 Viking image of Alba Mons. The volcano's relief is barely visible in orbital photographs. The broad system of fractures on the volcano's eastern side (right) is calledTantalus Fossae. The narrower fracture system on the western flank is Alba Fossae. (Viking colorMDIM 2.1) | |
| Location | NorthernTharsis Rise,Mars |
|---|---|
| Coordinates | 40°30′N250°24′E / 40.5°N 250.4°E /40.5; 250.4[1] |
| Discoverer | Mariner 9 |
| Eponym | Latin –White Mountain |
Alba Mons (formerly and still occasionally known asAlba Patera, a term that has since been restricted to the volcano's summit caldera;[2] also initially known as theArcadia ring[3]) is avolcano located in the northernTharsis region of the planetMars. It is the biggest volcano on Mars in terms of surface area, with volcanic flow fields that extend for at least 1,350 km (840 mi) from its summit.[4][5] Although the volcano has a span comparable to that of theUnited States, it reaches an elevation of only 6.8 km (22,000 ft) at its highest point.[6] This is about one-third the height ofOlympus Mons, the tallest volcano on the planet.[7] 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 largeTharsis volcanoes.[6][8] In broad profile, Alba Mons resembles a vast but barely raised welt on the planet's surface.[9] It is a unique volcanic structure with no counterpart on Earth or elsewhere on Mars.[6]
In addition to its great size and lowrelief, Alba Mons has a number of other distinguishing features. The central portion of the volcano is surrounded by an incomplete ring offaults (graben) and fractures, called AlbaFossae on the volcano's western flank andTantalus Fossae on the eastern flank. The volcano also has very long, well preservedlava flows that form a radiating pattern from the volcano's central region. The enormous lengths of some individual flows (>300 km (190 mi)) implies that the lavas were very fluid (lowviscosity) and of high volume.[10] 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.[11]
Alba Mons has some of the oldest extensively exposed volcanic deposits in theTharsis region. Geologic evidence indicates that significant volcanic activity ended much earlier at Alba Mons than atOlympus Mons and theTharsis Montes volcanoes. Volcanic deposits from Alba Mons range in age fromHesperian to earlyAmazonian[12] (approximately 3.6[13] to 3.2 billion years old[14]).
For years the volcano's formal name wasAlba Patera.Patera (pl.paterae) isLatin 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.[15] In September 2007, theInternational Astronomical Union (IAU) renamed the volcano Alba Mons (Alba Mountain), reserving the term Alba Patera for the volcano's two central depressions (calderas).[1] Nevertheless, the entire volcano is still commonly called Alba Patera in the planetary science literature.[16]
The term Alba is from theLatin word for white and refers to the clouds frequently seen over the region from Earth-based telescopes.[17] The volcano was discovered by theMariner 9 spacecraft in 1972 and was initially known as the Alba volcanic feature[18] or the Arcadia Ring[19] (in reference to the partial ring of fractures around the volcano). The IAU named the volcano Alba Patera in 1973.[1] The volcano is often simply called Alba when the context is understood.
Alba Mons is centered at40°28′N250°24′E / 40.47°N 250.4°E /40.47; 250.4 in theArcadia quadrangle (MC-3). Much of the volcano's western flank is located in the adjacentDiacria quadrangle (MC-2).[1] Flows from the volcano can be found as far north as 61°N and as far south as 26°N (in the northernTharsis quadrangle). If one takes the outer margin of the flows as the volcano's base, then Alba Mons has north–south dimensions of about 2,000 km (1,200 mi) and a maximum width of 3,000 km (1,900 mi).[6] It covers an area of at least 5.7 million km2[20] and has a volume of about 2.5 million km3.[12] The volcano dominates the northern portion of theTharsis bulge and is so large and geologically distinct that it can almost be treated as an entire volcanic province unto itself.[21][22]
Although Alba Mons reaches a maximum elevation of 6.8 km (22,000 ft) aboveMars’ datum, the elevation difference between its summit and surrounding terrain (relief) is much greater on the north side of the volcano (about 7.1 km (23,000 ft)) compared to the south side (about 2.6 km (8,500 ft)). The reason for this asymmetry is that Alba straddles thedichotomy boundary between the cratered uplands in the south and the lowlands to the north. The plains underlying the volcano slope northward[23] toward theVastitas Borealis, which has an average surface elevation of 4.5 km (15,000 ft)below datum (-4.500 km (14,760 ft)). The southern part of Alba Mons is built on a broad, north–south topographic ridge that corresponds to the fractured, Noachian-aged terrain ofCeraunius Fossae[12] (pictured left).
