Tag Archives: Geophysics

The lunar history

(Alternative title: The search for the origin of the Late Heavy Bombardment)

Hi there! It is a pleasure for me to present you today a multi-disciplinary study, which mixes celestial mechanics with geochemistry. If you want to know the past of a planetary body, you must go backward: you start from the body as you observe it nowadays, and from this you infer the processes which made it evolve from its formation to its present state. In The timeline of the Lunar bombardment – revisited, by A. Morbidelli, D. Nesvorný, V. Laurenz, S. Marchi, D.C. Rubie, L. Elkins-Tanton, M. Wieczorek and S. Jacobson, the authors exploit our observations of the craters and the chemistry of the Moon, and simulations of the motion of asteroids in the early Solar System, to give new constraints on the bombardment of the Moon between 3.9 and 3.7 Gyr (billions of years) ago, which is famous as the Late Heavy Bombardment (LHB). We will see that the results have implications for Mars. This study has recently been accepted for publication in Icarus.

The Lunar basins

Let us start from what we observe: the Lunar surface. This is a heavily cratered surface. Actually, the absence of atmosphere preserves it from erosion, and the small size of the Moon limits its heating, as a consequence the craters neither erode nor relax. Hence, the surface of the Moon is a signature of the activity in the early Solar System.

Let us focus on the largest structures, i.e. the maria and the basins. The maria are lava plains, which result from a volcanic activity of the early Moon. However, the basins are the largest impact craters. You can find below the largest ones, of course many smaller craters exist.

Basin Diameter (km)
South Pole-Aitken 2,600
Imbrium 1,100
Orientale 930
Serenitatis 920
Australe 880
Nectaris 860
Smythii 740
Crisium 740
Tranquillitatis 700
Tsiolkovsky-Stark 700
Fecunditatis 690
Mutus-Vlacq 690
Nubium 690

The early Moon was hot, because of the impact which created it. As a hot body, it stratified into a fluid core, a mantle and a crust, while cooling. The visible impact craters are younger than the crust, i.e. they are younger than 3.9 Gyr, and were created at least 600 Myr after the formation of the Moon… pretty late, hence due to the Late Heavy Bombardment.

Orientale Basin. © NASA
Orientale Basin. © NASA

Origin of the LHB: cataclysm or accretion tail?

Late Heavy Bombardment means that the inner Solar System have been intensively bombarded late after its genesis. But how did that happen? Two scenarios can be found in the literature:

  1. Cataclysm: the very young Solar System was very active, i.e. composed of many small bodies which collided, partly accreting… and became pretty quiet during some hundreds of Myr… before suddenly, a new phase of bombardment occurred.
  2. Accretion tail: the Solar System had a slowly decreasing activity, and the craters on the Moon are just the signature of the last 200 Myrs. The previous impacts were not recorded, since the surface was still molten.

The second scenario could be preferred, as the simplest one. The first one needs a cause which would trigger this second phase of bombardment. Anyway, many numerical simulations of the early Solar System got such an activity, as a dynamical phenomenon destabilizing the orbits of a group of small bodies, which themselves entered the inner Solar System and collided with the planets, accreting on them. The giant planets Jupiter and Saturn have a dominant dynamical influence on the small bodies of the Solar System, and could have triggered such an instability. One of the theories existing in the literature is the E-Belt, for extended belt. It would have been an internal extension of the Main Belt of asteroids, which would have been destabilized by a secular resonance with Saturn, and has finished as the impactors of the LHB. Why not, this is a theory.

When you model phenomena having occurred several billions years ago, you have so many uncertainties that you cannot be certain that your solution is the right one. This is why the literature proposes several scenarios. Further studies test them, and sometimes (this is the case here) give additional constraints, which refine them.

Thanks to the Apollo mission, samples of the Moon have been analyzed on Earth, and geochemistry can tell us many things on the history of a body. For the Moon, focus has been put on siderophile elements.

