NASA has visited the inner most planet to the Sun on more than one occasion and each has delivered incredible photos andMESSENGER (the more recent) delivered a full mapping of its surface. During its time there first the mapping mission, and then the extended time in order to stay for the solar maximum; NASA and by extension mankind was able to see a side of Mercury we never have before.
First let’s get acquainted with the basics. Mercury, like many planets, gets its name from one of the Roman gods. The Roman god Mercury was the swiftest of the gods and also the god that the merchants prayed to for successful commercial transactions. Since Mercury is a ‘swift’ planet in that it appears to move quicklyfrom night to night this moniker is appropriate. Interesting to note is that the ancient Greeks thought that Mercury was actually two planets – one visible in the morning sky and a totally different planet that was seen in the night sky. Due to this the Greeks referred to the night version as Hermes and the day planet as Apollo. When the ancient astronomer’s realized it was all the same planet, the name Hermes was stuck with however the Romans called it Mercury.
Still, even after being heavily debated, Mercury was shrouded in mystery due to its small size and proximity to the Sun. The largest of telescopes could glean only a hint of Mercury’s surface and so in 1974-5 when Mariner 10, NASA’s spacecraft made three fly-bys the mystery began to fade and understanding set-in.
Mercury was a pot marked and heavily cratered planet, much like the moon in many ways. But with the several fly-bys Mariner 10 was still only to glimpse 45% of Mercury’s surface. Enter MESSENGER.
MESSENGER was launched in 2004 and flew by Mercury three times before settling into an elongated orbit in 2011. Next two years were spent taking a myriad of pictures of the planet’s surface varying the distance or the illumination. In 2013 the science team in charge ofMESSENGER’s mission had enough data to produce a global map of Mercury’s surface.
So when a scientist looks at Mercury now what does he know?
Yes, the basics, such as an 88 day year (though from Earth it appears a 116 day orbital period). Or perhaps how although one might imagine with its proximity to the Sun, it might be hot, Mercury is unable to retain its heat due to a lack of atmosphere. Hence a variance of temperature is seen from −173 °C at night to 427 °C in the day. Although the feel might be similar to that of the Moon, in composition it is very different. Huge volcanic plains cover areas (such as the Caloris Basin) and though the craters fron the lunar highland when compared to Mercury might appear the same the truth is those of Mercury are much younger. Thus far NASA’s scientists have assigned names to 350 craters and other notable features, naming them after famous artists, musicians, painters and authors. Some of the crater names immortalize; Rachmaninoff-the Russian pianist and composer and also conductor, Hector Berlioz the French romantic composer, Alexander Calder the American sculptor, Truman Capote the American author, Enrico Caruso the Italian tenor, John Lennon an English songwriter (or perhaps you are more familiar with the Beatles?), and photographer Andre Kertesz. Obviously with 350 named there are quite a few more.
When MESSENGER honored John Lennon it released its own version of Imagine to the world:
“Imagine some ejecta
It isn't hard to do
Terraced walls and impact melt
Secondary craters too
Imagine central peaks
Rising above the floor...
You may say I'm a complex crater
But I'm not the only one
Someday more will join us
On the planet closest to the sun.”
Actually more interesting is the crater Kertesz. Pictured directly above the white stuff that can be seen on the floor of the crater may look like ice but it is not. This 31 kilometer crater offers irregular depressions that appear to show ice but is not ‘water ice’ as has been confirmed near Mercury’s poles, but does behave as ice might were it on another planet.
The daytime temperatures reached on this planet are so hot that rocks at many latitudes that would appear normal on other planets would quite literally evaporate on this planet. That actually leads to one theory about the formations of the odd features (called hallows) seen here on Mercury in many of her craters.
Here on Earth we consider it normal to have a basic pattern of temperature that is repeated each day. (recent sufferers of the polar vortex – make pretend!) The sunlight as experienced by Mercury varies to such a great extent due to not only their changing angle of elevation but also the apparent speed of the Sun in the Mercury sky. Add to that the great variance in the distance between Mercury and the Sun due to a rather elongated orbit and the intensity of the sunlight will not seem to form any pattern as far as the weather and a daily cycle.
Mercury does lay claim to a rocky interior. It is one of four planets (count Earth in there) that can call itself terrestrial. As stated before it is also the smallest. (equatorial radius of 2,439.7 km.)
It has a core with a high iron content, higher than other any other planet in the Solar System. Scientists theorize that that Mercury originally had a metal-silicate similar to most chondrite meteorites but early on was perhaps struck by a planetesimal of 1/6th its mass that striped most of the original crust away leaving the core behind. This giant impact hypothesis actually explains a lot and having a similar explanation is the formation of the moon.
What does Mercury consist of? First of all Mercury is the 2nd densest planet in the Solar system (second to Earth.) While Earth’s higher density is a result of gravitational compression and in fact if it were factored out Mercury would win the contest. It is comprised of 70 % metals and 30% percent silicate material. It’s core is large and rich in Iron. While it is not able to retain an atmosphere, it does have a tenuous surface-bounded exosphere that contains hydrogen, helium, oxygen, sodium, calcium, potassium and more. MESSENGER also found water vapor to be present – a result of the many comets which have struck its surface as well as creating water out of the hydrogen from the solar winds mixed with the oxygen in the rocks or perhaps from the frozen reservoirs at Mercury’s poles.
Mercury also can claim a magnetic field. According to data from Mariner 10, while it is only 1.1% as strong as Earth’s and though small is strong enough to create a magnetosphere and trap solar wind plasma. The spacecraft also encountered magnetic tornados also known as flux transfer events. (I like magnetic tornados better! More fun to conjure!) While the ‘tornados’ are not rare as it occurs in Earth’s magnetic field, MESSENGER did show the one’s that occurred on Mercury to have a reconnection rate at least ten times higher. (reconnection rate means that the magnetic topology is rearranged and converted to kinetic energy, thermal energy and/or particle acceleration.)
All and all while scientists still have data to pour over, we know a lot more about Mercury in 2014 than in 2011 or 1976 or 1900 or for that matter, ancient Greece.
Picture Mercury in the Middle
**TID BIT: Mariner 10 on Feb. 5, 1974, made history after making the NASA science probe became the first spacecraft ever to test out and execute the technique now known as a “planetary assisted fly-by” - used to alter its speed and trajectory so that it might reach another celestial body.