MESSENGER at Mercury (part I)

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of WashingtonI’m spending my morning in a session on Mercury and Messenger’s new results. There is no WiFi in here, so this is all getting posted in one large, time-delayed, lump. There are also no websites being noted so press can go get images, so this is going to be less visual then I would have liked. I’ll try to come in and add more images once I find power and can abuse my cellular connection with less fear of killing my battery.

All the talks below cover results from the January 2008 flyby. The next flyby will be October 6, 2008.

Talk 1: Gravity observations – David E Smith

There is a lot left to earn about Mercury. This little world close to the Sun doesn’t even have a well-mapped mass. This means that while we know about how much total mass it has, we don’t know how the mass is distributed. Within the planet, the core may not be completely centered, and on the planet, large mountains, deep vallys, and everything inbetween can make the average distribution of the planets mass not be centered on what we call the “core” of the planet. The best way to measure these features is to drop a test particle into the gravity field of the planet and watch how its motion matches and deviates from expected. The starting point – the calculated expectations – comes from assuming a spherical mass distribution. Any deviations from these expectations are called gravity anomalies.

In the case of Mercury, the test particle is the MESSENGER spacecraft. Our first “Drop” was the January 2008 flyby. At closest approach, Messenger was less then 300 km above the surface and moving at 5 km/s. Using Doppler measurements with 4 cm/s resolution, astronomers observed how MESSENGER passed Mercury and how its orbit was changed by Mercury’s gravity. Initial work done by David E. Smith and his collaborators found that flyby data could not be fit with a spherical mass distribution alone (Mariner found the exact same thing). To match the observed motions they needed to add a gravity anomaly to their data. This means they had to add the effects of a non-spherically distributed material. This anomaly didn’t match with the anomalies measured by Mariner, but that’s okay – it flew past a different part of the planet. To try and understand what on/in Mercury could be causing this gravitational “Mercury isn’t spherical” anomaly, imaging and additional flybys are needed. Both will happen, and this brings us to talk 2 of the Mercury Messenger Morning Session.

Talk 2: MESSENGER Mercury Laser Altimeter – Maria Zuber

The laser altimeter is a neat instrument that shots lasers at Mercury and then measures how long it takes the beam to reflect back. Changes in path length (caused by the beam going down into a basin and the reflecting back later than average, or by a beam hitting a ridge and bouncing back earlier than average) can be measured as changes in light round trip travel time.

During the January Flyby, the MLA began ranging 1 minute before closest approach (200km), and the laser traveled at an angle, measuring distances between 262km and1672km. Range The camera triggered 5532 times, tracing out features never before seen.

Not being seen is actually a bit problematic. During this first MLA run over the surface of Mercury, part of its track covered a section of the planet that hasn’t been seen in optical observations. During future flybys this image will be imaged, but … Until then there are several areas of surface that can’t be fully analyzed. In the places that have both visual and laser data, it is possible to use this data to measure the depths of basins, to start to discuss the surface roughness (they used different types of lasers, with different sized beams, and different beams reflect better off of different surfaces – by looking at what is reflecting off of different surfaces it is possible to judge the texture of the planet, which is just really cool).

This first flyby found surface features that appeared to match with the location of the gravitational anomaly found in the first talk. It also matched images (where they existed). Once MESSENGER moves into orbit, the MLA will map the sizes of surface features and determine the texture of the surface.

Talk 3: Hi Res imaging by the Mercury Dual Imaging System – L. Prockter

And here come the pretty pictures! The full catelogue will be released to the public this summer, but today we’re still in the embargo period – a 6 month period when the data is in the hands of the scientists who worked hard to make the mission possible and have earned first crack at getting the science (and the oh-so-important for the career journal articles) published.

The images are really nice. I have to admit, it looks a lot like the moon and my brain is going “Rocks, rocks, rocks,” so I’m probably not the right person to explain how nice the images are. The speaker is using a lot of geology terminology that is new to me (it’s like how they say Eskimos have many words for “snow” – geologists have many words for “rocks.”). All that said, there are over 1200 images (only a handful are being shown), with resolutions from 100 meters per pixel for the monochrome images and 500 meters per pixel in the color images (the used 12 bands spread across the visual and near infrared spectral bands)

In some craters there is high albedo (highly reflective) stuff in the floor of craters, but in other places there aren’t…) Why this is being seen is a mystery. Side by side craters with the same depth have very different crater floors – this isn’t just material being revealed when the craters clear off the surface.

