The Planet Mercury
Ready For Its Closeup

From Hermograph Press

In our last orbit, we explored Mercury through a telescope, a difficult activity. Only twice have we had a closeup view of the real face of this tiny world, in 1973 and then 35 years later. Let's see what we have learned.

Note: This page is always under construction.

First Closeups of the Little Planet

The decades of the 1960s and 1970s were the glory days of solar system exploration. Every planet but one was targeted for examination by robotic ambassadors with cameras for eyes and various sensors for every other sense. Mercury was given one shot in the NASA spotlight, the probe Mariner 10.

Mariner was launched in November 1973. The following February it passed by Venus. Using Venus' gravity to send it further into the inner solar system instead of back out towards Earth, Mariner arrived speedily at Mercury on March 29th, 1974. Mercury, already a practiced hand at synchronizing motions between it and other astronomical bodies, caused Mariner 10 to loop back to it in repeated encounters. The second time was September 21st, 1974 and the third was March 16th, 1975. The probe is still looping back to the planet but once the nitrogen gas supply ran out, NASA could no longer control which way the probe was oriented and no further photographs could be obtained.

Using the gravities of Venus and Mercury to change the probe's path was a first. This "slingshot" technique proven here paved the way for probes to use Jupiter's gravity to travel on to Saturn and beyond. Even today, in budget-crunched missions, probes are sent to other worlds and out of the plane of the planets by letting the Moon, Earth and other worlds change the trajectory of probes.

What Mariner Learned about the Planet

There were more than cameras on board the spacecraft:

Features of the Surface

The synchronous orbit Mariner 10 was put into by Mercury's gravity meant the probe would return again and again. In another of Mercury's playful use of coincidences, the probe would photograph pretty much the same side of Mercury each time it passed by. Thus Mariner 10 only saw a little more than one hemisphere of Mercury, agonizingly ironically similar to the old belief that we only saw one side of Mercury.

Mercury strongly resembles our Moon in that it is heavily cratered. However, it differs in that it has few if any dark plains that we call maria or seas on the Moon. There *are* large smooth plains on Mercury, they just don't look dark. These plains are called planitia.

Mountains are called montes but the only mountains that have great size are those that surround a great bulls-eye pattern called Caloris Basin, a huge but only (then) partially photographed pattern of rings of peaks surrounding an obvious point of impact (Planitia Caloris).

Other than the "hot basin," named because it is near one of Mercury's "hot poles" caused by its 2/3rds rotational resonance with its orbit and its eccentric orbit, and Borealis Planitia, the northern plain, all other planitia are named using the word Mercury in other languages of Earth. (Click here to detour briefly for a page on Mercury as written in other languages.)

Valleys are named for radar installations on Earth because it was radar that determined the real rotation period of Mercury. A valley is a vallis.

Scarps or cliffs are called rupes and are names for famous ships of discovery and exploration, an association honoring the mythological god's role in traveling and commerce. Ridges are called Dorsa and those honor astronomers associated with observing Mercury from Earth.

Craters are named for men and women who have made contributions to the arts and humanities. The only two major exceptions include the crater named for Gerard Kuiper, in honor of his contributions to planetary science. Kuiper was a bright spot seen even on distant photographs of Mercury, thus it was the first crater discovered on the planet. The other exception is a tiny indistinguishable crater that just happens to be at the right spot. Hun Kal, Mayan for "20", marks the 20th meridian of longitude on Mercury and thus marks the hermographic equivalent of the Greenwich Meridian of Earth.

In the twenty-plus years since Mariner 10, computers and digital enhancement techniques have improved. An interesting chemical analysis using these old photos has been done by Hawaiin astronomers Robinson and Lucy, and published in Science magazine in 1997. Their paper is also on a heavily graphic but colorful site at the Mercury Unveiled Website.

Return to the Planet

NASA, in August 2004, launched a probe called Mercury Messenger. Taking the long way and using multiple "slingshot" passes by planets, it arrived at Mercury in 2010 for a series of looping flyby's. It made three of these in 2008-9, photographing virtually every part of the little planet; now we know what the 'unknown' side of Mercury, and its polar regions, too, look like! It settled into an orbit around Mercury in 2011.

Messenger, which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, has taken thousands of photos over several Mercury days, and, most key, discovered evidence that there is water ice in continually shadowed craters in at least one, if not both, of Mercury's polar regions. The picture below shows craters where deposits have been indicated, marked with yellow coloration.

The probe is currently looking for evidence of a thin Mercurian atmosphere, and tiny moons.

(Photograph courtesy NASA/JHU/Carnegie Institute of Washington)

It  was deliberately crashed onto the planet's surface in April 2015. 

For more, see .

Some other sites worth looking at for Mercury information and images include:

All Mariner 10 pictures are from the Jet Propulsion Laboratory, the Lunar Planetary Institute, or NASA..

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