Why There Aren’t Any Spacecraft on Mercury Yet

NASA’s Messenger mission ended in 2015 and ESA-JAXA BepiColombo won’t reach Mercury until 2025.

These two spacecraft will coordinate measurements of solar wind, radiation and interplanetary dust; providing in-depth knowledge about Mercury’s magnetosphere and exosphere.

Mercury’s thin exosphere

Mercury lacks an atmosphere to protect it, leaving it exposed to the Sun’s constant solar wind. This steady flow of particles strikes the planet, altering its magnetosphere as they pass by and creating low-frequency waves that travel “upstream” within solar wind towards our sun.

Mercury is home to an immense field of magnetized neutral atoms which creates a tail stretching 1.2 million miles (2 million kilometers). Scientists have identified sodium and calcium as primary sources for these tail-trailing neutral atoms that form this cloud around Mercury’s surface.

Scientists have utilized the MESSENGER (Mercury Surface, Space ENvironment, Geochemistry and Ranging) satellite to track how these atoms are distributed throughout our planet. Oxygen and sulfur show strong local exosphere-surface correlations; in comparison, Na and Ca have weak correlations due to solar radiation acceleration that makes it harder for them to volatilize into space; thus making them more likely to be picked up by tail.

Mercury’s surface

Mercury lies so close to the Sun that its surface gets extremely hot, leading Mariner 10’s initial fuzzy images to show an intensely cratered planet that closely resembled Moon’s ancient highlands.

Mariner 10 also mapped Caloris, a massive impact basin named by researchers after one of their early astronauts, and discovered long, steep cliffs thought to have formed as the core cooled over billions of years, crumpling crustal layers to form steep long cliffs that may have led to long term crust crumpling as it collapsed under pressure from cooling core temperatures. Furthermore, Mariner 10 confirmed that Mars did not contain an extensive atmosphere – instead only having a thin layer composed of hydrogen and other gases covering its surface.

MESSENGER used its X-ray, gamma-ray and visible-infrared spectrometers to analyze the elemental composition of Mercury’s rocks. Furthermore, it discovered that some crater floors near Mercury’s poles remain always shadowed – possibly hiding any potential ice deposits there. When BepiColombo arrives in 2025 it will characterize Mercury’s magnetic field, exosphere and polar ice deposits before studying its inner structure and history in detail.

Mercury’s magnetic field

Magnetic fields are a special property found only on certain planets. Their creation comes about via a process known as dynamo, in which heat generated in the core is used to spin it and send waves of magnetism through space, producing magnetic fields in their wake. This phenomenon relies on its power being transmitted into space via spinning; and powers itself with intense heat generated within.

Mercury’s molten core is one of the strongest in our inner solar system, yet its magnetic field is weaker than expected. Scientists speculate that solar wind, with speeds reaching 105,000mph and blowing outward from its source can thwart Mercury’s dynamo mechanism and cause it to slow.

The MESSENGER mission has provided new data on Mercury’s magnetosphere, showing its magnetic field is axisymmetric with two poles and an offset magnetic equator, due to tail and plasma pressures. BepiColombo could offer greater coverage of Mercury’s magnetic field for detailed comparison with model predictions.

Mercury’s atmosphere

Mercury differs greatly from Earth in that it lacks much of an atmosphere; however, there is still a thin layer of gases called an exosphere containing hydrogen, helium, oxygen, sodium calcium potassium water vapor with only 1 trillionth of a pico-bar surface pressure (or one billionth of millionth of Earth’s air pressure at sea level).

Astronomers believe Mercury’s exosphere is replenished by particles streaming from the Sun as solar wind, micrometeoroids hitting its surface, and radioactive decay of elements from its crust; but is being depleted due to gravity as it moves through space. Therefore, scientists were shocked to see three separate occasions when MESSENGER observed its exosphere suddenly brighten up brightly with an increase of up to 20 times density on three separate occasions – which didn’t happen every time; its geometry and timing must have been perfect before; Japanese-European bepiColombo will perform detailed study of Mercury’s magnetosphere, exosphere, and polar ice deposits when arriving sometime between 2025-2027.