Lunar and Planetary Science XLVIII (2017) 2225.pdf

ICE OR VOLCANISM - INTERPRETATION OF PITTED MARTIAN TERRAIN Natalia Zalewska 1,2 ([email protected]) 1Institute of Aviation, Al. Krakowska 110/114, 02-256 Warsaw, Poland, ([email protected]) 2Space Research Center PAS, Bartycka 18A, 00-716 Warsaw, Poland

Until recently, a trend dominated that most of In the central part of Chryse 30.360°N, the oval depressions that we can see on planets and 323.373°E, (Fig.3) there are series of chains depres- terrestrial bodies are impact craters. In light of the ac- sions caused most likely by the collapse of land. Pingo quisition of images in progressively higher resolutions mechanism may be responsible for the formation of a we started to see the ground details which cast doubt sort of colapsed canyons or elongated forms of subsid- on our working explanaition of most events by meteor- ence land as a result of thawing of subsurface ice [4]. ite impacts. Mysterious pits often formed in chains These forms have been associated with an open pingo look like some kind of collapsed surface. Factors caus- type system additionally with assisted topography of ing the collapse has not been investigated. Cooling lava the area or tectonics and internal cracks in the rocks. or cryogenic processes have been suspected. HiRISE These are noticed on the slopes of craters or wherever camera provides us with pictures which show surface the area decline. Then flowing subsurface water or forms of unknown origin. On the images from Mars brine coming from the ice layer could while freezing within volcanoes and circumpolar areas there are de- accumulate and create a longitudinal hill that collaped pressions that can be explained in two ways, either by due to seasonal thawing forming gullies or canyons [5]. melting subsurface layer of ice or by cooling of lava At the end of these gullies remaining trace of the leak which forms branch intrusion and flank craters under- can be seen, as if there was a crack in the ground and neath. Since cooling causes a decrease in volume and liquid flew out on the surface. Thawing ice could cre- the area above the lava collapses. ate a kind of depression known to sedimentology or On many pictures from Mars similar cavities sometimes slightly winding depression with smooth are found on the slopes of Martian craters on Arsia slopes. Mons [1], Pavonis Mons on northern hemisphere and Today, the phenomenon is observed in Siberia Alba Patera on southern hemisphere. Such cavities can and Canada or Alaska, where due to melting perma- be compared to a Hawaiian type volcanoes. At the top frost, several meters deep trenches and pits are formed of Mauna Loa (Fig.1) linearly arranged craters can be [6]. seen, strikingly similar to those on Arsia Mons (Fig.2). Surprisingly similar structures can be ob- Basing on map ice content measured by Odyssey GRS served in the Copernicus crater and the Davy crater of apparatus, in this place of the volcanic cone, quite our moon or on Ganymede and Callisto - moons of small ice content can be observed that varies in the Jupiter (Fig.4), Fobos - moon of Mars and on Mercury. range of 2-4% hydrogen abundance. It is therefore dif- Structure on Ganymede [7] and Callisto have been ficult to explain these collapses by unfreezing of sub- often explained as a serial traces of a meteorite impact surface ice. In an infrared spectrum of these areas there but also they can be compared to the previously de- are no bands of water in the CRISM spectra, although scribed depressions on Mars [8]. It is known that the it does not say that the water in the form of ice couldn’t Galilean moons are icy moons, where crust is mainly have been there before. Data from Odyssey probe composed of ice and such phenomena can occur on (GRS) represents only the current state of water con- them. Davy crater is situated just around the equator of tent. our moon on the visible side. Serial depressions can be Mauna Loa craters (Fig.1) formed by magma seen on its surface. These depressions are called craters branch intrusion going off from the volcanic chamber and are explained not only by a meteorite impact but [2]. Currently, the craters are inactive but the last erup- also as volcanism although it may very well fit to those tion occurred in 1984. The first eruption was recorded mentioned depressions. In support of this theory, large in 1843, so Mauna Loa volcanoes are very young. It is quantities of water in the form of ice were discovered difficult to determine the exact age of the Martian by Clementine probe (1994) following by Cassini Tharsis volcanic area but it is estimated that it had his probe which detected a distinctive 4.6% signature over heyday in Hesperia about 3.5-3.7 billion years ago and the north polar region and a 3.0 % signature over the ended up probably about 1.8 billion years ago. Accord- south. The instrument could detect water to a depth of ing to [3] fan-shaped deposits are Amazonian-aged. about half a meter. It was a big surprise due to the pre- Chains of depressions on Arsia Mons just fit into that vailing view of dry basalt surface of our Moon [9], volcanic collapse. [10]. Lunar and Planetary Science XLVIII (2017) 2225.pdf

Is geological history of Mars functioning for Fig.2 Arsia Mons, 14,5 km chain of pittet terrain, peak of several years require amendments? Certainly yes. Cry- volcano is 16 km high. Chain South of Arsia Mons. HiRISE, osubsurface processes on Mars can support the hypoth- ESP_012600_1655, centered -14.285°N; 240.051°E These esis of geochemical origin of water, which separates depressions there are along faults and graben fractures, and from the magma, and its primary source comes from in diffuse patches that may reflect shallow magmatic intru- the protoplanetary disk. The water separated from the sions magma migrates up to the surface and if the tempera- ture is below zero the water deposits as a layer of ice in the case of Mars as a subsurface layer or in the case of moons of gas giants as an eruption through the surface. It can therefore ask whether such an artificial division of Martian eras: Noachian Hesperian and Amazonian is still valid? Did rivers ever flow on Mars? Or were they just seasonal melting of subsurface ice and often cata- strophic short outflows and its transgressions and re- gressions in the form of glaciers [11]. Fig.3 Pit Crater Chains in Chryse Planitia, HiRISE, PSP_008641_2105, centered 30.360°N, 323.373°E 15 km chain is caused propably by colapsed terrain above thawing ice layer.

Fig.1 Mauna Loa. 7,5 km chain of craters on Mauna Loa summit caldera: North Pit, Lua Poholo, Moku'āweoweo, South Pit, Hohonu Lua, Lua How lie in the line across the Fig.4. The only picture available of Callisto Gipul Catena. southern rift (SW rift zone), volcanic chamber is located at a Chain of craters on Jupiter’s moon, Callisto. This chain of 13 depth of 4.7 km. (Topographic Google Map) craters probably formed by a comet which was pulled into pieces by Jupiter’s gravity as it passed too close to the planet or by endogenic proces like by volcanic venting activity along a rift or colapsing ice layer. This chain is about 620 km long. Similar crater chains have also been identifed on Ga- nymede.

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