Publication Year 2015

Acceptance in OA@INAF 2020-05-05T14:39:34Z

Title Albedo Feature

Authors Kardevan, Péter; HARGITAI, HENRIK; ZINZI, ANGELO; ESPOSITO, Francesca

DOI https://doi.org/10.1007/978-1-4614-3134-3_461

Handle http://hdl.handle.net/20.500.12386/24519 Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:23:59 Page Number: 30

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Ruff SW, Christensen PR (2002) Bright and dark regions surroundings. Albedo features were traditionally on Mars: particle size and mineralogical characteris- identified by doing spectrally integrated observa- tics based on thermal emission spectrometer data. J Geophys Res 107(E12):5127. doi:10.1029/ tion of the reflected sunlight with a telescope in 2001JE001580 the visible spectrum range of light and having Schenk P, Hamilton DP, Johnson RE, McKinnon WB, adequate spatial resolution to resolve distinct Paranicas C, Schmist J, Showalter MR (2011) Plasma, parts of the surface of the object. Detection of plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites. the brightness variations is today extended to Icarus 211:740–757 photometric (radiometric) measurements with See TJJ (1910) On the craters, mountains, Maria and other modern satellite/spacecraft spectrophotometers phenomena observed on the surface of the Moon, and or spectroradiometers at separate monochromatic on the indicated processes of planetary growth. In: Researches on the evolution of the stellar systems. II: wavelengths or integrated observations in other the capture theory of cosmical evolution. Thos. wavelength regions as well (see spectral albedo, P. Nichols, Lynn, Mas https://archive.org/details/ narrowband albedo, and broadband albedo). researchesonevol02seetuoft Albedo features result from those brightness var- Soter S (1974) IAU colloquium 28, Cornell University. Cited by Tamayo et al. (2011) iations, that are due to variations of the reflective Spencer JR, Calvin WM, Person MJ (1995) Charge- properties (often referred to as albedo) of coupled-device spectra of the Galilean satellites: a planetary surface. molecular oxygen on Ganymede. J Geophys Res 100:19049–19056 Tamayo D, Burns JA, Hamilton DP, Hedman MM Variants (2011) Finding the trigger to Iapetus’ odd global Albedo pattern, albedo marking. albedo pattern: dynamics of dust from Saturn’s irreg- ular satellites. Icarus 215:260–278 IAU Definition Geographic area distinguished by amount of reflected light (IAU Gazetteer 2014). Albedo Feature Identification Pe´ter Kardeván1, Henrik Hargitai2, 3 4 Angelo Zinzi and Francesca Esposito The identification and mapping of albedo features 1 retired from Department of Environmental are achieved through observations of the relative Geology, Geological and Geophysical Institute of brightness variations of the planetary surfaces Hungary, Budapest, Hungary that can be carried out either by photographic 2 NASA Ames Research Center, Moffett Field, methods (analog photoplates or digital photo- CA, USA graphs), photometric (radiometric) spot measure- 3 ASI Science Data Center / INAF - Osservatorio ments and recording digital images by imaging Astronomico di Roma, Rome, Italy photometers, spectrophotometers, or 4 INAF-Osservatorio Astronomico di spectroradiometers. Capodimonte, Naples, Italy The delineation of albedo features on panchro- matic or monochromatic photography is based on the creation of photographic isodensity contours Definition (analog films) or isophotic contours (digital images) separating different categories of gray An albedo feature is a region on the surface of levels of the image. Special terminology has a nonluminous celestial body (e.g., planet, moon, been developed for making standard reference or small body) with distinct brightness (radiance) to the different gray levels of distinguished values or color, i.e., exhibiting observable/mea- albedo features (see “Classification of Albedo surable brightness- or color-contrast relative to its Categories”). Since these photographic density- based brightness scales are in nonlinear Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:23:59 Page Number: 31

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Albedo Feature, Fig. 1 This image pair illustrates the effects of illumination, or phase A angle, in recognizing different aspects of the same feature (a 200 m diameter crater) on the lunar surface (Plescia 2009). Left: M1046700 19L: incidence angle 50 (low sun), right: M1070 35386L incidence angle 25 (high sun). Scale bar 200 m. LROC Narrow Angle Camera, PIA12916 (NASA/GSFC/ASU)

functional relationship with the brightness or The albedo patterns in a terrestrial environ- albedo defined in photometry (radiometry) (see ment correspond to different surface cover clas- “Concept of Albedo”), the term relative albedo ses (vegetation, soil, etc.) having decisive role in contrast is used in such cases, and in all other climate forcing, and their reflective properties ones, when arbitrary scales are used often specific can be measured by satellite-, airborne-, or to the authors or to the research project. The field-measuring systems. photogeological interpretation calls however for Albedo features may or may not correspond to labeling the delineated albedo feature with the relief features; albedo features may not show any albedo values themselves that conform to its topography (e.g., albedo patterns such as ▶ swirls photometrical definition. Therefore, photometric or ▶ dust devil tracks). (radiometric) calibration is carried out. Albedo Albedo patterns are best visible at high sun value ranges from 0 (blackbody) to 1 (ideal (e.g., near full Moon). In contrast, relief features reflector). During classification, typically only are highlighted at low solar altitude angle (high few subtypes are defined: high albedo (bright), incidence angle) (near the terminator line) where intermediate, and low albedo (dark) (if needed, shadows are the longest and emphasize topogra- also very high and/or very low). The general term phy (Figs. 1 and 2). “albedo” in many photogeologic studies refers to relative, snow- or frost-free surface albedo (Prockter et al. 1998). The Concepts of Albedo The term “albedo feature” is used in connec- tion with spatial variation of surface brightness There are several types of albedo concept (often and called sometimes interchangeably as albedo referred as albedo products) used in different markings or albedo patterns. branches of science such as astronomy, remote The term albedo pattern, however, can be used sensing of Earth, climatology and oceanography, in connection with characteristic time variation etc. The detailed mapping of albedo features not of albedo in the sense of signatures of certain only serves the purposes of planetary geology, surface/atmospheric processes. but plays important role in the research of climate Albedo features can be permanent or variable. change of the Earth. Sagan et al. (1972) defined variable features as Different attributes (often more than one) are variable, snow-/frost-free land albedo patterns therefore attached to the term “albedo” to borrow that change with time (Sagan et al. 1972) specific meaning to it, establishing, thus, (▶ dark deposits (Mars) and ▶ wind streak). a precise terminology and making the plain term Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:23:59 Page Number: 32

