GD
Geodynamics

Solar System

Iron volcanism on metallic asteroids?

Metallic asteroid

This week, Francis Nimmo, professor in the Department of Earth and Planetary Sciences (University of California Santa Cruz), tells us about volcanism on metallic asteroids! Around and after the formation of the solar system (4.5 billion years ago onwards), volcanoes on some of the gigantic bodies of the asteroid belt might have erupted … iron. Explanations.

Francis Nimmo

Francis Nimmo

One way in which we can learn about the insides of planetary bodies is by looking for signs of volcanism. Volcanism transports molten material from the interior to the surface of a body, where it solidifies. Something similar may happen on icy bodies, where cryovolcanism is thought to occur. Together with Coby Abrahams, one of my grad students, I recently wrote a paper proposing “ferrovolcanism”, which is the eruption of metallic iron. We suggested that this may have happened in the past on metallic asteroids.

 

What are metallic asteroids? Some asteroids, such as 16 Psyche are thought to be made mainly of iron and nickel, on the basis of their spectra and radar signatures. They probably originated from a catastrophic collision. The picture is that a proto-planet, which had already formed a dense metallic core, was broken apart so violently that its core was fragmented, and the fragments then cooled to form free-floating metallic bodies. Such an event would also explain the large number of iron meteorites in our collections. So far we have never seen a metallic asteroid up close, but NASA is planning to send an orbiter which will reach 16 Psyche in 2026.

Iron meteorite

Artistic view of an iron meteorite with ferrovolcanism (Elena Hartley)

How would “ferrovolcanism” work? Immediately before the catastrophic collision, the core would have been liquid (because the silicate mantle is a good insulator). After the collision, the surface of a free-floating iron blob would have initially cooled very rapidly. Analysis of iron meteorites tells us that, in some cases, the blob developed a strong, cold iron crust. The melt (liquid iron) beneath the crust would have been less dense than the crust – just like molten magma on Earth. And so, just like magma on Earth, the liquid iron would tend to rise to the surface – hence ferrovolcanism.

There are a few factors that make ferrovolcanism less likely than silicate volcanism. One is that, as the iron crust cools, it contracts, putting the whole body into compression and making it more difficult for melt-filled fractures to open up. Another is that iron is both ductile and strong, so that fracture propagation is harder. And iron is also much more thermally conductive than rock, so that melt-filled fractures will cool and solidify rapidly. Nonetheless, we don’t think any of these factors is fatal. For instance, Mercury also experiences global compression but still shows signs of volcanism. Iron eruptions might be aided by volatiles coming out of solution, just as with terrestrial eruptions, although the catastrophic impact may have removed some volatiles.

How do we test this hypothesis? One way is to wait for images of 16 Psyche! If we see things that look like iron volcanoes, that would be interesting. But identifying signs of ancient volcanism is notoriously hard – this paper documents several proposed volcanic or cryovolcanic features which turned out to be something else on further investigation. And our calculations show that the volume of erupted material will be only about 0.1 percent of the total volume of Psyche, so these features may be hard to spot – especially after 4 billion years of erosion by impacts.

Another way is to look at the iron meteorites in our collections, because ferrovolcanism should produce anomalous-looking meteorites. For instance, one might find vesicle-rich samples, or ones happening late in the crystallization sequence but showing extremely rapid cooling (because they were erupted to the surface).

How did we write the paper? It was more or less accidental. Coby was working on how iron asteroids might lose their volatiles early when one day he turned to me and said “I think they’ll erupt!”. I thought this was a neat idea, and so we started to look at how the eruptive process might work.

The strangest part of writing our paper was that at the LPSC conference someone came up to me and said “I just invented iron volcanism!”. It turned out that another group had been working on ferrovolcanism completely independently. They suggested that pallasite meteorites might be evidence of ferrovolcanism (something that I had completely missed). This simultaneous but independent development of ideas seems to be quite common in the Earth Sciences – but that is a topic for another day.

 

Oceans on Mars: the geodynamic record

Oceans on Mars: the geodynamic record

Apart from our own planet Earth, there are a lot of Peculiar Planets out there! In this series we take a look at a planetary body or system worthy of our geodynamic attention, and this week we are back to our own solar system, more precisely to our neighbour Mars. In this post, Robert Citron, PhD student at the University of California, Berkeley, writes about the links between oceans, shorelines, and volcanism on Mars. A David Bowie approved blog post!

Robert Citron

Whether Mars was warm enough in its early history to support large oceans remains controversial. Although today Mars is extremely cold and dry, several lines of geological evidence suggest early Mars was periodically warm and wet. Evidence for ancient liquid water includes river channels, deltas and alluvial fans, lakes, and even shorelines of an extensive ocean.

Such features are carved into Mars’ ancient crust, which contains a remarkable geologic record spanning from over 4 billion years ago to present. How and where fluvial erosion takes place is highly dependent on topography. However, Mars is a dynamic planet and the topography observed today does not necessarily represent the planetary surface billions of years ago. Geological markers that seem misaligned today, such as river flow directions and sea levels, may be more consistent with ancient topography. Geodynamic models of how the planet has changed shape over time can therefore be used to test and constrain evidence of water on early Mars.

