Alba Mons (formerly and still occasionally known as Alba Patera, a term that has since been restricted to the volcano's summit caldera; also initially known as the Arcadia ring) is a volcano located in the northern Tharsis region of the planet Mars. It is the biggest volcano on Mars in terms of surface area, with volcanic flow fields that extend for at least from its summit.Cattermole, 2001, p. 85. Although the volcano has a span comparable to that of the United States, it reaches an elevation of only at its highest point.Carr, 2006, p. 54. This is about one-third the height of Olympus Mons, the tallest volcano on the planet. The flanks of Alba Mons have very gentle slopes. The average slope along the volcano's northern (and steepest) flank is 0.5°, which is over five times lower than the slopes on the other large Tharsis volcanoes.Boyce, 2008, p. 104. In broad profile, Alba Mons resembles a vast but barely raised welt on the planet's surface.See Carr, 2006, p. 54, Fig. 3.10 for MOLA profile of Alba Mons compared to Olympus Mons. The difference in relief is striking. It is a unique volcanic structure with no counterpart on Earth nor elsewhere on Mars.
In addition to its great size and low relief, Alba Mons has a number of other distinguishing features. The central portion of the volcano is surrounded by an incomplete ring of faults (graben) and fractures, called Alba Fossae on the volcano's western flank and Tantalus Fossae on the eastern flank. The volcano also has very long, well preserved lava flows that form a radiating pattern from the volcano's central region. The enormous lengths of some individual flows (>) implies that the lavas were very fluid (low viscosity) and of high volume. Many of the flows have distinctive morphologies, consisting of long, sinuous ridges with discontinuous central lava channels. The low areas between the ridges (particularly along the volcano's northern flank) show a branching pattern of shallow gullies and channels (valley networks) that likely formed by water runoff.
Alba Mons has some of the oldest extensively exposed volcanic deposits in the Tharsis region. Geologic evidence indicates that significant volcanic activity ended much earlier at Alba Mons than at Olympus Mons and the Tharsis Montes volcanoes. Volcanic deposits from Alba Mons range in age from Hesperian to early Amazonian (approximately 3.6 to 3.2 billion years old).
Name origin For years the volcano's formal name was Alba Patera. Patera (pl. paterae) is Latin for a shallow drinking bowl or saucer. The term was applied to certain ill-defined, scalloped-edged craters that appeared in early spacecraft images to be volcanic (or non-impact) in origin.Russell, J.F.; Snyder, C.W.; Kieffer, H.H. (1992). Origin and Use of Martian Nomenclature in Mars, H.H. Kieffer et al., Eds.; University of Arizona Press: Tucson, AZ, p. 1312. In September 2007, the International Astronomical Union (IAU) renamed the volcano Alba Mons (Alba Mountain), reserving the term Alba Patera for the volcano's two central depressions (calderas). Nevertheless, the entire volcano is still commonly called Alba Patera in the planetary science literature.A Google Scholar search of the astronomy and planetary science literature from 2007 to 2011 reveals 106 uses of Alba Patera versus 5 for Alba Mons (accessed May 7, 2011).
The term Alba is from the Latin word for white and refers to the clouds frequently seen over the region from Earth-based telescopes.Hartmann, 2003, p. 308 The volcano was discovered by the Mariner 9 spacecraft in 1972 and was initially known as the Alba volcanic feature or the Arcadia Ring (in reference to the partial ring of fractures around the volcano). The IAU named the volcano Alba Patera in 1973. The volcano is often simply called Alba when the context is understood.
Location and size Alba Mons is centered at in the Arcadia quadrangle (MC-3). Much of the volcano's western flank is located in the adjacent Diacria quadrangle (MC-2). Flows from the volcano can be found as far north as 61°N and as far south as 26°N (in the northern Tharsis quadrangle). If one takes the outer margin of the flows as the volcano's base, then Alba Mons has north–south dimensions of about and a maximum width of . It covers an area of at least 5.7 million km2 and has a volume of about 2.5 million km3. The volcano dominates the northern portion of the Tharsis bulge and is so large and geologically distinct that it can almost be treated as an entire volcanic province unto itself.Frankel, 2005, p. 134.
Although Alba Mons reaches a maximum elevation of above Mars’ datum, the elevation difference between its summit and surrounding terrain (relief) is much greater on the north side of the volcano (about ) compared to the south side (about ). The reason for this asymmetry is that Alba straddles the dichotomy boundary between the cratered uplands in the south and the lowlands to the north. The plains underlying the volcano slope northwardJager, K. M.; Head, J. W.; Thomson, B.; McGovern, P. J.; Solomon, S. C. (1999). Alba Patera, Mars: Characterization Using Mars Orbiter Laser Altimeter (MOLA) Data and Comparison with Other Volcanic Edifices. 30th Lunar and Planetary Science Conference, Abstract #1915. https://www.lpi.usra.edu/meetings/LPSC99/pdf/1915.pdf. toward the Vastitas Borealis, which has an average surface elevation of below datum (-). The southern part of Alba Mons is built on a broad, north–south topographic ridge that corresponds to the fractured, Noachian-aged terrain of Ceraunius Fossae (pictured left).