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, theMars Orbital Laser Altimeter (MOLA) instrument of theMars Global Surveyor spacecraft took over 670 million[24] precise elevation measurements across the planet. Using MOLA data, planetary scientists are able to study subtle details of the volcano's shape andtopography that were invisible in images from earlier spacecraft such asViking.[12]
The volcano consists of two, roughly concentric components: 1) an oval-shaped central body with approximate dimensions of 1,500 km (930 mi) by 1,000 km (620 mi) across surrounded by 2) a vast, nearly level apron of lava flows that extends an additional 1,000 km (620 mi) 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.[12][25] The central edifice has the steepest slopes on the volcano, although they are still only 1°.[6] The crest and upper flanks of the edifice are cut by a partial ring ofgraben that are part of the Alba andTantalus Fossae fracture system. Inside the ring of graben is anannulus of very low and in places reversed slopes[6] that forms a plateau on top of which lies a central dome 350 km (220 mi) across capped by a nestedcaldera complex.[25] Thus, the central edifice of Alba Mons resembles a partially collapsedshield 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 170 km (110 mi) by 100 km (62 mi) across at the center of the summit dome. A smaller, kidney-shaped caldera (about 65 km (40 mi) by 45 km (28 mi)) lies in the southern half of the larger one. Both calderas are relatively shallow,[4] reaching a maximum depth of only 1.2 km (3,900 ft).[7]
The larger caldera is bounded at the westernmost end by a steep, semicircular wall 500 m (1,600 ft) 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.[4] 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 ofsubsidence and/ormass wasting.[12] 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 50 km (31 mi) across. It is capped by a peculiar concentric circular feature 10 km (6.2 mi) in diameter[12][25] (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.[26] The shallowness of Alba's calderas compared to those seen onOlympus Mons and most of the otherTharsis volcanoes implies that Alba's magma reservoir was wider and shallower than those of its neighbors.[27]
Most of the central edifice of Alba Mons is mantled with a layer of dust approximately 2 m (6.6 ft) thick.[28][29] 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 smalllandslides. However, some isolated patches of dust appear smooth and undisturbed by the wind.[30]
Heavy dust cover is also indicated by the highalbedo (reflectivity) and lowthermal inertia of the region. Martian dust is visually bright (albedo > 0.27) and has a low thermal inertia because of its small grain size (<40 μm (0.0016 in)).[28][31] (See theMartian 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 ofduricrusts, sand, and rocks compared to the rest of the volcano.[31]
High thermal inertia can also indicate the presence of exposed water ice. Theoretical models of water-equivalent hydrogen (WEH) fromepithermal neutrons detected by theMars OdysseyNeutron Spectrometer (MONS) instrument suggest that theregolith just below the surface on Alba's northern flank may contain 7.6% WEH by mass.[32] This concentration could indicate water present as remnant ice or in hydrated minerals.[33] Alba Mons is one of several areas on the planet that may contain thick deposits of near-surface ice preserved from an earlier epoch (1 to 10 million years ago), when Mars’axial tilt (obliquity) was higher andmountain glaciers existed at mid-latitudes and tropics. Water ice is unstable at these locations under present conditions and will tend tosublimate into the atmosphere.[34] 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.[35]
The mineral composition of rocks making up Alba Mons is difficult to determine from orbitalreflectance spectrometry because of the predominance of surface dust throughout the region. However, global-scale surface composition can be inferred from theMars Odyssey gamma-ray spectrometer (GRS). This instrument has allowed scientists to determine the distribution ofhydrogen (H),silicon (Si),iron (Fe),chlorine (Cl),thorium (Th) andpotassium (K) in the shallow subsurface.Multivariate analysis of GRS data indicates that Alba Mons and the rest of theTharsis region belongs to a chemically distinct province characterized by relatively low Si (19 wt%), Th (0.58 pppm), and K (0.29 wt%) content, but with Cl abundance (0.56 wt%) higher than Mars' surface average.[36] Low silicon content is indicative ofmafic andultramaficigneous rocks, such asbasalt anddunite.
Alba Mons is an unlikely target for unmanned landers in the near future. The thick mantle of dust obscures the underlying bedrock, probably makingin 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 theViking 2 lander because the area appeared so smooth inMariner 9 images taken in the early 1970s.[37]
Much of the geologic work on Alba Mons has focused on the morphology of its lava flows and the geometry of the faults cutting its flanks. Surface features of the volcano, such as gullies and valley networks, have also been extensively studied. These efforts have the overall goal of deciphering the geologic history of the volcano and the volcano-tectonic processes involved in its formation. Such understanding can shed light on the nature and evolution of the Martian interior and the planet's climate history.
