The Geological History of Mars

Mars’ past is a fascinating one. I would say the largest reason for me saying that is because it’s so different to what most people would expect. Although saying that, if someone asked, “what was Earth’s geological history like?”, the majority of people would probably not have a clue. Therefore, when I say Mars’ past is different to what people “expect”, I mean that if someone was to give a slightly educated guess, I would be surprised if they were remotely close to the truth. But how do we analyse a planet’s history?

Analysing geological history 101

We can determine the history of Earth with relative ease (it’s certainly not trivial, but easy relative to Mars). This because we have physical access to it — we live on it! We can piece together the geological timeline by analysing rocks and minerals in laboratories (a well-known example for finding the age of organic material is known as radiocarbon dating). However, as you might guess, we can’t really do this on a planet that is, at its closest, 33.9 million miles away.

A sensible suggestion would be to use Martian meteorites that have landed on Earth, of which 120 have been discovered. Unfortunately, none of these meteorites hold samples of materials sufficient enough for a somewhat accurate analysis of the planet’s age as a whole.

Another suggestion is to send robots there to gather material samples. Well, that is a valid suggestion as we have attempted to do that. Notable examples being Sojourner, Spirit, Opportunity and Curiosity (the rovers). And all these rovers have successfully taken samples of the surface, there is the limiting factor that these are robots. Not only are they robots, but they are robots which had to be designed and constructed to a suitable degree for a 6-month journey in a spaceship across millions of miles. I’m trying to imply that, like with the meteorite samples, the “mini laboratories” we’ve sent on the robots have not been able to analyse the material samples sufficiently to measure the age of the red planet. Instead, there analyses have been for other research, such as detecting the signature of previous life. And even then, they produce an answer of “maybe” (not very binary!), as although there was some evidence, it was not sufficient for a definitive yes.

What is the moral of all these attempts? Without access to high quality samples and large, high tech labs, we cannot get absolute answers. So, our best option is to make predictions using what we can see with telescopes.

We do this by counting the number of visible craters on the surface. A higher number in a certain region (density) indicates an older terrain. We can then link this to the known impact history of the solar system to get some predictions. The southern highlands of Mars represent the oldest crust, having formed prior to 3.8 billion years ago. As we traverse up the planet, the crater density decreases — suggesting the northern plains are younger.

Then, we can associate these rough date estimates with other visual surface observations to build up a fairly good idea of how Mars’ geology has changed over time.

The eras

Great! Now we have a method to assess and make assumptions about Mars’ geological history… let’s actually use it.

Mars’ history has conveniently been divided into time frames or “eras”, of which there are three (well technically 4, but I’ll get onto that in a second). These periods are named after geographic regions on the red planet, where each region is estimated to have been formed in that time period. They are: Noachian, Hesperian, and Amazonian (that cheeky fourth one is the “Pre-Noachian”, which I’ll address first).

Pre-Noachian

The reason why I call this one “cheeky” is because sometimes it’s seen as a defined period, and sometimes it isn’t. This is simply due to the fact that so little is known about this time period. It was the first 400 million years of Mars’ existence (4.5–4.1 billion years ago), and thus was when its crust initially formed.

Like many planets when they are first “born”, they have very dense atmospheres. In Mars’ case, this was likely formed as a result of a high rate of asteroid impacts and the outgassing of the planet’s mantle. However, over the course of this time period, the atmosphere cooled which meant that much of the water vapour condensed and formed vast oceans on the surface. A blue mars? I bet you didn’t expect that one (albeit so hot that the oceans were the equivalent of pressure cookers). But eventually this very hot water did cool down, signalling the beginning of a period where Mars could have, in theory, supported life.

Since this point, the majority of the atmosphere has deteriorated, either escaping into space (due to its incredibly weak magnetic field, therefore not being able to retain it) or becoming incorporated with the surface as it cooled. So, what next?

Noachian

This is the first time period that we have some real, concrete evidence for.

The Noachian Period gets its name from an ancient highland region known as the “Noachis Terra”, which can be found between alongside the “Hellas” impact basin, one of the most significant regions in the southern hemisphere. I know, this is practically google Mars! Thinking about it, these are quite angry names for locations — I wonder if whoever came up with them was feeling a little on edge that day…

Huge basins such as Hellas, Isidis and Argyre were created during this period as a result of the continuous bombardment of asteroids and comets. These were of such a large magnitude, given the fact that they are visible on the planet today (4.1 billion years later).

