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Eugene Shoemaker with some of the first geologic maps of the Moon, Flagstaff AZ, mid-1960's

Many people are surprised when they learn that well before the first landing of Apollo in 1969, we already understood the geological history of the Moon.  The idea that such a thing was even possible drew considerable skepticism during early preparations for landing on the Moon.  The principles for the remote mapping of the geology of the Moon came from several closely related but distinct threads.  Eugene M. Shoemaker, a geologist with the U. S. Geological Survey (USGS) who founded the Branch of Astrogeology, laid out the methodology in broad outline from and through the systematic study of lunar surface images in the early 1960s.

One of the basic principles of geology is that younger rocks lie on top of (or intrude into) older rocks.  Interestingly, this relationship can be discerned from a photograph.  In the case of the Moon, images show the dark smooth plains of the maria (lava) and the rough, cratered highlands.  Some craters were found on top of the dark mare plains, while others were filled with mare.  Clearly, the craters on top of the mare formed after those plains existed and were thus younger than the maria.  On the other hand, dark mare that fills a crater must have formed after that crater existed and so in this case, the crater was older.

By following these simple relations over large areas, it is possible to determine the relative ages of mare and craters, both among themselves and to each other.  But such information is trivial unless we can relate these individual ages to some unit or event of regional significance.  In principle, if such a relationship can be defined we can extend relative age assignments over large areas, ultimately on a global basis.

The first effort to map the geology of the Moon was by the USGS, but not by the then-newly created Astrogeology Branch.  Branch of Military Geology scientists Arnold Mason and Robert Hackman produced the “Engineer’s Special Study of the Moon” in 1960.  This special one-off product documented the principal terrain types of the Moon (maria and highlands) and ordered features into three categories of relative age: post-mare craters (youngest), maria, and highlands (oldest).  Additionally, the map showed the distribution of linear features, presumed to be faults (fractures along which movement has occurred), and mare ridges (presumed to be folds) over the near side.  In this sense, the Engineer Special Study was a geological map because it showed the spatial distribution of rock types, their relative ages, and the inferred structure of the lunar surface.  This map was accompanied by a detailed text chart, which showed a region-by-region evaluation of the terrain and construction challenges for each area.  But a critical element was still missing.

On Earth, the geologist recognizes the rocks in the field, maps their locations and orientation, and documents the structure of the area under study.  But a key part of this work is to figure out where a particular area fits in the global column of geologic units.  On Earth, by documenting the slow, gradual nature of geological processes the stratigraphic column was developed slowly over the course of about a hundred years.  The terrestrial stratigraphic column also provided key evidence needed to show the gradual transition of life forms from simple invertebrate organisms in the earliest rocks, to the complex and varied life forms in succeeding strata.  With the development of a global stratigraphic system and accompanying geologic time scale for the Earth, a framework for understanding the history and processes of the Earth was created.

Gene Shoemaker recognized the need for an organized stratigraphy to aid in our understanding of the Moon.  He wanted to understand the Moon’s evolution and age, but also to correlate events on the Moon with events in Earth history.  He recognized that a major step forward to such an end was to define a formal stratigraphic system for the Moon – a clear succession of rock types with key regional units defining the system boundaries.  He began mapping the area around the crater Copernicus, which lies on the central near side of the Moon, recognizing that the rocks exposed there (from what had been discerned from images) represented all the distinct phases of lunar history.

Image of the area near Copernicus, showing how the relative ages of geologic features are determined from an image.

The basic sequence is easy to follow.  The oldest rocks (1) are those that form the highland units of the large, circular Imbrium impact basin.  These units are the mountains that make up the rim of the basin as well as the regional highlands around Copernicus, which are ejecta from the basin forming event.  Partial flooding by the dark, smooth maria followed (2), including both dark, ash-like materials and smooth flood-like plains (interpreted even then as flows of basalt, the most common volcanic rock type on Earth).  These eruptions were followed by the formation of impact craters, of which two kinds could be recognized:  an older group (3) that had slightly eroded and lost their bright rays (such as Eratosthenes) and a younger group (4) that preserved the bright rays and showed a fresh, unmodified form (such as Copernicus.)

