Retro-Reflection phenomena

Retro-Reflectivity on the moon

(glossary entry)

Description

Retro-reflectivity refers to the reflection of light back towards the source from which it originates. Like many natural surfaces, the Moon's surface reflects considerably more light back in this direction than would, say, a diffuse reflector like a sheet of paper or a surface sprayed with "flat" paint. One of the many manifestations of this is that the Full Moon (which is always viewed with the Sun "at our backs") is considerably brighter than might be expected from its surface brightness at other phases. Another is the "hot spots" seen in many photos taken from lunar orbit, where a bright patch is seen in the direction opposite the Sun (where one would expect to see the shadow of the spacecraft, if it were large enough to be visible). Yet another is the halo of light seen around the shadow of the head of an astronaut photographing his own shadow at close range.

The term retro-reflectivity can also be applied (less correctly) to sun-glints from large shiny surfaces. In this case, a strong flash of brightness (more properly called a specular reflection) is seen when the light source, surface and observer are at exactly the right angles (not necessarily 180°) to allow the surface to act as a mirror, shining the light into the observer's eyes. Specular reflections from flat or aligned surfaces on the Moon have been offered as a possible cause for some of the so-called Transient Lunar Phenomena. Although sun glints of this kind have been seen on Mars, none of the reported transient lunar brightenings has ever repeated when viewed again at the same combination of angles.

Additional Information

Causes

• The moon's surface is covered by a layer of dust, called the regolith. This dust layer is thought to be the primary cause of the Moon's retro-reflectivity (an optical effect sometimes called the Lunar Heiligenschein). The hiding of shadows from coarser structures, like surface rocks and boulders may also play a role.

The "classic" Heiligenschein on Terrestrial surfaces

• The term Heiligenschein (German for the "saintly glow"), sometimes used to describe the glow of the lunar retro-reflectivity, especially when seen around the shadow of an astronaut's head, is derived from a similar-appearing phenomenon observable on many of the Earth's surfaces. In particular, it was originally used to describe the diffuse colorless glow seen around the shadow of one's own head cast on dew-covered grass. It is sometimes also called the "Sylvanshine" and/or "Heiligenschein Streak" for the vertical column that is sometimes seen above the shadow of the observer's head. The so-called "Glory" (also around the anti-solar point) is a different phenomenon exhibiting spectral colors. The explanation of this rainbow-related effect can be found on many websites or books.

Lunar Retro-reflectivity Observed from Earth

• The condition of illumination responsible for the retro-reflectivity is most conveniently described by something called the phase angle (the angle between the observer and the Sun as seen from the lunar surface).
• The Moon exhibits both an increase in reflectance as the phase angle gets smaller, and an additional opposition surge in brightness as the phase angle approaches zero. The strength of the retro-reflectivity can be appreciated by comparing its behavior to an idealized diffuse reflector. If the Moon were a diffuse reflector, its total brightness (the intensity of the moonlight falling on the Earth) would be expected to increase by a factor of pi (3.14) in going from First Quarter (phase angle 90°) to Full (phase angle 180°). The actual increase is said to be more like a factor of 14 with perhaps as much as 40% of this, or so, occurring in the last 5°.
• When the Moon is observed from Earth, the entire lunar surface is at a similar phase angle (within about 0.5°); and the maximum amount of the Moon's retro-reflectivity is difficult to observe because when the Sun is directly behind the observer, the Earth's shadow is cast on the Moon, creating an eclipse.

