ENTABLATURE AND COLUMNAR JOINTING ON MARS

Although liquid water (and potential life) is the focus of NASA's Mars Exploration Program, the martian crust is dominated by volcanic rocks. Therefore, one of the most promising ways to locate past, near-surface water is to look for lava-water interaction features. Examples of such interactions in the form of entablature and columnar jointing have been discovered in the tilted and uplifted walls of impact craters in various locations on Mars, the first of which was discovered in Marte Vallis, a site of extensive past volcanic and water flooding. Images from the HiRISE camera in orbit at Mars reveal exposures of water-cooled, multi-tiered entablature and columnarly jointed lavas similar to those seen in the Columbia River Basalt Group.

Since water-cooled entablature is significantly more glassy than subaerially cooled lavas, an order-of-magnitude estimate of the amount of water necessary to form the entablature is to assume all latent heat of solidification of the lava was removed via phase-change from liquid water to water vapor. That is, the volume of water necessary to be vaporized to extract enough heat to completely solidify the lava, Vw, is given by: V_w = (V_l r_l L_l) / (r_w L_w), where V_l is the volume of lava, r_l and L_l are the density and latent heat of solidification of lava respectively, and r_w and L_w are density and latent heat of vaporization for water. The Marte Vallis exposure probably covered at least 200 km², based on the surface area of the crater that obliterated and exposed the entablature, and the columns were measured to have a height of 40 m. Given the estimate of 8x10⁹ m³ of lava and the thermo-physical properties of the lava and water, the water volume required for cooling is 4x10⁹ m³ or the equivalent of a 21 m deep layer. The multiple tiers of entablature indicate that this water was not introduced as a single standing body of water, but entered the lava in distinct pulses. While there are many problems with this estimate, some of which will be discussed in detail at the meeting, this seems reasonable as a first-order approximation and agrees to within a factor of two or so with recent modeling of the formation of terrestrial entablatures.