The debate over the history of water on Mars first began even before the first spacecraft flew by the planet. But after Mariner 4 scientists learned that Mars was currently devoid of liquid water. But more images returned by Mariner 9 revived the debate by revealing features resembling features on Earth formed by flowing water. Mars may not have liquid water now, but it seems clear that Mars had it in the distant pass.
But even though it is widely agreed now that Mars had water sometime in its past, there is still intense debate about how much water and how long it was liquid.
Two recent papers submitted by two different teams illustrate this debate. Lorena Moscardelli, Time Dooley, Dallas Dunlap, Martin Jackson, and Lesli Wood published a paper in GSA Today arguing that large polygon-like structures in the northern parts of Mars implies that a liquid water ocean once occupied that area. However, another group, R. Wordsworth, F. Forget, E. Millour, J. Head, J.-B. Madeleine, and B. Charney are publishing another paper in the journal Icarus suggesting that Mars could never have had significant amounts of standing water for long periods.
The Wordsworth paper describes the group’s method of creating a 3D General Circulation Model (GCM) of the early Martian atmosphere and found that under no reasonable set of circumstances could Mars have supported a large amount of standing water.
The model assumed a much thicker carbon dioxide atmosphere, a faint young Sun (the early Sun was much fainter than it is today); it took into account dynamical cloud formation by both CO2 and H2O; and it used a self-consistent model of a water cycle.
Wordsworth and his group also studied the effects of various conditions that might have influenced the early Mars climate, including atmospheric pressure on the surface, Mars’ orbital obliquity, the topology of the surface, and the amount of water the planet started with.
The group acknowledges that there is strong evidence of the presence of liquid water on the surface of Mars. They note in particular the extensive valley networks, fossilized river deltas, large amount of phyllosilicate (clay) and sulfate minerals on the surface. They even acknowledge that some scientists suggest evidence that the northern portion of Mars once supported an ocean, but they point out that this hypothesis is somewhat controversial.
Their conclusion is that no amount of CO2 in the atmosphere could have allowed the surface of Mars to remain warm enough to support liquid water for any length of time. There might have been some localized melting of water ice on hot summer days, or due to meteoric impacts or volcanism.
They consider and dismiss the possibilities that some other factor may have warmed Mars beyond what is possible for a CO2 atmosphere alone.
First, they consider the possibility of volcanically emitted sulfur dioxide, noting that the Tharsis volcanoes were at their most active phase during the Noachian era, which is what the paper considers. However, they argue that any sulfur dioxide-enhanced warming would only be effective for a few months at a time because the sulphate aerosols also emitted by the volcanoes would have induced an anti-greenhouse effect.
Second, they consider the presence of CO2 clouds that might have reflected infrared radiation back toward the surface, preventing the heat from escaping into space. But their simulations show that even if such clouds covered the entire planet, they could not have raised the temperature above the melting point of water.
They also examine a hypothesis put forth that impacts by large meteors (from 30-250 km in diameter) might have created an atmosphere of steam due to the energy of their impact. This would have created intense rains as the steam, which could have persisted for millennia condensed and returned to liquid form. However, the authors consider this inconsistent with o another written recently, Global modeling of the early Martian climate under a denser CO2 atmosphere: Water cycle and ice evolution by R. Wordsworth et al. the geomorphological evidence, in particular the lack of crater rim breaches in the Parana Valles region, which implies that there was no catastrophic flooding as would be required by that model.
Overall, their conclusion is that during the Noachian era, Mars was a frozen world, not a place where liquid water flowed freely, and certainly not a suitable home for a vast ocean.
But not all scientists agree on this point. Another group, led by Lorena Moscardell of the Bureau of Economic Geology at the University of Texas at Austin, has come to a far different conclusion.
Polygonal structures have been identified on the Martian surface as early as the Mariner flyby and their origin has been under debate ever since. The smaller polygons, those less than about 1 km in diameter, are generally believed to have formed due to thermal contraction, as similar features form on Earth in the permafrost environment.
But the larger polygons, mostly confined to the northernmost reaches of Mars, the great Vastitas Borealis plain and neighboring areas, are more mysterious. The paper includes an annotated graphic which could not be obtained as of this writing, but is available at the GSA Today website.
The Moscardelli team identified structures on Earth that closely resemble the great polygons of Mars, and they only form underwater.
Seismic surveys of the ocean floor have allowed a better examination of that environment. Such surveys, in the North Sea, for example, have identified kilometer-scale polygonal structures on the seabed. Surveys have identified a total of 20 examples of polygonal terrain.
The authors do not attempt to explain the cause behind such structures, as this is still a matter of debate. But they trace the geomorphological similarities between deep-water Earth polygons and the polygons of Mars.
The authors explain the criteria they identified to polygons to form on Earth: 1. Sequences of fine-grained sediment. 2. A water depth of at least 500 m. 3. Shallow burial depths.
They assert that the Martian polygons must have formed in a similar manner to those on Earth’s sea floor. They make their connection based on four pieces of evidence: 1. The geomorphological similarities between the polygons of Earth and Mars. 2. The geomorphological evidence of an ocean on the northern plains of Mars. 3. The need for large volumes of water to have formed certain outflow channels. 4. Similarities between Earth and Martian polygons to those created in a physical model.
These two papers are a microcosm of the debate about the geological history of Mars. Scientists continue to gather data and examine it in the hopes of coming to firm conclusions about the history of Mars and the history of water on Mars in particular.
The Curiosity rover, having just recently touched down safely at Gale Crater, is one of the tools scientists hope to use to discover more about Mars and any water that may have been there. The site that was selected for investigation by Curiosity was chosen partly because of NASA’s policy for picking sites for exploration: follow the water.
Curiosity is just one rover, and no one expects it to solve all of Mars’ mysteries, but scientists hope it will make headway, and prepare for the human exploration of Mars.