The Lunar Heiligenschein observed in orbit around the Moon

• Photographs taken from lunar orbit differ from those obtained from Earth in that the range of phase angles is typically quite large; and when looking a direction opposite the Sun, the shadow cast by the spacecraft is quite small. Hence one can observe the appearance of a small part of the lunar surface at essentially zero phase angle, and neighboring regions at increasing phase angles.
• If the counter-solar point (at an angle of 180° from the Sun, also known as the opposition point) is located on the Moon's surface, this point will be marked by a very bright diffuse spot (the "Heiligenschein" without central shadow!). Many of the orbital Fairchild metric (mapping) photographs, made during Apollo missions 15, 16, and 17 show this effect very well! To discover the lunar "Heiligenschein", one should look for shadow-less photographs (photographs which show only albedo differences).
• Looking at a single photo can easily give the incorrect impression that a crater is surrounded by a bright halo, when in fact it is only temporarily located at the center of the opposition surge.
• Because of the large range of phase angles available in a single image, the Apollo orbital photos gave the first quantitative estimates of the magnitude and angular range of the lunar opposition surge, although the results were not always consistent.
• The following sequence of oblique metric views from Apollo 15 is a typical one showing the bright retro-reflecting anti-solar point (the blue arrow) tracking the motion of the spacecraft over southern Mare Serenitatis and into the Littrow highlands area (the spacecraft actually took the photos while moving from right to left).

A Sequence of Oblique Metric Photos -- click to enlarge

• • It is not known if these photos all have the same processing, but they make it appear that the opposition surge varies both in brightness and angular size as it moves over different kinds of terrain. It looks to be at its most concentrated in the third view from the left, where it appears to be about the same size as the crater Clerke (just to the right of the tip of the red arrow). At a viewing distance of about 180 km, this 6 km apparent diameter translates into an angular size of about 2° (a cone angle of 1° around the anti-solar point). Note also how the Catena Littrow area (pointed to by the red arrow), flares strongly in brightness in the second and third frames (even though it is 5-10° away from the anti-solar point), and how the bright rays on the floor of Mare Serenitatis brighten and fade as the anti-solar point moves over them..

The Lunar Heiligenschein observed from the Moon's surface

• Photographs taken by astronauts or by spacecraft on the lunar surface show an even larger range of phase angles than those taken from orbit.
• When the Apollo astronauts walked on the moon's surface, they noticed a bright diffuse "glow" around the shadow of their head. The shadows of their heads were always located at the centre of the Heiligenschein (at 180° of the sun). Several of the Hasselblad photographs, made on the moon's surface, show this glow centered at the point where the phase angle approaches zero (the shadow of the camera taking the photo). A similar glow was noticed when the astronauts were inside the Lunar Module. The shadow of the LM's upper part was surrounded by the Lunar Heiligenschein.
• AS12-48-7026, made during the mission of Apollo 12, shows the Lunar Heiligenschein around the upper part of LM Intrepid's shadow.
• AS17-136-20744, made during the mission of Apollo 17, shows the shadow of an astronaut, surrounded by the Lunar Heiligenschein (around the upper part of the shadow). These photographs (AS12-48-7026 and AS17-136-20744) are included in the book Full Moon by Michael Light and Andrew Chaikin. The printed versions of these photographs (Plates 44 and 45) show much more brightness contrast than the online scans.
• AS17-140-21359 is another typical Hasselblad of an astronaut's Lunar Heiligenschein. Note the absence of shadows at small rocks around the astronaut's shadow, indicating one is looking along the direction of the sunlight. The increase in brightness at this viewing angle is in part due to this Shadow Hiding effect. This photograph is also included in the book Full Moon by Michael Light and Andrew Chaikin, as Plate 99.
• AS12-46-6841 is probably one of the most interesting photographs of the Lunar Heiligenschein. This photograph was made at Middle Crescent crater (Apollo 12 site). Note the long shadows of astronauts Pete Conrad and Alan Bean. The haze-like smudge in the centre of the photograph is due to dust on the Hasselblad-camera's lens.

