Workshop on Mars 2001 2541.pdf OCEANS ON MARS. J. W. Head, Department of Geological Sciences, Brown University, Providence, RI 02912 USA ([email protected]). Introduction: Understanding water, and its state, mapped contacts are ancient shorelines, then they distribution and history on Mars, is one of the most should also represent the margins of an equipotential fundamental goals of the Mars exploration program. surface, and if no vertical movement has occurred Linked to this goal are the questions of the formation subsequent to their formation, the elevation of each and evolution of the atmosphere, the nature of crustal contact should plot as straight lines. Preliminary accretion and destruction, the history of the cryos- analysis of the first 18 orbits showed that neither phere and the polar regions, the origin and evolution Contact plotted as a straight line, but that Contact 2 of valley networks and outflow channels, the nature of was a closer approximation than Contact 1 [5]. We the water cycle, links to SNC meteorites, and issues have now plotted data from Hiatus phase; SPO1, and associated with water and the possible presence of life SPO2, and later orbits, and produced a topographic in the history of Mars. One of the most interesting map of the northern hemisphere. Contact 1 as pres- aspects of recent discussions about water on Mars is ently observed is not a good approximation of an the question of the possible presence of large standing equipotential surface; variation in elevation ranges bodies of water on Mars in its past history. Here we over several km, an amount exceeding plausible val- outline information on recent investigatios into this ues of post-formation vertical movement. Contact 2 is question, and address the ways in which various types a much closer approximation to a straight line, and of present and future Mars missions can contribute to the most significant variations occur in areas where the debate, and gather data to test hypotheses. post-formation vertical movement is anticipated (e.g., Background: Abundant evidence exists for the Tharsis, Elysium, and Isidis). Derivation of the topo- presence of water on the surface and in the subsurface graphic map permits us to test for volumes of water in the past history of Mars [1]. Among the most dis- that might be contained in topographic basins of vari- tinctive pieces of evidence are the outflow channels ous scales. Assuming that the present topography is a that begin full-size at discrete sources and flow hun- reasonable approximation of the topography in Hes- dreds to thousands of km downslope into the northern perian and Amazonian time, we have measured the lowlands displaying a wide variety of bedforms on volume of the topography below Contact 2 and find their floors. An unusual characteristic of outflow that it is about 1.4 x 107 km3, a value lying between channels is that channel cutting does not continue far the minimum for all outflow channels (~0.6-0.8 x 107 into the northern lowlands even though downslope km3, [1,2]) and the maximum value for water- topographic gradients appear to continue. Where did containing megaregolith pore space (~5-20 x 107 km3) the water go? Did it spread out over the broad smooth [6]. This volume of the area below Contact 2 is lowlands and sink into the substrate, or could it have equivalent to a global layer about 100 m deep, and is ponded, creating lakes, seas or oceans? Some inves- within the range of estimates for available water [1]. tigators have hypothesized that outflow channels had The northern hemisphere topographic map also enough volume and occurred with sufficient simulta- permits us to assess what would happen if the low- neity and repetitiveness to produce large standing lands were flooded concurrently or if individual chan- bodies of water in the northern lowlands (Oceanus nels emptied into the lowlands at different times. We Borealis) at several times in the history of Mars [2]. sequentially flooded the northern lowlands in 500 m Specifically, Parker et al. [3-4] mapped two contacts increments and observed where the water would pond near and generally parallel to the highland boundary and how candidate seas and oceans might evolve with of the northern lowlands and interpreted these con- increased depth. The sequence of maps show that tacts to be shorelines, representing two separate high- there are two distinctive basins in the northern low- stands of a north polar ocean. Contact 1 is older and lands, the Utopia Basin and the North Polar Basin. corresponds approximately to the highland-lowland Individual channel-forming events may have flooded dichotomy boundary. Contact 2 is younger, lies only one of these basins, and volumes of the order of northward of Contact 1, and is more well-expressed 1-3 x 106 km3 are required to fill one of the basins to by a