What siderophile elements tell us

A siderophile element is a chemical element which has affinity with iron. Among these elements are iron, iridium, palladium, platinum, rubidium… When a planetary body is hot, it tends to differentiate, and its heaviest elements, i.e. iron, migrate to the core. This results in a depletion of highly siderophile elements (HSE). Since a very small abundance of these elements has been observed, then we have no problem, thank you…

NO NO NO there is actually a problem, since these siderophile elements should be present in the impactors, which are supposed to have accreted on the Moon AFTER its stratification… yes we have a problem.

But some of the authors have shown recently that on Earth, another phenomenon could remove the HSEs from the crust, well after the formation of the core: the exsolution and segregation of iron sulfide. In other words, the bombardment could have brought more HSEs than currently recorded. And this motivates to revisite the history of the Lunar bombardment.

Simulating the bombardment

So, the observations are: the craters, and the HSEs. The craters are not only the basins, but also the smaller ones, with diameters larger than 1 km. Even smaller craters could be used, but the data are considered to be reliable, i.e. exhaustive, for craters larger than 1 km. From that size to the large basins, we can fit a function of distribution, i.e. number of craters vs. diameter. Since there is an obvious correlation between the size of a crater and the one of the impactor, a population of craters corresponds to a population of impactors.

The authors dispose of statistics of collisions, which could be seen as mass accretion, between the Moon and small bodies during the early ages of the Solar System. These statistics result from numerical simulations conducted by some of them, and they can be fine-tuned to fit the crater distribution, their estimated ages, and the abundance of highly siderophile elements. Fine-tuning the statistics consist in artificially moving the parameters of the simulation, for instance the initial number of small bodies, or the date of the instability provoking the cataclysm, in the cataclysm scenario.

Cataclysm possible, accretion tail preferred

And here is the result: if the HSEs are only due to the mass accretion after the cooling of the Lunar crust, then the observations can only be explained by the cataclysm, i.e. the LHB would be due to a late instability. This instability would have resulted in a mass accretion from comets, and this raises another problem: this accretion seems to lack of primitive, carbonaceous material, while the comets contain some.

However, if the HSEs have been removed after the cooling of the crust, then the accretion tail scenario is possible.

We should accept that for this kind of study, the solution is not unique. A way to tend to the unicity of the solution is to discuss further implications, in examining other clues. And the authors mention the tungsten.

Tungsten is another marker

Tungsten is rather a lithophile than a siderophile element, at least in the presence of iron sulfide. In other words, even if it does not dislike iron, it prefers lithium (I like this way of discussing chemistry). Something puzzling is a significant difference in the ratios of two isotopes of tungsten (182W and 184W) between the Moon and the Earth. This difference could be primordial, as brought by the projectile which is supposed to have splitted the proto-Earth into the Earth and the Moon (nickname of the projectile: Theia), or it could be due to the post-formation mass accumulation. In that case, that would be another constraint on the LHB.

Implications for Mars

The LHB has affected the whole inner Solar System. So, if a scenario is valid for the Moon, it must be valid for Mars.
This is why the authors did the job for Mars as well. A notable difference is that Mars would be less impacted by comets than the Moon, and this would affect the composition of the accreted material. More precisely, a cataclysmic LHB would be a mixture of asteroids and comets, while an accretion tail one would essentially consist of leftover planetesimals. It appears that this last scenario, i.e. the accretion tail one, can match the distribution of craters and the abundance of HSEs. However, the cataclysmic scenario would not bring enough HSEs on Mars.

Predictions

This study tells us that the accretion tail scenario is possible. The authors show that it would imply that

  1. The quantity of remaining HSEs on the Moon is correlated with the crystallization of the Lunar magma ocean, which itself regulates the age of the earliest Lunar crust.
  2. For Mars, the Noachian era would have started 200 Myr earlier than currently thought, i.e. 4.3 Gyr instead of 4.1 Gyr. That period is characterized by high rates of meteorite and asteroid impacts and the possible presence of abundant surface water. Moreover, the Borealis formation, i.e. the northern hemisphere of Mars, which seems to be a very large impact basin, should have been formed 4.37 Gyr ago.

