Alba Mons
Alba Mons 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. Although the volcano has a span comparable to that of the United States, it reaches an elevation of only at its highest point. 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 flank is 0.5°, which is over five times lower than the slopes on the other large Tharsis volcanoes. In broad profile, Alba Mons resembles a vast but barely raised welt on the planet's surface. It is a unique volcanic structure with no counterpart on Earth or 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 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 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 show a branching pattern of shallow gullies and channels 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.
Name origin
For years the volcano's formal name was Alba Patera. Patera 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 in origin. In September 2007, the International Astronomical Union renamed the volcano Alba Mons, reserving the term Alba Patera for the volcano's two central depressions. Nevertheless, the entire volcano is still commonly called Alba Patera in the planetary science literature.Image:Alba Mons MOLA zoom 64.jpg|thumb|left|240px|MOLA topographic map of Alba Mons and surroundings. The main edifice appears in colors of red to orange; the surrounding apron is in shades of yellow-orange to green. The relief is greatest to the north because the volcano straddles the dichotomy boundary. Elevated terrain of Ceraunius Fossae, which underlies part of the volcano, extends southward like a handle.
The term Alba is from the Latin word for white and refers to the clouds frequently seen over the region from Earth-based telescopes. The volcano was discovered by the Mariner 9 spacecraft in 1972 and was initially known as the Alba volcanic feature or the Arcadia Ring. 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. Much of the volcano's western flank is located in the adjacent Diacria quadrangle. Flows from the volcano can be found as far north as 61°N and as far south as 26°N. 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.Although Alba Mons reaches a maximum elevation of above Mars’ datum, the elevation difference between its summit and surrounding terrain is much greater on the north side of the volcano compared to the south side. 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 northward 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.
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 instrument of the Mars Global Surveyor spacecraft took over 670 million 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.File:Alba Caldera THEMIS IR.jpg|left|thumb|240px|Central caldera complex of Alba Mons. The calderas are shallow compared to those on other Tharsis volcanoes. Within the larger caldera is a small shield capped by a concentric circular feature. Image is about across.
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, which give the volcano its elongation in the east–west direction. 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. 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 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.
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. 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.
Surface characteristics
Most of the central edifice of Alba Mons is mantled with a layer of dust approximately thick. The dust layer is visible in high resolution images of the summit. 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.Heavy dust cover is also indicated by the high albedo and low thermal inertia of the region. Martian dust is visually bright and has a low thermal inertia because of its small grain size. 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.
High thermal inertia can also indicate the presence of exposed water ice. Theoretical models of water-equivalent hydrogen from epithermal neutrons detected by the Mars Odyssey Neutron Spectrometer instrument suggest that the regolith just below the surface on Alba's northern flank may contain 7.6% WEH by mass. This concentration could indicate water present as remnant ice or in hydrated minerals. Alba Mons is one of several areas on the planet that may contain thick deposits of near-surface ice preserved from an earlier epoch, when Mars’ axial tilt was higher and mountain glaciers existed at mid-latitudes and tropics. Water ice is unstable at these locations under present conditions and will tend to sublimate into the atmosphere. Theoretical calculations indicate that remnant ice can be preserved below depths of 1 m if it is blanketed by a high-albedo and low-thermal-inertia material, such as dust.
The mineral composition of rocks making up Alba Mons is difficult to determine from orbital reflectance spectrometry because of the predominance of surface dust throughout the region. However, global-scale surface composition can be inferred from the Mars Odyssey gamma-ray spectrometer. This instrument has allowed scientists to determine the distribution of hydrogen, silicon, iron, chlorine, thorium and potassium in the shallow subsurface. Multivariate analysis of GRS data indicates that Alba Mons and the rest of the Tharsis region belongs to a chemically distinct province characterized by relatively low Si, Th, and K content, but with Cl abundance higher than Mars' surface average. Low silicon content is indicative of mafic and ultramafic igneous rocks, such as basalt and dunite.
Alba Mons is an unlikely target for unmanned landers in the near future. The thick mantle of dust obscures the underlying bedrock, probably making in situ rock samples hard to come by and thus reducing the site's scientific value. The dust layer would also likely cause severe maneuvering problems for rovers. Ironically, the summit region was originally considered a prime backup landing site for the Viking 2 lander because the area appeared so smooth in Mariner 9 images taken in the early 1970s.