Alba Mons is notable for the remarkable length, diversity, and crisp appearance of its lava flows.[37] Many of the flows radiate from the summit, but others appear to originate from vents and fissures on the lower flanks of the volcano.[38] Individual flows may exceed 500 km (310 mi) in length.[39] Lava flows near the summit calderas appear to be significantly shorter and narrower than those on more distal parts of the volcano.[40] The two most common types of volcanic flows on Alba Mons are sheet flows and tube-and-channel fed flows.
Sheet flows (also called tabular flows[39]) form multiple, overlapping lobes with steep margins. The flows typically lack central channels. They are flat-topped and generally about 5 km (3.1 mi) wide on the upper flanks of the volcano but become much wider and lobate toward their downstream (distal) ends.[38] Most appear to originate near the Alba and Tantalus Fossae fracture ring, but the actual vents for the sheet flows are not visible and may have been buried by their own products.[10] Flow thicknesses have been measured for a number of sheet flows based on MOLA data. The flows range from 20 m (66 ft) to 130 m (430 ft) thick and are generally thickest at their distal margins.[41]
The second major type of lava flows on the flanks of Alba Mons are called tube- and channel-fed flows, or crested flows.[39] They form long, sinuous ridges that radiate outward from the central region of the volcano. They are typically 5 km (3.1 mi)-10 km (6.2 mi) wide. An individual ridge may have a discontinuous channel or line of pits that run along its crest. Tube- and channel-fed flows are particularly prominent on the western flank of the volcano where individual ridges can be traced for several hundred kilometers. The origin of the ridges is uncertain. They may form by successive buildup of solidified lava at the mouth of a channel or tube, with each pulse of flowing lava adding to the length of the ridge.[42]
In addition to the two main types of flows, numerous undifferentiated flows are present around Alba Mons that are either too degraded to characterize or have hybrid characteristics. Flat-topped ridges with indistinct margins and rugged surfaces,[10][37] interpreted as lava flows, are common along Alba's lower flanks and become less sharp in appearance with increasing distance from the edifice.[12] In high resolution images, many of the flows on the volcano's upper flanks originally characterized as sheet flows have central channels with levee-like ridges.[43]
The morphology of lava flows can indicate properties of the lava when molten, such as itsrheology and flow volume. Together, these properties can provide clues to the lava's composition and eruption rates.[37] For example, lava tubes on Earth only form in lavas ofbasaltic composition.Silica-rich lavas such asandesite are too viscous for tubes to form.[10] Early quantitative analysis of Alba's lava flows[38] indicated that the lavas had low yield strength andviscosity and were erupted at very high rates. Alba's unusually low profile suggested to some that extremely fluid lavas were involved in the volcano's construction, perhapskomatiites, which are primitiveultramafic lavas that form at very high temperatures.[4] However, more recent work on the tube- and channel-fed flows indicates lava viscosities within the range of typical basalts (between 100 and 1 million Pa s−1).[44] Calculated flow rates are also lower than originally thought, ranging from 10 to 1.3 million m3 per second. The lower range of eruption rates for Alba Mons is within therange of the highest terrestrial volcanic flows, such as the 1984Mauna Loa,North Queensland (McBride Province), and theColumbia River basalts. The highest range is several orders of magnitude higher than the effusive rates for any terrestrial volcano.[43]
Since the late 1980s, some researchers have suspected that Alba Mons eruptions included a significant amount ofpyroclastics (and therefore explosive activity) during early phases of its development. The evidence was based on the presence of numerous valley networks on the volcano's northern flanks that appeared to be carved by running water (see below). This evidence combined withthermal inertia data, which indicated a surface dominated by fine-grained materials, suggested an easily erodible material, such as volcanic ash, was present. The volcano's extremely low profile is also more easily explained if the edifice were built largely from pyroclastic flow deposits (ignimbrites).[45][46][47]
More recent data fromMars Global Surveyor and theMars Odyssey spacecraft have shown no specific evidence that explosive eruptions ever occurred at Alba Mons. An alternative explanation for the valley networks on the north side of the volcano is that they were produced throughsapping or melting of ice-rich dust deposited during a relatively recent,Amazonian-aged glacial epoch.[12][48]
In summary, current geologic analysis of Alba Mons suggests that the volcano was built by lavas with rheological properties similar tobasalts.[49] If early explosive activity happened at Alba Mons, the evidence (in the form of extensive ash deposits) is largely buried by younger basaltic lavas.[12]
The immense system of fractures surrounding Alba Mons is perhaps the most striking feature of the volcano.[6] The fractures aretectonic features indicatingstresses in the planet'slithosphere. They form when the stresses exceed theyield strength of rock, resulting in the deformation of surface materials. Typically, this deformation is manifested as slip on faults that are recognizable in images from orbit.[51]