Further studies, explorations, space missions, lab experiments,… should give us new data, which would challenge these implications and refine these scenarios. So, the wording prediction can seem weird for past phenomena, but the prediction is for new clues.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

Resurfacing Ganymede

Hi there! After Europa last week, I tell you today on the next Galilean satellite, which is Ganymede. It is the largest planetary satellite in the Solar System, and it presents an interesting surface, i.e. with different terrains showing evidence of past activity. This is the opportunity for me to present you Viscous relaxation as a prerequisite for tectonic resurfacing on Ganymede: Insights from numerical models of lithospheric extension, by Michael T. Bland and William B. McKinnon. This study has recently been accepted for publication in Icarus.

The satellite Ganymede

Ganymede is the third, by its distance to the planet, of the 4 Galilean satellites of Jupiter. It was discovered with the 3 other ones in January 1610 by Galileo Galilei. These are indeed large bodies, which means that they could host planetary activity. Io is known for its volcanoes, and Europa and Ganymede (maybe Callisto as well) are thought to harbour a global, subsurfacic ocean. The table below lists their size and orbital properties, which you can compare with the 5th satellite, Amalthea.

Semimajor axis Eccentricity Inclination Radius
J-1 Io 5.90 Rj 0.0041 0.036° 1821.6 km
J-2 Europa 9.39 Rj 0.0094 0.466° 1560.8 km
J-3 Ganymede 14.97 Rj 0.0013 0.177° 2631.2 km
J-4 Callisto 26.33 Rj 0.0074 0.192° 2410.3 km
J-5 Amalthea 2.54 Rj 0.0032 0.380° 83.45 km

We have images of the surface of Ganymede thanks to the spacecraft Voyager 1 & 2, and Galileo. These missions have revealed different types of terrains, darker and bright, some impacted, some pretty smooth, some showing grooves… “pretty smooth” should be taken with care, since the feeling of smoothness depends on the resolution of the images, which itself depends on the distance between the spacecraft and the surface, when this specific surface element was directed to the spacecraft.

Dark terrain in Galileo Regio. © NASA
Dark terrain in Galileo Regio. © NASA
Bright terrain with grooves and a crater. © NASA
Bright terrain with grooves and a crater. © NASA

A good way to date a terrain is to count the craters. It appears that the dark terrains are probably older than the bright ones, which means that a process renewed the surface. The question this paper addresses is: which one(s)?

Marius Regio and Nippur Sulcus. © NASA
Marius Regio and Nippur Sulcus. © NASA

Resurfacing a terrain

These four mechanisms permit to renew a terrain from inside:

  • Band formation: The lithosphere, i.e. the surface, is fractured, and material from inside takes its place. This phenomenon is widely present on Europa, and probably exists on Ganymede.
  • Viscoelastic relaxation: When the crust has some elasticity, it naturally smooths. As a consequence, craters tend to disappear. Of course, this phenomenon is a long-term process. It requires the material to be hot enough.
  • Cryovolcanism: It is like volcanism, but with the difference that the ejected material is mainly composed of water, instead of molten rock. Part of the ejected material falls on the surface.
  • Tectonics: Extensional of compressional deformations of the lithosphere. This is the phenomenon, which is studied here.

Beside these processes, I did not mention the impacts on the surface, and the erosion, which is expected to be negligible on Ganymede.

The question the authors addressed is: could tectonic resurfacing be responsible for some of the actually observed terrains on Ganymede?

Numerical simulations

To answer this question, the authors used the numerical tool, more precisely the 2-D code Tekton. 2-D means that the deformations below the surface are not explicitly simulated. Tekton is a viscoelastic-plastic finite element code, which means that the surface is divided into small areas (finite elements), and their locations are simulated with respect to the time, under the influence of a deforming cause, here an extensional deformation.