Physical description
Alba's size and low profile makes it a difficult structure to study visually, as much of the volcano's relief is indiscernible in orbital photographs. However, between 1997 and 2001, the Mars Orbital Laser Altimeter (MOLA) instrument of the Mars Global Surveyor spacecraft took over 670 millionMOLA Shot Counter. MIT MOLA Website. http://sebago.mit.edu/shots// (accessed May 23, 2011). precise elevation measurements across the planet. Using MOLA data, planetary scientists are able to study subtle details of the volcano's shape and topography that were invisible in images from earlier spacecraft such as Viking.
The volcano consists of two, roughly concentric components: 1) an oval-shaped central body with approximate dimensions of by across surrounded by 2) a vast, nearly level apron of lava flows that extends an additional or so outward. The central body is the main topographic edifice of the volcano, marked by pronounced break in slope at the inner boundary of the apron. Extending east and west from the central edifice are two broad fan-shaped lobes (or shoulders), which give the volcano its elongation in the east–west direction.Ivanov, M.A.; Head, J.W. (2002). Alba Patera, Mars: Assessment of its Evolution with MOLA and MOC Data. 33rd Lunar and Planetary Science Conference. LPI: Houston, TX, Abstract #1349. https://www.lpi.usra.edu/meetings/lpsc2002/pdf/1349.pdf. The central edifice has the steepest slopes on the volcano, although they are still only 1°. The crest and upper flanks of the edifice are cut by a partial ring of graben that are part of the Alba and Tantalus Fossae fracture system. Inside the ring of graben is an annulus of very low and in places reversed slopes that forms a plateau on top of which lies a central dome across capped by a nested caldera complex. Thus, the central edifice of Alba Mons resembles a partially collapsed shield volcano with a smaller, summit dome sitting on top (pictured right). The summit dome has a distinct tilt to the east.
The caldera complex consists of a large caldera about by across at the center of the summit dome. A smaller, kidney-shaped caldera (about by ) lies in the southern half of the larger one. Both calderas are relatively shallow, reaching a maximum depth of only .
The larger caldera is bounded at the westernmost end by a steep, semicircular wall tall. This wall disappears at the northern and southern sides of the caldera, where it is buried by volcanic flows originating from the younger, smaller caldera. The smaller caldera is outlined everywhere by a steep wall that varies in height over a range of a few hundred meters. The walls of both calderas are scalloped, suggesting multiple episodes of subsidence and/or mass wasting. Two small shields or domes, several hundred meters high, occur within and adjacent to the large caldera. The shield within the large caldera is about across. It is capped by a peculiar concentric circular feature in diameter (pictured left).
Calderas form by collapse following withdrawal and depletion of a magma chamber after an eruption. Caldera dimensions allow scientists to infer the geometry and depth of the magma chamber beneath the summit of the volcano.Mouginis-Mark, P.J.; Harris, A.J.; Rowland, S.K. (2008). Terrestrial Analogs to the Calderas of the Tharsis Volcanoes on Mars in The Geology of Mars: Evidence from Earth-Based Analogs, M. Chapman, Ed.; Cambridge University Press: Cambridge, UK, p. 71. The shallowness of Alba's calderas compared to those seen on Olympus Mons and most of the other Tharsis volcanoes implies that Alba's magma reservoir was wider and shallower than those of its neighbors.Cattermole, 2001, p. 86.
Surface characteristics
Most of the central edifice of Alba Mons is mantled with a layer of dust approximately thick.Ruff, S. W.; Christensen, P. R. (2001). A Spectrally-based Global Dust Cover Index for Mars from Thermal Emission Spectrometer Data. First Landing Site Workshop for the 2003 Mars Exploration Rovers, Abstract #9026. https://www.lpi.usra.edu/meetings/mer2003/pdf/9026.pdf. The dust layer is visible in high resolution images of the summit (pictured right). In places, the dust has been carved into streamlined shapes by the wind and is cut by small landslides. However, some isolated patches of dust appear smooth and undisturbed by the wind.Keszthelyi, L.P. (2006). Dusty Top of Alba Patera Volcano. University of Arizona HiRISE Website. https://hirise.lpl.arizona.edu/PSP_001510_2195. (accessed May 18, 2011).
Heavy dust cover is also indicated by the high albedo (reflectivity) and low thermal inertia of the region. Martian dust is visually bright (albedo > 0.27) and has a low thermal inertia because of its small grain size (<).Putzig, N.E. et al. (2005). Global Thermal Inertia and Surface Properties of Mars from the MGS Mapping Mission. Icarus, 173 Tbl. 1, Fig. 5, p. 331. (See the Martian surface.) However, the thermal inertia is high and albedo lower on the northern flanks of the volcano and in the apron area farther to the north. This suggests that the northern portions of Alba's surface may contain a higher abundance of duricrusts, sand, and rocks compared to the rest of the volcano.