Shoemaker used these rock units to define the lunar time-stratigraphic systems:  the Imbrian, Procellarian, Eratosthenian and Copernican Systems were each assigned to represent an archetypical deposition event.  Rocks that existed before the formation of the Imbrium basin were assigned to an informal category, the pre-Imbrian.  Thus, Shoemaker created a geologic map that not only showed the distribution of rock units and the structure of a given area, but also classified these rock types into a stratigraphic column for the Moon, one that (because of the enormous extent of the Imbrium basin) could be applied to areas across the lunar near side.  With slight modification (the “Procellarian” System is no longer used and the pre-Imbrian has been subdivided into the Nectarian System and pre-Nectarian), this classification scheme subsequently has been applied to the entire Moon.

Shoemaker’s work on geologic mapping of the Moon gave us the ability to immediately put the lunar samples returned by Apollo into a regional and global context.  We found that most lunar events occurred very early in its history, with intense geological activity in the first 1-2 billion years and little activity since.  Thus, the Moon’s geological record perfectly complemented that of the Earth, whose traces of earliest activity have been erased over time by the active processes of erosion and plate tectonics.

The first geologic quadrangle map of the Moon showing rock units (basin, crater, and mare materials), structures, and their stratigraphic arrangement.

The 1960 Copernicus Prototype Chart LPC-58, the first true geological map of the Moon, was not formally published by the USGS, though a modified and updated version was published later in that decade.  By then, Gene had picked up a couple of co-authors for his effort, including one Harrison Hagan Schmitt (a young geologist with the USGS in the early 1960s), who in 1972 ultimately got the chance on the Apollo 17 mission to do what Gene Shoemaker originally got into the space business to do – check the interpretations of the remote lunar geologic mapping by doing field work on the Moon.

Click here to view Shoemaker’s LPC-58 geological map at full resolution.

President John F. Kennedy is briefed by Wernher Von Braun (partly hidden) and Hugh Dryden (speaking to the President) at Pad B, Complex 37, Cape Canaveral, Florida, November 1963. (John F. Kennedy Presidential Library)

As a memorial to honor Neil Armstrong’s contributions to aeronautics and astronautics, a bill (HR 6612) was recently introduced by Congressman Kevin McCarthy and passed by the House of Representatives to change the name of the NASA Dryden Flight Research Center (a field center proximate to Edwards Air Force Base in the Mojave desert north of Los Angeles) to the Neil A. Armstrong Flight Research Center.  While I take a back seat to no one in regard to my respect and admiration for Neil and his life of accomplishment, I think that this effort is both mistaken and inappropriate.

Who was this Dryden guy anyway?  Hugh L. Dryden was an American aeronautical engineer who became the last head of the National Advisory Committee for Aeronautics (NACA)* in 1947 and the first Deputy Administrator of the National Aeronautics and Space Administration (NASA) in 1958.  Dryden had a long research career in the complexities of airflow and the boundary layer, critical subjects in the science of aerodynamics.  Dryden’s published work in this field became standard texts for upcoming aeronautical engineers and aircraft designers.  Dryden, a quiet man whose life story is filled with notable achievements and roles, took the lead in establishing the National Academy of Engineering, the sister entity of the National Academy of Science.

In 1958, an act of Congress established NASA which absorbed the NACA and its aeronautical research facilities, including the field centers of Langley Aeronautical Laboratory near Hampton VA, Lewis (now Glenn) Research Center in Cleveland OH, and Ames Research Center next to Moffett Field in CA.  President Dwight D. Eisenhower tapped T. Keith Glennan to be NASA’s first Administrator.  Hugh Dryden was asked to join the new agency as its first Deputy.  In his new role, Dryden was a key link to the immediate past, providing both institutional memory and continuity of service.  The NACA had been involved in space research, including the X-15 project, a rocket-powered, piloted aircraft capable of supersonic transport to the outer fringes of the atmosphere.  Neil Armstrong, a NACA test pilot, flew seven X-15 missions before his career as a NASA Gemini and Apollo astronaut.

