MESSENGER and the Revival of Planet Mercury
With the exception of the Mariner 10, no other space probes had visited Mercury, our innermost planet. And even then, the Mariner 10 mission was just a few flybys in 1974-5 and not a chance for in-depth survey. But the Mercury Surface, Space Environment, Geochemistry, and Ranging probe, aka MESSENGER, was a game changer, for it orbited Mercury for several years. With this long-term exploration, our little rocky planet had the mysterious veil that surrounded it lifted and has proven to be just as fascinating a place as any other in the solar system.
Even though MESSENGER was only 1.05 meters by 1.27 meters by 0.71 meters, it still had plenty of room to carry high-tech instruments built by the Applied Physics Laboratory (APL) at the John Hopkins University (JHU), including:
- -MDIS: Wide and Narrow-Angle Color and Monochrome Imager
- -GRNS: Gamma Ray and Neutron Spectrometer
- -XRS: X-ray spectrometer
- -EPPS: Energetic Particle and Plasma Spectrometer
- -MASCS: Atmospheric/Surface Composition Spectrometer
- -MLA: Laser Altimeter
- -MAG: Magnetometer
- -Radio Science Experiment
And to help protect the payload, MESSENGER had a 2.5 meter by 2 meter sunshade. To power the instruments, two gallium arsenide solar panels 6 meters in length were required along with a nickel-hydrogen battery that would ultimately provide 640 watts to the probe once it reached Mercury orbit. To help maneuver the probe, a single bipropellant (hydrazine and nitrogen tetroxide) thruster was used for large changes while 16 hydrazine-fueled thrusters took care of the small stuff. All of this and the launch ended up costing $446 million, comparable to the Mariner 10 mission when taking inflation into account (Savage 7, 24; Brown 7).
But let’s look at some details about these impressive pieces of technology. MDIS made use of CCDs much like the Kepler Space Telescope, which collect photons and store them as an energy signal. They were able to view a 10.5-degree area and had the ability to look at wavelengths from 400 to 1,100 nanometers courtesy of 12 different filters. GRNS has the two previously mentioned components: the gamma ray spectrometer looked out for hydrogen, magnesium, silicon, oxygen, iron, titanium, sodium, calcium, potassium, thorium, and uranium through gamma ray emissions and other radioactive signatures while the neutron spectrometer looked for those being emitted from subsurface water being hit by cosmic rays (Savage 25, Brown 35).
XRS was a unique design in its functionality. Three gas-filled compartments looked at X-rays coming from Mercury’s surface (a result of the solar wind) and used it to gather data on the subsurface structure of the planet. It could look in a 12-degree area and detect elements in the 1-10 kilo eV range, such as magnesium, aluminum, silicon, sulfur, calcium, titanium, and iron, MAG looked at something else entirely: magnetic fields. Using a fluxgate, 3-D readings were gathered at all times and later stitched together to get a feel for the environment around Mercury. To ensure that MESSENGER’s own magnetic field did not disrupt readings, MAG was at the end of a 3.6-meter pole (Savage 25, Brown 36).
MLA developed a height map of the planet by firing IR pulses and measuring their return time. Ironically enough, this instrument was so sensitive that it was able to see how Mercury wobbles on its orbital z-axis, allowing scientists a chance to infer the internal distribution of the planet. MASCS and EPPS both made use of several spectrometers in an effort to uncover several elements in the atmosphere and what is trapped in Mercury’s magnetic field (Savage 26, Brown 37).
Orbital Manuever: Venus
MESSENGER was launched on a three-stage Delta II rocket from Cape Canaveral on August 3, 2004. In charge of the project was Sean Solomon from Columbia University. As the probe flew past Earth, it turned MDIS back to us to test out the camera. Once in deep space, the only way to get it to its destination was through a series of gravitational tugs from Earth, Venus and Mercury. The first such pull occurred in August of 2005 as MESSENGER got a boost from Earth. The first Venus flyby was on October 24, 2006 when the probe got to within 2,990 kilometers of the rocky planet. The second such flyby occurred on June 5, 2007 when MESSENGER flew within 210 miles, considerably closer, with a new velocity of 15,000 miles per hour and a decreased orbit around the sun that placed it within the possible bounds for a Mercury flyby. But the second flyby also allowed scientists at APL to calibrate their instruments against the already present Venus Express while collecting new scientific data. Such information included atmospheric composition and activity with MASCS, MAG looking at the magnetic field, EPPS examining the bow shock of Venus as it moves through space and looking at solar wind interactions with XRS (JHU/APL: 24 Oct. 2006, 05 Jun. 2007, Brown 18).
