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... This study found that the features seemed to be restricted to the SPLD and were not detected on the southern highlands or residual cap. Further study by G. V. Portyankina (2005) extended the survey of spiders to longitudes 10°-120° when later MOC NA images became available, and compared their presence with two sub-regions of the cryptic terrain: one where the mismatch of dark albedo and very cold temperatures (i.e., "cryptic behavior") occurred between L s = 90° and the complete sublimation of the seasonal ice cap (the layered deposits and the floor of the Prometheus impact basin), and another "more inclusive" definition which displayed cryptic behavior at any time during the defrosting season (Dorsa Argentia and the southern highlands). All identified spiders fell within the more inclusive version of the cryptic region and the majority fell within the more narrowly defined domain. ...
Plain Language Summary
Araneiforms, more colloquially coined “spiders,” are strange branched networks of troughs that are carved in the Martian regolith within the south polar regions, poleward of ∼70°. They have been proposed to form in spring, when sunlight passes through and heats the Martian seasonal CO2 ice layer, causing gas to build up beneath it and crack the ice, scouring squiggly and branched troughs on the surface and depositing the eroded material in the form of a plume. Such a process does not occur on Earth, so since their original detection, scientists have used creative approaches to understand the formation of araneiforms; comprising computational mathematical modeling, small‐scale experiments in planetary chambers to recreate the process, and even citizen science campaigns, where planetary enthusiasts have helped to map their locations on Mars. We review the work that has been conducted to understand the formation of these beautiful and puzzling surface features and discuss how they may help us to understand seasonal change on Mars in the present‐day and even the past. We discuss how our understanding of araneiforms can be applied to other icy planetary surfaces and finally present gaps in our knowledge and ways to address them.
... Starting portion of the cloud that closes to the Arsia Mons shows a lee wave pattern. MCC also captured the limb view of the AMEC tail portion that appears as a separate layer of cloud over the planet's edge (Kahn and Gierasch, 1982;Ganna Valeriyivna Portyankina,). The present work interprets AMEC as a lee wave cloud, and previous studies highly motivate us to study this unique feature during a dust storm season (Hernández-Bernal et al., 2020). ...
... The speed of light is very high compared to the wind's speed, and hence (such that u 2 /c 2 ≈ 10 − 3 ), the third term inside the square root will vanish. The above equation may be rearranged as follows (Portyankina et al., 2005), ...
The present analysis reported the lee wave interpretation on Arsia Mons Elongated Cloud (AMEC) that used to appear over the lee side of the Arsia Mons. The present work also reported the wavelength, the cloud's height, and wind speed of the observed lee wave structure. The estimated wind speed is 86 ± 4.9 m/s, with a wavelength of 60 ± 0.3 km at an altitude of 55 ± 7 km. This is the largest lee wave structure that appears during the dust storm season (Ls = 230 to Ls = 300) and strongly contributes to the planet's Albedo level. Calculated TOA reflectance varies from 0.05 to 0.15 for the blue channel and 0.02 to 0.09 for the red channel. Estimated AOD ranges from 1.8 (red channel) to 1.5 (blue channel), indicating the contribution of dust and water ice crystal. The estimated angstrom exponent (α) value signifies coarse mode particle presence in the cloud with an effective radius of 3.2 μm. The presence of water ice crystal contributes to the albedo level's increment to 0.8 and signifies the formation temperature for AMEC to be around 190 K.
... Thus, the likely reason why these araneiforms features were first identified by P4T is that these areas were covered in the CTX observations and not in the Piqueux et al. (2003) reviewed 540 observations. Bolstering this argument is the araneiform identifications from Portyankina (2005) which surveyed MOC NA observations from September 1997 to March 2004, including reviewing newer observations than what was available to Piqueux et al. (2003). Portyankina (2005) did not compare their araneiform identifications to the outline of the SPLD or the locations of other geologic units. ...
... Bolstering this argument is the araneiform identifications from Portyankina (2005) which surveyed MOC NA observations from September 1997 to March 2004, including reviewing newer observations than what was available to Piqueux et al. (2003). Portyankina (2005) did not compare their araneiform identifications to the outline of the SPLD or the locations of other geologic units. Although Portyankina (2005) do not resolve araneiform identifications to subimage positions, comparing their distribution, we find that there are several identifications outside of the SPLD (E0701468, R0902433, R0902028, R0901662, R0800241, R0904104, R0801805, R0903024, R0903639, R0701390, R0801195, M1101070, R0601143, R0903607, R0901019, R0601842, R0902403, R0903025, M1102723, M0905981, M1103774,M1000442, M1301816, and M0900454). ...
... Portyankina (2005) did not compare their araneiform identifications to the outline of the SPLD or the locations of other geologic units. Although Portyankina (2005) do not resolve araneiform identifications to subimage positions, comparing their distribution, we find that there are several identifications outside of the SPLD (E0701468, R0902433, R0902028, R0901662, R0800241, R0904104, R0801805, R0903024, R0903639, R0701390, R0801195, M1101070, R0601143, R0903607, R0901019, R0601842, R0902403, R0903025, M1102723, M0905981, M1103774,M1000442, M1301816, and M0900454). With a visual 550 inspection, we find many of these off SPLD MOC observations identified as containing araneiforms by Portyankina (2005) are heavily covered with dark seasonal fans, making it more difficult for a positive identification. ...
