 MOLA colorized relief map of Hesperia Planum region. Hesperia Planum has fewer impact craters than the surroundingNoachian highlands ofTyrrhena Terra andTerra Cimmeria. This indicates that the plain is younger than its more heavily cratered surroundings. | |
| Feature type | Lava plain |
|---|---|
| Location | Mare Tyrrhenum quadrangle,Mars |
| Coordinates | 21°25′S109°53′E / 21.417°S 109.883°E /-21.417; 109.883[1] |
| Diameter | ~1 600 km[1] |
| Eponym | PoeticHesperia |
Hesperia Planum is a broadlava plain in the southern highlands of the planetMars. The plain is notable for its moderate number ofimpact craters and abundantwrinkle ridges. It is also the location of the ancientvolcano Tyrrhena Mons (Tyrrhena Patera). TheHesperian time period on Mars is named after Hesperia Planum.[2][3]
Most place names on Mars are derived from sources in theBible orclassical antiquity.[4]Hesperia is a Greco-Latin poetic term for "lands to the west," which to theancient Greeks andRomans meantItaly, whileSpain was referred asHesperia Ultima.[5][6]Planum (pl.plana) isLatin for plateau or high plain. It is adescriptor term used inplanetary geology for a relatively smooth, elevated terrain on anotherplanet ormoon.[7]
The Hesperia region of Mars was named by Italian astronomerGiovanni Schiaparelli in 1877 for an intermediate-tonedalbedo feature centered at lat. 20°S, long. 240°W between two darker regions.[5][8] Believing the dark areas were bodies of water, Schiaparelli interpreted Hesperia to be afloodplain ormarsh bridging two adjacent seas, the Mare Tyrrhenum and Mare Cimmerium.[9] Although the existence of seas on Mars had been discounted by the early 20th century,[10] the true nature of the region remained obscure until thespace age. In 1972, theMariner 9 spacecraft showed that Hesperia was a cratered, wind-streaked plain.[11] TheInternational Astronomical Union (IAU) formally named the area Hesperia Planum in 1973.[12] The dark areas flanking Hesperia Planum were found to be heavily cratered uplands. In 1979, the IAU designated the upland area to the west asTyrrhena Terra and to the east asTerra Cimmeria.[13] (Terra is a Latin descriptor term meaning land or continent.)
Hesperia Planum is located along the broad northeastern rim of the giantHellas impact basin[14] and is centered at lat. 22.3°S, long. 110°E in theMare Tyrrhenum quadrangle (MC-22). A small part of this region in the south is found in theHellas quadrangle. It has a maximum width of 1,700 km (1,100 mi)[12] and covers an area of about 2 million km2 (770,000 sq mi).[15]
At large scales (>100 m or 330 ft), Hesperia Planum appears smooth and level,[16] having a relatively uniform surface elevation of 1.2 km (0.75 mi) aboveMars datum.[17] The plain's surface is 200–800 m (660–2,620 ft) lower in elevation than the surrounding uplands of Tyrrhena Terra and Terra Cimmeria and is slightly tilted to the south, with a mean regional slope of about 0.03°.[14] In high-resolution images (<19 m or 62 ft/pixel), the surface of Hesperia Planum is dominated by dust and fine-grained deposits. Few boulders or bedrock outcrops are visible. Abundant, shallow craters filled with smooth, flat-lying deposits are common. No vents or volcanic constructs are identifiable, although small (<10s meters wide) channels are present.[18]
Hesperia Planum is generally interpreted to be composed offlood lavas,[19] although layeredvolcaniclastic orlacustrine (lake-bed) sediments cannot be ruled out.[18] The lavas appear to partly fill a large, irregular topographic depression that existed inNoachian times. The rims of pre-existing impact craters are still visible in places, indicating that the lava deposits are 250–500 m in thickness. The volume of lavas within Hesperia Planum is comparable to that found inlarge igneous provinces on Earth, such as theColumbia River Basalt Group.[14]
The moderate amount of cratering on Hesperia Planum indicates that the plain has an intermediate age in Martian history. Inplanetary geology, the number density of impact craters is a measure of therelative age of a planetary surface. Heavily cratered surfaces are old, and sparsely cratered surfaces are young. Hesperia Planum is thetype locality for theHesperian System and time period. The lavas making up Hesperia Planum define the base of the HesperianSystem.[21] They erupted at the beginning of the Hesperian Period around 3700 million years ago.[22] (Mars itself, along with the other planets, formed about 4500 million years ago.) Hesperian lavas are younger than the rocks in the heavily crateredNoachian terrains but older than rocks formed during the more recent Amazonian Period. (SeeGeology of Mars.)
Wrinkle ridges are long, lineartopographic highs with a distinctive morphology that consists of a low, broad arch topped by a narrow crenulated ridge (pictured left). They are common features on the Moon where they occur exclusively within lava flow plains (the lunarmaria).[23] Their occurrence on Mars is thought to reflect a similar volcanic association. Thus, areas on Mars with abundant wrinkle ridges are interpreted as plains formed by very fluid basaltic lava (flood basalts). The ridges themselves are believed to be the surface expression ofthrust faults formed after the lava flows were emplaced.[24][25] They are not volcanic features, but secondary,tectonic structures that form in dense, competent rocks (such as layered basalts) that have undergone compressional stress. Hesperian-aged "ridged plains" like Hesperia Planum cover about 30% of the Martian surface.[19]
Tyrrhenus Mons (Tyrrhena Patera) is an eroded, low-lying volcano in the western part of Hesperia Planum. It is one of the oldest large central-vent volcanoes on the planet[27] and a member of a class of volcanoes calledhighland paterae, which erupted mainly in the LateNoachian and EarlyHesperian.[28] Tyrrhenus Mons stands only 1.5 km above the surrounding plains. At its center lies a 40 km diameter depression, orcaldera, from which radiate numerous flat-floored valleys and ridges that suggest the volcano has been highly eroded. The low relief of Tyrrhenus Mons combined with its degraded state indicate the volcano consists largely offriable and easily eroded material such asvolcanic ash. The ash was likely derived from the interaction ofmagma withgroundwater or ice.[29]