Orbital Manuevers: Mercury Flybys
But after these maneuvers, Mercury was firmly in the crosshairs, and with several flybys of said planet MESSENGER would be able to fall into orbit. The first of these flybys was on January 14, 2008, with a closest approach of 200 kilometers as MDIS took photographs of many regions that had not been seen since Mariner 10’s flyby from 30 years prior and some new ones including the far side of the planet. Even all of these preliminary photos hinted at some geological processes that went longer than anticipated based on lava plains in filled craters as well as some plate activity. NAC happened to spot some interesting craters than had a dark rim around them as well as well-defined edges, hinting at a recent formation. The dark part is not so easy to explain. It is likely either material from below brought up from the collision or it is melted material that fell back onto the surface. Either way, radiation will eventually wash out the dark color (JHU/APL: 14 Jan. 2008, 21 Feb. 2008).
And more science was being done as MESSENGER approached for flyby number 2. Further analysis of data gave scientists a startlingly conclusion: the magnetic field of Mercury is not a remnant but is dipolar, meaning that the interior is active. The most likely event is that the core (which was figured at 60% of the mass of the planet at the time) has an outer and inner zone, of which the outer is still cooling off and thus has some dynamo effect. This seemed backed up not only by the smooth plains mentioned above but also by some volcanic vents seen near the Caloris basin, one of the youngest known in the solar system. They filled in craters formed from the Late Heavy Bombardment Period, which also plummeted the moon. And those craters are twice as shallow as those on the moon based on altimeter readings. All of this challenges the idea of Mercury as a dead object (JHU/APL: 03 Jul. 2008).
And another challenge to the conventional view of Mercury was the weird exosphere it has. Most planets have this thin layer of gas that is so sparse that the molecules are more likely to hit the surface of the planet than they are with each other. Pretty standard stuff here, but when you take into account Mercury’s extreme ellipse of an orbit, the solar wind, and other particle collisions, then that standard layer becomes complex. The first flyby allowed scientists to measure these changes and to also find hydrogen, helium, sodium, potassium, and calcium present in it. Not too surprising, but the solar wind does create a comet-like tail for Mercury, with the 25,000-mile-long object being made mostly of sodium (Ibid).
The second flyby wasn’t much in terms of scientific revelations but data was indeed collected as MESSENGER flew by on October 6, 2008. The final one occurred on the 29th of September in 2009. Now, enough gravity tugs and course corrections ensured that MESSENGER would be captured next time instead of zooming by. Finally, after years of prepping and waiting, the probe entered orbit on March 17, 2011 after orbital thrusters fired for 15 minutes and thus cutting the speed down by 1,929 miles per hour (NASA “MESSENGER Spacecraft”).
A Changing Picture of a Planet
And after 6 months of orbiting and snapping pictures of the surface, some major findings were released to the public that began to shift the viewpoint of Mercury being a dead, barren planet. For starters, past volcanism was confirmed, but the general layout of the activity was not known, but a wide stretch of volcanic plains was seen near the northern pole. Altogether, about 6% of the surface of the planet has these plains. Based on how much of the craters in these regions were filled, the depth of the plains could be as much as 1.2 miles! But where did the lava flow from? Based on similar looking features on Earth, the solidified lava was probably released through linear vents that have now been covered by the rock. In fact, some vents have been seen elsewhere on the planet, with one being as long as 16 miles. Places near them exhibit a teardrop shape regions that can be indicative of a different composition that interacted with the lava (NASA “Orbital Observations,” Talcott).
A different kind of feature was found that left many scientists scratching their heads. Known as hollows, they were first spotted by Mariner 10 and with MESSENGER there to collect better photos scientists were able to confirm their existence. They are blue depressions found in close groups and frequently seen in crater floors and central peaks. There seemed to be no source or reason for their odd shading but have been found all over the planet and are young based on the lack of craters within them. The authors at the time felt it was possible that some internal mechanism was responsible for them (Ibid).
