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Figure 1. This is the first picture of Mercury returned by the Mariner 10 space probe.
All photos and drawings in this article courtesy of NASA, and in the public domain.
For most pictures, click on photo to see full resolution.

Mercury - Then and Now

Mariner 10 Revisited

By F. E. Harris

October 15, 2011

Introduction

On March 29, 1974, the Mariner 10 space probe made its first pass, photographing the surface of Mercury close up for the first time. 37 years to the day later, the Messenger space probe entered orbit around Mercury. I thought it would be interesting to compare the two missions, and also to compare Mercury then and now, looking for any changes on the surface, and what each mission has taught us about the smallest planet in our solar system, which is also the closest to the Sun.

Craters on Mercury are named for famous artists, poets, composers, and literary figures whose fame has lasted at least 50 years after their death.


Section 1: Mariner 10

The plan for the Mariner 10 mission was to have the spacecraft enter a solar orbit that made close passes by Mercury. After launch in November, 1973, followed by a flyby of Venus to adjust Mariner 10's orbit, three flybys of Mercury were completed, on March 29, 1974, September 21, 1974, and March 16, 1975. On each pass, Mercury would only be close enough to photograph for a short period: On Pass 1, pictures were taken for 10 hours, 23 minutes, and 48 seconds. On Pass 2, pictures were taken for 6 hours, 27 minutes, and 42 seconds. On Pass 3, the total time was 1 hour, 31 minutes, and 20 seconds. Not only were fewer pictures taken on each pass, there was also a noticeable degradation in the quality of the pictures, slight toward the end of Pass 2, but substantial for all of Pass 3.


Figure 2. Mariner 10 space probe.

Because of the fast flybys, pictures had to be taken with whatever scale was possible at the moment. This varied from about 1200 meters per pixel, down to about 120 meters per pixel. There was only one camera, set up for visible light only, so almost all pictures were black-and-white digital photos. A handfull of pictures were taken with an orange or blue filter, to provide only a few color frames of the surface. No infrared, UV, or other images were taken. Pictures are frequently at odd angles or upside down compared to the Messenger photos, which seem to always be oriented so that North is at the top.

Mission control for Mariner 10 at crucial points of the mission must have been very exciting, waiting with growing anticipation for 6 months, and then collecting a huge harvest of data in 10 hours or less.

Because Mariner 10's orbit around the Sun was almost exactly twice the period of Mercury's orbit, each pass viewed the same face of Mercury. Only about 45% of the planet's surface was photographed in the 3 passes. Total coverage was about equal to what we knew of the Moon, before space probes.

Mariner 10 was the first spacecraft to use an interplanetary gravitational slingshot maneuver, using Venus to alter its orbit, and reach Mercury. Similar maneuvers using Jupiter have set Pioneer and Voyager spacecraft on orbits that escaped the solar system entirely.


Figure 3. The Messenger space probe.


Section 2: Messenger

MESSENGER stands for MErcury Surface, Space Environment, GEochemistry and Ranging. From the name alone, it is obvious that Messenger is designed to do a lot more than simply take photographs. The dual camera and 2 spectrometers take pictures and spectra in visible light, UV, and infrared. Spectra are also gathered using X-ray, Gamma Ray and Neutron spectrometers. A plasma spectrometer measures the composition of charged particles and plasma that contacts Messenger.5 The spectrometers identify elements, ions, and chemical compounds by their radiation signatures.


Figure 4. Photo and X-ray spectra map of Beethoven Basin. Click to enlarge.

Perhaps most interesting is the X-Ray Spectrometer (XRS). The XRS uses x-ray illumination from solar flares given off by the sun. Ratios of different wavelengths gives information on the deep composition of the rock. Here a map of X-ray spectra has been overlaid on a photograph of Beethoven Basin. Green means a higher Mg/Si ratio than the surrounding highlands, giving clues to the composition of the rock.

