JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, E09006, doi:10.1029/2009JE003546, 2010 Explosive lava‐water interactions in Elysium Planitia, Mars: Geologic and thermodynamic constraints on the formation of the Tartarus Colles cone groups Christopher W. Hamilton,1 Sarah A. Fagents,1 and Lionel Wilson2 Received 16 November 2009; revised 11 May 2010; accepted 3 June 2010; published 16 September 2010. [1] Volcanic rootless constructs (VRCs) are the products of explosive lava‐water interactions. VRCs are significant because they imply the presence of active lava and an underlying aqueous phase (e.g., groundwater or ice) at the time of their formation. Combined mapping of VRC locations, age‐dating of their host lava surfaces, and thermodynamic modeling of lava‐substrate interactions can therefore constrain where and when water has been present in volcanic regions. This information is valuable for identifying fossil hydrothermal systems and determining relationships between climate, near‐surface water abundance, and the potential development of habitable niches on Mars. We examined the western Tartarus Colles region (25–27°N, 170–171°E) in northeastern Elysium Planitia, Mars, and identified 167 VRC groups with a total area of ∼2000 km2. These VRCs preferentially occur where lava is ∼60 m thick. Crater size‐frequency relationships suggest the VRCs formed during the late to middle Amazonian. Modeling results suggest that at the time of VRC formation, near‐surface substrate was partially desiccated, but that the depth to the midlatitude ice table was ]42 m. This ground ice stability zone is consistent with climate models that predict intermediate obliquity (∼35°) between 75 and 250 Ma, with obliquity excursions descending to ∼25–32°. For lava thicknesses ranging from 30 to 60 m and ground ice fractions ranging from 0.1 to 0.3, an ice volume of ∼4–23 km3 couldhavebeenmelted and/or vaporized by the time the lava solidified, and the associated hydrothermal systems could have retained temperatures >273 K for up to ∼1300 years. Citation: Hamilton, C. W., S. A. Fagents, and L. Wilson (2010), Explosive lava‐water interactions in Elysium Planitia, Mars: Geologic and thermodynamic constraints on the formation of the Tartarus Colles cone groups, J. Geophys. Res., 115, E09006, doi:10.1029/2009JE003546. 1. Introduction Fagents, 2001; Lanagan et al., 2001; Fagents et al., 2002; Head and Wilson, 2002; Fagents and Thordarson, 2007; [2] Volcanic rootless constructs (VRCs) are generated by Hamilton et al., 2010a, 2010b]. Integrated geological map- lava‐water interactions through repeated cycles of frag- ping of VRCs, age‐dating of VRC‐hosting lava surfaces, and mentation and pyroclastic dispersal [Thorarinsson, 1951, thermodynamic modeling of lava‐substrate interactions can 1953]. Terrestrial VRCs typically range from 1 to 35 m in establish where and when near surface groundwater (or ice) height, 2–450 m in basal diameter, and occur in groups with has been present in volcanic regions. This information is total areas of up to ∼150 km2 [Fagents and Thordarson, valuable for constraining models of global climate change on 2007]. VRC analogs on Mars are typically larger than ^ Mars [Head et al., 2003; Laskar et al.,2004;Head et al., those on Earth, with heights of 25 m, basal diameters of 2009] and for understanding the relationships between cli- 30–1000 m [Fagents et al., 2002], and cone group areas of mate, volcanism, volatile stability, and hydrothermal systems up to ∼1300 km2 [Hamilton and Fagents, 2009]. VRCs are [Carr, 1996; Head et al., 2009]. significant because they imply the presence of active lava [3] The Tartarus Colles are located in eastern Elysium flows and an underlying volatile phase (e.g., groundwater or Planitia, ∼750 km northeast of Grjótá Vallis. Lanagan et al. ice) at the time of their formation [Thorarinsson, 1951, 1953; [2001], Greeley and Fagents [2001], Fagents et al. [2002], Frey et al., 1979; Frey and Jarosewich, 1982; Greeley and Bruno et al. [2004, 2006], Baloga et al. [2007], and Hamilton et al. [2010b] used morphological and geospatial evidence to demonstrate that this region contains landforms that are 1Hawaii Institute of Geophysics and Planetology, University of Hawaii, analogous to VRCs in Iceland and, therefore, the Martian Honolulu, Hawaii, USA. cone groups provide evidence of near surface groundwater 2Department of Earth and Planetary Sciences, Lancaster University, Lancaster, UK. (or ice). [4] In this study, we map the western Tartarus Colles Copyright 2010 by the American Geophysical Union. region and present a thermodynamic model of lava‐substrate 0148‐0227/10/2009JE003546 E09006 1of24 E09006 HAMILTON ET AL.: LAVA‐WATER INTERACTIONS ON MARS E09006 interactions to: (1) better understand the geological history [8] Fuller and Head [2002] proposed that ascending dikes of Elysium Planitia, (2) constrain the depth to the ground ice in Elysium Planitia may have cracked the cryosphere and table when the VRC groups formed, (3) estimate the volume released groundwater to form floods that were overlain by of ground ice that could have been vaporized