19 July 2010

New Evidence for an Ocean on Mars?

Posted by Ryan Anderson

There’s a new Nature Geoscience paper that has made a big splash in the Mars community, reviving interest in the possibility of a northern ocean. This news was making the rounds a couple weeks ago, but I decided to hold off because at last week’s Mars Journal Club we discussed the paper.

The idea behind the paper is deceptively simple. The authors, Gaetano DiAchille and Brian Hynek, searched all over Mars for landforms that looked like deltas: fan-shaped features that form when flowing water encounters standing water and drops its load of sediment. By finding the elevation of the deltas, they could see if they all formed in the same big body of water.

A topographic map of Mars (red is high, blue is low). The white line marks the location of the putative shoreline. The red squares mark deltas that empty into the northern lowlands. Green triangles and blue diamonds mark deltas that are in smaller, closed basins. This image is Figure 2a in the paper.

Many of the delta-like features on Mars lead into closed basins, such as craters. These basins might have formed isolated lakes, and so their deltas were not counted when trying to infer the shoreline of the ancient ocean. On the other hand, DiAchille and Hynek found 17 deltas the did not empty into closed basins. These ones instead formed at the edge of the northern lowlands. Amazingly, when the authors measured their elevation, these 17 deltas -which were spread all around the planet – turned out to have the same elevation to within a few hundred meters! This suggests quite strongly that they all formed when flowing water encountered a large northern ocean. The inferred shoreline of the ocean matches pretty well with some stretches of the highland-lowland boundary that had previously been suggested as possible shorelines. The possible shore also is consistent with where the fluvial channels on Mars are observed. On a map of channels in ancient martian terrain, only 1% of the channels are at elevations below the elevation of the shoreline!

It’s also worth noting that some of the deltas that are in closed bases fall within the error bars on the possible shoreline. That might suggest that there was a global groundwater system, so that lakes near the ocean had the same water level.

There’s no doubt about it, this is a cool paper. I did have some questions about their methods. In particular, how do they know the “deltas” are actually deltas and not alluvial fans? Especially if these features formed three billion years ago, erosion could have significantly changed their shapes. It would be pretty easy to erode the edges of an alluvial fan to make something that looks more like a delta. Considering that the very best example of a delta on Mars – the Ebeswalde Delta – has been the subject of intense scrutiny and debate about whether it actually is a delta, I was surprised to see how nonchalantly the authors claimed that the fan-shaped features were deltas.

Still, this paper goes a long way toward resurrecting the idea of a northern ocean on Mars. It will be interesting to see how the community reacts to this in the coming years.  An ocean on Mars would be a huge discovery, but as the saying goes: “extraordinary claims require extraordinary evidence.” The extremely narrow error bars on the delta elevations are pretty extraordinary, but I’m sure many people will remain unconvinced.

Di Achille, G., & Hynek, B. (2010). Ancient ocean on Mars supported by global distribution of deltas and valleys Nature Geoscience, 3 (7), 459-463 DOI: 10.1038/ngeo891