Messenger's mission plan sounds similar to Mariner 10: Launch from Earth, pass Venus, orbit the Sun and fly by Mercury 3 times. But there are subtle differences, then a huge difference. Messenger conserves fuel by doing an Earth flyby to alter its orbit, then 2 Venus flybys to further alter and refine its orbit, then the 3 passes by Mercury, and then, finally, a rocket burn slows Messenger to orbit Mercury.

The many flybys saved a lot of fuel, but they made for a much slower mission. Mariner 10 got to Mercury in less than a year, but Messenger required 6 1/2 years between liftoff and Mercury orbit.


Section 3: The Photos

Beethoven, 20.8° South, 123.6° West

Since we have already seen Beethoven basin above, I suppose it would be best to start with Mariner's pictures of the same area. As usual, you really should click on the pictures to see them at full size and resolution.

Figure 5: a. Right hand side of Beethoven from Pass 2, rotated 180 degrees to match Messenger (North up). b. Beethoven seen early in the first pass, at a great distance, with horizon. c. Left side of Beethoven, rotated 144 degrees (but not rotated in full size image). Click images to enlarge.

The outlines of Beethoven Basin are very faint in the Mariner 10 pictures. The issue was the angle of the Sun: For Mariner, in these frames the sun was too close to directly overhead, so there are fewer shadows and highlights than in the Messenger photos.

Some of you might be wondering if these are really pictures of the same place. Besides the assurance given by the coordinates on the Messenger and Mariner 10 databases,2,4 there are also the 5 large craters within Beethoven, that form a distinctive "V" you can see in the enlarged pictures. There is also some distortion in the Mariner pictures because the camera was tilted about 31°, while the Messenger picture was from directly overhead.

Although two of the Mariner pictures are at higher magnification, there is more detail in the Messenger photo. Some of this is due to a better Sun angle, some to a better camera, and some due to better error correction protocols in modern data transmission.

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Tsai Wen Chi, ~25° North, ~22° East


Figure 6. Crater Tsai Wen Chi is near top center, with distinctive scarp running North-South, and double peak. Mariner 10, first pass, ~600 meters/pixel. Click to enlarge.


Figure 7. Crater Tsai Wen Chi is near bottom center, with North-South scarp and double peak more easily seen due to lighting from East. Messenger raw photo. Click to enlarge.


Figure 8. Crater Tsai Wen Chi is just to the right of center, with North-South scarp and double peak less easily seen due to lighting from West. Messenger map photo mosaic, 1000 meters/pixel. Blurred rectangles are due to mosaic. Click to enlarge.

You can see four very distinctive craters in these photos (figures 6, 7, and 8). The first is Tsai Wen Chi itself, with the fault running through it and the distinctive double peak. The second is the large concentric crater to the right. When craters are above a certain size, instead of forming a central mountain, they form a central lake of lava, with an inner ring wall.

The third distinctive crater is the small, new one with the very high central mountain, and two small, deep, dark craters within. It appears to be a relatively new crater, because of the crispness of the walls. (It is not visible in the second photo - off the bottom edge.) This is one of the rare cases where the Mariner 10 photo reveals information the Messenger photos do not: On the Mariner 10 photo you can see the rim walls of these small craters, and their shadows. If it were not for the Mariner 10 photo, one would think these were more examples of the much rarer phenoenon seen in the crater at the top left, of the first and third photos.

The fourth distinctive crater is the one at the top left of the first photo, and top left - center on the third photo. It is off the left edge of the second photo. It has a very large, deep hole in the center of the crater. There is a fault scarp running across the crater, and there is no rim wall around the central hole. The hole is not at all round. It is oblong in the direction of the fault. There is no rim wall at all around the central hole.

All this has led the Messenger team to conclude that there is a hollow or a cave, perhaps a lava tube, underneath this crater, and the central mountain of this crater has fallen into the hole. This is a rare phenomenon, but not unprecidented. Other examples have been found, elsewhere on Mercury.

In case you are wondering, Tsai Wen Chi was a Chinese poet of the Han dynasty.

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Weird Terrain

Figure 9. "Weird Terrain" on Mercury. Mariner 10, first pass.

