NASA Image MOLA Digital Altimetry Martian Oceans Evidence for a Northern Ocean on Mars

Overview Where could the water have come from? (Origin of water on Mars) Could the Martian climate have been favorable for a liquid ocean? (Climate conditions and obliquity simulations) Is there evidence that an ocean formed? (MOLA and MOC images)

What is the importance of possible oceans on Mars? Life on Earth formed in the ocean. If Mars had an ocean, this would be the best place to look for life on Mars. If Mars did have an ocean, then Earth is non-unique. It is possible there are other planets in the universe may have them as well which means extra-terrestrial life in the universe is possible.

Origin of Water on Mars Lunine et al. [1] discuss: It was too hot for water to form at distances of ~1 AU. The water must have been acquired from material that formed at larger distances from the sun. Earth was formed (and acquired its water) from planetary embryos which grew in the asteroid belt. Lunine et al. ran simulations where terrestrial planets are formed from Mercury-to-Mars-mass planetary embryos ranging in position from.5- 4 AU. Simulations in gas-free environment form massive planets. Inclusion of gas forms a number of very small planets. This doesn’t indicate failure of the model, but the stochastic (random) nature of terrestrial planetary formation.

Origin of Water on Mars (2) Lunine et al. conclude that: Mars is an embryo that escaped ejection by Jupiter or accretion of growing terrestrial planets. Mars did not acquire its water from collisions with planet-sized embryos like Earth. Mars collided with populations of comets and small asteroids and retained most of the water acquired from these collisions. Collisional history of water-laden asteroids with Mars expressed as cumulative fraction of “C-type” asteroids accreted vs. time (Ma) Probability of cometary collisions with Mars as a function of their initial semi-major axes

Climate Conditions Abe, Y. and Abe-Ouchi, A. [2] discuss: There are three climate regimes on a land planet, they depend on the obliquity and average surface temperature. The frozen regime is completely frozen and there is essentially no transport of water occurring, with a very low surface temperature due to high albedo. The upright regime occurs when the obliquity of the planet is smaller than the width of the Hadley cell and the summer temperature exceeds freezing temperature. The low-latitude area is always warmer than mid to high latitude area. The oblique regime occurs when the obliquity of the planet is greater than the width of the Hadley cell and the summer temperature is above freezing point. The mid to high latitude area is always warmer than the low-latitude area.

Climate Conditions (2) Mars is believed to have experienced a large change in obliquity, as much as 60°. Abe, Y. and Abe-Ouchi, A. [2] ran simulations for land and aqua planets with obliquities of 0° and 23.5° (upright regime) and 45° and 60° (oblique regime). They conclude that a land planet has a stronger resistance to complete freezing than an aqua planet. Both land and aqua planets in the oblique regime show stronger resistance to complete freezing than an upright planet. Also conclude that on a land planet in an oblique regime, low latitude area is more susceptible to freezing than mid-latitude area.

Evidence of an Ocean Parker et al. [4] discuss: Standing water forms an equipotential surface that intersects topography at fixed elevation around the margin of a depression. Abandoned shorelines are seldom level, though they often approximate a planar surface that has been tilted, faulted, or warped due to structural changes, isostatic rebound, or loading. Head et al. [3] point out that: Large outflow channels empty into the northern lowlands. Data from the Mars Orbiter Laser Altimeter (MOLA) instrument shows the unusual smoothness and and flatness of the northern lowlands.

Evidence of an Ocean (2) Parker mapped two contacts that are generally parallel to the southern boundary of the northern lowlands, which are interpreted to be ancient shorelines.

Evidence of an Ocean (3) Contact 2 is a better approximation to a straight line. The elevation range is ~4.7 km, with a mean value of km and a standard deviation of km. The most substantial variations occur in Elysium and Arabia where post- contact 2 activity has occurred, and near Tharsis, where uplift could have occurred.

Evidence of an Ocean (4) Parker and Banerdt [6] discuss the Mars Orbiter Camera (MOC) image at left as a pair of terraces winding around the inside rim and knobs with a large, degraded crater in northern Arabia Terra at the lowland/upland boundary. This is just one example of a shoreline they found. They conclude that Martian features exhibit a wide range of preservation states, suggesting geologic timescales. They also conclude that the shorelines suggest the involvement of water, and little/no evidence of fluvial or glacial scour.

Evidence of an Ocean (5) Head et al. [3] use the northern hemisphere topographic map to assess what would happen if i)individual channels emptied into the lowlands at different times and proceeded to fill them, ii)they were filled by a different mechanism (for the case of an ancient ocean that is older than outflow channels), and iii)if such an ocean were to recede. They flood the northern lowlands and observed where water would pond, and how the oceans might evolve with changing depth.

Flood depth of 500 m Flood depth of 1000 m Flood depth of 1490m (contact 1) Flood depth of1680 m, (mean depth of contact 1, level of contact 2 shown underneath)

Bimodal Distribution Similarity Smith et al. Science 1999 Image: Frank Kyte

Conclusions Water was brought to Mars by cometary and small asteroid impacts and was able to retain most of the water. It is possible that the climate could have supported liquid water at mid latitudes. Contact 2 forms an equipotential that could represent ancient shorelines. Mars probably had an ocean some time in its past!

References [1] Lunine, J. et. al., The origin of water on Mars, Icarus, 165(1):1-8, [2] Abe, Y. and Abe-Ouchi, A. (2003) 34th Annual Lunar and Planetary Science Conference, Abstract #1617. [3] Head, J. et al., Possible ancient oceans on Mars: Evidence from Mars Orbiter Laser Altimeter Data, Science, 286: , [4] Parker, T.J. et al., (2001) 32th Annual Lunar and Planetary Science Conference, Abstract #2051. [5] Parker, T.J. et al., (2002) 33th Annual Lunar and Planetary Science Conference, Abstract #2027. [6] Parker, T.J. and Banerdt W.B., (1999), International Conference on Mars 5, Abstract #6114.