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Messenger's Detective

by Perry Pezzolanella

Mercury is a desert world that seemed uninteresting and moonlike. NASA’s Mariner 10 and MESSENGER missions revealed much more and have made Mercury worthy of a new, sophisticated, flagship-class mission. Mercury is dominated by a large, solid iron core and a liquid outer core of iron, sulfur, and silicates. An iron-poor silicate mantle lies below a six-mile, thick silicate crust. The surface may be scorching hot, exceeding 800ºF on the dayside, but water ice has been detected in the permanently shadowed polar craters where temperatures fall well below -300ºF. Mercury has an inner core way too large for its size along with a surprising chemical composition. A detective is needed to solve these and other mysteries. The European Space Agency (ESA) – Japan Aerospace Exploration Agency (JAXA) BepiColombo mission is the detective that will make a dedicated effort.

Where Mercury formed is a mystery. At 3030 miles in diameter, Mercury is about midway in size between the Moon and Mars and orbits the Sun every 88 days at an average distance of 36 million miles. MESSENGER orbited Mercury from 2011-2015 and found that there is too much of the volatile chemical element potassium compared to thorium on the surface. Potassium evaporates very quickly in the intense heat while thorium survives. Planets closer to the Sun should have more thorium and less potassium, but Mercury’s potassium-thorium ratio is more like Mars. Did Mercury form further out in the Solar System and drift inward to its current position? Or did Mercury suffer from a huge impact that stripped away most of its crust leaving only the core and a thin crust? BepiColombo will investigate.

Another mystery is if there really is water on Mercury. MESSENGER confirmed ice at the poles, but where did it come from and is it primarily water ice? Mercury is not tilted on its axis so there are permanently shadowed craters that are always frigid and can maintain ice. BepiColombo will determine the chemical composition of the ice and where it originated. Comets are a theory as a source, but they are rare near Mercury. Asteroid impacts are another possibility and BepiColombo will investigate. Is Mercury geologically dead, or is it still active? MESSENGER found strange geological features that appear to be recent dents dotting areas inside and around a few craters. Called hollows, these dents may be the result of some volatile material outgassing from the outer layer of Mercury and sublimating. BepiColombo will image Mercury more than ten years after MESSENGER so it should be able to detect any changes in the hollows, either growing or shrinking. Mercury would then be classified as an active world if change is confirmed. Are there any active volcanic vents? MESSENGER found a few suspects and BepiColombo will be sure to investigate.

Mercury is a dark planet compared to the Moon since it reflects only about two-thirds as much sunlight as the Moon. BepiColombo will determine the mineral and element composition and distribution of the surface. It will image Mercury at a much closer range and more thoroughly than MESSENGER at higher resolution. Why does Mercury have a magnetic field while Venus and Mars do not? Mercury seems too small to have one, but it does, even though it is weaker than Earth’s. Mercury must have a partially molten core. BepiColombo will be orbiting Mercury close enough to detect any tides caused by the Sun. If it detects tides, then it is proof that Mercury has not cooled off to the point of being totally solid.

BepiColombo, launched on October 20, 2018, is well along on its journey to Mercury with arrival on December 5, 2025. It is a true detective almost befitting its name, but spelled differently from the famous TV detective, Columbo, because it is named after an Italian scientist, mathematician, and engineer, Giuseppe “Bepi” Colombo. Oh, there is just one more thing, mysteries are meant to be solved.