When we think about methods to monitor climate change, we often focus on temperature, atmospheric gases, and extreme weather events. However, in today's astrobito we will learn how the abundance of iron (Fe) on the surface of Mars, despite being so far away, can provide crucial information about its climate history
Paper details:
Title: Atmospheric oxidation drove climate change on Noachian Mars
Authors: Jiacheng Liu, Joseph R. Michalski, Zhicheng Wang, Wen-Sheng Gao
First-author Institution: Departamento de ciencias de la tierra y laboratorio de investigación espacial, Universidad de Hong Kong, Hong Kong, China
Status: Published in Nature Communications
Mars, the red planet, is a cold, arid world today, with temperatures varying greatly by latitude, that is, how far north or south a place is from the middle of Mars. However, this was not always the case. Scientific modelling suggests that early Mars had a different climate, with temperatures influenced more by elevation than by latitude. Several theories have been proposed to explain this dramatic shift in climate, including reductions in some atmospheric gases, or changes in cloud cover. Despite these efforts, the exact process driving this climatic shift remains unclear.
A research team from the University of Hong Kong and the China University of Geosciences recently shed some light on how Mars' climate has evolved. Today's astrobito delves into the findings of this study, exploring the role of iron (Fe) abundance on the surface of Mars, across its geological periods, as a marker of climate change.
Geological periods on Mars
The geological history of Mars is divided into three main periods (Figure 1): the Noachian (approximately 4.1 to 3.7 billion years ago), the Hesperian (3.7 to 3 billion years ago), and the Amazonian (3 billion years ago to present). Each period reflects significant changes in the surface and atmosphere of Mars. The Noean period saw substantial volcanic activity that decreased during the Hesperian and Amazonian periods, where geological activity stabilised considerably.
Figure 1. Mars geological periods.
Using data collected by the Mars Odyssey space mission's gamma ray spectrometer (GRS), the team behind this paper studied the abundance of iron (Fe) on the surface of different terrains on Mars dating to the Noachian, Hesperian, and Amazonian periods. Their results showed that Fe levels in Noachian terrains are relatively low compared to those from the Hesperian and Amazonian periods, which have higher Fe levels in comparison, but are similar to each other. This change in Fe abundance suggests that in the transition from the Noachian to the Hesperian period there was a change in the climatic conditions of Mars that drove the accumulation of Fe on its surface.
Spatial distribution of Fe on Mars
With this idea in mind, the team decided to study changes in the spatial distribution of Fe on the Martian surface throughout the Noachian period (Figure 2). They found that during the early and middle Noachian (approximately 4.1 billion years ago), there was a greater correlation between terrain elevation and its Fe abundance. Elevated regions of Mars exhibited greater Fe abundance, while lower elevations showed lower concentrations. On the other hand, during the late Noachian and early Hesperian (approximately 3.7 billion years ago), Fe abundance showed a stronger correlation with latitude, especially in the southern hemisphere, where the lands closest to the south pole of Mars showed a greater abundance of Fe, independent of their altitude.
Figure 2. Correlation between Fe abundance with elevation and latitude over the Noachian period. Figure adapted from Figure 3 in the main article.
These findings suggest that this variation in Fe distribution is due to changes in the temperature of Mars. The contact between Fe and liquid water drives a series of chemical reactions in which Fe in its elemental form is depleted to give way to the formation of different salts and minerals. Thus, at high temperatures, when water thaws, the abundance of Fe at the surface decreases considerably, while its abundance is greater at lower temperatures when the water is frozen. This suggests that during the Early Noachian, high terrains on Mars were always frozen, while low lands experienced freeze-thaw cycles, thus depleting the amount of available Fe.
But how does this explain that during the early Noachian there was more Fe in the highlands, but at the end of this period Fe was more concentrated at the poles? Well, the team suggests that this is due to a change in the Martian atmosphere.
The changing atmosphere of Noachian Mars
Over time, during the Noachian period, Mars' atmosphere became increasingly thin due to a decrease in its volcanic activity. This caused the abundance of atmospheric gases such as hydrogen (H2) to decrease, but that of other gases such as carbon dioxide (CO2) to increase. In chemical terms, this change is known as a transition from a reducing to an oxidising atmosphere.
This change in the Martian atmosphere greatly influenced how heat was distributed on Mars. With a reducing atmosphere, temperature was largely influenced by elevation. However, with the oxidation of the atmosphere, temperature began to be more influenced by latitude. That is, Noachian Mars went from being colder at high temperatures to being colder at the poles. This caused a shift of ice from the high regions to the poles, giving way to a constant melting that, finally, altered the concentrations of Fe in the high regions of Mars.
Thus, during the early Noachian, Mars enjoyed a reducing atmosphere that promoted the accumulation of Fe in its high regions due to its low temperatures. However, with the decrease in its volcanic activity and consequent oxidation of its atmosphere, the high regions on Mars heated up, exposing Fe to the mercy of liquid water, causing its depletion. This is why, during the middle and late Noachian, the concentration of Fe on the Martian surface increases with latitude and not with terrain elevation (Figure 2).
Iron as a climate marker
Analysis of Fe abundance on Mars provides crucial information about the planet's climate history, revealing that the oxidation of its atmosphere played an important role in driving climate change. This helps explain why Fe concentrations are higher in the Hesperian and Amazonian periods than the Noachian: since the transition from the Noachian to the Hesperian, there has been no other climate change that altered Martian conditions.
It is fascinating to learn that just by analysing the concentration of a chemical element (Fe in this case) on a planet millions of kilometres away, we can infer changes in the climate and atmospheric composition on that planet over time. These insights not only improve our understanding of Mars, revealing the complex history of our neighbouring planet, but also offer valuable lessons for studying climate change on other planets, including Earth.