Martian Ocean
Martian ocean: why is it interesting and controversial?
Observations have long shown that Mars had significant quantities of liquid surface water early in its history. However, whether or not it had a global-scale ocean remained controversial. In the 90s, ocean-scale shoreline features have been observed on the surface of the planet, but the great differences in elevations along them led some to dismiss them as ancient shorelines.
Invoking TPW, Jerry Mitrovica (my doctoral advisor) and colleagues demonstrated that the elevation differences are consistent with those of a deformed rotational bulge.
A few years later, di Achille and colleagues showed that the elevations of deltas already formed a consistent sea level without requiring TPW. Meanwhile, scientists studying the Martian atmosphere and climate argued that Mars never had enough water to host a global-scale ocean.
An illustration of Martian topography under the true polar wander scenario we found. The orange contours show the reconstructed shoreline of an ancient Martian ocean in this scenario. The colour gradient shows higher elevations in red and lower elevations in blue. The Tharsis volcanic province is visible on the left half of the globe, the Valles Marineris is in the middle-lower part, and the western side of Arabian Terra is on the right-hand side edge.
My contributions and findings:
In light of the debate of whether TPW contributed to deforming sea-level markers and whether there could even be sea-level markers, I looked at another set of data—the fluvial valley networks on Mars.
Using the elevation of the valley-network sink points (i.e., exit points of the water) along the limits of the northern lowlands (where the ancient ocean is widely believed to have been), and applying TPW theory, I showed that the location and elevations of the valley networks are consistent with an ancient ocean shoreline being deformed by TPW. I also determined the most likely location of the ancient rotation pole in this scenario.
This results independently agree well with the shoreline results of Mitrovica et al., reinforcing the TPW theory and the existence of an ancient ocean.
Interesting news…
The sea level inferred by the locations and elevations of the valley networks mean that roughly 120 million cubic kilometres of water is required to fill that ocean.
At the time of publication, estimates of present-day near-surface water on Mars amount to only 5 million cubic kilometres [Carr & Head, GRL, 2015]—1/24th of what is needed.
However, in 2021, a new study was published in the journal Science showing that previous investigations have underestimated the amount of water on Mars [Scheller et al., Science, 2021]. The new estimate ranges between roughly 15 to 220 million cubic kilometres of water, bringing our required water volume well within range!
Click here to read our published article.
The article on this page, all related graphics and picture (unless otherwise specified) © Ngai Ham Chan.
End of page image: Gale crater on Mars. Image credit: NASA/JPL-Caltech.