Dryden and the NACA worked with the U.S. Air Force on the MISS (Man-In-Space-Soonest) project, which ultimately became Project Mercury, our first human spaceflight program.  This program was being developed and managed out of Langley Aeronautical Laboratory, a NACA facility.  The Space Task Group at Langley was headed by Bob Gilruth (later center director of Johnson Space Center), with Max Faget as one of his young, bright engineers grappling with the problems of hypersonic and orbital flight.

Hugh Dryden performed admirably the job of technocrat and manager during these early, exciting years, but perhaps his biggest contribution to space history is barely known.  The fate of Project Mercury was unknown in early 1961.  Recently sworn in as the 35^th President of the United States, John F. Kennedy seemed supportive of bold new technical endeavors but had been largely silent on his plans, if any, for the civil space program.  Although Kennedy made much about a supposed “missile gap” with the Soviet Union, this policy discussion was focused entirely on our parity in ICBM deployment (or rather, the alleged lack thereof).

This all changed in April of that fateful year.  The Soviets launched Yuri Gagarin on his single orbit flight, once again beating America to the punch by putting the first man in space.  In the same month, the United States suffered a humiliating military and diplomatic setback with the very public failure of an American-instigated invasion of Cuba at the Bay of Pigs.  The new President eagerly sought a high-visibility field of endeavor (preferably technological) in which America could demonstrate its superiority over the USSR.  Initially, the desalination of seawater was a leading candidate among the many projects Kennedy considered.  However, at the height of the Cold War, that challenge didn’t quite fill the bill.

On April 14, two days after Gagarin orbited the Earth, Kennedy met with his new NASA Administrator James Webb and his deputy, the holdover from the Eisenhower Administration, Hugh Dryden.  During this meeting, Dryden pointed out that while the Soviets could beat America to many different space “firsts,” a near-term human landing on the Moon was out of reach for both nations – that while declaring a “contest” with the Soviets on virtually any space goal ran the risk of America losing, odds were even for the first manned lunar landing.  America could not go to the Moon now, but likely we could within a few years.  Thus, if space was to be the chosen field for a superpower contest, Dryden believed the goal of a human lunar landing was the challenge we could win.

Kennedy received a detailed memorandum outlining all his space options from Vice President Lyndon Johnson on April 29, 1961, but Dryden had already forcefully made his case for a lunar landing to the President two weeks earlier.  It is often thought that Wernher von Braun was the one who convinced Kennedy that the Moon was the proper goal for Apollo, but Dryden had digested and presented von Braun’s technical arguments in policy terms that Kennedy could understand.  In the public’s mind, von Braun was “Dr. Space,” largely because of his work with Walt Disney in the 1950s popularizing the idea of space travel.  But it was Hugh Dryden who helped turn the dream of landing people on the Moon into a political commitment from the President and ultimately, a reality.

Hugh Dryden remained the Deputy Administrator of NASA until his untimely death in 1965.  He has been honored with a crater named for him on the Moon and as the namesake of the NASA Dryden Flight Research Center, an entirely appropriate memorial given his contributions to aeronautics and his key role in the establishment of the Apollo program.  He was at the right place (the White House) with the right President (Kennedy) at the right time (when America needed a challenging yet achievable space goal).  His life was one of service and excellence.  I think it does a disservice to the memory of Hugh Dryden to re-name the Dryden Flight Research Center and what’s more, I believe that Neil – the consummate gentleman – would also view HR 6612, the congressional bill passed to drop Dryden’s name and insert his in its stead, as unnecessary and wrong-headed.

I certainly agree that we should name a major facility for Neil Armstrong.   May I suggest that the first manned lunar outpost be named for Neil Armstrong – the first man to set foot on the Moon.

* Pronounced by saying each individual letter: “N-A-C-A,” not as a single word, as we do for its successor agency, NASA.

——

Note Added Jan. 7, 2013:   I have been reminded that the NASA Authorization Act of 2008 had already designated the American portion of the then-planned international lunar outpost as the “Neil A. Armstrong Lunar Outpost” (sec. 404 b).  Thanks to both Bill Mellberg and Joel Raupe for jogging my memory on this.

Map of the Moon by Grimaldi and Riccioli, 1651. Most of the names on this map are still in use today.

