Just two weeks ago, NASA's Messenger spacecraft fired its braking rocket and settled into a polar orbit around the innermost planet — a historic first.

Messenger reaches Mercury

An artist's portrayal of the Messenger spacecraft after its arrival at Mercury.


Today key mission personnel provided an update to let everyone know that the spacecraft's instruments are already hard at work scrutinizing the innermost planet as never before.

Eric Finnegan, the team's engineering leader, proudly noted that the craft ended up remarkably close to its desired orbit, a highly elongated loop that brings the spacecraft to a point 129 miles (207 km) from the planet's surface every 12.07 hours. While a close-in circular orbit would have been optimal for science, it would have posed harsh engineering challenges (particularly repeatedly going into and out of the planet's shadow).

Messenger's orbit

Messenger's polar orbit around Mercury ranges in altitude from just 129 miles (207 km) to about 9,300 miles (15,000 km).


OK, OK, the spacecraft is healthy and functioning normally — important for mission success, to be sure. But what does Mercury look like, up close and personal? To the casual eye, it's got craters, craters, and more craters. If you didn't know better, you'd swear the spacecraft was really showing obscure areas of the Moon.

But the two could hardly be more different. Whereas the Moon has just a tiny core, Mercury is a cannonball world. Its iron-dominated core takes up 75% of the planet's radius and nearly half its volume. (In fact, after adjusting for compression effects, Mercury is actually denser than Earth.) Something happened at the dawn of solar-system history that set it distinctly apart from its terrestrial siblings.

As lead scientist Sean Solomon explained today, Messenger (short for Mercury Surface, Space Environment, Geochemistry and Ranging) is just going through the first laps of its planned year-long observation plan. In that time the planet will spin beneath the spacecraft's gaze six times. The entire globe will be imaged at eight different visible and near-infrared wavelengths by wide-field and telephoto cameras. Seven other instrument packages will assay its composition, map all of its highs and lows, measure its magnetic field, and scan the surrounding space for charged particles.

Mercury's Debussy crater

A bright spalsh of rays extend for hundreds of miles across Mercury's surface from the relatively fresh crater Debussy, 50 miles (80 km) across. This is color composite (using one violet and two near-infrared images) was the first returned by Messenger after it began orbiting the planet.
Click here for a larger version.

NASA / JHU-APL / Carnegie Inst. of Washington

Meanwhile, just by being in orbit, the spacecraft will be letting radio telescopes on Earth know how much its motion is sped up or slowed in response to subtleties in the planet's gravitational attraction — key to understanding how Mercury's interior is put together and, in particular, where all that iron came from.

One of three things happened: (1) somehow Mercury came together with hardly any lower-density silicates, the kind found abundantly in Earth's crust; (2) it endured a period of extreme heating from the young Sun, which caused many elements simply to boil away; or (3) something really big collided with Mercury and stripped away the lion's share of its crust and mantle.

Mariner 10, the only other spacecraft to make the long interplanetary trip inward, swooped past Mercury three times in the mid-1970s. But it could only observe about half the planet in detail (a consequence of Mercury's unique spin-orbit resonance), and it lacked the infrared spectrometer than could have coaxed out Mercury's mineral makeup.

Long shadows on mercury

A low Sun angle accentuates detail in this view near Mercury's terminator. Arrows mark one of the planet's many compression (thrust) faults, likely caused when the infant planet cooled and shrank slightly in volume.

NASA / JHU-APL / Carnegie Inst. of Washington

Messenger's spectroscopic observations should settle the argument. If (1) is correct, then the surface will be covered with a mixture of common minerals. By contrast, the evaporation model (2) would have left the planet depleted in "volatile" elements like potassium and sodium — but then why are these atoms oozing into space from its surface? And if the impact hypothesis is correct, then the silicate minerals on the planet's surface should have compositions more mantle-like than metal-poor crustal rocks.

The science team has promised to give us some answers by early May. Until then, let's just enjoy the images Or do your own flyovers of Mercury using the downloadable KML file for Google Earth.


