Thomas Watters on Researching the Geology of Mercury

Image courtesy of NASA

Back in September I wrote an article for Smithsonian Magazine about the discovery that Mercury has a system of plate tectonics. This gave me the opportunity to interview Dr. Thomas Watters about his discovery. Here is my interview with him in full.

As always, the ellipsis indicates that he was either talking too fast for me to type or was saying something that I knew I didn't need for the article. The only questions written down were those that I wrote in advance of the interview. My follow-up questions and comments didn't get typed in. A question mark mid-sentence means that I missed a single word.

This is super-specialized stuff that I realize most readers won't be able to follow. I am putting it online in case some day another writer might be working on a book or article about the history of space exploration and can use this as source material.

Am I correct in summarizing your evidence by saying that the presence of long escarpments means that there is geological activity, because otherwise they would have been erased by meteor impacts millions of years ago?

You're close. I can understand where it gets confusing. It's really what's new here is, the big news finding in these low-altitude final campaign Messenger campaign images is that we found very small versions of these big scarps that we've known were on Mercury since Mariner 10. Mariner imaged less than a full hemisphere, but a good chunk. There big thrust fault scarps that indicate that the crust had been fused together and contracted was evident in those images. With Messenger, we saw the other hemisphere and the scarps were there, too. Then we went into orbit and mapped them some more. One of these is as long as the San Andreas fault. These are monsters even by Earth standards. So we knew that Mercury had contracted to form these. The planet had literally been shrinking.

What we didn't know was how long this had been going on and more importantly if it was still going on. Because we really couldn't resolve even... sort of the smoking gun to determine whether this contraction is still going on is these very small scarps. And it sort of connects to the Moon. Because when Messenger was heading for Mercury, we had the lunar reconnaissance going around the Moon. Imaging the Moon with the highest resolutions ever from orbit. Down to a meter. In those, I found evidence of these very small fault scarps. We saw them globally on the moon, which means that the moon is contracting. Very slowly. …it indicated that, and the reason again, I'll preface it, that these small ones are so important is that the Moon and Mercury are airless bodies and there is nothing to protect them from constant meteorite bombardment. That bombardment will erode land forms very quickly. Seeing these small scarps means that these bodies are tectonically active...

...you had to keep moving the spacecraft. Burning fuel to keep the spacecraft from falling. And then we decided near the end to let the spacecraft go lower. ...when we started to reach this level of about 20 meters per pixel, suddenly these small scarps started to pop up. And that tells us what is happening on Mercury. In scale, they are identical to the ones we found on the Moon. The flux of meteors on Mercury is probably at least 3 times higher than around the Moon. ...this tectonic activity is still happening. Mercury is still forming new faults.

These very young faults occur in clusters. If you continue to contract the surface...

What is causing the contraction?

It is the cooling. It is the way Mercury is cooling down. We have known from even before we got Messenger there. We knew and had evidence that Mercury still had a molten core. We know that also because when Mariner 10 got there it had an active magnetic field, just like the Earth's. It was confirmed through other means and through Messenger's orbit that the liquid core was there. So you know you've got this heat source inside of Mercury which is slowly cooling. As it cools, the internal volume changes and the crust of the planet has to adjust to that change. That is what causes the contraction.

It has shrank by 1 to 2 kilometers in 3.9 billion years.

When the spacecraft descended that low, was it then doomed?

Yeah. We had a choice to either expend the available fuel to keep it in this mapping orbit or just allow it to become lower. The solar tides, there is no way you could keep a spacecraft in an orbit around Mercury that will just drag it down. The idea was that why don't we use that fuel to keep us in an orbit low where we can get these images of at least the northern hemisphere. ...it was highly eliptical. We couldn't put it in a circular orbit or it would get cooked. It was a decision to say this would give us an opportunity to see the surface at much higher resolution that we could see otherwise. …when we lowered the altitude we got down to 1 to 2 meters per pixels in some places. It was like a new mission. It meant that the spacecraft was doomed, but that was going to happen anyway.

The other thing I want to mention is that we got fortunate. My real goal was to find evidence for these young small scarps. But we got lucky and were able to image some of the bigger scarps in higher resolution. We imaged one particular large scarp at very high resolution and found evidence that that large scarp was currently active... the other thing is one of the really important observations we made with this low-altitude campaign was with the magnetometer. … what we were able to do was get low enough, close enough to the surface that now we could start measuring fossils of the magnetic field that got locked into the volcanic rock when it cooled. Sure enough, we detected in these ? billion year old rocks the remnants of a magnetic field, which was incredible. We thought that because of its size Mercury must have cooled quickly in the early days of the solar system. When the magnetic field was detected by Mariner 10, they thought it was a late stage, temporary thing. But finding this evidence of a magnetic field in 3.6 billion year old rocks, means that like Earth's magnetic field it has been around for billions of years. Together with the tectonic history, it paints a whole new picture of what Mercury's history must have been like. ...it puts Mercury very close to Earth in terms of very slow cooling that allows the outside to remain cool and the inside hot.