Then scientists began looking at the chemical makeup of the planet. Using GRS, a respectable amount of radioactive potassium was seem, which surprised scientists because it is quite explosive at even small temperatures. With follow-ups by XRS, further deviations from the other terrestrial planets were seen such as high levels of sulfur and radioactive thorium. Taking these into account destroys most theories about how Mercury formed and left scientists trying to figure out different ways Mercury could have a higher density than the rest of the rocky planets. What is interesting about these chemical findings is how it relates Mercury to metal-rich chondritic meteorites, which are thought of as the left overs of the solar systems formation (NASA “Orbital Observations”).
And when it comes to the magnetosphere of Mercury, a surprise element was spotted: sodium. How the heck did that get there? After all, sodium is known to be on the surface of the planet. As it turns out, the solar wind travels along the magnetosphere towards the poles, where it is energetic enough to break sodium atoms off and create an ion that flows freely. Also seen floating around was helium ions, also a likely product of the solar wind (Ibid).
Extension Number One
With all of this success, NASA decided on November 12, 2011 to extend MESSENGER a full year past its March 17, 2012 deadline. For this phase of the mission, MESSENGER moved into a closer orbit and went after several topics, including finding the source of surface emissions, a timeline on the volcanism, details on the density of the planet, how electrons change Mercury, and how the solar wind cycle impacts the planet (JHU/APL 11 Nov. 2011).
One of the first findings of the extension was that a special physics concept was responsible for giving Mercury’s magnetosphere motion. Called the Kelvin-Helmholtz (KH) instability, it is a phenomenon that forms at the meeting place of two waves, similar to what is seen on Jovian gas giants. In Mercury’s case, gases from the surface (caused by solar wind interaction) meet the solar wind again, causing vortices that further drive the magnetosphere, according to the study done in Geophysical Research. The result came only after several flybys through the magnetosphere gave scientists the required data. It seems that the dayside sees a greater disturbance due to the higher solar wind interaction (JHU/APL 22 May 2012).
Later in the year, a study published in Journal of Geophysical Research by Shoshana Welder and team showed how areas near volcanic vents differ in than older areas of Mercury. XRS was able to show that older regions had higher amounts of magnesium to silicon, sulfur to silicon, and calcium to silicon but that the newer places from volcanism had higher amounts of aluminum to silicon, indicating a different origin for the surface material possibly. Also found was high levels of magnesium and sulfur, with levels nearly 10 times that seen in other rocky planets. The magnesium levels also paint a picture of hot lava as a source, based off comparable levels seen on Earth (JHU/APL 21 Sept. 2012).
And the magma picture grew even more interesting when features reminiscent of tectonics were found in the lava plains. In a study by Thomas Watlens (from the Smithsonian) published in the December 2012 issue of Science, as the planet cooled off post-formation, the surface actually started to crunch against itself, forming fault lines and graben, or raised ridges, that were made more prominent from then-molten lava cooling off as well (JHU/APL 15 Nov. 2012).
Around the same time, a surprise announcement was released: water ice was confirmed to be on Mercury! Scientists had suspected it was possible because of some polar craters that are in permanent shadow courtesy of some fortunate axis tilt (less than a whole degree!). On top of this, radar bounces found by the Arecibo radio telescope in 1991 looked like water ice signatures. MESSENGER found that the two ideas were indeed correlated by reading the number of neutrons bouncing off the surface as a product of cosmic ray interactions with hydrogen, as recorded by the neutron spectrometer. Other evidence included differences in laser pulse return times as recorded by MLA, for those differences can be a result of material interference. Both support the radar data. In fact, the northern polar craters mainly have water ice deposits 10 centimeters deep below a dark material which is 10-20 centimeters thick and keeps temps just a bit too high for the ice to exist with it (JHU/APL 29 Nov. 2012, Kruesi “Ice”).
Extension Number Two
The success behind the first extension was more than enough evidence for NASA to order another on March 18, 2013. The first extension not only found the above findings but also showed that the core is 85% the diameter of the planet (compared to Earth’s 50%), that the crust is mainly silicate with a later of iron between the mantle and the core, and that the height differentials on the surface of Mercury are as big as 6.2 miles. This time, scientists hoped to uncover any active processes on the surface, how materials from volcanism have changed during time, how electrons impact the surface and the magnetosphere and any details about the thermal evolution of the surface (JHU/APL 18 Mar. 2013, Kruesi “MESSENGER”).