sharply defined smooth, lobate, or arcuate con- spill over into the adjacent one. Detailed simulations tact and associated features interpreted to be related to of flooding events from individual channels are un- shorelines and basinward deposition and evoution. derway [7]. Results: The new MOLA data permit us to test Several other geologic features are thought to have theses hypotheses in several ways. First, if the been associated with the presence of bodies of water Workshop on Mars 2001 2541.pdf OCEANS ON MARS: J. W. Head or residual ground ice remaining from them, and the [5], and the implied ocean volume is within the range new topographic data can be used to assess their lo- of estimates of available water on Mars. In addition, cations. Lucchitta et al. [8] examined the locations of detailed topographic maps of the northern lowlands a variety of features in the northern lowlands using reveal two major basins (Utopia and North Polar); Viking image data in an attempt to identify the loca- features thought to be related to the evolution of tion and characteristics of sedimentary deposits that standing bodies of water (polygons, lobate impact might have resulted from the debouchment of the craters) show a high degree of correlation with basin large outflow channels into the adjacent plains. They topography. New slope maps reveal evidence for brought strong support to the sedimentary layer hy- subtle terraces that may be related to regression of pothesis by pointing out that the polygonal ground such a standing body of water. These new data are occurred in close proximity to major channel systems, consistent with, but do not prove, the hypothesis that that the outflow channels and the fractured plains de- the northern lowlands of Mars was occupied by posits have similar ages, that Antarctic analogs re- standing bodies of water ranging in scale from seas to vealed many similarties to this process, and that po- perhaps as large as oceans in earlier Mars history. lygonal ground occurred elsewhere on Mars in similar In addition to the possible presence of large stand- situations. We digitized the global map of the poly- ing bodies of water in the northern lowlands in the gonally fractured terrain on Mars of Lucchitta et al. past history of Mars, other workers have identified and superposed it on our MOLA topography map; we numerous regions elsewhere on Mars where evidence found that there is a strong correlation between the exists for former standing bodies of water at the lake location of the polygonal ground and the position of and sea scale [e.g., 11-16, and see discussion in 1]. the Utopia and North Polar basins. Furthermore, consideration of the hydrosphere and Martian impact craters in the 2-50 km diameter cryosphere [17] in the past history of Mars has led to range commonly have ejecta deposits with distinctive the proposal that large standing bodies of water in the lobe and rampart morphology, interpreted [9] to be Noachian were an inevitable consequence of the pres- due to the presence of ground water or ground ice in ence of outflow channels later in history [18]. All of the target area which mobilizes the ejecta material. It these observations and hypotheses show that explora- is also observed that craters on Mars smaller than a tion plans should be testing various aspects of these few km do not have ramparts, and thus the onset di- questions at all scales and should be complementary ameter of ramparts may be an indication of the depth in their approach [e.g., 19, 20]. where ground water or ground ice is encountered. On General exploration goals and objectives: On the basis of this concept, Kuzmin et al. [10] assessed the basis of the observations and proposed hypothesis, the onset diameter globally and found that in equato- what are the types of questions that might be ad- rial regions the diameter was 4-6 km but toward the dressed and measurements that can be made? pole it was 1-4 km. We have digitized the Kuzmin et 1) What is the origin of smooth plains deposits in al. global onset-diameter map and superposed it on craters and intercrater areas? How can one distin- our MOLA topographic map; we find that there is a guish among volcanic, eolian, fluvial and aqueous strong correlation between the smallest onset diame- deposits? What are the criteria for orbital remote ters and the position of the two large basins. sensing and lander/rover exploration? If there was a standing body of water earlier in the 2) What types of evaporites are predicted for Mars history of Mars, it is not there now. In order to ex- and in what configurations might they be found?: amine the fate of a possible ocean as it regressed, we What are the starting conditions, how do such depos- produced slope maps for the interior of the northern its evolve, can they be recognized after eolian modifi- lowlands.