There are also weird things being found, including one publicly captivating feature (image above left) temporarily called “The Spider.” This feature consists of radial features – called Radial Graven – and a crater located roughly centered on the radial crack. It is possible (probable?) this crater is serendipitous. What ever it is (more on that to come), this feature is fascinating and is only one of many features that will keep planetary scientists busy for decades to come.

Talk 4: The Tectonics of Mercury – Tom Watters presenting
Let the hardcore geology begin! I feel like I need to read a book on geology. The terminology is flying fast a furious, and science results are cool but this is a language I’m not entirely fluent in, and I have to pay attention closely and type less.

This team looked at geologic features to assess the tectonics (motion of the surface caused by planetary evolution) of Mercury. Features like thrust faulting (cracking) and folding (hill formation) were shown in Mariner orbits on features of all ages.

In the new images they see features called lobar scarps, which are cracks caused by the planets contraction as it cooled. Total extent of contraction indicated by all the scarps visible in this new data indicates the amount of contraction is greater than was estimated by Mariner.

Talk 5: Mercury Color and Albedo – Mark Robinson presenting

One of the things people keep talking about is how the composition of the Moon (a well understood object) compares to Mercury (our unknown). This is complicated by the Sun’s weathering of Mercury (by blasting it with particles and radiation it is causing chemical changes on Mercury that the Moon doesn’t to experience. To try and compensate for the weathering, they compared newer (they call it “immature”) material on Mercury to the Moon. They find that Mercury’s immature material has a 20-40% lower albedo than the Moon – this is likely due to actual compositional differences between the two small worlds.

To better understand what they were looking at, they acquired images in 11 different color bands spread across the Visual and Near IR wavelengths. In non-stretched images, Mercury still looks like just a gray ball, but if you stretch the color contrast in the image enough you start to see detailed differences in colors. Color distribution in both hemispheres is the same (ignoring a large crater). They find three Mercury’s surface has three major material regions: smooth plains, low albedo blue crustal material, and average (grey) crustal material.

Talk 6: Origin of Plains of Mercury – Ian Head presenting
Analysis of the images shows that the surface shows definite signs of what is called weathering. In the context of Mercury, weathering refers to the effects of solar winds, craters on top of craters, and lava flows. In the images, weird radial features, called radial graven appear on Mercury in regions like the Central Caloris basin. These may be from what is called a “Radial Dike Swarm” that is caused by a volcanic magma reservoir coming up near the surface (this is a shallow magma emplacement). As the lava releases into surroundings, these radial features are formed. We see similar here on Earth and also on Venus. They also find in the images, volcanic vents around south circumference of the Caloris Basin’s interior margin.

By looking at these features and others, they find many examples of escarpment formation and radial dikes. There are also examples of flooding crater floors. They use these features to say, “embayment and flooding relationships show plains formation and crater filling is from intrusion and subsurface Dikes”

More of this to come in this afternoons talks. . .

Talk 7: Spectroscopic Observations – W. McClintock presenting
Half the room just left. The guy behind me (who said to no one inparticular, “And now all the geologists leave” just as the exodus began) suspects they are heading over to the Mars session Rebecca is in. One of the sad things about all conferences is the necessity to pick and choose what you see. With 3.5 hour long sessions, it is very very hard to stay focused. I have to admit I’m struggling not to think about how uncomfortable this chair is.

With this speaker, the session has just moved into detailed science results based on spectroscopy, and now the pretty pictures are gone for a while. This instrument, however, is cute enough to almost make up for it. The data being shown comes from a 3.45 kg spectragraph that works at wavelengths from 115nm to 600nm with 0.5-1nm resolutions. The smallest spectrograph I have seen prior to this was a 50+ pound camera mounted on the 107” telescope at McDonald Observatory, and the spectrograph I used in grad school filled a very large room!

Continuing the theme of comparing Mercury and the Moon (the far side of the moon in this case) Spectrum, they find Mercury is bluer. This is may be caused by thermal emission (Mercury is hotter than the moon). But blue light is most likely caused by opaque materials – something like Iron Titanium Oxide. This is just speculation at this point. Reducing spectral data is hard tedious work. Results are still coming. They’re earned my respect by being able to reduce the analysis they have so soon after getting their data. At this stage, all we can really say is in spectra Mercury has more blue light then the moon.