Do you think that Mercury has a system of tectonic plates like Earth's, or are there differences that need to be pointed out?

There's very definitely a difference. The Earth, with this mosaic of plates, everyone immediately thinks of plate tectonics. Earth's shell is broken up among about a dozen plates that cause most of the tectonic activity on Earth. On Mercury, we don't have any evidence for a series of plates. Mercury seems to be a one-plate planet. That shell is uniformly contracting. We don't really understand why the Earth developed this mosaic of plates. But it's what keeps the Earth from contracting. ...

How recently has there been volcanic activity on the surface of Mercury?

It's not really, I think, well-known. There isn't any evidence in the Messenger data of what you could describe as recent volcanic activity. Mercury had its last really massive global scale volcanic resurfacing event back where it locked in the fossil magnetic fields over 3 billion years ago. But there's probably been more recent events than that. There's evidence of localized pyroclastic eruptions but it's really hard to figure out if those are very recent. ...there are studies going on looking at that very thing.

Given the extreme temperature fluctuations, do you think that it will ever be possible to send probes to the surface of Mercury for more data?

Yeah, I think so. There's a couple of ways you could approach it. Its certainly a challenge. One thing I haven't talked about is the implication for seismic activity... all these young faults are all growing, developing, directly with slip events, connected with Mercury-quakes. So if you wanted to put a lander down or something similar to what we're going to do on Mars in 2018 with INSIGHT to put a real seismometer. It's going to be a challenge because the day side of Mercury gets up to 800 degrees F. And the other side gets down to minus ? degrees F. … it's going to be tough but it's not impossible. And there are areas on Mercury near the poles that are permanently shadowed like we have on the Moon. So you could find areas in craters [that aren't so hot] where you could put landers that wouldn't be as thermally challenged. But we're getting better at that. We're talking about probes that would land on Venus. They wouldn't last for a terribly long time, operating for a decade or better like on Mars. …but the technology is getting better and better. In the future it will be possible to do this and put probes on the surface. I'm very hopeful that this paper might stimulate thinking about putting a seismic network on the planet.

Where do we go from here to gather more data about Mercury? What else could be done?

We've got a mission, the space crafts are built and being prepared to be launched for an ESA mission called Bepicolombo. It has slipped a few times and I don't know where they are on the launch date. ...it's going to continue, if everything is successful, the studies that Mariner 10 and Messenger have begun and one thing in particular that I'm looking forward to is higher res imagining in the southern hemisphere because that's where we have the poorest imaging.

Is it hard to get attention and funding for Mercury?

Funny you bring this up. On Monday, NASA, I was happy they thought this was significant enough to do a piece on NASA's website. I thought great, not a big science news-worthy day so maybe Mercury will get the spotlight! But later that day, I found out that NASA just made an announcement that Hubble had found evidence of plumes coming from Europa. So it was like, great, Mercury is not going to compete with water vapor plumes coming from a deep ocean that might have life. I get this picture of geysers on Europa ejecting the poor life forms way up in the air...

The one thing Mercury has, is, we're pretty confident that we've got water in these permanently shaded craters. But Mercury is not a habitable world and we have a lot of focus on habitable worlds. Mars is certainly the most hospitable for past life or current life. And of course its a much easier place to, you're not going to have to deal with the kind of extreme temperatures that you're looking for on Venus or Mercury. It's a challenge to get missions to planets that don't have this habitability element.

What we've found now from Mercury is that there's no other planet we know of that is tectonically active. Trying to understand how rocky planets evolve in this solar system. This whole notion of what is the spectrum of evolution on a rocky body? Is plate tectonics a necessary element of developing life on a rocky planet? There are some really important things to learn about. What the evolution of rocky bodies are. Mars, at the opposite end of this, its core froze very early in its history. Its magnetic field, also recorded as a fossil, is billions of years old. From what we can tell volcanic activity shut down on Mars a long time ago. The freezing of the core and the loss of the magnetic field is probably connected to when Mars's climate changed. Because it no longer had a magnetic field to protect its atmosphere from being swept away by the ? wind. …it really does beg the question of how much we really know about how rocky planets form and evolve.