The authors used two kinds of data, that we would call initial conditions for numerical simulations: simulated terrains, and real ones.
The simulated terrains are fictitious topographies, varying by the amplitude and frequency of deformation. The deformations are seen as waves, the wavelength being the distance between two peaks. A smooth terrain can be described by long-wavelength topography, while a rough one will have short wavelength.
The real terrains are Digital Terrain Models, extracted from spacecraft data.

The authors also considered different properties of the material, like the elasticity, or the cohesion.

A new scenario of resurfacing

It results from the simulations that the authors can reproduce smooth terrains with grooves, starting from already smooth terrains without grooves. However, extensional tectonics alone cannot remove the craters. In other words, if you can identify craters at the surface of Ganymede, after millions of years of extensional tectonics you will still observe them. To make smooth terrains, you need the assistance of another process, the viscoelastic relaxation of the lithosphere being an interesting candidate.

This pushed the authors to elaborate a new scenario of resurfacing of Ganymede, involving different processes.
They consider that the dark terrains are actually the eldest ones, having remaining intact. However, there was indeed tectonic resurfacism of the bright terrains, which formed grooved. But the deformation of the lithosphere was accompanied by an elevation of the temperature (which is not simulated by Tekton), which itself made the terrain more elastic. This elasticity itself relaxed the craters.

Anyway, you need elasticity (viscoelasticity is actually more accurate, since you have energy dissipation), and for that you need an elevation of the local temperature. This may have been assisted by heating due to internal processes.

In the future

Ganymede is the main target of the ESA mission JUICE, which should orbit it 2030. We expect a big step in our knowledge of Ganymede. For this specific problem, we will have a much better resolution of the whole surface, the gravity field of the body (which is related to the interior), maybe a magnetic field, which would constrain the subsurface ocean and the depth of the crust enshrouding it, and the Love number, which indicates the deformation of the gravity field by the tidal excitation of Jupiter. This last quantity contains information on the interior, but it is related to the whole body, not specifically to the structure. I doubt that we would have an accurate knowledge of the viscoelasticity of the crust. Moreover, the material properties which created the current terrains may be not the current ones; in particular the temperature of Ganymede is likely to have varied over the ages. We know for example that this temperature is partly due to the decay of radiogenic elements shortly after the formation of the satellite. During this heating, the satellite stratifies, which alters the tidal response to the gravitational excitation of Jupiter, and which itself heats the satellite. This tidal response is also affected by the obliquity of Ganymede, by its eccentricity, which is now damped… So, the temperature is neither constant, nor homogeneous. There will still be room for theoretical studies and new models.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

Plate tectonics on Europa?

Hi there! Jupiter has 4 large satellites, known as Galilean satellites since they were discovered by Galileo Galilei in 1610. Among them is Europa, which ocean is a priority target for the search for extraterrestrial life. Many clues have given us the certainty that this satellite has a global ocean under its icy surface, and it should be the target of a future NASA mission, Europa Clipper. Meanwhile it will also be visited by the European mission JUICE, before orbital insertion around Ganymede. Since Europa presents evidences of tectonic activity, the study I present you today, i.e. Porosity and salt content determine if subduction can occur in Europa’s ice shell, by Brandon Johnson et al., wonders whether subduction is possible when two plates meet. This study has been conducted at Brown University, Providence, RI (USA).

Subduction on Earth

I guess you know about place tectonics on Earth. The crust of the Earth is made of several blocks, which drift. As a consequence, they collide, and this may be responsible for the creation of mountains, for earthquakes… Subduction is a peculiar kind of collision, in which one plate goes under the one it meets, just because their densities are significantly different. The lighter plate goes up, while the heavier one goes down. This is what happens on the west coast of South America, where the subduction of the oceanic Nazca Plate and the Antarctic Plate have created the Andean mountains on the South America plate, which is a continental one.

Even if our Earth is unique in the Solar System by many aspects, it is highly tempting to use our knowledge of it to try to understand the other bodies. This is why the authors simulated the conditions favorable to subduction on Europa.

