MESSENGER, the Largest Spacecraft on Mercury, Observes the Planet’s Core and Exosphere

Scientists want to better understand how Mercury formed and evolved during its 4.5 billion-year existence, so they are studying both its interior and exterior features.

Without major rocket advancements like solar sails or solar electric propulsion, reaching Mercury was unlikely. But researchers found a way to use its gravity as an aid.

Mercury’s Surface

Mercury’s proximity to the Sun makes it challenging for astronomers on Earth to study. Even at its closest approach, it measures only three times larger than our Moon and seven times brighter; optimal viewing occurs between January and April and late August to December.

Mariner 10 made three flybys of Mercury during its three flybys in the 1970s, but more recently MESSENGER was able to orbit Mercury until running out of fuel and colliding into its surface in 2015. MESSENGER revealed numerous fascinating facts about Mercury – such as that its polar deposits are composed mainly of water ice.

MESSENGER data also revealed that Mercury’s rocky terrain continues to change billions of years after an asteroid strike, as seen by its surface striations known as “crater rays”, which appear brighter due to their finer grain crushed rock composition that reflects more light.

Mercury’s Atmosphere

Mercury differs significantly from Earth in that it lacks a thick atmosphere; instead, its tenuous exosphere consists of oxygen, hydrogen, sodium and helium held together by gravity in an irregular distribution sphere.

Mercury is constantly losing and creating gases as micrometeoroid impacts and subatomic particles from the sun hit its surface, freeing atoms to escape into space and thus creating the exosphere – an ever-evolving balance of creation and loss.

For better insight, scientists analyzed data collected during MESSENGER’s three flybys. MESSENGER measured variations in concentration of three key exospheric elements – sodium, calcium and magnesium. Researchers discovered each element experience its own seasonal variation tied to its orbit – for instance the sodium concentration increases as the planet gets closer to the Sun while diminishing as it moves further away.

Mercury’s Magnetosphere

Mercury’s magnetosphere is heavily affected by both solar wind dynamics and internal dynamo effects, with particularly dramatic impacts during interplanetary coronal mass ejections (ICMEs), expected to occur frequently during this period of high solar activity.

MESSENGER was an extraordinary witness to these events, collecting an unparalleled dataset that not only provided an unprecedented view of Mercury’s magnetic field structure but also revealed important insights into the physical processes that drive magnetosphere collapse and can be controlled by solar wind conditions.

On a recent flyby of Mercury, PICAM and MIPA measured ion densities just downstream the planet’s bow shock. Their data reveal a consistent signal with an unexpectedly higher average density, likely caused by an extremely large ICME Pram event that collapsed Mercury’s dayside magnetosphere on this orbit. They also demonstrate an unexpectedly large tail current sheet; an increase in current density leads to greater magnetic pressure on either northern or southern tail lobes of Mercury’s tail lobes resulting in an increased magnetic pressure increase on both northern or southern tail lobes of its tail lobes of Mercury’s tail lobes of its tail lobes.

Mercury’s Core

Mercury, as the second-smallest planet in our Solar System, is also its densest. Its thick metallic core comprises over 50% of its volume; much thicker than Earth’s. Geologists use data gathered by MESSENGER to understand what constitutes Mercury’s core.

This spacecraft discovered anomalous patterns on Mercury’s surface craters, such as ridges and troughs not present elsewhere, as well as mapping in unprecedented detail and discovering carbon-containing organic compounds.

MESSENGER’s gravity measurements allowed scientists to calculate Mercury’s mass and density, which, combined with topographic data, enabled researchers to assess its internal structure. For instance, this revealed its solid inner core resting beneath an outer layer of iron sulfide; and suggested its mantle and crust are less dense than anticipated – supporting one theory on how our Solar System’s smallest rocky planet developed its massive inner core; researchers may use this discovery as evidence against alien stars with which it orbits.