We present the results of a systematic mapping of seasonally sculpted terrains on the South Polar region of Mars with the Planet Four: Terrains (P4T) online citizen science project. P4T enlists members of the general public to visually identify features in the publicly released Mars Reconnaissance Orbiter CTX images. In particular, P4T volunteers are asked to identify: 1) araneiforms (including features with a central pit and radiating channels known as 'spiders'); 2) erosional depressions, troughs, mesas, ridges, and quasi-circular pits characteristic of the South Polar Residual Cap (SPRC) which we collectively refer to as 'Swiss cheese terrain', and 3) craters. In this work we present the distributions of our high confidence classic spider araneiforms and Swiss cheese terrain identifications. We find no locations within our high confidence spider sample that also have confident Swiss cheese terrain identifications. Previously spiders were reported as being confined to the South Polar Layered Deposits (SPLD). Our work has provided the first identification of spiders at locations outside of the SPLD, confirmed with high resolution HiRISE imaging. We find araneiforms on the Amazonian and Hesperian polar units and the Early Noachian highland units, with 75% of the identified araneiform locations in our high confidence sample residing on the SPLD. With our current coverage, we cannot confirm whether these are the only geologic units conducive to araneiform formation on the Martian South Polar region. Our results are consistent with the current CO2 jet formation scenario with the process exploiting weaknesses in the surface below the seasonal CO2 ice sheet to carve araneiform channels into the regolith over many seasons. These new regions serve as additional probes of the conditions required for channel creation in the CO2 jet process. (Abridged)
DefinitionA spider is a topographical feature of several dendritic troughs radially converging to a common central depression exclusively found in the southern polar areas of Mars.SynonymsSpider; Black or dark spider; Cold jet flow channels; Geyser flow channels; Martian spiderDescriptionIts troughs are irregular, radiating away from the center and merge, branch, and anastomose in a seemingly random pattern. Spiders were found either isolated or in groups of sometimes overlapping individuals. During ice-free summer, spiders show no albedo difference to the surrounding substrate: they are solely topographical features. Early in local spring however their albedo often contrasts with the surroundings: either spider’s bluish-bright (standard processed HiRISE false colors) troughs stand out against red substrate or dark troughs against bright substrate. Spiders should not be confused with dark fan deposits that appear in spring in the same area (Fig. 1) (
Dark polygons associated with fans and spots appear during the spring on the southern seasonal cap. The basal sublimation of the translucent cap and the venting of the CO2 gas are responsible for their formation, as previously proposed for the spots and fans. Dark polygons appear when dark material emerges from elongated vents, whereas spots and fans form from point sources. A class of erosive features (etched polygons) is associated with depressions a few meters to tens of meters in diameters connected to a network of radiating troughs (“spiders”). Spiders are shaped by the scouring action of the confined gas converging toward point sources, whereas the etched polygons result from the forced migration of the CO2 gas over longer distances. The minimum age of the spiders is 104 years. They result from one of the most efficient erosive processes on Mars, displacing 2 orders of magnitude more dust per year than a typical dust storm or than all the dust devils during the same time period. In the north, parts of the seasonal cap are translucent between Ls = 355° and Ls = 60° and are associated with spots, fans, dark polygons, and possibly spiders, suggesting that the basal sublimation and venting of the cap triggers a subice gas and dust flow that is modifying the morphology of the surface layer. However, perennial features are extremely uncommon on the north regolith, indicating that the conditions for their formation or conservation are not met. The reduced basal energy budget of the north cap compared to the south and the shorter seasonal life time of the north translucent ice may explain the relative scarcity of features in the north. The polar layered deposits contain the stratigraphic record of climatic changes and catastrophic events. Both polar deposits may have been locally disrupted by the seasonal subice gas flow and the stratigraphic record may have been partially lost.
The High Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has been used to monitor the seasonal evolution of several regions at high southern latitudes and, in particular, the jet-like activity which may result from the process described by Kieffer (JGR, 112, E08005, doi:10.1029/2006JE002816, 2007) involving translucent CO2 ice. In this work, we mostly concentrate on observations of the Inca City (81°S, 296°E) and Manhattan (86°S, 99°E) regions in the southern spring of 2007. Two companion papers, [Hansen et al. this issue] and [Portyankina et al. this issue], discuss the surface features in these regions and specific models of the behaviour of CO2 slab ice, respectively. The observations indicate rapid on-set of activity in late winter initiating before HiRISE can obtain adequately illuminated images (Ls < 174° at Inca City). Most sources become active within the subsequent 8 weeks. Activity is indicated by the production of dark deposits surrounded by brighter bluer deposits which probably arise from the freezing out of vented CO2 [Titus et al., 2007. AGU (abstract P41A-0188)]. These deposits originate from araneiform structures (spiders), boulders on ridges, cracks on slopes, and along linear cracks in the slab ice on flatter surfaces. The type of activity observed can often be explained qualitatively by considering the local topography. Some dark fans are observed to shorten enormously in length on a timescale of 18 days. We consider this to be strong evidence that outgassing was in progress at the time of HiRISE image acquisition and estimate a total particulate emission rate of >30 g s−1 from a single typical jet feature. Brighter deposits at Inca City become increasingly hard to detect after Ls = 210°. In the Inca City region, the orientations of surficial deposits are topographically controlled. The deposition of dark material also appears to be influenced by local topography suggesting that the ejection from the vents is at low velocity (<10 m s−1) and that a ground-hugging flow process (a sort of “cryo-fumarole”) may be occurring. The failure up to this point to obtain a clear detection of outgassing though stereo imaging is consistent with low level transport. The downslope orientation of the deposits may result from the geometry of the vent or from catabatic winds. At many sites, more than one ejection event appears to have occurred suggesting re-charging of the sources. Around Ls = 230°, the brightness of the surface begins to drop rapidly on north-facing slopes and the contrast between the dark deposits and the surrounding surface reduces. This indicates that the CO2 ice slab is being lost completely in some areas at around this time. By Ls = 280°, at Inca City, the ice slab has effectively gone. CRISM band ratios and THEMIS brightness temperature measurements are consistent with this interpretation.
