contact information but just double check with David and maybe enter that into the chat window. And so do mark your calendars for our next webinar on Wednesday, May 11th. So when we're here from Dr. Stephen Levin, the project scientist for the Juneau mission to Jupiter and don't forget to sign up early on the morning or to get up early on the morning of Monday, May 9th for the transit of Mercury. So thank you again Dr. Larry Knitler for joining us this evening. It was an absolute delight to have you with us Larry and I learned a whole lot about Mercury and it's amazing the science that was returned by actually a fairly inexpensive mission as mission well. So keep looking up and we'll see you next month. Thank you so much, we'll come and see you. Thank you all. Thank you Larry. Bye bye. Bye bye everyone.