Comet Messengers Nearing ☿ Mercury

MESSENGER spacecraft has been orbiting the planet Mercury since March 17, 2011. The instrument payload has provided, and continues to provide, a wealth of information. It is the first spacecraft to orbit that planet. It has given us views of Mercury that mankind has never seen. The spacecraft has acquired more than 150,000 images and a vast amount of other data. The entire surface has been mapped. MESSENGER is scheduled to continue orbital operations until late March 2015. Click the next image for a more detailed view.

The spacecraft’s seven scientific instruments and radio science investigation are unraveling the history and evolution of the innermost planet. In early November, the spacecraft turned its attention to the approaching new comet C/2012 S1 (ISON) and the well-known short-period comet 2P/Encke. It has returned the first images of the two comets from orbit around Mercury. This is a very different perspective on the comets than ours from Earth. There will be more images in the days to come.

Three images of Encke on November 6 – 8 are on the left. Three images of ISON are shown on November 9 – 11 are on the right. The faint comets are within the >< brackets. Both comets appear to brighten a little each day when compared top to bottom. Encke was 0.5 AU from the Sun and 0.2 AU from MESSENGER. Comet ISON was 0.75 AU and 0.5 AU, respectively. An astronomical unit AU is the distance between the Sun and Earth. More images will be obtained as the comets get brighter and closer to Mercury. Details here.

How Do You Get to Mercury?

The animation below illustrates how MESSENGER followed a path through the inner solar system, including one flyby of Earth, two flybys of Venus, and three flybys of Mercury. The fullscreen button makes it easier to watch. Several repeat viewings helps, too. Keep your eye on the spacecraft as it is passed by each planet. You will notice it slows down with each encounter and falls into a different orbit.

Each time the craft made a flyby of a planet, its speed and direction were altered by what is known as the slingshot effect. The speed of MESSENGER changed due to the gravitational pull of the planet as it was passed by the planet. With the new speed and direction, the craft was set on a new orbit around the Sun that intersected the orbit of the next inner planet. As it fell inward toward the Sun in these stages, it gradually settled into a final orbit that intersected the orbit of Mercury. On March 17, 2011, the on-board engine was able to slow the craft into a final eccentric orbit around Mercury. It will remain in orbit for the duration of the mission into March 2015.

Six Important Questions

The MESSENGER mission hopes to  answer several questions about Mercury.

1. Why is it so dense? The metal-rich core has 60% of the planet’s mass, twice the %-age as for Earth. Why so different?
2. What is its geologic history? The suite of 7 instruments will map the surface history of the entire planet as well as some of the interior structure.
3. What is the nature of the magnetic field? Earth and Mercury have magnetic fields. Venus and Mars do not. What accounts for the difference?
4. What is the nature of its core? Measurements will determine the size of Mercury’s core and whether it is surrounded by a liquid layer.
5. What are the highly reflective materials at the poles? Is water ice able to exist in the permanently shadowed craters?
6. What volatile gases are in its exosphere? Hydrogen, helium, oxygen, sodium, potassium, calcium, and magnesium are known. Are there others? What is their origin?

What Instruments Are Used?

Selecting the scientific instrumentation for a mission is a balance between resources for mass, power, space, schedule, and cost. In the case of MESSENGER, it was especially difficult. Mass was limited to 50 kilograms (110 pounds). It needed extra rocket fuel for final orbit insertion burns. The instruments had to be mounted where Mercury was visible to them, but the Sun was not. The nearness to the Sun causes high temperature and radiation values that would shorten the life of the instruments.

Some Image Gallery Highlights

  

  

  

Rotation animation showing the south pole over a 176 day period. Craters here are deep enough to never get sunshine in the bottoms. Ice exists there. (The planet does not stop halfway and pause.)

Some Findings So Far 

Surface Details

Imaging maps are providing the first global look at the planet. They reveal broad expanses of smooth plains near Mercury’s north pole, likely among the largest expanses of volcanic deposits on Mercury. Volcanism shaped much of Mercury’s crust.

Higher resolution observations at up to 10 meters per pixel reveal light patchy deposits of rimless, irregular pits varying in size from hundreds of meters to several kilometers associated with central peaks, peak rings, and rims of craters. These appear to be venting spots of some sub-surface gases.

Composition

Magnesium/silicon, aluminum/silicon, and calcium/silicon ratios averaged over large areas of the planet’s surface show that Mercury’s surface is not dominated by feldspar-rich rocks. There are large amounts of sulfur at Mercury’s surface as well as radioactive isotopes of potassium and thorium.

“The abundance of potassium rules out some prior theories for Mercury’s composition and origin,” says Larry Nittler, a staff scientist at the Carnegie Institution of Washington. “Moreover, the inferred ratio of potassium to thorium is similar to that of other terrestrial planets, suggesting that Mercury is not highly depleted in volatiles, contrary to some prior ideas about its origin.”

Topography

The north polar region of Mercury, for instance, is a broad area of low elevations.

Tests for polar ice deposits preserved on the cold, permanently shadowed floors of high-latitude impact craters are being done. Evidence is strong for abundant amounts of water ice in the permanently shadowed craters at the poles. Details here.