Journey to the center of Mars: Underground lakes, large core and a radioactive crust?

Recently there has been many studies released by planetary scientists covering our closest neighbour, and my favourite planet, Mars! Covering everything from biosignatures, to interior make up, this past month has been full of interesting Martian News!

Liquid water on Mars?

Liquid water is one of the key biomarkers used when deciding if a planet could of sustained life. It is known that some time in its past Mars had vast oceans of liquid water however these no longer exist. Evidence for these oceans are widespread, from definitive features like lake beds and river deltas viewed from space by remote orbiters, and geological evidence gathered by rovers on the ground. For example in september of 2012, MSL Curiosity discovered evidence of "vigorous water flow" in gale crater. However, although there may be no liquid water visible, there might be whole lakes hiding just below the surface.

This was first proposed back in 2018, when a ground-breaking (see what I did there!) paper was released announcing the discovery of vast reservoirs of water under Mars' south polar ice cap, also known as the south polar layered deposits (SPLD). This was backed up by another paper in 2020 detailing three underground lakes in the same region

These discoveries were made by the MARSIS instrument on the ESA's Mars express orbiter. This is a ground penetrating radar that's used to peek a couple miles under the surface. The instrument returned strong, bright signals that is consistent with liquid water.

However there are many problems with this hypothesis, for one the temperature on mars is extremely cold and any water would immediately turn to ice without external heating. And the known volcanically active Tharsis region is far away.

A new study released on July 15th provides a much more plausible answer, Isaac Smith and his team from York university suggest that frozen clays called smectites would give of a similar signature and would be more consistent with current planetary models of Mars. Smectites are common here on earth and have also been discovered on many parts of gale crater by MSL. 

However this isn't a bad thing in the search for extraterrestrial life, as clays provide all the minerals for life and some even suggest that clays on earth were the birthplace of life as we know it. It also poses interesting questions about their formation. Smectites are formed by the erosion of volcanic rock by flowing water. This suggests that long ago, there was volcanic activity at Mars' poles and a thermal environment suitable for water.

To test their theory, the team cooled smectites to the average temperature on Mars (7 degrees celsius) and using a wave propagation model, it was observed that it produced similar radar reflections as picked up by MARSIS.

Mars InSight returns valuable data on the inner structure of Mars!

Ever since it touched down on Elysium Planitia back in November 2018, NASA's InSight lander hasn't had and easy time with the failure of its "Mole" heat probe and dust accumulating on its solar panels, greatly reducing its power production capabilities. However since 2019 its SEIS (Seismic Experiment for Interior Structure) instrument has detected over 700 "Mars-quakes". 

These Mars-quakes are very weak compared to terrestrial quakes, only being felt a couple miles around the focus. This is due to their cause, unlike Earth, Mars has no plate tectonics, so these quakes are likely caused by the less energetic cooling of the planet's core, meaning the crust shrinks and deforms. Luckily the SEIS instrument is very sensitive so can pick up weak tremors from long distances. The main challenge for the team has been to filter out background noise caused by the lander itself and other environmental factors.  

InSight being prepared for launch at NASA JPL

Once the tremors are detected and recorded, we can measure the speed and direction of these waves to determine the size of the core, mantle and other components of the planet. This can refine our models and hopefully give clues on why mars turned from a potentially habitable earth like environment to a bleak dessert devoid of life.

Any tremor produces 2 types of shockwaves, P-waves and S-waves. P-waves move through particles via compression and therefore can travel through any medium, S-waves are shear waves and move up and down like waves on the surface of water. These can only transfer through solids. This difference proved key to detecting Mars’s core, since P-waves can pass through a solid mantle and into a liquid core, but S-waves can’t.

So when three distinct phases of Mars-quakes were identified, the P-wave, followed by the main S-wave, and a second, smaller S-wave a few hundred seconds later with the correct orientation to be a reflection, this meant these secondary waves must of been reflected off the dividing line between the solid mantle and liquid core. And by measuring the time it took for these reflections to be detected, the depth of this boundary can be measured. 

The Mars InSight team therefore estimates that the core is around 3,6000 km in diameter, slightly larger than previously thought. This larger volume, but similar weight means the core has a lower density and must be made out of lighter materials like carbon, oxygen and hydrogen. However the prevailing light element is sulfur taking up 10-15% of the cores mass. This can explain Mars' weak magnetic field as less of the core is made of ferrous metals like iron.

The findings also make clear that Mars’s mantle doesn’t reach the depths and pressures needed to make a distinct lower mantle, the type of geologic layer that within Earth is a hot, dense region of solid rock that begins some 410 miles (660 kilometers) beneath our planet’s surface. The high-pressure minerals found in Earth’s lower mantle help insulate our planet’s core, so the lack of one on Mars likely means that its core had a much easier time cooling off, meaning early Mars had a much larger magnetic field than it has currently.

The death of Mars’s magnetic field has been linked to the loss of much of its atmosphere, so pinning down more details of the field’s demise may help scientists understand when and why Mars became the dry, seemingly barren world it is today. The lack of a magnetic field resulted in the martian atmosphere being unprotected from solar radiation, slowly wearing it down until it reaches pressures less than 1% of Earth's seen today.

The model also suggests that there is a much higher concentration of decaying radioactive isotopes in the crust than the mantle beneath. The concentrated heat released due to nuclear decay could of caused the planet's volcanic activity despite a lack of global plate tectonics.

These exciting discoveries about our cosmic neighbour can potentially prepare us for its eventual and nearing colonisation as we look to a future amongst the stars. But it can also hold up a mirror to our own planet and past, helping us understand its formation, in a time where humans seem hell bent on turning our precious oasis into a desert similar to our Martian brother.

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