After more than 10 years in space and over 4 years orbiting Mercury, the MESSENGER mission ended last week when the spacecraft crashed into the surface of the planet on April 30th. While the impact was not visible from Earth, MESSENGER managed to send back 5 final images at an altitude of about 40 km from the surface. Travelling at a speed of 8,750 mph (or 14,000 km/hr) the impact would have produced a new crater about 15 m wide on the Mercury’s surface.

One of the final images sent back by MESSENGER from an altitude of about 40 km before colliding with the surface of Mercury of 30 April 2015. (Credit: NASA/JHU Applied Physics Laboratory/Carnegie Institution of Washington)

MESSENGER was launched 3 August 2004 and travelled for  more than six and a half years going into orbit around Mercury on 18 March 2011, becoming the first spacecraft to orbit  the innermost planet in our Solar System. During its journey to Mercury, MESSENGER completed a flyby of Earth (August 2005), two flybys of Venus (October 2006 and June 2007) and three flybys of Mercury (January 2008, October 2008 and September 2009).

The primary mission was successfully completed in March 2012 and MESSENGER continued to complete two additional extended missions, XM1 and XM2. Finally on 24 April 2015 MESSENGER ran out of propellant, which prevented it from maintaining its altitude and so it finally succumbed to gravity and crashed into the surface of Mercury on 30 April 2015.  Unfortunately the impact was not visible as the collision was on the side of the planet facing away Earth and space telescopes cannot look at Mercury due to its proximity to the Sun, which would damage sensitive telescope optics.

During its mission, MESSENGER provided an enormous amount of data and has greatly changed our understanding of Mercury. The primary goals of the mission were to study Mercury’s chemical composition, geology and magnetic field to better understand the formation and evolution of the planet, as well as its interaction with the Sun. Some of the key science highlights of the MESSENGER mission include:

  • Being the first satellite to map the entire surface of Mercury. The surface is dominated by impact craters, but also be volcanism. This was clearly demonstrated by MESSENGER when it imaged volcanic vents near the rim oftheCaloris Basin, one of the largest and youngest impact crater in the solar system

    False-colour image of the 1,500 km-wide Caloris impact basin. The orange areas are lava that flooded the original basin, and subsequent impacts are shown in blue, revealing original basin floor material. (Credit: NASA, JHU APL, Arizona State U., CIW)


  • MESSENGER confirmed that water ice exists in the polar regions of Mercury.  The day side of Mercury canexceed300C, but due to the lack of obliquity (or axial tilt) in Mercury’s orbit, the floor of solar polar craters never receive any sunlight and temperatures are kept at achilly-170C.  MESSENGER detected water ice in the polar regions covered with an as-yet mysterious dark organic material.

    Water ice in the northern polar region of Mercury, seen in yellow inside craters that are in constant darkness. (Credit: NASA/JHU Applied Physics Laboratory/Carnegie Institution of Washington)

  • A detailed understanding of Mercury’s global cooling, which produced huge cliffs known as lobate scarps.  These form when the giant core of Mercury cools and effectively causes the entire planet to shrink.  The core comprises about 65% of Mercury by mass, and as it cools it contracts (and it is thought that the cooling of Mercury’s core caused it to shrink by 1-2 km in radius), causing the overlying layers to similarly contract. This results in wrinkly lobate scarps on the surface.

Y-shaped lobate scarp in a large old crater on Mercury. The right-hand side of the Y shape crosses the crater floor and the crater rim is a classic lobate scarp seen in almost all areas of Mercury. (Credit: NASA/JHU Applied Physics Laboratory/Carnegie Institution of Washington)

  • Amazing new details have also been uncovered about Mercury’s incredibly thin atmosphere, known as an “exosphere”.  The exosphere is so thin that atoms and molecules in the this atmosphere are actually more likely to collide with the surface than other particles in the exosphere!  The material found in the exosphere, which comprise volatiles like hydrogen, helium, sodium, potassium and calcium, are through to result from sputtering of the surface of Mercury, kicked up by solar radiation and solar wind, as well as by meteorite impacts.  Due to interactions with the strong solar wind, the exosphere stretches out into an amazing 2 million km tail away from the Sun.

    The giant sodium exosphere tail of Mercury comparison during the second and third Mercury flybys (compared with models). The changes in The sodium derives from material being sputtered from the surface at high latitudes on the day side of Mercury. Interactions with the solar wind carry the sodium atoms “downstream” of the solar wind. The changes seen are through to result from variations in Mercury’s exospheric “seasons.” (Credit: NASA/JHU Applied Physics Laboratory/Carnegie Institution of Washington)

  • A further mystery of Mercury which MESSENGER has helped shed light on in the global magnetic field of the planet. The Mariner 10 mission detected Mercury’s magnetic field in the 1970s, which was puzzling to astronomers as the huge iron core of Mercury was through to have cooled long ago, preventing a global magnetic field.  While only about 1% the strength of the Earth’s magnetic field, a global magnetic field is difficult to understand. The field was thought to a ‘relic field’, frozen into the rocks of the outer surface of the planet when the core cooled and the global magnetic field presumably died away.  However, MESSENGER data confirms the global magnetic field of Mercury and there is now consensus that Mercury indeed hosts a global active magnetic dynamo in the core similar to the Earth.  Mercury’s magnetic field interacts with the interplanetary magnetic field and charged particles from the solar wind, both of which distort the shape of the field like a windsock.

    Mercury’ global magnetic field interacts with the solar wind, resulting in a windsock type shape, similar to other global planetary magnetic fields in the solar system (Credit: NASA/JHU Applied Physics Lab/Carnegie Institute of Washington)

While the MESSENGER mission has ended, there is still a wealth of data for astronomers to work through and we can expect more exciting results from the mission in the coming years.

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[Sarah Maddison]

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