The Moon is remarkable for the variety and unusual nature of the names of its surface features.  The dark, smooth maria are named for weather or states of mind (Sea of Rains, Sea of Tranquility) while many of the abundant craters of the Moon are named for famous scientists, philosophers, mathematicians and explorers.  Before the advent of the space age, only the near side of the Moon was visible, although most scientists believed that the far side probably looked exactly like the one facing Earth. (How wrong they were!)  Naturally, once we had the ability to see uncharted lunar territory, a new era of name assignment commenced.  But even now, many lunar craters and features await something more than mere coordinates.

The drawings by Galileo of the Moon in 1610 show craters and mountain ranges but he did not assign names to them.  As telescopes improved, revealing finer surface details, several maps appeared with names bestowed by their astronomer authors to flatter patrons or express their nationalism.  Most of those early names have been forgotten to history.  In 1651, an influential map by Jesuit astronomers Grimaldi and Riccioli became the foundation for the official naming reference guide that we use today.

With the flight of the Luna 3 probe in 1959, the Soviet Union was the first nation to image the far side of the Moon.  To the surprise of most, large regions of maria (so prominent on the near side) were mostly missing from the far side.  Although the first images were of very low quality, the Soviets couldn’t resist the urge to name newly discovered features for a variety of Russian heroes and place names, such as Tsiolkovsky and the Sea of Moscow.  Some new “features” were misidentified because of the low resolution – the name “Soviet Mountains” (no longer used) was given to a bright linear streak across the far side globe (a feature that turned out to be a long ray from the fresh crater Giordano Bruno and not a mountain range).

Over subsequent years, as both American and Soviet spacecraft filled in the far side coverage with increasingly higher quality images, most major far side craters received names of various scientists and engineers.   From around the world, a mixed bag of names were submitted to the International Astronomical Union (IAU – the body of scientists who authorize the names of planetary surface features) for consideration and approval.  Although some were historically significant, many were people with whom few were familiar.

Though NASA does not have the authority to assign names to features on the Moon, an informal practice of naming landmarks was common during the Apollo missions.  Names were given to the small craters and mountains near each landing site (e.g., Shorty, St. George, Stone Mountain) but official names were used as well (e.g., Hadley Rille).  NASA adopts informal names for the same reason that names are given to geographical features on Earth – as shorthand to refer to landmarks and other mapped features.  The most recent illustration of this practice occurred on December 17, 2012 when NASA named the location where the deliberately de-orbited GRAIL spacecraft crashed onto the Moon near the crater Goldschmidt (73°N, 4°W) the Sally K. Ride Impact Site.  Sally thus joins other women of science and note who have lunar features named for them – Hypatia, Caroline Herschel and Marie Curie, among others.  Most of the informal names assigned during Apollo were later given “official” status by the IAU.

The Apollo basin (a 540 km diameter crater on the southwestern far side) was named to honor the Apollo missions – the only crater on the Moon so designated.  Within a few years of their missions, smaller craters were named for the living crews of Apollo 8 (Borman, Lovell and Anders) and Apollo 11 (Armstrong, Aldrin and Collins).  Also located around the Apollo basin are craters named for deceased astronauts and NASA employees, including the lost crews of Apollo 1 and the lost crews of the final missions of the Challenger and Columbia Space Shuttles.  It is appropriate that some feature honors humanity’s first efforts to reach the Moon, as well as others who gave their lives pioneering space.  In a similar vein, craters near the poles of the Moon tend to be named for famous polar scientists and explorers, such as Nansen, Shackleton, and Amundsen.

Other than these exceptions, the location of specifically named craters has little rhyme or reason.  Neither scientific prominence nor contribution guarantees any crater-endowed immortality.  Copernicus and Archimedes are rightly honored with spectacular craters named for them.  But Galileo and Newton (titans in the history of science) are fobbed off with insignificant or barely detectable features.  One of the most prominent craters on the Moon is named for the astronomer Tycho Brahe, an eccentric who spent most of his career trying to validate a variant of the Earth-centered, Ptolemaic model of the Solar System (Ptolemy also has a prominent crater in the center of the near side named for him).  It’s not clear why Riccioli assigned the names he did to these craters, though he cannot be blamed for giving Newton short shrift, as the future Sir Isaac was only nine years old when the Grimaldi and Riccioli map was published.

