View of planet Mercury from space. Space, nebula and planet Mercury. This image elements furnished … [+]
While Mars and Venus hog the headlines, what little attention our tiny planet Mercury gets is usually an afterthought. Perched on a highly elliptical 88-day orbit around the Sun, our innermost planet is both bizarre and poorly understood.
About a third of the width of Earth, at first glance, the cratered surface of Mercury looks a bit like our Moon. But its gently rolling surface is pockmarked by craters and punctuated by mile-high cliffs that snake across its surface. Mercury’s surface is dominated, however, by the 1550km-wide Caloris Basin, the largest impact basin in our solar system.
The planet is hardly habitable. With surface temperatures that range from 180 to 450 degrees Celsius, few if any planetary scientists think it was ever hospitable, even during our Sun’s faint early history.
Above all, Mercury remains mysterious, arguably because it’s so difficult to explore. Getting there at all is no easy task. Newton’s laws of physics make it extraordinarily difficult for spacecraft to catch up to the tiny planet in its very fast, short orbit around our star. And once a given spacecraft does make it to Mercury, it must slow down enough so that it can be gravitationally captured into a stable orbit.
NASA’s Mariner 10 probe made three successful flybys of Mercury in the mid-1970s. But it only imaged less than half the planet. Beginning in 2011, NASA’s MESSENGER spacecraft orbited the planet 4000 times beginning Mariner 10 in the 1970s. And it only managed to image less than half of the planet.
Yet beginning in 2011, NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft —- the first mission to visit the planet in 35 years —- orbited the planet 4000 times. Before running out of propellant and crashing into the planet in 2015, MESSENGER was able to characterize Mercury’s surface composition and chemistry, as well as verify the fact that its polar deposits are dominated by water ice.
As for the MESSENGER mission’s primary take away?
Mercury formed with less oxygen which affected how it melted and separated and what minerals can form in its mantle and crust, Larry Nittler, a cosmochemist and planetary scientist at Arizona State University, told me via email. Any accurate model of Mercury’s formation will have to adequately explain both and thus far no one does, he says.
The European Space Agency’s (ESA) BepiColombo mission in cooperation with the Japanese Space Agency is now en route to Mercury. One of its goals is to characterize Mercury’s internal structure. What’s known is that the planet contains a higher percentage of iron (totaling some 85 percent of its volume) than any other planet in our solar system.
And no one seems to know why.
—- Is Mercury’s large iron core the product of stripping caused by collisions, or simply the fact that it formed in a very iron-rich area of our planetary disk?
Computer models have shown that it is difficult to strip that much material off a planet without having most of it re-accrete back onto its surface, says Nittler.
How will BepiColombo determine Mercury’s internal structure?
Launched in 2018, BepiColombo is set to fly by Mercury six times before its two complementary spacecraft, the Mercury Magnetospheric Orbiter and the Mercury Planetary Orbiter, begin circling the planet in late 2025.
By measuring how the planet’s gravity field pulls on both orbiting spacecraft, the ESA team should be able to compare Mercury’s gravity field with predictions made from models of the planet’s interior structure, says Nittler.
This is a 3d render of mercury in what It would look like in colour.
Nittler says other puzzles include:
—- The origin of Mercury’s strange surface hollows
These hollows are thought to resemble karst-like landforms here on Earth, that are created by the dissolution of soluble rocks, mostly limestone or dolomite.
They probably form by the loss of some easily evaporated volatile component which caused the rocks to sink below the surface, says Nittler. But we haven’t figured out what this component is, he says.
—- An abundance of graphite beneath the surface.
While all of Mercury’s surface is quite dark, we have found that material excavated by deep impacts is even darker, says Nittler. It is likely caused by a high abundance of carbon, probably in the form of graphite, he says.
Sometimes referred to as black lead, graphite is a naturally produced, crystalline form of carbon.
There may have been an original crust made entirely of graphite that later got covered by the silicate lava flows that make up the planet’s crust today, says Nittler. One intriguing idea is that shortly after Mercury formed its mantle was completely molten and carbon crystallized into graphite which floated to the top like icebergs, he says.
—- Did Mercury form where it is now, or farther out in our solar system?
One of the proposed explanations for its large core is that it could have undergone a hit-and-run-collision with something else (like Venus?) and left much of its mantle behind, says Nittler. This would require it to have spent some time farther from the Sun, but doesn’t require that it formed there, he says.
As for what Nittler finds most puzzling about our own Mercury?
Its origin, he says. Basically, any model that explains Mercury’s origin has to explain its large core, says Nittler.
After a half century of robotic exploration, Mercury still guards its secrets. And at the current pace of exploration, unfortunately, its mysteries will endure for generations to come.