The satellite Europa

Europa is the smallest of the four Galilean satellites of Jupiter. It orbits Jupiter in 3.55 days at a mean distance of 670,000 km, on an almost circular and planar orbit. It has been visited by the spacecraft Pioneer 10 & 11 in 1973-1974, then by Voyager 1 & 2 in 1979. But our knowledge of Europa is mostly due to the spacecraft Galileo, which orbited Jupiter between 1995 and 2003. It revealed long, linear cracks and ridges, interrupted by disrupted terrains. The presence of these structures indicates a weakness of the surface, and argues for the presence of a subsurface ocean below the icy crust. Another argument is the tidal heating of Jupiter, which means that Europa should be hot enough to sustain this ocean.
This active surface shows extensional tectonic feature, which suggests plate motion, and raises the question: is subduction possible?

Numerical simulations of the phenomenon

To determine whether subduction is possible, the authors performed one-dimensional finite-elements simulations of the evolution of a subducted slab, to determine whether it would remain below another plate or not. The equation is: would the ocean be buoyant? If yes, then the slab cannot subduct, because it would be too light for that.

The author considered the time and spatial evolution of the slab, i.e. over its length and over the ages. They tested the effect of

  1. The porosity: Planetary ices are porous material, but we do not know to what extent. In particular, at some depth the material is more compressed, i.e. less porous than at the surface, but it is not easy to put numbers behind this phenomenon. Which means that the porosity is a parameter. The porosity is defined as a fraction of the volume of voids over the total volume investigated. Here, total volume should not be understood as the total volume of Europa, but as a volume of material enshrouding the material element you consider. This allows you to define a local porosity, which thus varies in Europa. Only the porosity of the icy crust is addressed here.
  2. The salt content: the subsurface ocean and the icy crust are not pure ice, but are salty, which affects their densities. The authors assumed that the salt of Europa is mostly natron, which is a mixture essentially made of sodium carbonate decahydrate and sodium bicarbonate. Importantly, the icy shell has probably some lateral density variations, i.e. the fraction of salt is probably not homogeneous, which gives room for local phenomenons.
  3. The crust thickness: barely constrained, it could be larger than 100 km.
  4. The viscosity: how does the material react to a subducting slab? This behavior depends on the temperature, which is modeled here with the Fourier law of heat,
  5. The spreading rate, i.e. the velocity of the phenomenon,
  6. The geometry of the slab, in particular the bending radius, and the dip angle.

And once you have modeled and simulated all this, the computer tells you under which conditions subduction is possible.

Yes, it is possible

The first result is that the two critical parameters are the porosity and the salt content, which means that the conditions for subduction can be expressed with respect to these two quantities.
Regarding the conditions for subduction, let me quote the abstract of the paper: If salt contents are laterally homogeneous, and Europa has a reasonable surface porosity of 0.1, the conductive portion of Europa’s shell must have salt contents exceeding ~22% for subduction to occur. However, if salt contents are laterally heterogeneous, with salt contents varying by a few percent, subduction may occur for a surface porosity of 0.1 and overall salt contents of ~5%.

A possible subduction does not mean that subduction happens. For that, you need a cause, which would trigger activity in the satellite.

Triggering the subduction

The authors propose the following two causes for subduction to happen:

  1. Tidal interaction with Jupiter, enhanced by non-synchronous rotation: Surface features revealed by Galileo are consistent with a crust which would not rotate synchronously, as expected for the natural satellites, but slightly faster, the departure from supersynchronicity inducing a full rotation with respect to the Jupiter-Europa direction between 12,000 and 250,000 years… to be compared with an orbital period of 3.55 days. So, this is a very small departure, which would enhance the tidal torque of Jupiter, and trigger some activity. This interpretation of the surface features as a super-synchronous rotation is controversial.
  2. Convection, i.e. fluid motion in the ocean, due to the variations of temperature.

No doubt Europa Clipper and maybe JUICE will tell us more!

