Chapter 4 -- SURFACE FEATURES OF MARS
The dark Martian surface markings, called "maria" or "albedo features," were once thought by some astronomers to be great lakes, oceans, or vegetation, but space probes in the 1970’s revealed them to be vast expanses of rock and dust. Windstorms sometimes move the dust, resulting in both seasonal and long-term changes in these markings. These features seem to darken during early Martian spring in such a manner that a "wave of darkening" appears to sweep from the thawing polar cap towards the equator. This event, which occurs during each hemisphere’s spring season, lent credence to the theory that the maria were composed of vegetation, which was replenished when water flowed from the melting polar cap towards the equator.
Now we know that this concept is false. In fact, C.F. Capen showed that the wave of darkening is in actuality a "wave of brightening" [Michaux,1972, Capen,1976, Dobbins,1988]. The albedo features only appear to darken because the adjacent ochre desert areas have brightened during early spring. This has been confirmed by Viking Lander photos, which reveal a fresh, bright layer of dust appearing on the ground during early spring. Light and dark surface features tend to change in albedo and color contrast diurnally and more slowly as the seasons change. Seasonal variations are usually predictable, but secular or long-period changes are unpredictable.
Among the areas where yearly variations have been recorded are Trivium-Elysium, Solis Lacus, Syrtis Major, and Sabaeus-Meridiani. The Syrtis Major is the planet’s most prominent dark area. Classical observations have revealed seasonal variations in the breadth of this feature: maximum width occurring in northern mid summer (145° Ls), and minimum during early northern winter, just after perihelion (290° Ls) [Antoniadi,1930, Capen,1976]. However, recent observations by ALPO astronomers and by the Hubble Space Telescope (HST) suggest that no such variations have occurred since 1990 [Lee,et al.,1995. Troiani, et al.,1997]. For a look at some Mars charts and the official IAU names for Mars features see:
1954 ALPO Mars Section Mars Chart:
New ALPO Mars Section Mars Chart: Mars_Map_Venable-Melka.jpg
ALPO Mars Section Mars Chart: Alpomarsmap.jpg
Ebasawa Mars Chart: Ebasawamars.jpg
USGS Planetary Nomenclature: USGS Planetary Nomenclature: MARS
Seasonal Changes. Several regions that display seasonal changes are:
Syrtis Major is the planet’s most prominent dark area. Classical observations indicated seasonal variations in the breadth of this feature: maximum width occurring in northern mid summer (145° Ls), when its eastern edge expands eastward to about 275° W. longitude [Dollfus, 1961]. Minimum width classically occurs during early northern winter, just after perihelion (290° Ls) [Antoniadi,1930, Capen,1976]. However, recent observations by A.L.P.O. astronomers and the Hubble Space Telescope (HST) suggest that no such variations have occurred since 1990 [Lee, et al.,1995.].
The Syrtis Major area has also undergone some rather dramatic long-term, or "secular," changes over the years. During recent apparitions it has become narrower and more blunted in appearance compared to the 1950’s. After the 2001 dust storm this feature appeared thinner and more tapered to the north than it was before the storm [McKim, 2002]. Osiridis Promontorium became very dark in 1984, appearing as a dark bar jutting out into Libya from the northeast border of Syrtis Major. This feature was conspicuous in 1879, 1909, and during the 1940’s and 1950’s. The broad "canal," Nilosyrtis that curves northeast from the northern tip of Syrtis Major, was inconspicuous in 1984 [Parker et al., 1999].
The Nepenthes-Thoth (268° W, 08° N) feature, lying to the west of the Elysium shield, so prominent in the 1940’s, and 1950’s, decreased in size in 1960 and began fading in 1971. It was virtually undetectable in 1984. Nodus Laocoontis (246°W, 25°N), first described by S. Kibe in 1935, had faded during the 1970’s and was not seen during the 1983-1985 apparition.
Hellas. One of the most active areas on Mars is the Hellas Basin (292° W, 50° S), not only because of its dynamic meteorology but also for its never-ceasing albedo changes. Surface structure becomes apparent in this area when its darker center (Zea Lacus) seems to extend its arms or canals (Alpheus) to the north, and connect Mare Hadriacum (265° W, 40° S) and Yaonis Fretum (318° W, 43° S) eastward to the western edge of Peneus. As the Martian southern summer solstice approaches, the basin often becomes flooded with dust if a violent storm begins. Hellas was the initial site of the great planet-encircling dust storm of 2001 and is a region that bears careful scrutiny during the 2003 apparition, since Mars will then be in its "dusty season."