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variations can be carried out by using narrow- band albedo or spectral albedo products. The definition of albedo given above implies that the quantification of the incident and reflected radiation power can be made either locally, using surface densities of radiation power, or globally, characterizing the incoming radiation power over the whole surface using surface-integrated values of surface densities, that is, the radiation power values themselves. The albedo of a celestial body as a whole is used in planetology and astronomy or even in calibration procedures of the brightness values belonging to albedo features as well. One may find two versions of such albedo products with their surfaces modeled either as a plain disk or a sphere. Careful distinction should therefore be made, when using those types of albedo inte- Albedo Feature, Fig. 2 Lineaments of the Jovian moon grated to the whole surface of a celestial body, Europa “transform” from albedo features (seen under high solar altitude angle/low incidence angle) into topographic between bond albedo also known as spherical features (ridges) (observed in low sun conditions, near the albedo and the geometric albedo also known as terminator) (Lucchitta et al. 1981). Scale bar ca. 100 km. physical albedo. Also, distinction between the Voyager 2, PIA01504 (NASA/JPL) brightness concepts of a celestial body and that of a surface should be made. The former is in “albedo” ambiguous. Yet, the definition of functional connection to the magnitude of a star albedo, as an instruction of measurement, is gen- and the latter being synonym of the term specific eral and commonly applicable in all specific intensity or radiance. albedo concepts. The mapping of albedo features applies the local characterization, i.e., surface densities of radiation power having the physical dimension Definitions of Albedo of electromagnetic power/m2. Thus, the term “albedo” that is used in connection with albedo According to a commonly accepted formulation, features is defined as a characteristic reflective “Albedo is defined as the ratio of reflected solar quality of a surface element modeled locally as shortwave radiation from a surface to that inci- plain surface. These special albedo products are dent upon it” (Strugnell and Lucht 2001). called sometimes material albedo or inherent Reflected radiation in the whole solar spectrum albedo in planetology, indicating that it is an is referred namely as shortwave radiation intrinsic property of the surface element that (0.3–5.0 mm). This type of broadband albedo can be referred in remote sensing unambiguously governs the amount of shortwave radiation that as true intrinsic surface albedo. is effectively absorbed by the material of which The terminology of the radiometrical quanti- a surface element is composed (Dumont ties used in terrestrial environments is quite dif- et al. 2011). While broadband albedo products ferent, because both incident and reflected (also bolometric albedo of a celestial body radiation power can be measured on the Earth’s referring to the whole solar spectrum) are used surface. Therefore, a more specific definition has rather in climatology and astronomy, planetary been accepted: “Albedo at some level of a geo- geological interpretation of brightness physical system is defined as the ratio between the upward flux density . . . exiting that particular Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:23:59 Page Number: 33

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level and the downward flux density impinging However, in the case of diffuse radiation on that same level” (Pinty et al. 2005). This resulting from scattering by the surface and the definition incorporates the term surface albedo atmosphere above it, the surface density of radi- A again, as that measured at the bottom of atmo- ation power having though the same physical sphere, i.e., at the level of the Earth’s surface, dimension as flux density is referred rather as seemingly in a different meaning as defined for- irradiance or exitance, depending on whether merly in connection with albedo features in plan- incoming or reflected diffuse radiation is charac- etology. The problem is that in this case surface terized, and the radiation power is a double inte- albedo is not an intrinsic feature of the surface. It gral of the surface and solid angle. In lighting is referred therefore as apparent surface albedo, industry, “albedo” is defined simply as diffuse contrary to intrinsic surface albedo that can be reflectance, and in oceanography, the quotient calculated in terrestrial environment by modeling of exitance/irradiance is used as albedo, of atmospheric radiation transfer (Liang corresponding to the definition by measurement et al. 1999) (also see below). The “geophysical instruction. system” means here a complex system of an The intrinsic reflective feature of a surface is unisotropically scattering surface with arbitrary referred in radiometry either as reflectivity or topography and the consequent shadow casting reflectance, although the former term is used in and mutual view shadowing that is coupled with modern terminology rather in connection with an inhomogeneous atmosphere of changing com- specular reflections (Fresnel reflectivity) position above it scattering both upward and depending solely on material parameters and the downward radiation anisotropically as well. optical constants of participating media, and the Therefore, the measured surface albedo depends latter is used rather in connection with scattering on the spectral and angular distribution of both when a surface and a medium reflects/transmits incident and reflected radiation power. Coupling radiation diffusively. The amount of reflected of surface and atmosphere is incorporated by radiation depends also on the physical parameters multiple scattering, which is often neglected in (such as particle size, density, porosity, etc.) several albedo products (e.g., blue-sky albedo, describing the inner structure of the surface etc.). This definition is special inasmuch as the material. power density is referred as flux density, and no Planetary surfaces can be modeled either distinction is made to the irradiance or exitance using single or multiple scattering models (Coakley 2002). This is acceptable only in the (Hapke 2012). case of directional radiation component when Broadband measuring instruments include photons travel in a single direction. pyrheliometers (for direct solar radiation), The diversity of albedo products arises even pyranometers, and photometers (for direct plus from the fact that the nature of radiation is differ- diffuse radiation in the visible wavelength range), ent for directional and diffuse radiation. In the while spectroradiometers (spectrophotometers) former case, the amount of radiation power arriv- are used for spectral measurements resulting ing at a unit surface element is referred as flux in different kinds of data as a consequence of density, and the radiation power is simply the their differently specified measuring parameters. surface integral of the flux density. Flux and These differences are discussed under the term irradiance concepts lead to the same value of parameters of radiation geometry including these physical quantities in this case, as radiation illumination and viewing geometry, which com- arrives from a single direction. The same is true prise angles “i” and “e” of incident and reflected for reflected radiation in the case of specular radiation, the phase angle “g” (Fig. 3), and the reflection. The term “flux” is used, therefore, in solid angles “Ω”, a measure of the collimation of planetology and in all other cases when the sole radiation (not represented in Fig. 3). The types of incoming radiation component is Sun’s radiation reflectance according to different radiation geom- that can well be modeled as a unidirectional one. etry are best summarized by Hapke (2012)as Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:00 Page Number: 34