Also during this time, volcanic activity was rife in the Tharsis region (as well as parts of the highlands). Consequently, some of the largest volcanoes in the solar system were born: Arsia Mons, Pavonis Mons, Ascraeus Mons, and my personal favourite with a 674 kilometre diameter and 25 kilometre height… Olympus Mons (the tallest planteray mountain in the solar system).

The eruptions of these volcanic mammoths played a major role in the early natural terraforming of Mars. As they poured ash and gases into the atmosphere, it thickened (much like our controversial CO2 emissions on Earth) considerably, trapping solar heat and global warming the red planet. Unlike in our case where global warming is “killing” the planet, in young Mars’ case, this is what gave it life (metaphorically… as far as we know).

Clouds likely developed, thus leading to precipitation, which in turn led to the development of lakes and more shallow oceans. This is when a large number of the valley networks and basins visible on the surface today were formed. And, seemingly contrary to what I began by saying, surface rovers have been able to perform some analysis that’s given us conclusive evidence which indicates that many rocks on the surface of Mars were subject to the effects of long, non-acidic water exposure, leading the formation of clay minerals.

Unfortunately, this was also the beginning of the end for Mars’ “earth-like” period. As the interior of the planet began to cool, its magnetic dynamo deteriorated, causing its magnetic field to disperse. As a result, most habitable environments began to shrink, and the chances of life drew thin. However, it must be remembered that these geological processes take places over hundreds of millions of years, and so there was a monumental amount of time where life may or may not have developed (especially with these Noachian surface conditions, where the emergence of life would seem favourable for a time longer than it took for life to develop on Earth.)

Hesperian

The proceeding period is known as the “Hesperian” period, and is named after Hesperia Planum, a very rugged area just north of the Hellas Planitia impact basin.

We can see from the fewer impact craters in this area, this was a time of relative calmness (well, barbaric by present Earth standards, but calm when compared to the Noachian period). The placid nature also extended to the volcanism, where the overall global geological activity was slowing down at a fair rate. However, what activity there was resulted in continued volcanic eruptions which emitted vast amounts of sulphur dioxide and water. These two compounds reacted to form sulphuric acid, which subsequently rained onto the surface. The sulphate deposits left as a result of this is what allows us to categorise this period from today’s observations.

As the atmosphere continued to disband (due to the lacking magnetic field), the planet cooled rapidly, leading to much of the water becoming locked up as permafrost or subsurface ice. But this was not the end of “blue Mars”: what asteroid and comet impacts there were caused this subsurface ice to melt and erupt on the surface in catastrophic torrents which led to the formation of magnificently massive outflow channels which caused erosion unlike anything seen on Earth. It literally reshaped parts of the surface of the planet, causing “chaotic terrain”, as can be seen in our images of the Kasei Valles mosaic (which comprises of 67 images stitched together to capture land that spans 1550 kilometres across)

Amazonian

The “Amazonian” period is the most recent, and current period of Mars’ life. It gets its name from the smooth plains of Amazonis Planitia. It began when the Hesperian ended, about 2.9 billion years ago, making it the longest out of all the eras (covering over half of Mars’ lifetime).

The Amazonian era is defined by its continued decline in geological activity, causing Mars to mostly be cold, dry and barren, as it is today. Occasionally, there have been some short spouts of water eruptions (like described above), which allowed for small scale returns to the warm and wet conditions.

The continued disbanding of the atmosphere (becoming roughly 1/100 the thickness of Earth’s), from the lack of substantial magnetic field, has caused two large issues for any form of life to be present on the surface: One, the planet has become very cold, as it has effectively lost its “blanket”, causing an average temperature of -60 degrees Celsius. And two, there is very little protection from the ionising cosmic rays from the sun, meaning the surface is the equivalent of a kitchen counter whose cook has OCD (i.e. the rays sterilise the surface 24/7).

Although, there has been some geological activity to note. The main example the eruption of Olympus Mons (as mentioned earlier) which caused widespread lave flows. Furthermore, aeolian (wind) erosion has shaped large areas of Mars (mostly broad plains near the poles).

The main takeaway from this period is that it leads to the present, where Mars has no liquid water on the surface. For the most part, liquid ice can only be found at the poles, or as evidence seems to suggest, underground. Thus, it is entirely possible that this water may be released in the future if we can terraform Mars by global warming it. If this is the case, Mars will be blue once again.

Originally published at http://thephysicsfootprint.com on February 24, 2022.