and/or melted lava once the dikes reached the surface. In this model, at the time of the cone‐forming eruptions, (4) infer the rootless eruptions initiated when lava flows interacted with obliquity‐driven climate conditions required for ground ice surface water and/or groundwater that had infiltrated into the stability during the emplacement of the Tartarus Colles lava substrate during the antecedent aqueous floods. In other flow, and (5) explore the potential for lava to generate models, dikes were not the cause of aqueous floods and lava hydrothermal systems that could have provided habitable flows, but rather groundwater and magma traveled toward niches for life on Mars. the surface through zones of weakness in the cryosphere that were generated by regional tectonic stresses [Burr et al., 2002; Berman and Hartmann, 2002; Plescia, 2003]. 2. Geologic Setting [9] The youngest of the landforms on Mars to have been [5] The western Tartarus Colles cone groups are located interpreted as VRCs are located in eastern Cerberus Palus, on Mars between approximately 25–27° North and 170– near the mouth of Marte Vallis [Lanagan et al., 2001]. In 171° East (Figure 1). This region is situated between this region, lava surface ages are <1–10 Ma [Hartmann and northeastern Elysium Planitia and southern Arcadia Planitia, Berman, 2000; Berman and Hartmann, 2002]. Strati- and includes the Nepenthes Mensae and Elysium rise units graphically, the Tartarus Colles cone groups are older than [Tanaka et al., 2005] as well as a younger VRC‐hosting the VRCs in Marte Vallis, but if the VRCs in the Tartarus volcanic unit. Colles region volatilized ground ice associated with a [6] The Nepenthes Mensae unit includes knobs and mesas northeast component of an aqueous flood from Grjótá with intervening slope‐ and plains‐forming materials that Vallis, then the Tartarus Colles cone groups may be on the outcrop in the Tartarus Colles and were emplaced during the order of several tens of millions of years old. If rootless early Hesperian to late Noachian [Tanaka et al., 2005]. The eruptions occurred in the Tartarus Colles region ∼10–40 Ma Tartarus Colles are surrounded by the Elysium rise unit ago then ice and fossil hydrothermal systems may be pre- (Figure 1b), which consists of lava flows that were erupted served to this day. These structures could potentially pro- from Elysium Mons, Hecates and Albor Tholi, and local vide information about the development of habitable niches sources during the early Amazonian to late Hesperian on Mars. However, if the volatile source for the rootless [Tanaka et al., 2005]. eruptions in the Tartarus Colles region was part of the global [7] The Cerberus Fossae are located in Elysium Planitia cryosphere, then the timing of these rootless eruptions and are associated with Amazonian age lava flows inter- would not depend on the age of floodwater from Grjótá spersed with aqueous flood deposits, tectonic features (e.g., Vallis. In this case, the Tartarus Colles cone groups could be fissures), and mantling deposits [Keszthelyi et al., 2004]. significantly older than the Cerberus Fossae 2 unit, which Lava flows and aqueous floods associated with the Cerberus would reduce the likelihood that fossil ice and hydrothermal Palus originated from the Cerberus Fossae fissure system structures would be preserved. The geological evidence in [Burr et al., 2002; Fuller and Head, 2002; Plescia, 2003; the Tartarus Colles region is examined to distinguish Head et al., 2003]. Deposits related to Cerberus Fossae between these two hypotheses. divide into three units. The oldest of these units is the early Amazonian age Cerberus Fossae 1 unit [Tanaka et al., 3. Methodology 2005], located in southern Elysium Planitia. This unit is overlain to the north by the late to middle Amazonian age 3.1. Integrating Geologic and Thermodynamic Cerberus Fossae 2 unit. Source regions for the Cerberus Constraints Fossae 2 unit are primarily located in Grjótá Vallis and, [10] Constraints on the formation of the Tartarus Colles in this region, the youngest aqueous flood deposits are cone groups are established using a threefold methodology 10–40 Ma old [Burr et al., 2002]. Mapping of scoured involving: (1) geologic mapping, (2) surface age estimations channels, streamlined forms, and longitudinal lineations based on crater size‐frequency distributions, and (3) ther- suggests that floodwater was released from the northernmost modynamic modeling of lava‐substrate heat transfer. Geo- fissures of Cerberus Fossae to form north and south branches logic mapping is used to identify geologic units, measure (Figure 1b), which flowed southeast toward Cerberus Pla- their areas, and constrain their stratigraphic relationships. nitia [Burr et al., 2002; Plescia, 2003; Burr and Parker, This information is important for calculating VRC group 2006]. Burr and Parker [2006] also noted small stream- areas, estimating lava thicknesses, and identifying the lined forms that suggest a portion of the floodwater may have properties of the substrate beneath the VRC‐hosting Tartarus traveled northeast (Figure 1), toward the Tartarus Colles.