It is possible to both suggest a name and to propose a crater for that name, though the IAU is not obliged to accept either.  Often, a suggested name is approved but assigned to a different crater.  Currently, the guidelines for submission and assignment of new names for lunar craters are: 1) a scientist or explorer who has made some significant contribution, preferably to the study of the Moon and planets; 2) deceased for at least three years before a crater name becomes official; 3) it cannot duplicate any existing lunar name.

In 2005, I proposed the name Ryder (to honor my colleague Graham Ryder, a lunar scientist who passed away in 2002) and suggested a small, bright crater on the far side to carry his name.  Both suggestions were adopted.  We have since found that Ryder crater is actually quite a geologically spectacular feature (Graham would be proud of his namesake).  In a truly singular event, the crater Shoemaker (named in 2000 and located near the south pole of the Moon) actually contains some of Gene Shoemaker’s remains – a small portion of his ashes was carried aboard the Lunar Prospector spacecraft in 1998.  At the conclusion of that mission, the vehicle was crashed into the south polar crater that was subsequently named for him.

We don’t know what the IAU will do concerning the designation of the Sally K. Ride Impact Site but as history suggests, granting of official status is not guaranteed.  No matter – we will continue to assign names to features as needed and the IAU will do what they do.  In the early 1970s, the IAU (by fiat) abolished the famous Mädler nomenclature system (wherein a small, nearby crater is given the name of a large neighbor plus a letter, such as Copernicus H).  Most working lunar scientists stubbornly refused to accept this decision and continued using the old crater names.  After 30 years of bureaucratic intractability, the IAU finally surrendered and formally adopted the Mädler system.

Official or not, with the passage of time, named lunar landmarks will become familiar to those visiting and working on our nearest neighbor.  Perhaps interesting monikers will be attached by those locals, as is done here on Earth when we assign nicknames to places – like the Big Apple, the Windy City, the Big Easy and the City by the Bay.

Just published:  The Clementine Atlas of the Moon, Revised Edition, an updated atlas and reference guide to lunar features, by Ben Bussey and yours truly.

Map of the gravity gradients of the Moon as recorded by the GRAIL spacecraft (NASA)

The NASA mission GRAIL (Gravity Recovery And Interior Laboratory) has been orbiting the Moon since last spring.  The mission consists of two identical small spacecraft (dubbed Ebb and Flow) that very carefully keep track of their relative position from each other.  By tracking both of these spacecraft with high precision from Earth, we can monitor any small variations (caused by variations in the Moon’s gravity field) away from their predicted orbital paths.  If the satellite is flying over an area on the Moon with less material than normal (for example, over a deep crater, a hole in the Moon’s crust), it will be less attracted to the Moon because of this mass deficiency and will therefore fly away from the Moon.  If, on the other hand, it flies over an area of excess mass, such as a thick stack of dense lava flows, the excess mass pulls the satellite slightly toward it, increasing its speed and pulling it downwards.  As Ebb and Flow orbit the Moon, they conduct a delicate “dance.”  These movements are caused by variations in the Moon’s gravity (largely a reflection of variations in the density of its crustal rocks).  When combined with the high-resolution, precision topography of the Moon (currently being gathered by the Lunar Reconnaissance Orbiter), we are able to reconstruct the structure and thickness of the lunar crust from orbit.

GRAIL has unveiled a new global gravity data set, very high in resolution and precision and greater than ten times better than our previous version of the global gravity from the Japanese mission SELENE (or Kaguya).  One interesting result shows unusual structure – long, quasi-linear gravity features appear in a variety of locations associated with lunar impact basins.  Basins are very large craters that formed during asteroid collisions prior to 3.8 billion years ago.  Some of these linear features extend on great circles across the lunar globe for distances of more than 500 km. These results suggest that solidified intrusions of once-molten rock may form a dense, criss-crossing network within the upper crust.

