How lava flows on Olympus Mons tell us about the volcanic history of Mars

When we think about big mountains on Earth, the first thing that comes to mind is almost always Mt. Everest. Everest, however, is absolutely miniscule in comparison to the solar system’s largest volcano, Olympus Mons.

Standing at 21,229 meters above the Mars global datum (which can be thought of like sea level here on Earth), Olympus Mons is about two and half times as tall as Mount Everest. Unlike Everest, which is the result of two continental plates slamming into each other, Olympus Mons has a volcanic origin. It can be classified as a shield volcano, similar to those that make up the Hawaiian Islands.

Shield volcanoes are characterized by broad, gentle slopes (4-8 degrees) and tend to erupt nonviolently out of fissures on the flanks of the volcano (think the recent lava flows in Hawaii). The tops of shield volcanoes tend to have calderas, which are large collapsed structures found at the top of volcanoes. The magma in the chamber provides pressure that effectively holds up the top of the volcano.The magma chamber is literally inflating the top of the volcano, sustaining its dome-like shape. When the volcano is done erupting and the chamber is partially empty, the top of the volcano collapses and forms a caldera. Things can get a bit more complicated, however, as has been recently discovered at the caldera at the top of Olympus Mons.

Figure 1: The caldera atop Olympus Mons. Colors correspond to elevation, where warmer colors are higher and cooler colors are lower. The blue shapes extending outwards in almost all directions are lava flows. From Mouginis-Mark and Wilson (2019).

The caldera atop Olympus Mons, like the volcano itself, is huge. Its horizontal dimensions are 60 X 80 km, and it has a depth of about 3 km. Lava flows radiate outwards from the rim of the caldera (Fig.1). The strange thing about this caldera, however, is that some of these lava flows appear to flow uphill (Fig. 2). This is not normal, even on Mars. Lava can sometimes travel small distances uphill due to confining pressure or momentum. The lava flow on Olympus Mons, however, didn’t travel a short distance uphill -- it appears to have gone several kilometers!

Figure 2: Zoomed in from figure 1 to show the anomalous lava flow. The left and right images show the same lava flow. The left is CTX image of the flow and the right is an interpreted sketch of the flow with contour lines. The arrow denotes flow direction. Notice how the flow travels uphill. From Mouginis-Mark and Wilson (2019).

Scientists have instead interpreted this to mean that the lava flow was emplaced and flowed downhill normally, cooling as it did so. Later, new magma was brought up to the near surface through cracks in the rock in a sheet-like intrusion, called a dike. This dike caused a localized inflation to occur within the caldera which tilted certain areas. One of those areas was near the lava flow, making it appear to be flowing uphill. But why is this significant? This tilting shows scientists that volcanism didn’t end when previously thought, when the caldera collapse occurred. Instead, there must have been renewed volcanism closer to the present. By understanding the timing of volcanism on Mars, we can better understand Mars’s evolution as a planet, and therefore what past conditions on Mars could have been like. Was Mars more volcanic in its past? Could this volcanism provide a greenhouse effect to allow life to exist? This work takes us one step closer to fully answering these questions.

Original paper:

Mouginis-Mark, P. J., & Wilson, L. (2019). Late-stage intrusive activity at Olympus Mons, Mars: Summit inflation and giant dike formation. Icarus, 319(September 2018), 459–469. https://doi.org/https://doi.org/10.1016/j.icarus.2018.09.038

Buried liquid water on Mars?

The question of whether there is water on Mars has been hotly debated for decades. It has generally been proven that water exists on Mars, though it’s mostly trapped in its ice form. For life to exist as we know it, liquid water is a vital component. Because Mars has such a thin atmosphere, liquid water on the surface isn’t stable and will immediately boil off. This makes it very tricky to even begin to try to observe liquid water. Recently however, using radar instruments, Italian researchers have discovered evidence of a large, liquid water lake beneath the glaciers at the southern pole of Mars.

Aboard the EU’s Mars Express spacecraft is a special instrument called a radar sounder. Radar sounders work by shooting specific frequencies of radar pulses at the surface of a planet and then observing the reflected signal. This can be thought of like clapping in a cave and listening to the echo that comes back to you. Depending on what the echo “sounds” like, scientists can gain a wealth of information about whatever surface they aimed the instrument at. Differing frequencies used by the instrument contain information about the surface rock, ice, or even what lies beneath the surface.

Figure 1: Radargram of the subglacial lake. In this figure, the x-axis is horizontal distance on the planet, and the y-axis is the time is takes the radar signal to bounce back to the spacecraft, which can be interpreted to understand subsurface topography. This figure can be thought of as a cross section. The subglacial lake can be seen at the basal reflection label. The SPLD label shows the South Polar Layered Deposit, which is an area of the south pole where pure water ice is formed in layers. Figure 2A from Orosei et al. (2018).