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Albedo Feature, Fig. 3 Radiance I from a surface element DA. Incident (i), emission (e), and phase angles (g) are shown. J: irradiance (After Fig. 8 from Hapke (2012))

follows: in the modern usage (Nicodemus illumination angle, referred as black-sky albedo et al. 1977), the word “reflectance” is preceded (see “MODIS Reflectance Products”) derived by two adjectives, the first describing the degree from measurements by atmospheric corrections. of collimation of the illumination source and the Subsequent integration of DHR over all illumi- second that of the detector, thus characterizing nation angles gives the bihemispherical reflec- the differences in solid angles or beam geome- tance (BHR) referred as white-sky albedo as tries. The term “reflectance” of a surface is used well, which is a measurable quantity. The blue- here just as being the quotient of the reflected to sky albedo can be derived for given ratio of the incoming radiation power. The usual adjectives directional to diffuse radiation power, neglecting are “directional” (denoted by index “d”), multiple scattering effects. These albedo prod- “conical,” or “hemispherical” (denoted by the ucts are used in terrestrial environment. index of “h”). For example, the directional- In the case of telescopic observations and in

hemispherical reflectance, rdh, is the total fraction several satellite and airborne systems or even of light scattered into the upward hemisphere by field measurements, often unidirectional obser- a surface illuminated from above by highly col- vations are carried out; therefore, the irradiance limated source. or the incoming radiance cannot be measured at Current mathematical tools for specifying the the level of reflecting surface, but estimated only anisotropic behavior of the diffuse reflection pro- using several BRDF-models, most often the cess in the presence of anisotropic diffuse illumi- Lambertian model treating diffuse radiation as nation for different radiation geometries are isotropic scattering. Actually, the theoretical based on the concepts of radiance and the Bidi- BRDF value of an isotropically scattering ideal rectional Reflectance Distribution Function surface can be used as normalization factor. Nor- (BRDF), incorporating both direct and diffuse malization of the true BRDF can be introduced, radiation (Nicodemus et al. 1977). The instanta- e.g., using the same geometry as in the target neous BRDF is a theoretical kernel function measurement, resulting in the concept of reflec- connecting incoming and reflected radiance that tance factor. The bidirectional reflectance

can only be modeled/estimated (not measured). factor RBI(n, i, e, g), abbreviated as usual as The multi-angle (directional) measurements BRF, results from the bidirectional reflectance are producing the sampling values of a physical specially for the Sun’s directional illumination as

variable, denoted here as rBI(n, i, e, g) and called bidirectional reflectance, a concept different RBIðÞ¼n,i,e,g rBIðÞn,i,e,g =ðÞcosðÞ i =p , from but often mixed with BRDF. Integration of BRDF over all view angles gives the directional- where the reflectance of the perfectly diffuse

hemispherical reflectance (DHR), rdh, for given (isotropic) surface is cos(i)/p (Hapke 2012). Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:00 Page Number: 35

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The directional-hemispherical reflectance of rðÞ¼n,i,e,g pI=F an isotropically scattering surface is In this case the formal definition of phase A rdh ¼ I=F ¼ AL cosðÞ i =p function does not change, so that

where I is the reflected radiance (brightness), F is rðÞ¼n,i,e,g AnfðÞn,i,e,g : the (solar) irradiance, and AL is called the Lambertian albedo, which is the directional- Calculation of albedo for planetary bodies. hemispherical reflectance factor of an Albedo is generally calculated starting with the isotropically scattering surface. Lambert(ian) radiance factor r of the planet: r(n,i,e,g)= pI/F albedo refers to radiation scattered isotropically (Fig. 3). (in all directions; from a Lambertian surface), Several authors derived semiempirical photo- irrespective of the incidence angle. It character- metric functions from physical laws. The most izes the material’s albedo regardless of its widely used are the Hapke (1981) and Shkuratov geometry. (Shkuratov et al. 1999) functions, as they well Normalization can be carried out with the the- reproduce both laboratory and planetary data. oretical reflectance value of an ideally diffuse Nevertheless, McEwen (1991) demonstrated surface with the illumination angle i = 0, rather that if not used near the limb or terminator, sim- than with the same illumination angle of the pler functions, such as that of Lambert and target. This quantity being used extensively is Minnaert (Minnaert 1941), show negligible devi- called bidirectional radiance factor, RADF(n, ations from the more complex Hapke and i, e, g), a similar quantity as BRF (Hapke 1981): Shkuratov models. Using the semiempirical expression of RADFðÞ¼n,i,e,g rBIðÞn,i,e,g =ðÞ1=p , Minnaert for the photometric function, the radi- ance factor can be written as Note that in planetology the factor of p is often omitted, and RADF r (n,i,e,g)= I/F is kðÞn, g kðÞn, g 1 BI rðÞ¼n,i,e,g B0ðÞn,g ðÞcos i ðÞcos e referred as radiance factor. It is not correct, the ¼ B0ðÞn,0 fMinnðÞn,i,e,g term so defined is only proportional to it (Thomas (1) 2005). The normal albedo is the radiance factor at where fMinn is the Minnaert photometric function g = 0 phase angle and at i = e: and B0(n, g) and k(n, g) are the Minnaert empir- ical coefficients. B equals A at g = 0, while k is ¼ ðÞ=ðÞ= 0 N An rBI n,e,e,0 1 p related to the limb-darkening effect. Indeed, at zero-phase angle, the values 0, ½, and 1 for the The dependence of viewing geometry is k parameter correspond to opposition limb effects expressed by the photometric function, f(i,e,g), of cosine brightening, uniform disk brightness, in connection with the brightness (radiance) and the cosine darkening brightness of a perfect values observed at fixed “e” and varying “i” and Lambert surface, respectively. In the visible spec- “g,” after normalization (see “Reflectance tral range, the full Moon has k = 0.5 (uniform Factors”): disk brightness) (Harris 1961). For k = 1, the Minnaert expression (1) reduces to fðÞ¼n,i,e,g rBIðÞn,i,e,g =rBIðÞn,e,e,0 ¼ rBIðÞn,i,e,g =An rðÞ¼n,i,e,g ALðÞn,g cos i (2)

The definition of radiance factor can be based where A is the Lambert albedo. Expression (2) is = L on rdh I/F, denoted as r(n, i, e, g) given as the Lambert model, or the cosine model. This Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:00 Page Number: 36

