MESSENGER mission scientists have confirmed that the probe impacted the surface of Mercury on April 30th, as predicted, at 3:26 p.m. EDT (12:26 p.m. PDT, 7:26 p.m. GMT, 9:26 p.m. CET). Traveling some 8,750 mph (3.91 km per second), MESSENGER made an unseen crater on Mercury’s far side.
This colorful view of Mercury was produced by using images from the color base map imaging campaign during MESSENGER’s primary mission. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up the planet’s surface. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington.
MESSENGER was launched on August 3, 2004, and it began orbiting Mercury on March 18, 2011.
The spacecraft completed its primary science objectives by March 2012. Because its initial discoveries raised important new questions and the payload remained healthy, the mission was extended twice, allowing the probe to make observations from low altitudes and capture images and information about the planet in unprecedented detail.
Last month, the MESSENGER team embarked on a hover campaign that allowed the spacecraft at its closest approach to operate within a narrow band of altitudes, 5 to 35 km above the planet’s surface.
On April 28, the team successfully executed the last of seven orbit-correction maneuvers (the last four of which were conducted entirely with helium pressurant after the remaining liquid hydrazine had been depleted).
With no way to increase its altitude, the probe was finally unable to resist the perturbations to its orbit by the Sun’s gravitational pull, and it crashed into the planet, creating a new crater up to 52 feet (15.8 m) wide.
“Today we bid a fond farewell to one of the most resilient and accomplished spacecraft ever to have explored our neighboring planets,” said Dr Sean Solomon of Columbia University’s Lamont-Doherty Earth Observatory, who is MESSENGER’s Principal Investigator.
“Our craft set a record for planetary flybys, spent more than four years in orbit about the planet closest to the Sun, and survived both punishing heat and extreme doses of radiation.”
“Among its other achievements, MESSENGER determined Mercury’s surface composition, revealed its geological history, discovered that its internal magnetic field is offset from the planet’s center, taught us about Mercury’s unusual internal structure, followed the chemical inventory of its exosphere with season and time of day, discovered novel aspects of its extraordinarily active magnetosphere, and verified that its polar deposits are dominantly water ice.”
In the mid-1970s when Mariner 10 flew past Mercury three times, the probe imaged less than half the planet. Until MESSENGER arrived, the rest of Mercury was a land of mystery.
MESSENGER was the first spacecraft to view the entirety of the mighty Caloris basin – one of the biggest and youngest impact features in the Solar System.
Moreover, the probe spotted volcanic vents around the rim of the basin, proving that volcanism – and not only impacts – has shaped the surface of the innermost planet.
Mercury would seem to be an unlikely place to find ice. But the tilt of Mercury’s rotational axis is almost zero – less than one degree – so the floors of craters at the planet’s poles never see sunlight.
Planetary researchers suggested decades ago that there might be frozen water trapped there. From Mercury orbit, MESSENGER was able to look down on the planet’s poles like no other spacecraft or telescope, and it confirmed the unlikely: permanently shadowed craters near Mercury’s poles have temperatures less than minus 280 degrees F (minus 173 degrees C), and water ice is stable on their dark inner surfaces. Some of the polar ice is covered by a mysterious dark organic material that scientists still do not understand.
The dominant tectonic landforms on Mercury are huge cliffs called ‘lobate scarps.’ Even before MESSENGER, scientists thought these scarps were signs of global shrinkage, like wrinkles on a raisin. Why would Mercury shrink? The planet’s core makes up a whopping 60-70 percent of its mass. Cooling of this oversized core has led to a remarkable contraction of the planet. MESSENGER’s images of lobate scarps show that the total contraction is 2 – 7 times greater than researchers previously thought.
Until Mariner 10 discovered Mercury’s magnetic field in the 1970s, Earth was the only other terrestrial planet known to have a global magnetic field. Earth’s magnetism is generated by the planet’s churning hot, liquid-iron core via a mechanism called a magnetic dynamo. Scientists have been puzzled by Mercury’s field because its iron core was supposed to have finished cooling long ago and stopped generating magnetism. Some scientists thought that the field may have been a relic of the past, frozen in the outer crust. MESSENGER data show otherwise: Mercury’s field appears to be generated by an active dynamo in the planet’s core. It is not a relic.
Orbiting Mercury, MESSENGER also made the first in situ observations of Mercury’s unique exosphere, an ultra-thin atmosphere where atoms and molecules are so far apart they are more likely to collide with the surface than with each other. This material is derived mainly from the surface of Mercury itself, knocked aloft by solar radiation, solar wind bombardment and meteoroid vaporization.
MESSENGER was able to determine the chemical composition of the planet’s exosphere (hydrogen, helium, sodium, potassium, and calcium) and monitor the material as it was stretched out into a comet-like tail as long as 2 million km by the action of the solar wind.
This tail, as well as Mercury’s magnetic field, was often buffeted by solar activity during MESSENGER’s long mission, giving the spacecraft a point-blank view of the roughest space weather in the Solar System.
“A resourceful and committed team of engineers, mission operators, scientists, and managers can be extremely proud that the MESSENGER mission has surpassed all expectations and delivered a stunningly long list of discoveries that have changed our views not only of one of Earth’s sibling planets but of the entire inner Solar System,” Dr Solomon said.
“Going out with a bang as it impacts the surface of Mercury, we are celebrating MESSENGER as more than a successful mission,” said Dr John Grunsfeld of the Science Mission Directorate at NASA Headquarters in Washington.
“Now, we begin the next phase of this mission – analyzing the exciting data already in the archives, and unraveling the mysteries of Mercury.”
A future Mercury mission, such as the ESA/JAXA BepiColombo mission, might be able to identify the impact crater left behind by MESSENGER.
The MESSENGER team has acquired images of the entire planet, so a future mission will have the probe’s observations of the region before the impact to use as a baseline for comparison with subsequent imaging data sets to help pinpoint MESSENGER’s impact site.
The impact crater should be one of the youngest on Mercury and should have exposed fresh material from Mercury’s subsurface that will have been exposed to the effects of space weathering for only a limited and precisely known time, so multispectral observations of MESSENGER’s crater will provide an important constraint on rates of optical maturation of Mercury’s surface material.