Alba's tectonic features are almost entirely extensional,[52] consisting of normalfaults,graben andtension cracks. The most common extensional features on Alba Mons (and Mars in general) are simplegraben. Graben are long, narrow troughs bound by two inward-facing normal faults that enclose a downfaulted block of crust (pictured right). Alba has perhaps the clearest display of simple graben on the entire planet.[53] Alba's graben are up to 1,000 km (620 mi) long, and have a width on the order of 2 km (1.2 mi)–10 km (6.2 mi), with depths of 100 m (330 ft)–350 m (1,150 ft).[54]
Tension cracks (orjoints) are extensional features produced when the crust is wrenched apart with no significant slippage between the separated rock masses. In theory they should appear as deep fissures with sharp V-shaped profiles, but in practice they are often difficult to distinguish from graben because their interiors rapidly fill withtalus from the surrounding walls to produce relatively flat, graben-like floors.[53] Pitcrater chains (catenae), common within many graben on Alba's flanks, may be the surface manifestation of deep tension cracks into which surface material has drained.[51]
The graben and fractures around Alba Mons (hereafter simply called faults unless otherwise indicated) occur in swarms that go by different names depending on their location with respect to Alba's center.[51] South of the volcano is a broad region of intensely fractured terrain calledCeraunius Fossae, which consists of roughly parallel arrays of narrow, north–south oriented faults. These faults diverge around the flanks of the volcano, forming an incomplete ring about 500 km (310 mi) in diameter.[6] The set of faults on Alba's western flank is called Alba Fossae and the one on the eastern flankTantalus Fossae. North of the volcano, the faults splay outward in a northeasterly directions for distances of many hundreds of kilometers. The pattern of faults curving around Alba's flanks has been likened in appearance to the grain of a piece of wood running past a knot.[55] The entire Ceraunius-Alba-Tantalus fault system is at least 3,000 km (1,900 mi) long and 900 km (560 mi)–1,000 km (620 mi) wide[56]
Several causes for the faults have been suggested, including regional stresses created by the Tharsis bulge, volcanic dikes, and crustal loading by Alba Mons itself.[6] The faults of Ceraunius and Tantalus Fossae are roughly radial to the center ofTharsis and are likely a crustal response to the sagging weight of the Tharsis bulge. The faults ringing Alba's summit region may be due to a combination of loading from the Alba edifice and magma uplift or underplating from the underlying mantle.[52][54] Some of the fractures are likely the surface expression of giganticdike swarms radial to Tharsis.[57][58] An image from High Resolution Imaging Science Experiment (HiRISE) on theMars Reconnaissance Orbiter (MRO) shows a line of rimless pit craters inCyane Fossae on the Alba's western flank (pictured right). The pits likely formed by the collapse of surface materials into open fractures created as magma intruded the subsurface rock to formdikes.[59]
The northern slopes of Alba Mons contain numerous branching channel systems orvalley networks that superficially resemble drainage features produced on Earth by rainfall. Alba's valley networks were identified inMariner 9 andViking images in the 1970s, and their origin has long been a topic of Mars research. Valley networks are most common in the ancientNoachian-aged southern highlands of Mars, but also occur on the flanks of some of the large volcanoes. The valley networks on Alba Mons areAmazonian in age and thus significantly younger than the majority of those in the southern highlands. This fact presents a problem for researchers who propose that valley networks were carved by rainfall runoff during an early, warm and wet period of Martian history.[60] If the climate conditions changed billions of years ago into today'scold and dry Mars (where rainfall is impossible), how does one explain the younger valleys on Alba Mons? Did Alba's valley networks form differently from those in the highlands, and if so, how? Why do the valleys on Alba Mons occur mainly on the northern flanks of the volcano? These questions are still being debated.[61]
InViking images, the resemblance of Alba's valley networks to terrestrialpluvial (rainfall) valleys is quite striking. The valley networks show a fine-textured,parallel to dendritic pattern with well-integrated tributary valleys anddrainage densities comparable to those on Earth'sHawaiian volcanoes.[11][62] However, stereoscopic images from the High Resolution Stereo Camera (HRSC) on the EuropeanMars Express orbiter show that the valleys are relatively shallow (30 m (98 ft) or less) and more closely resemblerills orgullies from intermittent runoff erosion than valleys formed from sustained erosion.[63] It seems likely that the valleys on Alba Mons formed as a result of transient erosional processes, possibly related to snow or ice deposits melting during volcanic activity,[63][64] or to short-lived periods of global climate change.[12] (See Surface characteristics, above.) Whether the eroded material is an ice-rich dust orfriable volcanic ash is still uncertain.