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expression provides an easier method of deriving with the fine-grained and high-albedo terrains albedo from surface measurements. and lower values with rougher and low-albedo The Minnaert model also allows the derivation material. of information about the roughness degree of the observed surfaces, starting from the evaluation of the limb-darkening parameter k. Formation It has been demonstrated by several authors (Veverka et al. 1978a, b, 1986; Goguen 1981; Albedo features seen in identically illuminated McEwen 1991; Erard et al. 1994; Esposito surfaces (Helfenstein and Wilson 1985) indicate et al. 2007) that the Minnaert model well material units that are horizontally distinct, e.g., describes the scattering properties of most partic- in the following: ulate materials and planetary surfaces, especially (1) Composition (e.g., dark basalt, bright frost) at small-phase angles (<40). The model cannot (2) Physical properties: particle size (e.g., dust be used with observations of the limb and at the vs. sand), macroscale roughness, and poros- terminator of a planet and with mirror-like ity, these properties may also affect radar surfaces. backscatter and thermal infrared emission; Veverka et al. (1978a) observed experimen- (▶ radar features, ▶ dust devil track, ▶ fine tally that k is related to multiple scattering as well ejecta halo), crystal structure (e.g., due to as albedo and texture/roughness, especially for weathering), due to e.g., bright materials. They also demonstrated that (3) Emplacement, modification, and age (e.g., for phase angles close to zero and surfaces with due to solar radiation, impact comminution) similar texture and roughness, the Minnaert parameters are linearly related with k, increasing as B increases (see also Esposito et al. 2007). 0 Regional Variations The limb-darkening parameter varies with the wavelength of the detected radiance. Generally, Mercury in the near IR, k varies from 0.5 to 1 going Mercury has circular ▶ high reflectance plains, from very dark to bright planetary surfaces which are interpreted as volcanic plains (e.g., (De Grenier and Pinet 1995; Erard et al. 1994; Prockter et al. 2010) similar to lunar maria. Esposito et al. 2007). This is due to the increased contribution of multiple scattering on the radi- Venus ance factor when the albedo increases. In the Surface (clear-sky) albedo is not known for visible wavelength range, the limb-darkening Venus, because its surface cannot be investigated parameter k is generally independent on surface by optical remote sensing due to its optically albedo and for Mars is 0.6 (De Grenier and opaque atmosphere, which can be only pene- Pinet 1995). This is due to the negligible contri- trated by radar (▶ radar feature). bution of multiple scattering at these wavelengths (dominance of single scattering processes). Moon Esposito et al. 2007 produced a map of the (▶ Mare, volcanic, ▶ swirl) (Fig. 6). Low-albedo Minnaert and Lambert albedo (Fig. 4) and of the regions (at 750 nm) are interpreted as titanium- geographical distribution of the limb-darkening rich dark mare lavas and mantle deposits or parameter k over the surface of Mars, starting mature mare soils. Dark patches are thought to from the data obtained by the Short Wavelength be produced by explosive volcanic activity Channel of the Planetary Fourier Spectrometer (▶ dark mantling deposit, regional, ▶ dark man- (PFS) on board Mars Express spacecraft tling deposit, annnular, ▶ dark/bright halo pit), (Fig. 5). This k map provides a relative except for smaller, impact-related ▶ dark halo granulometry map of the Martian surface and craters. confirms the association of higher values of k Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:00 Page Number: 37

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A

Albedo Feature, Fig. 4 Lambert albedo map obtained the spectrometer TES at a resolution of 8 per pixel by from PFS/MEX at 7,030 cm-1 (bottom panel), compared considering only the pixels corresponding to geographical with TES/MGS albedo (top panel). The top panel has been points observed by PFS (Esposito et al. 2007) extracted from the full resolved albedo map acquired by

Bright regions are believed to be immature to 1 cm, rather than bedrock (Ruff and soils on bright ejecta blankets of fresh impact Christensen 2002)(▶ dark deposits, Mars). craters or are rich in anorthosite (and poor in High albedo corresponds to low inertia materials iron). The brightest area of the Moon is at the (dust particles of 2–40 mm) (Fig. 8). Thermal fresh, rayed Giordano Bruno crater and the ray- inertia is the ability of a material to conduct and less Sharonov crater which likely exposes crustal store heat, e.g., during the day, and reradiate it anorthosite (McEwen and Robinson 1995). during the night (▶ aeolian dust deposit). The albedo contrast of Mars is greatest in red Mars color (centered near 0.6 mm) (Barlow 2008, The disk of Mars is dominated by three surface p. 73). albedo domains: (1) dark, (2) orange-red bright, and (3) white bright (at polar latitudes) (Fig. 7). Albedo Classes of Mars and Their Interpretation On frost-free surfaces, low albedo correspond to Note that when referring to albedo features, the high thermal inertia materials dominated by classical albedo nomenclature is used: high- and mafic particulate materials ranging from 0.1 mm low-albedo regions on Mars are defined as those Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:00 Page Number: 38

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Albedo Feature, Fig. 5 Limb-darkening parameter maps, obtained at 7,030 cm-1 from PFS/MEX measurements (Esposito et al. 2007)

Albedo Feature, Fig. 6 Clementine NIR albedo mosaic of the Moon (70N–70S latitudes) (USNRL/BMDO/USGS)

higher and lower, respectively, than 0.2 (Ruff and regions (Fenton et al. 2007): Tharsis/ Christensen 2002) or 0.15 as measured by Amazonis, Arabia, and Elysium/Isidis MGS-TES (Rogers et al. 2007). (albedo: 0.27–0.30) (Christensen 1988). For comparison, the “light-toned” sedimentary (1) Very high albedo: areas of seasonal frost and outcrop of White Rock has an albedo of perennial polar ice caps. 0.18 (▶ light toned deposit). (2) High albedo: associated with a thick (>1m) (3) Low albedo: most Martian low-albedo fea- layer of fine-grained particles (2–40 mm) tures correspond to areas covered by bedrock (dust) (Ruff and Christensen 2002). These outcrops or coarse grains (710–1,000 mm) of are also known as three low thermal inertia Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:00 Page Number: 39

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Albedo Feature, Fig. 7 Three units with distinct albedos on Mars, as seen from Earth (Photo by A Efrain Morales, Jaicoa Observatory, Jan 9, 2002)

Albedo Feature, Fig. 8 Regional albedo variations on Mars, MGS-TES (Christensen et al. 2001). Values range from 0.08 (shown in black) to 0.32 (shown in white). http://www.mars.asu.edu/data/tes_albedo/ (NASA/JPL/ASU)

basaltic and andesitic materials (Ruff and Southern High-Latitude Band (Mare Christensen 2002). Typically, dust-free Chronium–Mare Australis) (Geissler 2005). regions of Mars include Syrtis Major (4) Intermediate albedo (0.15–0.19) (Rogers (a basaltic volcano) and Mare Acidalium, et al. 2007): on Mars, an intermediate albedo the Southern Tropical Dark Band (Mare type is also observed between dark and bright Cimmerium–Mare Sirenum), and the (intermediate values exist for both albedo and thermal inertia) which is not a transitional Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:01 Page Number: 40