Variation with types of topography

• It seems obvious that different kinds of lunar surface features respond differently to the changing illumination during the lunar cycle. For example, the bright lunar rays are most readily visible near Full Moon, implying that they brighten more than the rest of the surface.
• Wildey (1978) produced a black and white image of the whole Moon that purports to highlight the regions that exhibit the greatest (and least) surge in brightness in the hours immediately around Full Moon. Plates at a phase angle of about 2 degrees were apparently compared to others taken at around 4 degrees. He claims the greatest opposition surges are around Aristarchus and at Cassini’s Bright Spot. Bright rays in general were not found to have an unusually large opposition surge (although evidently the have a greater overall brightening than most of the Moon).
• Others have followed Wildey, typically forming ratios of Clementine images taken at zero and non-zero phase angles.

Observations of reflected sunlight in shadowed craters

• On page 18 of H.P.Wilkins's and P.Moore's book The Moon:
  • "...after all, it is not surprising that detail can occasionally be seen on the unilluminated portion, so much light is sometimes reflected from the surrounding sunlit cliffs or walls that Dr. W.H. Steavenson, using a shaded eyepiece, has seen the central mountains of the craters Agrippa and Godin when the interior of the crater was filled with shadow, and even noted the short shadow cast by the central mountains owing to the light reflected on to them by the cliffs".

Other things attributed to lunar retro-reflectivity

• O'Keefe (1957) suggested that the brightness in lunar rays was due to their being composed of transparent glass-spherules, producing a retro-reflection effect like a bead-coated projector screen.
• The ability to sometimes see detail in the shadowed parts of an Apollo astronaut's suit while walking on the Moon has sometimes been attributed to the shadowed part of the white suit receiving retro-reflected light from the bright Lunar Heiligenschein around the astronaut's shadow on the retro-reflective regolith. A typical Hasselblad photograph which shows the "not shadow-like" or "not dark shadow" on an Apollo astronaut's suit is AS16-117-18825 (Apollo 16's John Young, near the Lunar Roving Vehicle). But this explanation cannot be correct for several reasons. The light from the opposition surge on the lunar soil is not like the beam from a flashlight. Even if the reflected light were 50% stronger than normal over a 5° angle, that little zone is no more than 0.4% of the total hemisphere into which light is reflected and hence it augments the overall reflectivity by only about 0.2%. The remaining 99.8% of the light reflected by the lunar soil is spread over a larger range of angles and similar to what one would expect from an ordinary diffusely reflecting surface. The detail visible in the shadowed parts of the astronaut's suit is presumably lit by a variety of sources including this diffuse reflection from the surface. Note that photos of astronauts walking in space, above the Earth or Moon, where their shadows are lost in space (and hence there cannot be reflections from the anti-solar point) show similar detail in the shadowed areas. In part this is because of the whiteness of their suit compared to the relatively low reflectance surfaces on which many of the other shadows fall. As further proof, many Apollo photos show detail in the shadowed side of small rocks and boulders lying directly on the lunar surface. If the shadowed sides of these rocks were lit primarily by small-angle retro-reflections we would expect to see a bright fringe around the sunlit edge (where the returning retro-reflected light would strike) and a dark interior (where no retro-reflected rays could strike). Instead, when the face of the rock is flat we see the entire shadowed side illuminated by a uniform light, evidently produced by the general wide-angle reflections from the lunar surface.