Hellas also was involved in both the December 11, 1983, the January 5, 1984, dust storms [Beish et al.,1984]. As these apparitions progressed and southern hemisphere winter got underway, Hellas and the high basin Argyre (30°W, 50°S) appeared brilliant white on the southern limb. Both of these great basins are the water-ice reservoirs of the southern hemisphere and are often covered with frost or with low clouds. These features were often confused with the South Polar Cap (SPC) or its winter hood, owing to their foreshortened appearance due to the planet’s axial tilt.
Solis Lacus is called the "Eye of Mars" because, with the surrounding light area called Thaumasia, it resembles the pupil of an eye. Centered at 90° W, 30° S, Solis Lacus is notorious for its variability. Small and relatively inconspicuous in 1971, it underwent a major dark secular change in 1973, perhaps as a result of the major dust storms occurring during those years [Dobbins et al.,1988]. During the ensuing two decades it remained large dark oval with a north-south orientation [McKim,1992]. During the 1992-1993 apparition Solis Lacus presented as a small, dark oval, but it enlarged and elongated in 1975 and has remained a large dark oval feature oriented slightly east-west until late 2001. At that time, after the massive dust storm had subsided, it appeared smaller than it had before the storm and the "canal" Nectar had all but disappeared [McKim,2002].
Just west of Solis Lacus other areas have undergone changes are Daedalia-Claritas and Mare Sirenum. In 1973, the normally light region located between Sirenum M. and Solis Lacus, Daedalia-Claritas, underwent a dramatic darkening, which persisted through 1980. In 1984, this region had returned to its normal light intensity. However, during March and April 1984, A.L.P.O. observers reported that northeastern M. Sirenum had weakened considerably, possibly as a result of dust deposition from the storms sighted earlier in that region [Capen,1986].
Early in 2001, after the dust had cleared from the 2001 storm, IMP observers reported a significant darkening in Daedalia-Claritas that extended eastward into Thaumasia near the site of the Phasis "canal." Both this region and nearby Solis Lacus bear careful watching in 2007.
Trivium-Cerberus (210° W, 22° N), lying on the southern rim of the Elysium shield, is another feature of great interest to professional Mars researchers. During the 1950s it was a classically dark feature 808 x 249 miles (1,300 x 400 km) in size, but it weakened somewhat in the 1960s. During the 1970s it varied in size and intensity from prominent to near invisibility. This area appears to have been covered over with dust during February and March of 1982 [Parker et al.,1990]. A generally "washed out" appearance was reported during the remainder of that apparition and very low contrast has been observed ever since. Dust storms during 1983 and 1984 appeared to further lower the contrast of the Elysium and Trivium Charontis region [Parker et al., 1999]. On May 14, 1984, A.L.P.O. observers reported that the Trivium Charontis--Cerberus was very difficult to see or missing from the face of Mars [Beish,1984 and Troiani,1996]. Except for a brief darkening in 1995 it has remained nearly invisible, appearing as two or three dots on a half-tone background [Moersch et al.,1998. Troiani et al.,1998].
In 1977 A.L.P.O. Mars observers reported a new dark area on the western side of the Elysium shield volcanoes [Capen and Parker,1980] Astronomers reported that the normally insignificant "canal" Hyblaeus (240°W, 30°N) had darkened and expanded westward into Aetheria. Termed the "Hyblaeus Extension" by Capen, this change has persisted to the present. Interestingly, it was subsequently found on Viking Orbiter photographs taken in 1975, apparently undetected by Viking scientists. This is an example of the importance of ground-based observations of Solar System objects. On June 10, 1984 (162° Ls), A.L.P.O. observers photographed a further darkening in this region, located in Morpheos Lacus (228°W, 37°N). This darkening persisted into the 1980’s & 1990’s along with other changes near Elysium, notably the lightening of the wedge-shaped feature, Trivium Charontis. The entire region near the huge Elysium volcanoes appears to be in a state of flux and should be monitored often.
Cerberus III. A recent surface change is the appearance of a very conspicuous dark band across Hesperia. This has been named for the faint "canal" Cerberus III and was first detected in 1986 [Beish et al.,1989].
In 1990 A.L.P.O. observers reported a bright streak running east-west from 160°W to 260°W at 50-60°N. At 220°W longitude, another streak extended at right angles from it and extended southward into Elysium. These streaks, also observed in 1995 and 1997, appeared bright through all filters, and their nature is not known. This entire region bears careful scrutiny and will be well placed for observation during the Perihelic apparitions of the early 21st Century.
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