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Albedo Feature, Fig. 9 Phobos albedo units. HiRISE PSP_007769_9010 (NASA/JPL/UA)

type, but likely corresponds to well-indurated occurs only when the electrostatic force is larger duricrust (Christensen et al. 2001 and refer- than gravity and cohesive force together. Dust ences therein). particles accelerate and may follow a ballistic (5) Other albedo features: dark terrains, particu- trajectory. The night side (or shadowed area) is larly at high latitudes, are dominated by negatively charged, because without UV radia- ▶ dust devil tracks where bright dust have tion, there are no photoelectrons produced. Also, been removed from the surface. Dark the emitted electrons may be deposited on the (mafic) sand covers large areas in the north- night side (Zakharov 2012). This dust migration ern circumpolar ▶ erg and occurs in smaller may produce a dust belt (ring) around Phobos and patches in intracrater dune fields. Exposures a dust torus around Deimos (Krivov and of subsurface ice and underlying dark mate- Hamilton 1997)(▶ dust pond). rial in fresh impact craters produce small high- and low-albedo spots (▶ splotch), Gaspra respectively. ▶ Wind streaks may appear (Main belt asteroid): there are bluer terrains at the darker or brighter than their surroundings. high slope angle surface units and fresh craters, The southern seasonal polar cap displays where the regolith is interpreted to be younger a distinct low-albedo area termed ▶ Cryptic and less weathered than at other locations (Clark Region. The seasonal cap hosts various local et al. 2001). scale seasonal ▶ dark dune features. Io Phobos Distinctly colored plains on Io occur in geograph- Is not homogeneous and consists of a bluer and ically constrained distributions: the two most redder unit (Fig. 9) (unlike Deimos). The bluer extensive units are red-brown plains (8.6 % of material is draped over the south-eastern rim of surface), which occur >30 latitudes, and white Stickney crater and is thought to be relatively thin plains (6.9 % of surface), mostly in the equatorial (Thomas et al. 2011). According to the dynamic anti-Jovian region (<30 latitudes, dust model, the blue markings are produced by 90–230W). Red-brown plains result from long- horizontal wind-like motion of dust particles term accumulation of ▶ red diffuse deposits and induced by electrostatic levitation. Solar are interpreted to be the result of alteration from UV-radiated slope surfaces are positively condensed sulfur gas induced by exposure to charged: high energy UV radiation produces pho- radiation coming in from the poles. White plains toelectrons that escape (emit/detach) from the accumulated white diffuse material that is

surface and thus charge the dust positively. This interpreted as condensed SO2 (coarse- to Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:01 Page Number: 41

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moderate-sized grains of SO2 snow and frost) and occurs in the south polar terrain, in the ▶ Tiger its contaminants and possibly indicate regions of stripes region. These plains are 10 % brighter cold traps (Williams et al. 2011 and references than the average reflectivity of Enceladus possi- A therein). bly due to particle fallout and also display the greatest albedo contrast (20 %) between the Europa darker, coarser-grained stripes and the plains Low-resolution Voyager imagery showed (Porco et al. 2006). smaller and larger darks spots and linear features. Small spots were named ▶ lenticulae, larger Dione, Rhea ones, ▶ maculae. Collections of uneven splotches See ▶ Wispy Markings. were informally termed ▶ mottled terrain. Linear features were named ▶ lineae, of which one spe- Titan cific type was termed ▶ triple band. Higher- Titan appears gray at visible wavelengths (Vixie resolution Galileo images revealed the true et al. 2012). On Titan’s surface, low- and high- morphology of these features. Lenticulae and albedo regions can be distinguished (Fig. 10). maculae (albedo features) are both a type of These units have sharp contact in several loca- ▶ chaotic terrain (topographic feature), and tions (e.g., in Western Xanadu) (Fig. 11). some were officially named ▶ chaos. Triple The following models have been put forward bands (albedo feature) were revealed to be to explain albedo variations (Porco et al. 2005 ▶ double ridges (topographic feature) after and references therein, Langhans et al. 2012 and higher-resolution, low-sun images became avail- references therein): able (Greenberg 2008, p. 20; Fig. 2). The descrip- (1) Dark regions are liquid or solid hydrocarbons tor term ▶ regio is used for bright plains precipitated from the atmosphere, while crisscrossed by ridges (e.g., Falga Regio) and bright regions are water-ice bedrock chaos-like darker regions (mottled terrains) outcrops. (e.g., Dyfed Regio) (Doggett et al. 2009). (2) Dark areas are hydrocarbon liquids, whereas One of the darkest and reddest features on bright ones are ethane mist overlying the Europa is Castalia Macula (informally known as liquid. This theory requires the change of “the dark spot”) (Prockter and Schenk 2005) most surface patterns that are not confirmed which is interpreted to be a young structure. by observations. Observations show that For hemispheric albedo features of Europa, bright regions are at a higher topographical see ▶ Albedo/Color Dichotomy. level than dark ones. For regional scale albedo features of Gany- (3) Dark areas are composed of an easily deform- mede see ▶ Albedo/Color Dichotomy. able, probably organic material precipitated For hemispheric albedo features of the mid- from the atmosphere. A thin, brittle crust of sized Saturnian satellites, see ▶ Albedo/Color bright material overlies it and exposes dark Dichotomy. regions when pulled apart. Enceladus Spectral mapping by Cassini’s Visible and The visual geometric albedo of Enceladus is 1.4, Infrared Mapping Spectrometer, covering which makes it one of the brightest satellites in a spectral range from 0.35 to 5.2 microns, could the Solar System. Its reflectance spectrum is distinguish three spectral surface units that may dominated by pure water ice. The surface may reflect differences in composition or grain size. be coated by particles from Saturn’s E ring, (1) Bright unit forms islands and large continents whose impacts comminuted the regolith into (e.g., Xanadu), interpreted to be older and fine-grained bright material, combined with elevated as compared to dark surfaces. a fallout of particles from venting or geyser-like activity. The brightest of the ▶ bright plains Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:01 Page Number: 42

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Albedo Feature, Fig. 10 938-nm methane window albedo map of Titan. Cassini ISS mosaic, February 2009 (NASA/ JPL/SSI)

Albedo Feature, Fig. 11 The boundary between Shangri-La (dark) and Xanadu (bright, right), Titan (Porco et al. 2005). Cassini ISS mosaic (NASA/JPL/SSI)

(2) Dark brown unit broadly correlates with Significance Titan’s equatorial dune fields. (3) Dark blue unit often occurs at the eastern Monitoring albedo variations with time allows boundaries of bright terrains and is thought for the study of the following important aspects to have higher water-ice content compared to of terrains: (1) resurfacing phenomena and redis- the other surface units. tribution of dust on the surface, (2) the nature of the surface materials in terms of granulometry, See also “▶ Mid-latitude Dark Linear and (3) the contribution of surface–atmosphere Feature.” interaction to geologic and climatic evolution of the planet (indeed albedo changes could affect Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:01 Page Number: 43