Alba's well-preserved lava flows and faults provide an excellent photogeologic record of the volcano's evolution. Usingcrater counting and basic principles ofstratigraphy, such assuperposition andcross-cutting relationships, geologists have been able to reconstruct much of Alba's geologic and tectonic history. Most of the constructional volcanic activity at Alba is believed to have occurred within a relatively brief time interval (about 400 million years) of Mars history, spanning mostly the late Hesperian to very early Amazonian epochs. Faulting and graben formation in the region occurred in two early stages: one preceding and the other contemporaneous with the volcano's formation. Two late stages of graben formation occurred after volcanic activity had largely ended.[22]
Based on Viking Orbiter images, the volcanic materials related to the formation and evolution of the volcano have been grouped into the Alba PateraFormation, which consists of lower, middle, and uppermembers.[12][65] Members low in the stratigraphic sequence are older than those lying above, in accordance withSteno'slaw of superposition.
The oldest unit (lower member) corresponds to the broad lava apron surrounding the Alba Mons edifice. This unit is characterized by sets of low, flat-topped ridges that form a radial pattern extending for hundreds of kilometers to the west, north, and northeast of the main edifice. The ridges are interpreted to be lava flows,[65] although the flow margins are now degraded and difficult to delineate. Broad lava flows with flat-topped ridges are characteristic features of lavaflood provinces on Earth (e.g.,Columbia River basalt) that were formed at high eruption rates.[66] Thus, the earliest phase of volcanic activity at Alba Mons probably involved massive effusive eruptions of low viscosity lavas that formed the volcano's broad, flat apron. Lava flows of the apron unit straddle the early Hesperian-late Hesperian boundary, having erupted approximately 3700 to 3500 million years ago.[12][14]
The middle unit, which is early Amazonian in age, makes up the flanks of the main Alba edifice and records a time of more focused effusive activity consisting of long tube- and channel-fed flows. Volcanic spreading occurred in a northward direction forming the two flanking lobes. (SeeOlympus Mons andTharsis for a discussion of volcanic spreading on Mars.) Faulting and graben formation at Alba and Tantalus Fossae occurred penecontemporaneous with the lava flows. Any early explosive activity on the volcano may have occurred during the culmination of this middle phase of activity, which ended about 3400 million years ago.[12][14][67]
The youngest unit, also early Amazonian, covers the summit plateau, dome, and caldera complex. This period of activity is characterized by relatively short-length sheet flows and construction of the summit dome and the large caldera. This phase ended with an eastward tilting of the summit dome, which may have initiated additional graben formation in Alba Fossae. The last volcanic features to form were the small shield and caldera at the summit. Much later, between about 1,000 and 500 million years ago, a final stage of faulting occurred that may have been related to dike emplacement and the formation of pit crater chains.[12][14][67]
The classification of the Alba Mons volcano is uncertain. Some workers describe it as ashield volcano,[12][52] others as a lowland patera[68] (in contrast tohighland paterae, which are low-lying ancient volcanoes with furrowed ash deposits located in the southern Martian highlands), and still others consider it a one-of-a-kind volcanic structure unique to Mars.[6][10] Some researchers have compared Alba Mons tocoronae structures on the planetVenus.[69][70] Alba Mons shares some characteristics with theSyrtis Major volcanic structure. (SeeVolcanism on Mars.) Both volcanoes areHesperian in age, cover large areas, have very low relief, and large shallow calderas. Also like Alba, Syrtis Major displays ridged tube- and channel-fed lava flows.[71] Because Alba Mons liesantipodal to the Hellas impact basin, a few researchers have conjectured that the volcano's formation may have been related to crustal weakening from the Hellas impact, which produced strongseismic waves that focused on the opposite side of the planet.[72][73][74]
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