Paint-By-Numbers Mercury

October 30, 2008

When there's a birthday party at my house a half-eaten cake usually lies around and gets nibbled at until it's just a mess of colored frosting and intermixed cake layers.

The MESSENGER spacecraft’s latest released images reveal a similar mess on the innermost hot rock planet Mercury. They are from the second of three planned flybys. The planet’s surface is so rototilled, sprayed and flooded, that any resemblance to the comparatively simple geology of Earth’s moon is purely superficial.

This wasn’t terribly obvious in natural color images of Mercury that show a bland, grey surface. But a series of images taken through 11 different color filters that span from visible through infrared light reveal an abstract art surface that tells of a chaotic history. The subtle color differences are exaggerated in these images to reveal contrast and detail.

The MESSENGER team doesn’t know the mineralogy behind these colors just yet. But a sort of paint-by-numbers geologic scheme can be deduced from matching the colors to the underlying terrain.

For example, the fresh impact craters have ejecta material from below the surface that is newly exposed to sunlight. It appears bright yellow. Regions that look like they have been filled in with lava flows appear reddish and comparatively smooth. 

The planet is mottled with mysterious blue terrain. Shall we call it the Smurf Lands? The blue may indicate a different kind of subsurface material that was volcanically ejected from deep in the interior. Some of the blue stuff has also been excavated by impacts because they form blue ray streaks across the surface (seen in high definition, of course). And some impact craters look like they punched though a subsurface blue layer.

By far the weirdest feature of all in the image is a pure dark blue small y-shaped area. There simply is nothing else quite like in the color-enhanced images. What geologic event could have shaped it?

Mission scientists conclude the Mercury’s interior must be variegated with different layers or simply a different mixe of material that is exposed by volcanism and impact cratering. The latest flyby data indicate volcanism was much more rampant on Mercury than on the moon. In some places surface impact craters are flooded to a mile deep with frozen lava.

Images from the Mariner 10 probe in 1974 made Mercury look dull and moonlike. That’s probably why we didn’t decide to return to the planet for nearly 30 years.

But a new generation of space probe is unveiling a once rambunctious world with a tortured and chemically diverse surface. This planet has suddenly got a lot more interesting. And there are no doubt equally new surprises awaiting us at next MESSENGER flyby in 2009.

Postcards From Mercury

October 07, 2008

Hot off the spacecraft! NASA’s MESSENGER probe made its second flyby of Mercury on Monday. The images released today by the scientists at the Johns Hopkins University Applied Physics Lab, unveil a largely never-before seen face of the planet.

One of the more seminal photos from the sequence was taken on the probe’s outbound leg. This whole globe view shows the entire planet draped in the “mother-of-all” impact ray patterns that wrap around the planet like fingers clutching a baseball.  A comparatively recent meteorite impact splattered stringy linear bright features across the planet. It reminds me a little bit of the “Cover the Earth” logo for Sherwin Williams paint.

I always marveled at the ray features on the moon from the 100 million year-old impact crater Tycho near the south pole. But this feature is even more dramatic, and nothing like it has been seen on dozens of moons photographed across the solar system.

Earth based radar mapping constructed the first crude picture of this hemisphere in the early 1990s. The young crater that is the source of the rays was simply called North Giant Crater.

A stunning view over the planet's limb shows two very long scarps that crosscut each other. Scarps are unique to Mercury. They formed after the iron core cooled and the crust shrank, causing a folded apple skin texture on the surface.  This happened later in Mercury’s geological evolution as evident in the view of the scarp on image right. It cuts through a crater, showing that it formed after the impact that created the crater. Unlike the moon, many of the craters floors appear to be resurfaced with smooth ancient lava flows.

My favorite image is from MESSENGER looking down into Machaut crater, which is big enough to swallow Rhode Island. Ever-smaller craters pepper the floor and intricate linear features reminded me of some of Galileo's drawings of the moon as first spied through his telescope in 1609.

The largest crater within Machaut appears to have been partly flooded with lava flows, and then peppered with more meteorites. The adjacent slightly smaller crater it markedly younger. It appears to have burrowed below the lava-flooded surface, as evident by the deeper shadow.

The steep sunlit terraced wall is striking. Imagine standing on the rim of the younger inner crater and surveying the much larger basin around you. The huge blinding sun is low on the horizon and is roughly the angular size of half-dollar held at arm’s length.