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solar heating and the global circulation of winds below a thin atmosphere (McCord et al. 1971) across the planet). and determined to be the color of an optically Changes in surface albedo indicate active sur- thick atmosphere by polarimetric measurements A face processes. Regolith maturity (which also (Veverka 1973). involves compositional changes) can be inferred from albedo. Temporal, reoccurring albedo changes might imply seasonal changes. Albedo Terrestrial Analog changes affect the absorption of solar energy by the surface and impact the global climate and Earth’s surface has three major albedo domains: circulation of winds (Fenton et al. 2007; Geissler (1) low-albedo oceans and seas, (2) intermediate- et al. 2008). Simulations show that a runaway albedo continents, and (3) high-albedo seasonal ice-albedo mechanism can result in an or perennial snow and/or ice blankets and sea ice ice-covered Earth (Poulsen 2003). (Fig. 12). Ocean Surface Albedo (OSA) is “the Differentiation of surface albedo features ratio of the upwelling to down-welling solar irra- from atmospheric features is essential in photo- diance (flux) at the air-sea boundary.” Clear-sky geological interpretation of low-resolution (cloudless) ocean albedo varies between 0.03 and (telescopic or spacecraft imagery) observations. 0.4 depending on solar zenith angle. Its value While highly variable albedo features are likely greatly depends on wind at low sun (Jin associated with atmospheric phenomena (i.e., et al. 2004). clouds), permanent albedo patterns more proba- bly represent surface features. Recurrent albedo features are likely associated with seasonal Alteration changes on the surface (e.g., extension of the polar cap) or near-surface (e.g., high-albedo Albedo features may change their albedo and fogs in Hellas, Mars (Leonard and Tanaka shape on seasonal and decadal time scales. 2001)) or atmospheric features related to topog- Changes in albedo are now believed to be gener- raphy (e.g., orographic clouds near Tharsis ated by erosion and deposition of the bright Mar- Montes, Mars), which rotate with the diurnal tian dust by the wind and not by the displacement rotation of the planet (Beish 1999). Quasi- of mobile dark sand (Geissler and Mukherjee permanent albedo features may be associated 2010; Geissler 2004, 2005). In some places, dust with atmospheric phenomena (e.g., The Great is stripped away episodically or albedo bound- Red Spot of Jupiter). Reappearance of confirmed aries advance gradually at speeds of up to tens of surface albedo patterns can be used to determine kilometers per Martian year (Geissler and rotation period (Beer and M€adler 1830; Clerke Mukherjee 2010; Fig. 13a). 1885[2010], p. 320). This is also essential for Temporal behavior of albedo features has establishing a surface-bound coordinate been classified into the following subtypes: (longitude) system. (a) By Beish (2011): (1) seasonal and (2) secular Surface features may be misinterpreted as (b) By Geissler and Mukherjee (2010): (1) fea- atmospheric features and vice versa. Schro¨ter tures showing gradual changes, (2) features interpreted dark albedo features of Mars as rela- showing episodic changes that typically took tively stable clouds influenced by underlying place during the perihelion season, and aerographical features (Clerke 1885[2010], (3) features showing changes on a quasi- p. 320). The Great Red Spot was thought to be continuous basis (e.g., Solis Lacus) reflections from an active volcano or a topographic feature fixed to the planet as On the Moon, both high-albedo fresh impact suggested by N. Green in 1887 (Rogers 1995, ejecta (albedo ca. 0.15–0.17) and low-albedo p. 255). The apparent color of Titan was initially fresh maria (0.8–0.11) surfaces change their theorized to be the color of the visible surface albedo with time, eventually reaching the average Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:02 Page Number: 44

A 44 Albedo Feature

Albedo Feature, Fig. 12 Clear-sky (cloudless) albedo variations on Earth for January 1987 (northern winter). White dots: no data (Budikova 2013). Earth Radiation Budget Experiment (NASA)

albedo of mature highlands (0.11) (Wood 2003). Albedo variations during a dust storm cannot With time, surface albedo decreases in visible to be always imputed to real surface albedo near-IR: mafic absorption features are attenuated, changes, as the augmented number of dust parti- while a strong positive slope is introduced, caus- cles suspended in the atmosphere tends to raise ing spectral reddening. Lunar albedo changes are the albedo value measured by orbiting instru- due to space weathering (the bombardment by ments. Zinzi et al. (2010) demonstrated that solar wind ions and micrometeorites) (Cassidy using the Dust Cover Index (DCI) developed by and Hapke 1975) which creates amorphous coat- Ruff and Christensen (2002), it is possible to ings on grain surfaces and tiny blebs of metallic correct data from this bias. Zinzi pointed out (nanophase) iron (Kramer et al. 2011). that the dust transported and deposited by On Mars, low-albedo materials exhibit the least a storm does not significantly affect high-albedo amount of effects from alteration. Cerberus is areas, whereas major changes of surface albedo a dark feature that almost disappeared under are registered for dark regions. a newly emplaced dust mantle between 1982 and Using TES data collected between 1999 and 1995 (between Viking and MGS observations) 2005, Zinzi et al. (2010) demonstrated that the (Geissler 2004; Lee et al. 1996;Fig.13b). Solis surface albedo of Syrtis Major raised to an inter- Lacus (in Solis Planum) shows dramatic seasonal mediate value (i.e., roughly 0.2) for months after and secular variations; this is an area of bright and a dust storm has ended. The atmospheric dust dark wind streaks. Seasonal changes are explained opacity returned lower than 0.2 on June 2002, by the following cyclical mechanism: during early whereas the surface albedo returned to the typical southern summer, dust is removed by dust storms value of 0.12 only on March 2003 (Zinzi which results in the dark Solis Lacus feature. Fol- et al. 2010; Fig. 14), thus demonstrating that the lowing the dust storm season, in late summer, dust deposited on the ground during the storm enhanced deposition occurs in the lee sides, remained well after the end of the storm there, forming bright streaks. Later, dust is deposited effectively changing the surface albedo. from the air over the entire region that decreases On Europa, many features, including bands, the contrasts of albedo features (Lee 1985). appear to brighten with age. The youngest forms Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:02 Page Number: 45

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A

Albedo Feature, Fig. 13 Surface changes over 20 Earth (Geissler et al. 2008). (c, d). Surface changes in Cerberus, years. (a, b) Changes in Utopia Planitia. The isolated dark Mars. Scale bar 300 km (Geissler 2005). (a, c): Viking; patch (A) is Alcyonius, at 33N, 94E. Scale bar 500 km (b, d): MOC (NASA/JPL/MSSS)

are commonly of lower albedo than their sur- roundings, while older bands show little or no albedo contrast relative to their surroundings 0.3 (Collins et al. 2010). The oldest surfaces, ridged plains, have the highest albedo. Brightening may be due to frost deposition or chemical alteration (Prockter and Schenk 2005). Crater rays are 0.2 usually bright and darken with age. TES albedo History of Investigation

