Mercury – Small Planet, Big Mystery
Mercury – the smallest planet in the solar system, but the biggest mystery.
Mars and Venus always take the spotlight in big news articles and
so little attention is paid to the tiny planet closest to the sun. It’s often
an afterthought. Mercury, taking only 88 days to complete its rather elliptical
orbit around the sun has many strange and unclear points.
Mercury’s diameter is about a third
of Earth’s and, at a glace, its surface covered in craters is a little similar
to that of the Moon. The slowly rotating Venus is covered in craters and running
across the surface are jagged cliffs of about 2km high. On top of this, what
makes Mercury’s surface so unique is that it has the Caloris Basin, the largest
impact crater in the solar system with a diameter of 1550km.
The surface of this planet is
pretty much unsuitable to hosting life. As its surface temperature reaches 180
to 450 degrees Celsius, there aren’t many planetary scientists who believe it
would have been suitable for life – even in the early days when the sun was
dark.
More than anything, the reason that
Mercury’s mysteries have been left unsolved is likely due to the fact that it
is extremely difficult to investigate. No matter the method, reaching Mercury is
just not that simple. To land a probe on such a tiny planet, travelling at such
high speeds in very small orbit around the sun is according to Newton’s laws of
physics would be extraordinarily difficult. On top of this, as soon as a probe
were to reach Mercury it would have to decelerate enough to enter a stable
orbit after entering Mercury’s gravity.
NASA’s Mariner 10 probe succeeding
is passing by Mercury 3 times in the mid 1970’s. However the Mariner 10 was
only able to photograph less than half of the surface of Mercury.
35 Years later, NASA’s probe
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) visited
Mercury and began its observation in 2011, succeeding in making over 4000
orbits of the planet. In 2015, by the time it used the last of its propellant
and crashed onto the surface of Mercury, MESSENGER succeeded in ascertaining
the surface composition and chemical properties of Mercury’s surface as well as
confirming that over half of Mercury’s polar region was covered in frozen
water.
The Mystery of Mercury’s Metallic
Core. Was a collision with a celestial body the cause? Or was it the location of
formation?
-The main points of Messenger’s plan.
America’s Arizona State University’s
cosmochemist Larry Nittler, in a reply to the reporter’s email, explained that as
Mercury was formed in a state of having very little oxygen, and that this had
an effect on the processes of dissolution and separation, and on what minerals
would be able to form in the crust and mantle of the planet. According to
Nittler, in order to produce a model of the precise composition of Mercury, it’s
necessary to first accurately explain these two points and until now, no one
has been able to do this.
The probe BepiColombo, of the
international cooperative Mercury investigative mission shared by JAXA and ESA,
is currently heading to Mercury. One of this mission’s goal is to shed light on
the characteristics of the composition of Mercury’s inner layers. We currently
know that Mercury, compared to the other planets in the solar system, has a high
percentage of iron – 85% of it’s volume.
It seems that nobody seems to know
the reason for this, though.
-How was Mercury’s large iron core
formed?
Nittler points out that in computer
models, they discovered that if you strip away large amounts of matter from a
planet, it’s difficult to stop most of that from resettling onto the planet’s
surface.
Just how will the BepiColombo
ascertain the inner layer composition of Mercury?
After launching in 2018 and making
6 flights past Mercury, is expected to launch 2 supplementary research probes,
MPO (Mercury Planetary Orbiter) and MMO (Mercury Magnetospheric Orbiter) into Mercury’s
orbit at the end of 2025.
Nittler states that by measuring
the gravity working on both orbital probes MPO and MMO the ESA team should be
able to compare the data from Mercury’s gravitational field and the predicted
values taken from the model of the planet’s inner composition.
-The cause of the strange cavities
on Mercury’s surface.
It is thought that these cavities
are reminiscent of the Karst landscapes on Earth. Karst is formed from the erosion
of rocks easily eroded by water like Limestone and Dolomite.
These cavities are probably formed as rocks sunk below the surface due to the disappearance of some volatile, easily vaporised substance. However, Nittler says that just what this substance is, is not yet clear.
What holds the key to solving Mercury’s various mysteries?
-The abundance of Graphite beneath
the surface.
According to Nittler, most of Mercury’s
surface is rather black but, due to a substance brought up from large-scale astronomical
collisions, it has become even blacker. It is highly possible that this is due
to the large presence of carbon in the form of graphite.
Graphite is the naturally occurring
crystallised form of carbon.
Nittler explains that it is
possible that the entire crust was initially formed from graphite but was later
covered by silicate lava flow, making the current crust. There’s an incredibly
interesting theory that states that directly after Mercury’s formation, its
mantle was completely molten and the graphite formed from the crystallisation
of carbon floated to the surface like icebergs.
-Was Mercury formed where is
currently is now, or was it further out in the solar system?
According to Nittler, one suggested
explanation for why Mercury has such a large core is that Mercury (like Venus?)
was grazed in collisions with other astronomical bodies and lost a large amount
of its mantle as a result. In this theory, it would be necessary for Mercury to
be much further from the Sun for some time, but it would not have had to be
formed there.
-Mercury’s biggest mystery
Mercury’s biggest mystery is its origin,
Nittler believes. He says that fundamentally, any model that explains the
origins of Mercury needs to explain why it has such a big core.
Even with half a century of probe
investigations, Mercury’s secrets still elude us. At the current pace of progression
it seems that these mysteries will continue for the next few generations
unfortunately.
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