"Weird Terrain" was the Mariner 10 scientists' name for the hilly, lineated stuff on the left of the picture. The ground on the right is pretty strange also: It looks as if it were being stretched by plate tectonic forces. In both cases we are looking at geological processes on Mercury that have little to do with cratering.

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Ray Craters

Figure 10. A ray crater, a common phenomenon on the Moon and Mercury. This picture is from Mariner 10, first pass.

Messenger Wide Angle Camera picture of Debussy

Figure 11. Messenger Wide Angle Camera (WAC) picture of Debussy. Taken from orbit, September 19, 2011.

Ray crater Debussy, named after the French composer, was one of the few features of Mercury that were imaged from Earth prior to the arrival of Mariner 10. As such, it played a crucial role in determining Mercury's rotational period. It was originally imaged by radar,6 and named "Feature A." Debussy's rays are one of the largest features on the surface of the planet.

Ray craters are relatively recent craters, and there seem to be more of them on Mercury than there are on Earth's Moon.

I decided to show a picture of Debussy taken with Messenger's Wide Angle Camera (WAC). WAC helps Messenger's science team to see the big picture. This is important both for mapping the planet and putting together large scale mosaics of the terrain, and also for seeing global geological processes. Mercury appears to have a liquid iron core, like Earth, and may have some plate tectonic processes operating that still effect the surface. The "Weird Terrain" shown in Figure 9, may be evidence of this.


Section 4: Conclusions

I started this little project, hoping that if I could find enough pictures of the same ground from Mariner 10 and Messenger, I would be able to discover a new small crater, or some other change on the surface of Mercury, in the 37 years between Mariner 10 and Messenger. I found that, besides cratering, there are volcanic and land slumping phenomona that are operating on Mercury today. Between these three processes, it is inevitable that eventually, active change in the surface will someday be observed. What I also discovered during my search, was that the Mariner pictures are not as good as they should be. Mariner pictures that declare the same number of meters per pixel are about half as sharp as Messenger pictures. A little snooping into the noise and error correction methods in use then, revealed why.

Figure 12. Mariner 10's last picture from Pass 1. Click on this picture to view at full size, and the noise (black specles) become apparent.

This is the last picture from Mariner 10's Pass 1. The ray crater in the picture may be Debussy, but what interests me her is the noise, the black speckles on the picture. It could be that the camera or the electronics is overheating, or it could be a problem with transmission.

Figure 13. Typical picture from Mariner 10, Pass 3.

Here is a typical picture from Pass 3. The picture is about 1/4 as wide as the pictures from Pass 1 and Pass 2. This appears to have been ordered to deal with noise. My guess is that they transmitted the smaller pictures, multiple times, and combined the images to eliminate noise by some sort of voting algorithm.

Figure 14. Typical raw Mariner 10 image. The specle noise on this picture, visible in the expanded image, appears to have been corrected by a less accurate method than the CRC algorithm used on later space probes, starting with Voyager.

Noise , perhaps due to solar radiation, made speckle like this on all "raw" frames. It appears the Mariner crew dealt with it just by averaging neighboring pixels to replace the specle. The result was much better looking, but a lot of detail was lost.

Figure 15. Same image as Figure 14, after error correction.

References

1. "Mariner 10 space probe," Wikipedia
http://en.wikipedia.org/wiki/Mariner_10
2. "Mariner 10," NASA Reports, NSSDC ID: 1973-085A, 19 August 2011.
http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1973-085A
3. "MESSENGER," Wikepedia,
http://en.wikipedia.org/wiki/MESSENGER
4. "MESSENGER - Mission to Mercury," NASA Mission Pages.
http://www.nasa.gov/mission_pages/messenger/main/index.html
5. "MESSENGER - The Payload Instruments ," MESSENGER web site, Johns Hopkins University.
http://messenger.jhuapl.edu/instruments/index.html
6. "Debussy (crater)," Wikipedia, 11 May, 2011
http://en.wikipedia.org/wiki/Debussy_%28crater%29
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