Image of Rex


March 31, 2011 at 10:21 am

I believe the fourth possibility is that Mercury as we see it now, is the core of a dead star. Recently a new M class of star was announced which caused me to think that Mercury was a star or nearly so and that the iron core we are seeing is what remains. Also not too long ago, a theory as to the current planets positions was published where Jupiter was closer to the Sun and moving towards it while Saturn's presence stopped this action and reversed it to where the planet are located today. With so many other stars having companions, some much closer to the Sun than Mercury currently is, why not think of Mercury as the core of a dead star? Not only that, but Mercury has a tail of sorts. What planets has that?
I don't have the math, physics or chemestry background to prove my theory so all I can rely on is some common sense. If Mercury were wacked by something, then how come no huge collision scars are seen? Why not a huge spin rate? or wild spin motion?

I will leave it to others to take my idea a step further and come up with reasons why it could not be a former dwarf or midget star or almost star...whatever.

It seems to me that I read where a star that has used up all of its hydrogen, helium and other gases, that all the compression forces cause the core to become dense and eventually producing an iron core. Mercury in my opinion was not massive enough to become a nova, just plain burned out.

I will look forward to other's ideas on this.

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Image of eanassir@gmail.com


April 1, 2011 at 12:03 pm

Mercury stopped its axial rotation a long time ago, when its core became cold, then its gravity weakened --> so it lost its atmosphere into the outer space, and it contracted leading to increasing its density and fissuring of its surface.
Moreover, it lost is water; and a large number of craters appeared on its surface because of the falling oa a large number of comets on it when it became cold.
Now it does not spin around itself: one side has a perpetual day which is facing the sun and is very hot; while the opposite side has a continuous night and is very cold.


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Image of Andrew


April 2, 2011 at 2:27 am

What a picture! But what is that object about halfway to the edge of the photo from crater Debussy, just below the nine o'clock position? It looks like a crater with an inverted V leading away from it. Superficially it looks like a shadow but it would have to be a very high and bifid mountain.

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Image of Phil


April 2, 2011 at 4:31 pm

Eanassin, you should get your "facts" from scientific publications and not religious books. Mercury does in fact rotate 3 times for every 2 revolutions around the Sun. This was unexpected (it was thought to rotate 1:1) and discovered only during the Mariner 10 mission.

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Image of Phil


April 2, 2011 at 4:35 pm

Rex, it is unlikely that Mercury is a "dead star". If it was ever large enough to ignite (and extinguished almost immediately), it wouldn't have cooled down enough to be a (relatively) cold, solid body in the 4.5 billion years it has existed. That much iron and other metals suggests that it would have been a fairly good sized star to begin with (assuming roughly the same metalicity as the Sun), so I can't see how it would have cooled down so much in such a short time (as well as losing all gases and volatiles).

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Image of Graham W. Wolf

Graham W. Wolf

April 4, 2011 at 8:33 pm

The probe's safely made it into Mercurian orbit... phew! Now for the awesome mind-blowing photos to come. I await these with great pleasure, as the forthcoming photos from the Pluto Mission in a few year's time. That's the OTHER planet that needs to be extensively imaged.

Go hard NASA, go well!


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Image of Rex


April 5, 2011 at 2:56 pm

I am glad to see that others have added their thoughts regarding my idea about Mercury. As I recall, it was around the mid-1960's, 1965 I think, that Mercury was found to rotate in a 2:3 manner. Anyway, I like all the rest of you, look forward to information gained from Messenger regarding the magnetic field structure, heat loss, gravitational influence and so forth. How do we know how hot Mercury is? How much is the heat caused from the Sun compared to any heat generated from Mercury itself? This is the first time we have had a probe linger around the planet and I am anxious to learn what we can. If my idea is correct, then I would expect Mercury to have a static magnetic field, one that was etched in place. If it varies, then that would tell me that the interior is still active much like the Earth. Don't forget, Mercury is the only planet to orbit the Sun very close to the equator, about 5 degrees as I recall. A good spot for a double-star to start to form. Just a thought.

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Image of Larry Geary

Larry Geary

April 8, 2011 at 6:27 pm

Will Messenger's mission controllers have to deal with any small effects of General Relativity given the proximity to the sun? After all, Mercury's orbit is very slightly effected by GR compared to the predictions of Newtonian mechanics.

On a different subject, the many announcements of close-in exoplanets assume the planet is tidally locked. Since Mercury's rotation is NOT locked into a 1:1 ratio with its orbit, maybe these exoplanets are not locked either. In the case of many close-in exoplanets around a single star, there could be complex resonances among the rotation periods, or they may even rotate freely, or even chaotically.

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