They can see some differences from feature to feature overall trends will be mapped out as more data is acquired and more analysis takes place. They can say there are “no obvious 1 micron Iron features observed in the Average Mercury” spectrum (this is a prominent Iron line). Fresh material also appears brighter and a less red.

More to come with more flybys…

Talk 8: UV Surface Spectra – F. Vilas presenting
Remember what I said about spectra being hard to reduce? This data defines difficult. This is a scanning grating spectroscope. This means it takes spectra such that each wavelength corresponds to one location, and the next wavelength corresponds to the next location. The are doing dashed lines of complete spectrum across the surface, such that every tens of kilometers they’ll get a new point on the surface in a given wavelength. In some cases, features like crater walls, shadows, and ridges cut across individual spectra effecting intensities and creating all sorts of data analysis chaos to be sorted out. From their first fly by they only get 3.5 spectra, and their signal wasn’t great. Many many more spectra are planned, and once they are in hand, hopefully a way to make sense of this will become more apparent.

Talk 9: X-Ray and Gamma Ray Spectroscopy – Larry Nitter presenting

This group looked at models of Mercury’s surface composition (including materials such as Feldspar, Iron and volatile rich materials) and they considered how emitted X-Ray and Gamma-Ray photons do and don’t match expectations based on their surface models.

Solar X-Rays hitting the surface and X-Ray fluorescence releasing a new X-ray photon skyward induce the X-rays they observe. Unfortunately, during the flyby, the Sun was anomalously low in X-Ray flare activity, so they didn’t see any surface X-Rays. They did see low-energy electrons emitting X-Rays that were probably triggered by the planet’s magnetic field. Using gamma-rays (often from radioactive decay) they did detect Silicon but not Potassium on the surface of the planet. This ruled out highly volatile-rich models at the region sampled by GRS. This means says that while there may be Potassium on mercury, it is not at the same high levels seen in some meteors.

Talk 10: Mercury’s Exosphere – Bill McClintock presenting
Chemistry. Chemistry. Chemistry! This team finds the exosphere (a fancy word for the thin layer of material that around they planet) consists of a diversity of atoms, including H, HE, O, and Metals (Na, K, and Ca). It may also contain Mg, Si, Al, Fe? S?, OH? and other atoms.(Question marks are the scientist’s way of noting weaker results with higher errors). This tells us about surface materials of Mercury. These atoms are knocked from the surface by the Solar Wind and Radiogenic Decay. The solar wind pushes the escaping material into an exotail producing a neutral tail like a comets tail. The tail has a velocity of 5 km/s away from the planet.

This team was able to measure the densities of material in this “tail”/exosphere. They find a 2.7×10^7 Na atoms/cm^2 sodium. There is also Hydrogen present with 1.2×10^11 atoms/cm^2. (Reading their graph I had a bad moment when I read Mercury as an element instead of as the planet – I need more coffee). Both elements are enhanced in the northern hemisphere. They also find both Na and Ca in exosphere are also asymmetric between poles. This is the first-ever hydrogen observation of the tail and the amount of hydrogen is a surprise. This Hydrogran, conventional wisdom argues, comes from a solar wind source that implants protons in Mercury’s regolith that then escape.

Talk 12: The Magnetic Field of Mercury – B.J. Anderson presenting
Talk 13: Ion Plasma Measurements in the Mercury Magnetosphere – Tom Zurbuchen presenting
These two talks are on Magnetic Fields and I’m combining my write up.
First: More atoms: Na, Mg(?), O, Si, Cl,. And Ca(?) with ions are all found in the magnetosphere of Mercury.
Second: Mercury’s magnetic field is tilted about 5 degrees relative to the North rotational pole and rotated 51 degrees in longitude. Mariner’s initial measurements are within error of these measurements indicating no magnetic field changes between the two missions. The magnetic field isn’t a perfect dipole with perfect symmetry. There are quadruple terms as well and longitude structures, but more data is needed from future flybys to confirm the details.

And now I’m going to get lunch…

One Comment

  1. michael cassidy March 10, 2008 at 4:45 pm #

    I like pretty pictures.
    The gravity Observations and laser altimeter are very interesting.

    If we can results such this and the Mars Rovers maybe th expense of sending people is silly.

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