0.1 The lunar brightness scales. Early selenographers used a three-degree scale for 0 0.2 0.4 0.6 0.8 1 brightness values (Beer and Madler 1838). The 10 scale of brightness values was established by Albedo Feature, Fig. 14 Syrtis major albedo observa- Schro¨ter (1791,61}24) to help visual observa- tions before and after a dust storm showing two different trends with atmospheric opacity. Black dots are the tions of the Moon. This brightness scale was before-storm observations, while white dots are after- applied and refined by late-nineteenth–early- storm ones. TES albedo: albedo from MGS Thermal twentieth-century observers (e.g., Lohrmann, Emission Spectrometer; t: opacity (After Zinzi Beer and M€adler, Elger). In the 10 scale, “zero et al. 2010) (Reprinted from Icarus, 208, Zinzi A, Palomba E, Rinaldi G, D’Amore M, Effect of atmospheric value corresponds to the shadow which is dust loading on Martian albedo features, 590–597. Copy- projected by the mountains. The first three right (2010), with permission from Elsevier) degrees may be denominated grey, the fourth and fifth, light grey, the sixth and seventh, white, and the last three, shining white. The 1st, Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:02 Page Number: 46

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Albedo Feature, Fig. 15 Aristarchus crater, the brightest spot of the Moon. (a) Clementine UVVIS 750 nm mosaic (USNRL/BMDO/USGS), (b) Bright material of Aristarchus central peak, LROC NAC mosaic (NASA/GSFC/ASU)

9th, and 10th degrees are found only on small Elger (1895) remarked that “Proctor, parts of some spots. It is in the craters, and in the discussing this question on the basis of Zollner’s annular mountains, that we find the last three experiments respecting the light reflected by var- degrees of brightness. There is only one annular ious substances, concludes that the dark area just mountain, viz. Aristarchus (Fig. 15), and one mentioned must be notably darker than the dark point in Werner which reach the 10th and highest grey syenite which figures in his tables, while the degree of illumination” (Beer and Madler 1838). floor of Aristarchus is as white as newly fallen Elger (1895) added more details: “The most snow.” brilliant object on the surface is the central peak The 10 scale was standardized by the of the ring-plain Aristarchus, the darkest the floor Selenographical Society and the Selenographical of Grimaldi, or perhaps a portion of that of the Journal provided a list of lunar features for each neighbouring Riccioli. Between these extremes, value (Elger 1895). They were correlated to there is every gradation of tone.” albedo values by Fessenkov (1962; Table 1). Neison (1876) referred to brightness as “the In the 1920s, photoelectric photometry pro- general tint.” vided objective measurements of the albedo of The lunar brightness scale values were diffi- any selected spots, and modern CCD detectors cult to correlate to brightness values of known are able to measure the albedo of broader areas. terrestrial materials. John Herschel maintained A spectrometer is a device measuring properties that “the actual illumination of the lunar surface of the electromagnetic radiance. is not much superior to that of weathered sand- While the brightness scale was specifically stone rock in full sunshine.” “I have,” Herschel designed for lunar observations, having values explained further, “frequently compared the from the darkest, shadowed areas (0) to the moon setting behind the grey perpendicular brightest spot of the near side of the Moon facade of the Table Mountain, illuminated by (10), albedo values measure the reflectivity of the sun just risen in the opposite quarter of the the surface from 0 (incident light is completely horizon, when it has been scarcely distinguish- absorbed) to 1.0 (100 % of incident light is able in brightness from the rock in contact with reflected) (Wood 2007). it” (Herschel 1893). Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:03 Page Number: 47

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Albedo Feature, Table 1 The lunar brightness scale, after Elger 1895 and Fessenkov (1962) Brightness Selenographical scale Society (Elger Feature, Selenographical Feature, Fessenkov Albedo, Fessenkov A (degree) 1895) Journal (Elger 1895) (1962) (1962) 0 Black/real Black shadows shadow of lunar mountains 1 Grayish black Darkest portions of the floors Grimaldi and Riccioli 0.061 of Grimaldi and Riccioli floors 1.5 Interiors of Boscovich, Billy, Boscovich floor 0.067 and Zupus 2 Dark gray Floors of Endymion, Le Julius Caesar and 0.074 Monnier, Julius Caesar, Endymion floors Cruger, and Fourier 2.5 Interiors of Azout, Vitruvius, Pitatus and Marius floors 0.081 Pitatus, Hippalus, and Marius 3 Medium gray Interiors of Taruntius, Plinius, Taruntius, Plinius, 0.088 Theophilus, Parrot, Flamsteed, Theophilus, Flamsteed, and Mercator Mercator floors 3.5 Interiors of Hansen, Hansen, Archimedes, 0.095 Archimedes, and Mersenius and Mersenius floors 4 Yellowish gray, Interiors of Manilius, Ptolemaeus, Manilius, 0.102 average light Ptolemaeus, and Guerike and Guericke floors 4.5 Surface round Aristillus, Aristillus environs 0.109 Sinus Medii 5 Pure light gray Walls of Arago, Landsberg, Arago, Landsberg, and 0.115 and Bullialdus. Surface round Bullialdus walls, Kepler Kepler and Archimedes environs 5.5 Walls of Picard and Picard and Timocharis 0.122 Timocharis. Rays from walls, rays of Copernicus Copernicus 6 Light whitish Walls of Macrobius, Kant, Macrobius, Kant, Bessel, 0.129 gray Bessel, Mosting, and Mo¨sting, and Flamsteed Flamsteed walls 6.5 Walls of Langrenus, Lagrange, Mons La Hire, 0.135 Theaetetus, and Lahire and Theaetetus walls 7 Grayish white Theon, Ariadaeus, Bode B, Theon Junior, Ariadaeus, 0.142 Wichmann, and Kepler Behaim, and Bode B walls 7.5 Ukert, Hortensius, Euclides Euclides, Ukert, and 0.149 Hortensius walls 8 Pure white Walls of Godin, Bode, and Godin, Copernicus, and 0.156 Copernicus Bode walls 8.5 Walls of Proclus, Bode A, and Proclus, Bode A, and 0.163 Hipparchus C Hipparchus C walls 9 Glittering white/ Censorinus, Dionysius, Mersenius and Mosting 0.169 Proclus Mosting A, and Mersenius A walls B and C 9.5 Interior of Aristarchus, La Aristarchus interior 0.176 Peyrouse DELTA 10 Dazzling white/ Central peak of Aristarchus Aristarchus central peaks 0.183 Aristarchus brightest Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:03 Page Number: 48