Shorelines are one example where geodynamic models have helped interpret the geological record. Perhaps the most compelling evidence for ancient Martian oceans are the hypothetical palaeo-shorelines that border Mars’ northern lowland basin. However, the shorelines fail to follow an equipotential surface, or contours of constant elevation, which would be expected if they formed via a standing body of liquid water. One explanation is that the shorelines used to follow an equipotential surface, but subsequent changes to the planet’s shape warped them to their present-day elevations, which vary by up to several kilometers.

Two geodynamic processes that likely changed the global shape of Mars are surface loading and true polar wander. True polar wander occurs when a planet reorients relative to its spin axis, which reshapes the planet because it changes the location of the equatorial bulge produced by the planet’s rotation. Large scale true polar wander on Mars was examined by Perron et al. (2007), which found that it could have warped past shoreline profiles to their present-day topographic profiles.

Another possibility is that Martian shoreline markers were deflected by flexure associated with surface loads. The emplacement or removal of material on a planet’s surface can cause flexure and displacement of nearby crust. This is observed on Earth, where melting of glaciers has unburdened underlying crust, allowing for rebound and displacement of past shoreline markers. Similar processes could be at work on Mars, but on a global scale.

Mars topography: The massive Tharsis volcanic province (red) is situated on the boundary between the southern highlands (orange) and northern lowlands (blue). The lowlands may have been covered by one or more ancient oceans, and are bordered by palaeo-shorelines. Two of the most prominent shorelines are the older Arabia shoreline (dashed line) and younger Deuteronilus shoreline (solid line). Image constructed by R. Citron using MOLA data. Shoreline data from Ivanov et al. (2017) and Perron et al. (2007).

The largest load on Mars is the Tharsis rise, a volcanic province that dominates the topography and gravity of the planet. Tharsis was built by volcanic activity over hundreds of millions to billions of years. Its emplacement changed Mars’ shape on a global scale; in addition to the Tharsis rise, there is a circum-Tharsis depression and an antipodal bulge.

In recent work (Citron et al. 2018), we found that the present-day variations in shoreline elevations can be explained by flexure from Tharsis and its associated loading. Of the two most prominent Mars shorelines, the older (~ 4 billion years old) Arabia shoreline corresponds to pre-Tharsis topography, deformed by almost all of the flexure associated with Tharsis. The younger (~ 3.6 billion years old) Deuteronilus shoreline corresponds to late-Tharsis topography, requiring only ~17% of Tharsis loading to explain its variations in elevation. This suggests that the Arabia shoreline formed before or during the early stages of Tharsis, and the Deuteronilus shoreline formed during the late stages of Tharsis growth. The match between the present-day shoreline markers and ancient equipotential surfaces supports the hypothesis that the markers do indicate shorelines formed by an ancient ocean.

The timing of ancient Martian oceans is consistent with recent work by Bouley et al. (2016), which found that the Mars valley networks (ancient river channels) also better fit Mars’ pre-Tharsis topography. In the topography of Mars prior to Tharsis, the flow direction of the channels are more consistent with the topographic gradient, and the channels occur at latitudes and elevations where climate models predict water ice (resulting in ice melt) to form.

The timing of the shorelines and valley networks relative to Tharsis volcanism suggests a close link between the stability of water on Mars and volcanic activity. Atmospheric models predict a cold and icy early Mars, however it is possible that oceans may be more sustainable during periods of heightened volcanism. Tharsis activity has also been associated with outflow channels indicative of catastrophic flooding that may have inundated the northern plains with water. Further examination of the link between Tharsis volcanism and oceans could increase our understanding of early Mars habitability.

Further reading:

Bouley, S., Baratoux, D., Matsuyama, I., Forget, F., Séjourné, A., Turbet, M., & Costard, F. (2016). Late Tharsis formation and implications for early Mars. Nature531(7594), 344.

Citron, R. I., Manga, M., & Hemingway, D. J. (2018). Timing of oceans on Mars from shoreline deformation. Nature555(7698), 643.

Ivanov, M. A., Erkeling, G., Hiesinger, H., Bernhardt, H., & Reiss, D. (2017). Topography of the Deuteronilus contact on Mars: Evidence for an ancient water/mud ocean and long-wavelength topographic readjustments. Planetary and Space Science144, 49-70.

Matsuyama, I., & Manga, M. (2010). Mars without the equilibrium rotational figure, Tharsis, and the remnant rotational figure. Journal of Geophysical Research: Planets115(E12).

Perron, J. T., Mitrovica, J. X., Manga, M., Matsuyama, I., & Richards, M. A. (2007). Evidence for an ancient martian ocean in the topography of deformed shorelines. Nature447(7146), 840.

Ramirez, R. M., & Craddock, R. A. (2018). The geological and climatological case for a warmer and wetter early Mars. Nature Geoscience11(4), 230.