The MESSENGER spacecraft is sending back the highest resolution images of the planet Mercury every taken. Many of these images are of portions of Mercury’s surface that have never been seen up close before. The Mariner 10 spacecraft that visited Mercury over three decades ago never got to see all of the surface, so many of the features being seen now are being seen for the first time, so they have no names attached to them, yet.
This image, taken during MESSENGER’s flyby of Mercury three days ago, shows surface features typical of Mercury. The first thing that you notice are the craters. Mercury has craters. Without a significant atmosphere, erosion does not cover up the craters as it does on Earth. Mercury lacks plate tectonics and significant volcanism, so geologic processes also do not erase craters. In this respect, Mercury is much like the Moon. But, you may notice that there are fewer craters on Mercury than on the Moon. On our Moon, the craters are packed so close together that craters are on top of craters. On Mercury, there is actually some planetary surface between the major craters. This surface, called the intercrater plains tells us something about the different impact histories of Mercury and the Moon. Convention explanations are that many of the impacting objects originate from the asteroid belt, so the farther a planet is from the asteroid belt, the fewer impacts it will receive. Alternate explanations exist, however. It has been suggested that Mercury cooled slower than the Moon, so volcanic activity could have been involved in erasing some of the earliest craters. Other suggestions are that many of the Moon’s craters are secondary impacts due to its own violent formation history. Until recently, we have had little new data on either Mercury or the Moon. Now, MESSENGER is sending back more information on the Mercury, and a whole flotilla of spacecraft have begun to study the Moon again. We may soon have some answers to these questions.
Notice also, though, that the craters have some interesting features. Some craters are simply bowl shaped features. Larger craters have a rebound in them that creates a central peak. Still larger craters have a multiple ring structure. The multiple ring craters are believed to result from impacts so violent that the ground begins to behave like a liquid. The rings are like ripples. A few impact basins on the Moon exhibit features like this. Most of the largest craters on Mercury have smooth flat floors. These features are called smooth plains. They are believed to be features analogous to the Moon’s maria (seas), which are volcanic plains that formed in many of the Moon’s largest impact basins. If an impact is large enough, it can crack the surface of the planet down to a depth deep enough for the magma to rise through the fractures to the surface. That magma on the Moon is basaltic in nature. Basalts are dark colored rocks, so the lunar seas are dark. However, there is little albedo difference between Mercury’s smooth plains and the intercrater plains. Does that mean that the interior of Mercury is different in nature than the interior of the Moon? Or, is there a surface layer of regolith on the smooth plains that is similar in nature to that on the smooth plains? Or, perhaps, the intense solar radiation has degraded the rocks to the point that they are all similar albedo? Or, as it has been suggested, was the impact that produced these largest basins simply so powerful that the ground was merely liquified and was able to flow into such smooth surfaces. These questions were not answered by Mariner 10, so perhaps MESSENGER can give us some answers.
But, one of the most interesting features is the lobate scarp than runs diagonally across this entire picture. Lobate scarps on Mercury are one of its most unique features. They are cliffs that can be extremely long and very high. This scarp is one of the longest and highest on Mercury. The scarps are believed to be the result of thrust faults in Mercury’s crust. Mercury has an unusually high density. It is believed to have a fantastically large percentage of the planet composed of a mostly iron core. As the planet cools, the interior of the planet shrinks faster than the crust. This puts extreme compressional tension on the crust, and it cracks. Portions of the crust slide over one another, creating these scarps. For decades, this has been our explanation for Mercury’s lobate scarps. Now, maybe, MESSENGER can help us find out if it is right.
But, concentrate, if you will, on the lower right of the image above. Here is a higher resolution image zoomed in on that part of the planet:
You can see the scarp running in a large arc across the picture. It cuts right across a large impact basin in the lower right of the picture. But, notice that near the top of this particular picture there is a crater that sits right on top of the scarp. The scarp cuts through the walls of the larger basin, and it is clearly visible as a giant cliff running across the floor of that basin. This tells us that the scarp came after the basin had formed. If the massive impact that created the basin happened after the scarp was already there, then it would have obliterated all traces of it at that location. This image also tells us that the smooth plain in the basin also came first. All models for how the smooth plains formed require some sort of liquid-like behavior. That means that the material would simply flow off of the cliff, and so you would only get the smooth plain at the lower elevation portion of the basin. But, this image also tells us that whatever geologic event that crafted the scarp, though more recent than the massive impact creating the giant basin, was not terribly recent in the planet’s history. The crater at the upper portion of the picture lies on top of the line of the scarp, yet there is no clear indication of the scarp on the crater’s floor, and the crater wall is intact. This tells us that the impact causing that crater came later. Since the ejecta (material thrown out from the crater when it formed) has already degraded to the point that it is similar in color to the surrounding rock (it is lighter in color at first, and then darkens), this means that the crater itself is rather old. There are even smaller craters inside of it an on its rim, also an indication that it has been there for quite a while. So, that tells us that whatever mechanism causes the scarping behavior, it has been inactive for a long time. Careful analysis of the floor of the crater does show a faint line that may, or may not, be continuous with the scarp. That may be some indication that small geologic shifts have occurred, or it may be some artifact due to the fault that runs through the crust at the location of the scarp. Planetary scientists will be studying this to see if this tells us that the thrust faulting continues, moves at a slow pace, or has ended.
Also in this lower image, you can see large valleys extending away from the giant impact basin. These are like grabens. They are fissures in the ground. They may be due to rifting, a pulling apart of the crust, perhaps due to the formation of the scarp nearby. But, more likely they are associated with the formation of the giant impact basin. Graben-like features are seen elsewhere on Mercury near such large impacts.
Many of the features being seen by MESSENGER have never been seen before, so they do not yet have names. They will soon be getting names, though. But, they don’t get just any name. There are certain conventions for naming planetary features. On Mercury, craters are named after people who have made contributions to the humanities (artists, playwrites, musicians, composers, etc). The one exception to that is Crater Kuiper, named for Gerald Kuiper, who was on the Mariner 10 imaging team, but died before the spacecraft arrived at Mercury. Scarps on Mercury are named for ships of discovery or exploration. Valleys and graben are named after radio telescopes. Plains (with a couple of exceptions) are named after various historic names for Mercury in different cultures and languages. Mercury also has a couple of ridges. These are named for astronomers who have studied Mercury. I will be interesting in the new names. Once the new features are named, they will be listed in the USGS Gazetteer of Planetary Nomenclature.
Images courtesy of NASA/Johns Hopkins University APL/Carnegie Institution of Washington