Each new image from MESSENGER gives Mercury a unique identity all its own. Like the moon, these images speak to us over billion years of evolution. They give us a hint that Mercury truly must have been a spectacular place 4 billion years ago when planet wide fireworks saw rivers and lakes of lava, and volcanic fountains spewing hot embers high into blazing sky.

Return to Mercury, Once is Not Enough

October 03, 2008

Next Monday October 6, NASA’s MESSENGER spacecraft (with the tortured acronym MErcury Surface, Space ENvironment, GEochemistry and Ranging) will make its second flyby of the innermost planet Mercury. MESSENGER will swoop just 125 miles above the ancient pockmarked surface and then hurtle away along its solar orbit.

We first visited Mercury in 1974 with NASA’s Mariner 10 space probe. But it only photographed a little less than half of the planet. We’ve had to wait another 34 years to see the uncharted territory on the opposite side of Mercury. But the wait has been worth it. With today’s computers, laser altimetry and imaging systems MESSENGER will snap off more than 1200 digital pictures in just a few precious hours.

During the first MESSENGER flyby on January 14, 2008 its cameras photographed approximately 20 percent of Mercury's surface that was never before seen by space probes. Before that flyby Mercury was pretty much shrugged off in textbooks as superficially resembling the crater-faced moon.

But on its first pass MESSSENGER found a shield volcano, frozen lava flows, and more complex terrain that means a more complex geologic history than our moon’s.

About one-third of the color photos from the second flyby will show a never-before-seen area of Mercury's surface that is opposite from the side of the planet we saw during the first flyby.

We have had such a flood of images of the planets and moons over the past decade it’s easy to overlook the fact that the innermost of the terrestrial planets still has something new and likely unexpected for us to see. I can’t wait!

As in the previous flyby, a laser altimeter on the spacecraft will measure the planet's topography. But this time scientists will be able to make three-dimensional visualizations by combing the altimetry data with overlapping high-resolution photographs. We’ll be able to construct videos of flying across Mercury’s surface as armchair astronauts.

One of the strangest features photographed so far, called Pantheon Fossae, looks like a giant squashed spider on Mercury. The feature has an uncanny resemblance to the imprint of a BB pellet shot into a window. More than 200 spidery cracks radiate outward like the spokes on a wheel. The webs are really troughs located inside the 3.8 billion year-old giant Caloris impact basin.

The basin’s interior appears to have been flooded by volcanic material in a similar way to the lunar mare basins. But the spidery cracks are something else. They seem to radiate outward from a 25-mile diameter impact crater called Apollodorus (after the architect of the Pantheon temple in Rome) at the center of the web. Is this coincidence or not?

What fascinates me is that nothing like it has been seen on dozens of cratered moons and the other terrestrial planets. When you find only one of something it is, well, very mysterious. What’s going on at Mercury that hasn’t happened elsewhere in the solar system?

Several geological models have been proposed for the spidery formation, including uplift of the basin due to heating from below, pressure building up from of surrounding plains, or a flow of the crust. However, to date, none of these models can explain the radial pattern observed.

Three-dimensional computer models by MESSENGER lead scientist Sean Solomon and colleagues simulated the effects of a meteorite impact. They found that if the basin floor was under stress, the blasting out of the crater relieved the stress, weakening the central floor. This caused troughs triggered by the impact to spread out like cracks in a window. The results were reported last week at the European Planetary Science Congress in Munster, Germany.

The Apollodorus crater looks like it overlies the spidery troughs suggesting that the Pantheon network formed at the same time as the crater did.

Competing ideas say that that the Pantheon troughs were simply caused by magma oozing out the center of the basin and forming a radial network of dykes. But it’s curious that similar features aren’t seen in the mare impact basins on the moon.

During the flyby MESSENGER will rob momentum from Mercury to place the craft onto a trajectory that will allow it to go into orbit around Mercury in March 2011. Before then the craft will make one more flyby on September 29, 2009.

On March 18, 2011, MESSENGER will burn off one-third of its propellant to essentially come to a virtual stop relative to Mercury’s velocity. This will put the craft into an elliptical orbit about Mercury. The lowest point will be only 125 miles above the surface. It will take 12 months for the planet’s entire surface to be photo mapped, making one of the most elusive planets in the solar system one of the best know.