Using a radar sounder named MARSIS, Italian scientists made several flybys of the south pole region, which is mostly covered in water-ice glacier year round. What they found were bright subsurface radar reflections where, ordinarily, there should be nothing (fig.1). When you shoot a radar wave at a material, the target’s atoms will react by becoming electrically polarized to varying degrees (the atoms will have one side become more positive than the other, or vice versa). This degree of polarization can be unique for different materials, so it’s a great way to identify materials hidden in the subsurface. This quality of materials is known as dielectric permittivity. By analyzing the MARSIS radar reflections, the scientists discovered that the dielectric permittivity of the anomalous reflection matched that of water-bearing materials. By making several flybys of the area, the scientists were able to map out the subglacial lake and found it to be about 20 km wide and approximately 1.5 km beneath the surface (fig.2) 

Figure 2: This image shows the estimated size of the lake. Radar tracks are colored depending on subsurface elevations with warmer colors corresponding to higher elevations and cooler corresponding to lower elevations. The perceived lake is marked by the bold, black line. Figure 3A from Orosei et al. (2018).

Does this confirm that there’s a subglacial lake at Mars south pole? It’s very likely. (SHARAD, another radar sounder orbiting Mars, didn’t see the lake, though this may be to due to the different set of frequencies SHARAD uses). Does this confirm that there’s life on Mars? Not yet. For all we know, the lake could be completely inhospitable, or might not even be there in the first place! What this study does provide is compelling evidence that the secrets to extraterrestrial life in our solar system might be hiding under some ice, just one planet away!

Orosei, R., Lauro, S. E., Pettinelli, E., Cicchetti, A., Coradini, M., Cosciotti, B., … Seu, R. (2018). Radar evidence of subglacial liquid water on Mars. Science, 361(6401), 490 LP-493. Retrieved from http://science.sciencemag.org/content/361/6401/490.abstract

How Can Molten Rock Transform into Geometric Columns?

Figure 1: (A) Columnar jointing on mars. (NASA, 2007). (B) Tops of columnar joints at Giant’s Causeway, Northern Ireland. (Wikimedia: Code Poet, 2005). (C) Sides of columnar joints at Devil’s Tower, Wyoming. (Wikimedia: Jonathunder, 2017).

Marte Vallis of Mars (Figure 1a), Giant’s Causeway of Northern Ireland (Figure 1b), and Devil’s Tower of Wyoming (Figure 1c) give us just a few examples of the beautiful features formed when lava flows cool and contract to produce geometric columns, a phenomenon called columnar jointing. When columnar joints form, solidified igneous rocks crack in a particular way that causes them to develop columns that resemble  hexagonal  honeycombs from above and a bundle of posts from the side (Figure 2a and 2b). These cracks, called joints, form in lava as it cools and act as pathways for water to flow through the hot rock. Water can flow into the spaces between the columns and react with the hot rocks, even forming valuable mineral deposits such as copper and zinc. This makes them a valuable resource for geothermal and hydrothermal energy, as well as mineral deposits. Columnar joints are common features that can form in any kind of igneous rock, but the conditions under which they develop aren’t perfectly understood.

Figure 2: (A) Close up view of the tops of columnar joints at Giant’s Causeway, Northern Ireland (Wikimedia: Mayer, 2003). (B) Close up view of the sides of columnar joints at Devil’s Tower, Wyoming (Wikimedia: Konstantin, 2003).

Most materials expand when they’re heated up and then contract when they’re cooled down. For a molten lava flow, this means that it takes up less space after it cools. Cracks or joints form in order to compensate for that change in volume. If the contraction happens under specific  conditions, columnar joints will form. But what exactly are these conditions?

Researchers, Lamur et al. 2018, brought rock samples from Iceland’s Eyjafjallajökull volcano into the lab and performed specialized heating and cooling experiments. Their goal was to understand how and why columnar jointing takes place by focusing on the temperature window in which they form. To do this, they heated the field samples to simulate the natural pressure conditions expected in a typical lava flow. They observed how the rocks changed as they cooled to figure out how temperature affects the formation of columns and the size of the gaps between them. Lamur and his team found that columnar jointing happens at 840-890ºC (1544-1634ºF). This means that columnar joints only form when the  molten rock has begun to solidify, but is still hot enough for the rock to be somewhat flexible. They also used their observations to predict how wide the gaps between columns would become under different temperature conditions. The size of those gaps tells us about how well fluids would be able to circulate within the rocks. The larger the gap, the greater its contribution would be to secondary mineral deposition and to geothermal and hydrothermal resources.

References

Code Poet. "Giant's Causeway, Co. Antrim, Northern Ireland." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 28 May 2005. https://commons.wikimedia.org/wiki/File:Causeway-code_poet-4.jpg

Jonathunder. "Devils Tower National Monument at sunset in Wyoming, United States." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 23 August 2017. https://commons.wikimedia.org/wiki/File:SquareDevilsTower.jpg

Konstantin, Phil. "Closeup photo of the columns on Devil's Tower." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 24 May 2003. https://commons.wikimedia.org/wiki/File:DevilsTowerCloseupByPhilKonstantin.jpg

Lamur, Anthony, et al. "Disclosing the temperature of columnar jointing in lavas." Nature communications 9.1 (2018): 1432. https://www.nature.com/articles/s41467-018-03842-4

Mayer, Matthew. "Close up of Giant's Causeway." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 25 April 2003. https://commons.wikimedia.org/wiki/File:Giants_causeway_closeup.jpg

NASA. “Marte Vallis, Mars.” Nasa Earth Observatory, 31 October 2007. https://earthobservatory.nasa.gov/IOTD/view.php?id=38904