In order to understand the significance of these gravity features, it is necessary to understand some elementary facts about planetary geology.  Planets generate heat and this heat must be dissipated.  Typically, the heat generated from both the original energy release during formation (accretion) and from the decay of radioactive elements (e.g., uranium) melts the interiors of planets, forming bodies of liquid rock called magma.  This magma is usually less dense than the rocks from which it forms and thus, rises upwards towards the surface.  Sometimes, the molten rock cannot ascend any higher from the deep locations where it comes from and freezes in place – geologists call this type of frozen rock body an intrusion, because it intrudes into pre-existing rock as a liquid and then solidifies by crystallizing.  When magma actually reaches the surface of a planet, it can erupt onto its surface as lava; this activity is called extrusive because the molten rock extrudes onto the surface and then solidifies as lava flows.

Clearly, all erupting lava must have at one time been an intrusive magma body, at least during the time it was ascending upwards toward the surface.  Although many magma bodies reach the surface and create lava flows (such as the dark, smooth maria of the lunar lowlands), sometimes this magma cannot reach the surface and freezes in place within the crust as a linear or tabular body.  Such features (called dikes) are an essential part of the underground, igneous plumbing of volcanoes on all of the terrestrial planets.  We knew that they must have formed on the Moon because we saw the evidence of vents and structures in the maria that are the surface expression of such features.

The Crisium basin; gravity gradient map at left, topographic map at right. Linear feature at far left extends over 300 km (NASA)

For the first time, the new GRAIL data show us direct evidence for these buried igneous dikes within the lunar crust.  One particularly prominent dike occurs near the Crisium basin, on the eastern near side of the Moon.  This dike extends over 1000 km in a quasi-radial direction northwest of the Crisium rim, disappearing beneath the mare lavas of that basin.  The fact that it is not clearly aligned with the basin structure suggests that it may predate it; we estimate that Crisium basin is older than 3.9 billion years.  This long linear feature may have been formed when molten magma from the deep interior of the Moon oozed its way toward the surface, before “freezing” at some intermediate level.  Its presence, evident now only by a faint gravity signature (those denser areas “tugging” on the GRAIL satellites “Ebb” and “Flow”), is a tell-tale remnant of its existence deep inside the Moon’s crust.

Many other linear and circular features are evident in the gravity gradient map produced by GRAIL.  Most of these seem to be associated with the large basins of the lunar highlands, the largest impact craters on the Moon. These features both excavate large amounts of crustal material during formation, and serve as topographic lows and structural traps for the accumulation of subsequent erupted lavas.  The gradient structures show a complex network of density patterns in the shallow subsurface of the Moon; this area is a morass of crushed rock, fractures, large faults and collapse features.  The entire outer portion of the lunar crust has been shattered and broken by an impact barrage of almost unimaginable violence.  The crust has since been partly annealed together by heat, re-fractured by additional impacts, intruded by large bodies of molten rock, resurfaced by the eruption of lavas from the deep interior, and finally has had its outermost surface pulverized into a fine powder by the micrometeorite bombardment.

The Moon may look like a silent, dead world but its past (which is Earth’s past) is testament to an early history of extreme violence and chaos.  The results from the GRAIL mission are helping us understand this complex story.

Composite image of the north pole of Mercury. Red are the areas of permanent shadow; yellow delineates radar bright deposits mapped from Earth. Data are plotted on a photomosaic of MESSENGER images. NASA

Mercury – the planet, not the element – was in the news this past week.  For some time, we had suspected that the poles of Mercury might harbor deposits of water ice.  This – on a planet so close to the Sun that the surface temperature at the equator is hot enough to melt lead!

Yet like the Moon, Mercury’s spin axis is perpendicular to the plane in which it orbits the Sun.  This means that large craters near Mercury’s poles lie in permanent shadow (“shivering” around -170° C), unaffected by the Sun’s searing heat (equivalent to more than eleven times the solar flux we get on Earth).  As on the Moon, these permanently shadowed areas get heat from only two sources – the 3 K background heat of space, created during the Big Bang some 15 billion years ago, and whatever heat is being generated now from the deep interior (a quantity that geophysicists call the heat flow of a planet).

