# Composition of Martian Crust

Does the composition of the crust of Mars roughly correspond to that of the crust of Earth. That is, is the elemental abundance in decreasing order silicon, oxygen, aluminum, etc.?

The most abundant elements are:

Element  Approximate % by weight

Oxygen        46.6
Silicon       27.7
Aluminum       8.1
Iron           5.0
Calcium        3.6
Sodium         2.8
Potassium      2.6
Magnesium      2.1
All others     1.5


Based on data from a number of NASA and ESA spacecraft and rovers, (Fine summary on wikipedia: https://en.wikipedia.org/wiki/Composition_of_Mars). The four top elements are most likely the same. They are roughly the same on Mars and Earth and may be in the basic proportions

• Oxygen

• Silicon

• Aluminum

• Iron

I would expect at some point that the relative abundance of elements on Mars to deviate from that on Earth based on the following differences:

• Earth and Mars were most likely formed at two very different distances from the center of our solar system. This may have significantly affected the elemental abundances between the two planets.
• Earth has had significant amounts of water since from very early in its history, as recorded by some of the oldest rocks on Earth which are meta-sedimentary deposits in Australia.
• We do not known how much water has ever been on Mars. Mars is currently very dry. Water plays a big role in element distribution on Earth and my guess is water will have played a smaller role on Mars.
• Lastly, I am becoming convinced that biologic activity has also significantly affected element distribution on Earth. Bacteria/Archaea have been found in deep rocks in South African gold mines, which is remarkable. Bacteria and Archaea may play a larger role in various geological processes than traditionally thought. The Mantle is no longer viewed has dry and sterile.

• Life on Mars (even as Bacteria/Archaea) is yet to verified.

Although the Martian crust has has a similar set of the most-abundant elements as Earth's -- for instance, on both planets oxygen is #1 and silicon #2 -- there are some differences. Compared with Earth's crust, the Martian version has more iron and magnesium, and less aluminum, calcium, and alkali metals. The top five elements on Mars are shown here: oxygen, silicon, iron, aluminum, magnesium where the elements in bold rank above their placements in the Earthly crust composition. Calcium and the alkali metals sodium and potassium are further down the list, lower-ranked on Mars than on Earth.

Looking for America (and Eurasia, Africa, ...)

Except for mermaids, mermen (their masculine counterparts), and maybe some islanders, we all do not just live on Earth's crust. We live on continental crust, with its granite-rich rock composition. This is distinguished from the ocean crusts based primarily on basalt and other mafic rocks.

Mars, despite probably once being a wet world, shows no evidence of any similar continental crust formation [1]. A study of rocks from Gale Crater suggests that apparently granite-like material is not from a continental crust but from a localized fractionation process, and Mars more generally has a primarily basaltic crust. Similarly, McSween et al. [2] report in their abstract:

Surface rocks on Mars are dominantly tholeiitic basalts formed by extensive partial melting and are not highly weathered. Siliceous or calc-alkaline rocks produced by melting and/or fractional crystallization of hydrated, recycled mantle sources, and silica-poor rocks produced by limited melting of alkali-rich mantle sources, are uncommon or absent.

Cream rises to the top

The presence or absence of continental crust impacts crustal composition because, while ocean crust is primarily mafic, granite-rich continental crust is primarily felsic, meaning its igneous rock components are mostly minerals rich in aluminum and alkali metals. This is in contrast with mafic rock which contains more calcium, iron and magnesium. Felsic rock is less dense than mafic rock, and geological processes on Earth concentrated the felsic rock at the outer surface of Earth. Calcium-rich mafic rock, which is less dense than rock containing higher amounts of magnesium and iron (which are generally mixed together in igneous-rock minerals), would also rise to the top. Thus on Earth, calcium, aluminum and alkali metals have been enriched by forming a continental component in our crust; while on Mars these geologic processes were apparently lacking and the crust remained closer to the heavy, mafic, iron- and magnesium-rich composition of the bulk of the planet's rock.

References

1. Arya Udry, Esteban Gazel, and Harry Y. McSween Jr., "Formation of Evolved Rocks at Gale Crater by Crystal Fractionation and Implications for Mars Crustal Composition", Journal of Geophysical Research: Planets 123 (2018), Issue 6, pp. 1525-1540. Link
2. Harry Y. McSween Jr.1,G. Jeffrey Taylor, and Michael B. Wyatt, "Elemental Composition of the Martian Crust", Science 324 (2009), Issue 5928, pp. 736-739. Link
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