Later in April, one of the most successful NASA missions will come to an explosive close. Scientists and engineers at the NASA Applied Physics Laboratory at Johns Hopkins University will accelerate the robotic Mercury Surface, Space Environment, Geochemistry and Ranging spacecraft – shortened as MESSENGER – downward, like a bullet at Mercury.

MESSENGER was launched in 2004 to study the Solar System’s innermost member. After its primary mission ended in 2012, NASA extended it twice, each fetching more and more discoveries. In 2015, the adventure ends – which is not as tragic as it sounds. Here are 10 feats/discoveries the spacecraft will be remembered for.

Like Frodo’s journey to Mordor

At their closest, Earth and Mercury are separated by 92 million km. If MESSENGER had sped from Earth to Mercury in a straight line, the Sun’s gravity would’ve made it impossible for it slow down to get into orbit around Mercury. Instead, the mission’s engineers at the APL first got MESSENGER into a large orbit around the Sun, then swung it into orbit around Earth, then twice around Venus, and then finally around Mercury. The journey spanned 7.9 billion km and took more than 6 years.

MESSENGER's loopy path from Earth to Mercury. Credit: JHU APL
MESSENGER’s loopy path from Earth to Mercury. Credit: JHU APL

Brimming with brimstone

MESSENGER found Mercury had almost 10-times the amount of sulfur as on Earth or Mars, as well as high levels of the metals magnesium and calcium, and thorium. These elements are usually dredged up from the planet’s insides through volcanoes, so scientists inferred Mercury was a hotbed of volcanic activity. The molten lava from these eruptions has also solidified on the surface to form smooth plains surrounded by rugged terrain, giving it a look much like our moon’s.

Colored maps showing the surface composition of Mercury. Credit: JHU APL
Colored maps showing the surface composition of Mercury. Credit: JHU APL

The surprisingly molten core

Before the MESSENGER mission, astronomers weren’t easily convinced that Mercury’s core could be molten – the planet was too small for its core to have remained liquid for billions of years. However, the probe was able to confirm a partly liquid core based on how the planet’s gravitational field varied. MESSENGER also found the magnetic field due to the flow of the core inside the planet was lopsided. On Earth, the center of the magnetic field is at the center of the core, but on Mercury, it was offset to the north by 484 km.

The nature of Mercury's magnetic field, illustrated. Credit: NASA
The nature of Mercury’s magnetic field, illustrated. Credit: NASA

It probably won’t rain on Mercury, but…

The Sun-facing side of Mercury is scorched to 427 degrees Celsius, and its wheeze of an atmosphere does nothing to cool the surface. In this hell, MESSENGER found the uppermost reaches of its air to contain water vapor. Scientists think powerful bursts of hydrogen from the nearby Sun carved out oxygen from Mercury’s rocks, and then combined to form water. Thanks to the heat, the water vaporized and floated to the top of the atmosphere.

Cold enough for ice

Who’d have thunk it? While boasting of the second-hottest surface of a planet in the Solar System, Mercury also has ice. MESSENGER found 20 billion Olympic skating rinks’ worth of it lurking in the shadowed bottoms of craters near the planet’s north pole. They could’ve got there because, unlike Earth, Mercury’s rotation is not tilted along an axis. As a result, the bottoms of these craters remain shielded from the Sun for long periods of time. MESSENGER also spotted dark patches around the ice that scientists think could be hydrocarbons expelled from comet and asteroid impacts.

Locations of ice around Mercury's north pole, imaged with radar. The brighter it is, the more water there is. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
Locations of ice around Mercury’s north pole, imaged with radar. The brighter it is, the more water there is.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

The other family portrait

On February 14, 1990, the Voyager 1 space probe paused just beyond the orbit of Pluto, turned around and took a selfie that was photobombed by Venus, Earth, Jupiter, Saturn, Uranus and Neptune. This became the iconic Voyager Family Portrait. In November 2010, MESSENGER snapped a similar portrait, this time from near Mercury. The photobombers this time were Mercury, Venus, Earth, Mars, Jupiter and Saturn. Between the two selfies is a picture of our solar neighborhood.

MESSENGER's family portrait, labeled, as taken in 2010. Credit: NASA
MESSENGER’s family portrait, labeled, as taken in 2010. Click for larger image. Credit: NASA

A particle only MESSENGER could study

A lot of what we know about the Sun comes from studying the particles it ejects – protons, neutrons, electrons, photons, gamma rays, etc. However, scientists appreciate uncharged particles because only those are undeterred by magnetic fields, which alter their paths and can also fiddle with their properties. So, we’ve studied solar photons in detail. The problem with the solar neutron, also called the fast neutron, is that within 15 minutes of being thrown into space, it decays into other particles. Near Mercury, the story is different: MESSENGER was able to study them in detail in 2014, using its Neutron Spectrometer instrument.

A solar flare erupted on the far side of the sun on June 4, 2011, and sent solar neutrons out into space. Solar neutrons don't make it to all the way to Earth, but NASA's MESSENGER, orbiting Mercury, found strong evidence for the neutrons, offering a new technique to study these giant explosions. Credit: NASA/STEREO/Helioviewer
A solar flare erupted on the far side of the sun on June 4, 2011, and sent solar neutrons out into space. Solar neutrons don’t make it to all the way to Earth, but NASA’s MESSENGER, orbiting Mercury, found strong evidence for the neutrons, offering a new technique to study these giant explosions. Credit: NASA/STEREO/Helioviewer

Mercury is the densest inner planet

Subtract the compression due to its own gravity, and Mercury’s density is a whopping 5,300 kg/m3 – higher than the three other rocky planets in the Solar System (Venus, Earth and Mars). This implied then that 60% of the planet’s mass was actually its core’s. And a core that heavy would have to take up 75% of the planet’s volume. MESSENGER supplemented this information with the discovery of abundant sulfur, magnesium and calcium on the surface and a paucity of silicates. Piecing all this together has prompted the hypothesis among scientists that Mercury formed with different starting ingredients from the three other rocky planets.

An artist's impression of how much space Mercury's core takes up inside the planet. Credit: NASA
An artist’s impression of how much space Mercury’s core takes up inside the planet. Credit: NASA

Smell something burning?

In 2011, MESSENGER spotted a strange feature: small, rimless depressions on Mercury’s surface that looked nothing like craters but pocked the planet all over. Since called hollows, they have resulted in a terrain that looks distinctly like Swiss cheese. Astronomers think they form when pockets of volatile substances like sulfur are boiled off by the Sun’s heat, leaving behind the fresh wounds. Given the environment in which these hollows form, they’re also likely to be found nowhere else in the Solar System.

The 'hollows'. Credit: NASA/Johns Hopkins APL/Carnegie Institution Of Washington
The ‘hollows’. Credit: NASA/Johns Hopkins APL/Carnegie Institution Of Washington

Plunge to death – for science!

MESSENGER’s running out of fuel. As its orbit shrinks and it slowly spirals downward, the solders holding the spacecraft together face more of the heat being reflected off the surface and start to melt. But before it disintegrates, the scientists operating it have another idea. On April 30, they plan to crash the spacecraft on Mercury’s surface at 14,000 km/hr. The fatal plunge will blow it to smithereens – and in the process give scientists insights into how crash-landing objects like comets and asteroids form craters. Then, the BepiColombo mission to Mercury, destined to launch in 2017, could study the impact in detail.

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