A new image from the ESA’s Mars Express Orbiter shows exactly how different regions in Mars are from one another. From the cloudy northern polar region all the way to the Helles Planitia down in the south, Mars is a puzzle of different terrain types. At the heart of it all is what’s known as the Martian dichotomy.

Mars from pole to pole as imaged by the Mars Express orbiter. Image Credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

Mars’ northern pole region is obscured by clouds, with tendrils reaching south into the northern hemisphere. The clouds block the polar region from view, but we know that it’s always covered in ice, thicker in the winter and thinner in the summer. We also know that it’s sunken in elevation. The northern hemisphere is 2 km lower than the southern hemisphere. In Mars’ ancient past, the northern hemisphere may have been covered in water.

The northern hemisphere is characterized by low-lying plains without many impact basins. This leads scientists to think that it’s much younger. It certainly appears younger next to the southern hemisphere, which is heavily marked with all sizes of craters.

The plain geography that dominates the northern hemisphere is called the Vastitas Borealis, and it wraps all the way around the planet. The topographic map of Mars puts it into focus.

<Click to Enlarge> A topographic mercator projection map of Mars from MOLA (Mars Orbiter Laser Altimeter) data. Blue is low elevation, red is high elevation. Mars’ northern hemisphere is about 2 km lower than the southern hemisphere. Image Credit: By NASA / JPL / USGS – http://mola.gsfc.nasa.gov/images.html and http://photojournal.jpl.nasa.gov/catalog/PIA02993, Public Domain, https://commons.wikimedia.org/w/index.php?curid=32873138

A rugged band of terrain separates Mars’ northern hemisphere from its southern hemisphere. The band features flat-topped mesas, cliffs, fractures, canyons, and valleys. Scientists think many of these features were carved by water and ice, while some were likely carved by lava flows from Mars’ massive volcanoes.

One representative area from that band of terrain is called Cydonia Mensae, a much-studied region. It contains the infamous “Face on Mars” that looks roughly like a human face.

Part of the Cydonia Mensae region on Mars, in the transition region between the heavily cratered southern highlands and the smooth northern lowlands. This image is from the High Resolution Stereo Camera on the ESA’s Mars Express orbiter. Image Credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

The northern plains just below the polar cap are dark grey and dusty. Together with a prominent escarpment, they create a dark swathe on the planet’s mostly tan-colored surface. Travelling south, next comes the vast orange highlands, pock-marked with all sizes of craters. Scientists think that the southern highlands are ancient and that some of these craters date back to the planet’s very early history.

The part of the southern highlands visible in the main image are Arabia Terra (towards the upper left) and Terra Sabaea (to the middle and lower right. At the bottom and almost out of sight is the massive Hellas Planitia, a large plain inside the Hellas impact basin.

A topographic slice of Mars with labels. Image Credit: NASA/MGS/MOLA Science Team, FU Berlin

The Martian dichotomy, the split between the northern terrain and the southern terrain is one Mars’ largest mysteries. Why are the two regions so different? What caused the discrepancy?

Mars might have had tectonic plates similar to Earth’s in its past. Maybe some geological processes in the planet’s mantle are responsible. Perhaps some unimaginably colossal impact in the past caused it. Maybe a combination of factors.

The single-impact hypothesis has gained ground in recent years, as we get better and better looks at Mars. One mark against it is that an impact of the necessary magnitude would’ve blanketed large parts of Mars in ejecta. But there’s no evidence of that on the surface. On the other hand, if it happened long enough ago, perhaps as long ago as 4.5 billion years, erosion could’ve removed evidence of that blanket.

The Martian dichotomy could have been created by some type of plate tectonics that moved massive amounts of material around on the planet. Since our understanding of that process on Earth is still very limited, it makes it difficult to conjecture about the same processes on Mars. But it is possible that cells or plumes of material from the planet’s mantle rose up to the surface in one hemisphere, while material subsided in the other. But since there’s no observed evidence of plate tectonics on Mars, it makes it unlikely.

This image shows a slice of the Red Planet from the northern polar cap downwards, and highlights cratered, pockmarked swathes of the Terra Sabaea and Arabia Terra regions. Image Credit: NASA/Viking, FU Berlin

There’s even a hypothesis that multiple large impacts created the dichotomy. But the problem with all of these proposed explanations for the dichotomy is that we just don’t have enough evidence. Not to confirm one, nor to really rule on out. Maybe NASA’s InSight Lander will shed some light.

Currently, there are six orbiters at Mars, and on the surface there’s the Curiosity rover and the InSight lander. And there are several other missions being planned.

Over time, our scientific knowledge of Mars will continue to grow and eventually, maybe in the extremely distant future, we’ll solve the Martian dichotomy.

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