Alba Mons

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. 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 (and steepest) 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 nor elsewhere on Mars. 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. 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).

thumb|left|240px|[[Mars Orbital Laser Altimeter|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 (in reference to the partial ring of fractures around the volcano). The IAU named the volcano Alba Patera in 1973. and has a volume of about 2.5 million km<sup>3</sup>.

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 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 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 central edifice has the steepest slopes on the volcano, although they are still only 1°. 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

thumb|right|Dust mantle at the SW edge of small caldera on Alba Mons ([[HiRISE).]]

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 (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.

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 (<). (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. 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 (1 to 10 million years ago), when Mars’ axial tilt (obliquity) 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 (GRS). This instrument has allowed scientists to determine the distribution of hydrogen (H), silicon (Si), iron (Fe), chlorine (Cl), thorium (Th) and potassium (K) 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 (19 wt%), Th (0.58 pppm), and K (0.29 wt%) content, but with Cl abundance (0.56 wt%) 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.

Geology

left|thumb|Sheet flows on northwestern flank of Alba Mons. Note multiple overlapping lobes ([[Thermal Emission Imaging System|THEMIS VIS)]]

right|thumb|300px|Lava flows extending north and northwest of Alba Mons. The sinuous ridges are tube- and channel-fed flows. Faint, degraded flows and ridges in the north are part of Alba's broad lava apron ([[Mars Orbital Laser Altimeter|MOLA).]]

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.

Lava flows

Alba Mons is notable for the remarkable length, diversity, and crisp appearance of its lava flows. Individual flows may exceed in length. Lava flows near the summit calderas appear to be significantly shorter and narrower than those on more distal parts of the volcano. 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

The second major type of lava flows on the flanks of Alba Mons are called tube- and channel-fed flows, or crested flows.

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,

The morphology of lava flows can indicate properties of the lava when molten, such as its rheology and flow volume. Together, these properties can provide clues to the lava's composition and eruption rates. Calculated flow rates are also lower than originally thought, ranging from 10 to 1.3 million m<sup>3</sup> per second. The lower range of eruption rates for Alba Mons is within the

range of the highest terrestrial volcanic flows, such as the 1984 Mauna Loa, North Queensland (McBride Province), and the Columbia River basalts. The highest range is several orders of magnitude higher than the effusive rates for any terrestrial volcano.

More recent data from Mars Global Surveyor and the Mars 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 through sapping or melting of ice-rich dust deposited during a relatively recent, Amazonian-aged glacial epoch.

In summary, current geologic analysis of Alba Mons suggests that the volcano was built by lavas with rheological properties similar to basalts. If early explosive activity happened at Alba Mons, the evidence (in the form of extensive ash deposits) is largely buried by younger basaltic lavas. (THEMIS IR daytime mosaic).]]

thumb|right|240px|Graben are formed by extensional stresses (red arrows) in the crust. Graben consist of flat-floored valleys bound by opposite-facing normal faults, and are often separated by upland blocks called horsts.

Tectonic features

The immense system of fractures surrounding Alba Mons is perhaps the most striking feature of the volcano.

Alba's tectonic features are almost entirely extensional, consisting of normal faults, graben and tension cracks. The most common extensional features on Alba Mons (and Mars in general) are simple graben. 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. Alba's graben are up to long, and have a width on the order of –, with depths of –.

Tension cracks (or joints) 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 with talus from the surrounding walls to produce relatively flat, graben-like floors. The entire Ceraunius-Alba-Tantalus fault system is at least long and – wide

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. An image from High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) shows a line of rimless pit craters in Cyane 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 form dikes.

Valleys and gullies

thumb|left|High resolution view of valley network on NW flank of Alba Mons. Younger fault crosscuts the valleys. Image is about across. ([[Mars Global Surveyor, MOC-NA)]]

The northern slopes of Alba Mons contain numerous branching channel systems or valley networks that superficially resemble drainage features produced on Earth by rainfall. Alba's valley networks were identified in Mariner 9 and Viking images in the 1970s, and their origin has long been a topic of Mars research. Valley networks are most common in the ancient Noachian-aged southern highlands of Mars, but also occur on the flanks of some of the large volcanoes. The valley networks on Alba Mons are Amazonian 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. If the climate conditions changed billions of years ago into today's cold 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.

In Viking images, the resemblance of Alba's valley networks to terrestrial pluvial (rainfall) valleys is quite striking. The valley networks show a fine-textured, parallel to dendritic pattern with well-integrated tributary valleys and drainage densities comparable to those on Earth's Hawaiian volcanoes. However, stereoscopic images from the High Resolution Stereo Camera (HRSC) on the European Mars Express orbiter show that the valleys are relatively shallow ( or less) and more closely resemble rills or gullies from intermittent runoff erosion than valleys formed from sustained erosion. 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, or to short-lived periods of global climate change. Members low in the stratigraphic sequence are older than those lying above, in accordance with Steno's law 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, 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.

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. (in contrast to highland 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. Alba Mons shares some characteristics with the Syrtis Major volcanic structure. (See Volcanism on Mars.) Both volcanoes are Hesperian 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. Because Alba Mons lies antipodal 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 strong seismic waves that focused on the opposite side of the planet.

References

External links

View of Albra Mons in context in view extending from the North pole down, taken by Mars Express in 2017