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Albedo Mapping continents, ocean basins, and tectonic environ- Earth-based telescopic observations are not capa- ments) and a second based on a Moon-like Mars ble to resolve topographic features on other (with features equivalent to the lunar maria, ter- planets and moons (except for our Moon), due rae, and rays) (Ronca 1970)(▶ dark splotch, to low resolution and the lack of discernible ▶ dark deposits (Mars)). shadows, particularly on Mars and outer Solar It was observed by several astronomers in the System bodies, which are always observed as early twentieth century that during the southern a full or nearly full disks from the Earth. Topo- spring, albedo features darken gradually from the graphic shading is also absent in orbital images of South towards the equator and vice versa during Titan, because of the scattering of light by Titan’s the northern spring. This phenomenon was called atmosphere (JPL 2007). the “Wave of darkening” and was explained by the Topographic information is generally obtained activity of vegetation during the spring, associated from close/targeted flybys or from orbital mis- with the melting of the polar ice cap. In the 1960s sions. Before such data becomes available, albedo to 1980s, several workers expressed their doubts (spectro-/photometric) maps can be constructed on its existence (Kieffer 1992, p. 42), while others through the following methods and sources: showed that actually a wave of brightening occurs (1) rotation light curves, (2) light curves from as bright fresh dust is deposited in areas adjacent to mutual events (occultations, eclipses, and transits) dark features (Beish 2011). (e.g., Pluto albedo maps, Buie et al. 1992), (3) dis- tant (Earth-based) telescopic observations (e.g., Albedo Features (Moons of Jupiter and early Mars maps or Hubble Space Telescope’s Saturn) albedo map of Vesta, Li et al. 2010), and Brightness variability of the Jovian and Kronian (4) low-resolution spacecraft imagery from moons has been demonstrated by Guthnick (e.g., distant/nontargeted flybys (e.g., Pioneer images 1906). Based on his results (See (1910)), he pro- of the Galilean satellites, Fimmel et al. 1977, posed that satellites of Jupiter and Saturn have p. 180). lunar-like maria on their surfaces. Guthnick’s On Mars, Viking infrared thermal mapper observations could show in 1925 that the Galilean (IRTM) and MGS-TES provided detailed tempo- moons are tidally locked (Ulivi and Harland ral and spatial albedo information (Christensen 2007, p. xli). 1988). The first digital measurements of lunar The first photoelectric photometry-based albedo were provided by Clementine at 750 nm. information on the Galilean moon’s brightness Today, high-resolution reflectance and emissivity was published by Stebbins (1927) which also spectra (hyperspectral data curves) are used for confirmed synchronous rotation. the estimation of mineralogical compositions of Thomas Lee suggested that the high albedo surface materials, instead of a single, averaged from VIS to 3.4 mm of Io is compatible with albedo value (▶ Dark deposits). a sulfur compound (Lee 1971). The drop in the curves for Europa and Ganymede confirmed Albedo Features of Mars Kuiper’s earlier suggestion that H2O ice is pre- Albedo and shape of features of the Martian disk sent on these satellites (▶ bright plains) as seen from the Earth have been recorded in all (Cruikshank and Nelson 2007). apparitions since the early twentieth century and still are recorded by amateur Mars observers. The Darkening Process Martian surface was classified into dark areas, Gold (1955) proposed that the lunar surface light areas, canals, and oases (all albedo features) darkens with time, as inferred from the existence during much of the twentieth century. Even in of bright rays only at young craters. Optical 1970, Ronca proposed two possible interpreta- effects of space weathering were thought to be tions: one based on an Earth-like Mars (in this caused by impact vitrified glass in agglutinates, model, albedo features corresponded to and submicroscopic iron was believed to result Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:03 Page Number: 49

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from impact melting of minerals whose surfaces transferred into the names of corresponding are saturated with solar wind-transported hydro- or nearby topographic features (Coprates ! ! ! gen. However, vacuum-melted glasses are not Canal Coprates Chasma Hellas Hellas A dark. Hapke et al. (1975) suggested that the spec- Planitia!Syrtis Major!Syrtis Major Planum; tral effects are due to vapor condensates, which etc.) (de Vaucouleurs et al. 1975), but about became accepted in the 2000s (Hapke 2001). 400 have not been carried forward (Gangale and Dudley-Flores 2013). The albedo features of Mercury were named Origin of Term originally by Antoniadi. Large bright regions have no descriptor term, while dark regions are The term albedo was introduced into optics by termed solitudo in the nomenclature (Dollfus Lambert (1760). Since the illuminating light et al. 1978). source is the Sun, which appears white for visual inspection, the portion of reflected Sun’s radia- tion appears the whiter, the larger portion of See Also radiation power is reflected by the surface. This is the origin of the term “albedo”, construed from ▶ Albedo Dichotomy the Latin word “albus,” meaning “white.” ▶ Crater Ray ▶ Dark Deposits (Mars) ▶ Dark Halo Crater IAU Descriptor Term ▶ Dust Devil Track ▶ Lenticula Descriptor terms of albedo features without ▶ Mare (Volcanic) genetic implications include macula (dark spot), ▶ Palimpsest facula (bright spot), regio (large area), and linea ▶ Red Spot (Moon) (linear feature). ▶ Slope Streak Mare and terra as descriptor terms were first ▶ Swirl applied to lunar dark and bright regions, respec- ▶ Wind Streak tively, by Langrenus in 1645. The presently used system of lunar nomenclature was introduced by Riccioli in 1651. Low-albedo regions are now References known to be composed of mare materials (basaltic plains). High-albedo regions (highland Barlow NG (2008) Mars: an introduction to its interior, terrains), used to be called terrae, are today surface and atmosphere. Cambridge University Press, New York unnamed. Several albedo features on the Moon Beer W, M€adler JH (1830) Physische Beobachtungen have no corresponding topographic landform des Mars bei seiner Opposition im September 1830. (e.g., Reiner Gamma). Berlin. http://reader.digitale-sammlungen.de/de/fs1/ The presently used system of nomenclature of object/goToPage/bsb10060422.html?pageNo=7 Beer W, Madler JH (1838) Survey of the surface of the both dark and bright “classical albedo features” moon. Edinb New Philos J 25:38–67. (English trans- on Mars was developed by Schiaparelli in the lation, condensed) late nineteenth century. Additional features Beish JD (1999) Discrete topographic and orographic were identified and named by Antoniadi and clouds of Mars. Association of Lunar and Planetary Observers. http://www.alpo-astronomy.org/mars/dis- Lowell at the beginning of the twentieth century, crete.htm some of which were observational artifacts. About 100 of their albedo names were Comp. by: Udayasankar Stage: Revises1 Chapter No.: Title Name: EPL_214584 Date:27/2/15 Time:05:24:03 Page Number: 50

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