Later in the year, it was reported that lobate scarps aka graben, or sharp divides in the surface that can extend far above the surface, prove that Mercury’s surface shrunk more than 11.4 kilometers in the early solar system, according to Paul Byrne (from Carnegie Institution in DC). Mariner 10 data had only indicated 2-3 kilometers, which was well below the 10-20 theoretical physicists were expecting. This is likely because of the huge core transferring heat to the surface in a more efficient manner than most planets in our solar system (Witze).
By mid-October, scientists announced that direct visual evidence for water-ice on Mercury was found. By making use of the MDIS instrument and the WAC broadband filter, Nancy Chabot (the Instrument Scientist behind MDIS) found it was possible to see light reflected off the crater walls which then hit the crater bottom and back to the probe. Based on the level of reflectivity, the water ice is newer than the
Prokiev crater that hosts it, for the boundaries are sharp and organic-rich which implies recent formation (JHU/APL 16 Oct. 2014, JHU/APL 16 Mar. 2015).
In March of 2015, more chemical features were revealed on Mercury. The first was published in Earth and Planetary Sciences in an article entitled, “Evidence for geochemical terranes on Mercury: Global mapping of major elements with MESSENGER's X-Ray Spectrometer,” in which the first global picture of magnesium-to-silicon and aluminum-to-silicon abundance ratios was released. This XRS data set was paired with previously collected data on other chemical ratios to reveal a 5 million square kilometer stretch of land which has high magnesium readings which could be indicative of an impact region, for that element is expected to reside in the planet’s mantle (JHU/APL 13 Mar. 2015, Betz).
The second paper, “Geochemical terranes of Mercury's northern hemisphere as revealed by MESSENGER neutron measurements" published in Icarus, looked at how low-energy neutrons are absorbed by the mainly silicon surface of Mercury. Data collected by GRS shows how elements which take in neutrons like iron, chlorine, and sodium are distributed over the surface. These too would have resulted from impacts digging into the mantle of the planet and further imply a violent history of Mercury. According to Larry Nittle, the deputy principal investigator of MESSENGER and a co-author for this and the previous study, it implies a 3-billion-year-old surface (JHU/APL 13 Mar. 2015, JHU/APL 16 Mar. 2015, Betz).
Just a few days later, several updates were released about previous MESSENGER findings. It was a while ago, but remember those mysterious hollows on the surface of Mercury? After more observations, scientists determined that they form from sublimation of surface materials that once gone create a depression. And small lobate scarps, which hinted at a contraction in the surface of Mercury, were found alongside their larger cousins, which are 100s of kilometers in length. Based on the sharp relief at the top of the scarps, they cannot be older than 50 million years old. Otherwise, meteoroid and space weathering would have dulled them (JHU/APL 16 Mar. 2015, Betz).
Down with MESSENGER
Thursday, April 30, 2015 was the end of the road. After engineers squeaked out the last of the probe’s helium propellant in an effort to give it more time past the planned March deadline, MESSENGER met its inevitable end as it crashed into the surface of Mercury at about 8,750 miles an hour. Now the only evidence for its physical existence is a 52-foot-deep crater which was formed as MESSENGER was on the opposite side of the planet from us, meaning that we missed the fireworks. In total, MESSENGER:
- -Orbited 8.6 Mercury days aka 1,504 Earth days
- -Went around Mercury 4,105 times
- -Took 258,095 pictures
- -Traveled 8.7 billion miles (Timmer, Dunn, Moskowitz)
Post-Flight Science, or How the Legacy of MESSENGER Continued On
But despair not, for just because the probe is gone does not mean that the science based off the data it collected is. Just a week post-crash, scientists found evidence for a much stronger dynamo effect in Mercury’s past. Data collected from an altitude of 15-85 kilometers above the surface showed magnetic fluxes corresponding with magnetized rock (JHU/APL 07 May 2015, U of British Columbia).
And because of the proximity to Mercury, detailed data on its libations, or gravitational interactions with other celestial objects, was collected. It showed that Mercury spins about 9 seconds faster than Earth-based telescopes were able to find. Scientists theorize that libations from Jupiter may tug on Mercury long enough for the hang up/speed up, depending on where the two are in their orbits. Regardless, the data also shows that the libations are twice as large as suspected, further hinting at a non-solid interior for the little planet but in fact a liquid outer core that accounts for 70 percent of the mass of the planet (American Geophysical Union, Howell, Haynes).
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© 2016 Leonard Kelley
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