Other examples of Retro-Reflectivity

• "Cat's eye" reflectors on bicycles and cars (also called cube-corner reflectors).
• Similar (but much more precise) arrays of cube-corner reflectors (LRRR's) were left on the Moon by three Apollo and two Russian missions, and form the basis for an on-going Lunar Laser Ranging Experiment which monitors changes in the distance between Earth and Moon to unprecedented accuracy. The locations of the LRRR's are also among the most precisely known positions on the Moon. See the photograph of the Apollo 11 LRRR on the lunar surface at Statio Tranquillitatis. The fact that the reflections from the aluminized(?) surfaces of these carefully crafted quartz corners can be detected against the background of the lunar soil clearly demonstrates that the general retro-reflectivity of the Moon is of a much lower order.
• Movie projector screens, made of a white plastic sheet with millions of small transparent glass-spherules on it (an equivalent of the dew-drop Heiligenschein on grass) (see Bibliography).
• Road traffic signs (made of minuscule cube-corner reflectors, or transparent glass-spherules).
• White paint on roads (contains millions of small transparent glass-spherules, the same effect as the Movie screens).
• The temporary brightness enhancement of Saturn's rings when this planet is at opposition (a Shadow Hiding effect). This effect (a temporary brightness enhancement) is noticeable on all the outer solar system's planets when they are at opposition. Not on Mercury and Venus because they are behind the sun when they are at "Full".
• The Gegenschein (the Opposition effect) on the solar system's cloud of asteroids and meteoroid-dust (a diffuse glow at 180° away of the sun).
• The visibility of the moon's near side when there's a Total Solar Eclipse (!). This is the so-called Ashen-light which is also visible before First Quarter Moon ("young moon") and after Last Quarter Moon ("old moon"). During a Total Solar Eclipse, the moon's near side is like a "Full Moon" (not illuminated by the Sun, but rather by the reflected sunlight of the "Full Earth"!).
  • Note: the Earth probably does not show a large opposition surge as viewed from the Moon, so the intensity of the Earthshine during a Total Solar Eclipse is probably not much greater than at any other time near a New Moon. The Earthshine gives the Moon's nearside a constant, but very dim, Full Moon-like component as viewed from Earth, whose intensity varies during the lunar cycle, going to near zero near Full Moon (when the Earth is dark as seen from the Moon). The Earthshine component always enjoys the full retro-reflectivity of the lunar surface, since we are viewing from less than 1° away from the light source (the sunlit parts of the Earth, as seen from the Moon). It can most readily seen a few days before or after New Moon, when it is still relatively bright, but can be seen (near Moonrise/Moonset) in a relatively dark sky. - Jim Mosher
  • The Apollo astronauts orbiting the Moon were easily able to see the lunar landscape illuminated by the Earthshine, and were able to detect its "terminator" (which is always located at the extreme limit of what we can see from Earth). The part of the Moon that was illuminated by neither sunlight nor Earthshine was extremely dark and showed no detail. - Jim Mosher

Specular reflections from the Moon?

• The most creditable search for shiny reflective surfaces on the Moon seems to be part of the joint ALPO/BAA transient effects effort directed by British astronomer Anthony Cook. Cook publishes monthly forecasts of places and times when past reports of localized lunar brightenings (and other kinds of events) can be re-observed under a nearly identical viewing geometry. The results are sometimes reported in ALPO’s The Lunar Observer newsletter. To the best of my knowledge nothing that could be attributed to a repeating specular reflection from a shiny surface on the Moon has ever been observed. - Jim Mosher

LPOD Articles

Bright Targets (Full Moon photography)

APOD Articles

Saturn near Opposition (the bright appearance of Saturn's rings, depicted on a very sharp detail-rich telescopic photograph made by several top-astrophotographers).
The Crown of the Sun (with retro-reflection of the "Full Moon", seen during the great Total Solar Eclipse of August 2017, across the United States).

EPOD Articles

Earthshine from Eclipse Lunar Crescent and Earthshine

Bibliography

Retro-Reflection/ "Heiligenschein" (general)

• Les Cowley's Atmospheric Optics website (explaining the dewdrop form of the Heiligenschein).
• M.G.J. Minnaert's Light and Color in the Outdoors (De Natuurkunde Van 't Vrije Veld).

Lunar Retroreflectivity

(see also opposition surge)

• John O'Keefe. 1957. Lunar Rays. Astrophysical Journal, 126:466 (an excerpt of this article was printed on pages 214 and 215 of the book Mysterious Universe; a handbook of astronomical anomalies (1979), by the underestimated explorer and investigator of odd scientific discoveries: W.R.Corliss).
• Robert Wildey. 1978. The Moon in Heiligenschein. Science Vol. 200, pp. 1265-1267.

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