Large planets (like Earth) generate heat mostly from the decay of radioactive elements deep inside them.  This heat is lost largely through the phenomenon of volcanism, in which melted rock from the interior is erupted onto a planet’s surface as lava and ash.  Smaller planets and moons likewise experience this heating and volcanism, but because they are have lower overall contents of heat-producing elements, their volcanic episodes occurred in the distant past.  Much of the heat of these smaller planets has been largely dissipated.  Thus, on Mercury, we suspect that the overall heat flow is very low, resulting in extremely cold temperatures on the floors of its permanently shaded polar craters.

For many years, astronomers have studied Mercury with radio telescopes from Earth (using radar to make images of its surface).  Because the orbital inclination of Mercury is relatively high (about 7°), we can get a fairly good look into the interiors of the polar craters.  Interestingly, even though Mercury is much farther away than the Moon, we can see more of the mercurian polar areas because of this relatively high orbital inclination (the Moon’s orbital plane is inclined only 5°).  These radar pictures showed an amazing and unexpected feature – the dark areas are filled with material that is highly reflective at radio frequencies, properties similar to the surfaces of the icy moons of Jupiter (Europa, Ganymede and Callisto).

These results were so unexpected and startling that debate raged for many years whether these deposits really were what they appeared to be: water ice.  Facts are stubborn things and few materials have radio properties similar to ice.  Some suggested that sulfur might be an alternative explanation, but provided little evidence for such behavior.  Moreover, another moon of Jupiter, Io, which has a surface largely composed of sulfur, does not show the radar brightness or “glint” seen on the other, ice-rich Jovian moons.

The debate on the nature of the Mercury polar deposits has now been settled with the release of new data from the MESSENGER mission.  Launched on August 3, 2004, with insertion into obit around the planet on March 18, 2011, the spacecraft has been taking pictures and making measurements of Mercury for the last two years.  We have mapped the extent of darkness near the poles, measured the temperatures of the surface inside these regions, and detected the presence of significant amounts of hydrogen there.  All of these results are strongly supportive of the water ice interpretation.

The existence of ice near the poles of Mercury supports the case for water ice on our own Moon, although there are some significant differences between the two occurrences.  Like Mercury, the Moon’s spin axis is nearly perpendicular to the plane of its orbit around the Sun.  The similarity of the terrain of both bodies results in deep holes that hide large expanses of terrain from the glare and heat of the Sun.  Both objects have been volcanically active in the past, but not today, meaning that the average rates of heat flow on both are low.  These properties result in the creation of polar “cold traps” in which any entering volatile substance (such as water molecules) cannot escape.

The solid bodies of the inner Solar System are constantly hit by debris from comets and asteroids.  This material contains water, both in free form and bound within hydrous minerals.  On smaller objects (like the Moon and Mercury), most of this water is lost to space, but we suspected that some of it might be retained within these dark cold traps near the poles.  Now we know that such a process does occur.

Differences between the Moon and Mercury result in differing amounts and settings for their polar deposits.  Being much closer to the Sun, one might expect Mercury to contain less water ice, but a variety of evidence suggests that the opposite is the case.  The polar ice of Mercury appears to be greater in extent and thickness than comparable deposits on the Moon.  This probably results from two factors.  First, Mercury is a bigger object, with a surface gravity about twice that of the Moon.  Thus, it is more difficult for water to “escape” from Mercury.  Second, the closeness of Mercury to the Sun (the edge of biggest gravity well of the Solar System) results in a higher flux of cometary impacts there than experienced in the Earth-Moon system.  So more water is being added to Mercury, where it is more easily retained.

Nonetheless, both Moon and Mercury have similar polar environments and processes.  The long debate – a scientific controversy for over 50 years – about water at the poles of these objects has been resolved.  The next steps will be to characterize these deposits in situ using a soft lander and selected instruments to measure the amounts, states and distributions of water in the polar areas.  Because of the great difficulty in even getting into orbit around Mercury (let alone landing there), doing this first on the Moon will mostly likely happen first.  So, here again is another rationale for sending a robotic surveying lander and rover mission to the poles of the Moon – in addition to characterizing these areas for our future presence there, by inference, we will also learn about the polar processes on and environment of Mercury.

A planetary “two-fer.”  Let’s get on with it.