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

The mountainous equator of Iapetus

Hi there! You may have heard of Iapetus, a large satellite of Saturn which orbits very far. Today I will tell you about its equatorial ridge. This is the opportunity for me to present you Testing models for the formation of the equatorial ridge on Iapetus via crater counting, by Amanda L. Damptz, Andrew J. Dombard & Michelle R. Kirchoff, which has recently been accepted for publication in Icarus.

The satellite Iapetus

The satellite of Saturn Iapetus was discovered by Giovanni Domenico Cassini in October 1671. It orbited then on the western side of Saturn. During many years, i.e. until 1705, he was unable to observe it on the eastern side, since it was two magnitudes fainter. This has two implications:

  1. Iapetus has a two-tone coloration, i.e. a dark and a bright hemisphere,
  2. its rotation is synchronous. Like our Moon, it is locked in the synchronous 1:1 spin-orbit resonance, constantly showing the same face to Saturn.

Beside this, Iapetus is a large body (diameter: 1,470 km), which orbits at 3.5 millions km from Saturn (for comparison, Titan orbits almost thrice closer), with an orbital eccentricity of 0.028, very close to the one of Titan. It has an unexpectedly high orbital inclination, i.e. 15.47° with the equator of Saturn, and 8.13° with the Laplace Plane. We should imagine the proto-Saturn nebula, from which Iapetus has probably been formed. It was pretty much like a disc, but distorted by the Sun if you were far enough from Saturn, which is the case for Iapetus. What I mean is that the gravitational action of the Sun tends to shift the equilibrium orbital plane from the equatorial one, this is why we need to distinguish it, and we call it the Laplace Plane. In that case, the orbital inclination of Iapetus with respect to the Laplace Plane should be very small, but it is not. This probably contains an information on the history of Iapetus, but we do not know which one yet.

Let me go back to the rotation. Iapetus is so far from Saturn that it needs almost 80 days to complete one revolution, and 80 days for a rotation, since it is synchronous. This is the largest known spin period for a natural satellite in the Solar System.

As most of the satellites of Saturn, our knowledge of Iapetus made invaluable progress since 2004 thanks to the Cassini spacecraft, which imaged it. It confirmed the two-tone coloration, and detected a mountainous equatorial ridge.

Iapetus seen by the Cassini spacecraft. © NASA
Iapetus seen by the Cassini spacecraft. © NASA

The equatorial ridge

The Cassini images showed a 20 km-wide mountainous ridge, which is very close to the equator. So close that it is hard to believe it appeared there by chance. It is present on the dark hemisphere, while isolated equatorial mountains can be seen on the bright side. Some peaks reach 20 km.

Since its discovery late 2004, this ridge is a matter of investigation, and several competing explanations can be found in the literature:

  1. A signature of its past, fast rotation (the measured oblateness of Iapetus is consistent with a rotation period of 16 hours)
  2. A signature of a past critical spin state, i.e. close to provoke disruption of Iapetus,
  3. Upwarping of the lithosphere from below,
  4. Cryovolcanism,
  5. Planetary contraction,
  6. Material from an ancient ring system,
  7. Material from impact generated debris.

We can see that some of these scenarios propose an inner (endogenic) cause, while others propose an outer (exogenic) one. Almost all of them suggest an early formation of the ridge, except the last one.
One way to date a geological feature is to count its craters, and this is where this study intervenes. Its first product is a database of 7,748 craters ranging from 0.83 to 591 km in diameter.

Counting the craters

When an impactor reaches a planetary surface, it creates a crater. If one day geological processes are strong enough to create a tectonic feature, then it may at least alter the crater, or even hide it. If we see an uncraterized geological feature, that means that it is pretty young. We could even try to give it an age in estimating the evolution of the cratering rates over the evolution of the Solar System. By the way, the early Solar System was very intensively bombarded, with an episode of Late Heavy Bombardment occurring between 4.1 and 3.8 billions years ago. Bombardments still happen nowadays, but are much less frequent.

In this study the authors worked from Cassini and Voyager images of the surface of Iapetus, and considered different zones: central ridge, peripheral ridge, and off ridge. Moreover they classified the craters following their diameters, so as to estimate a distribution law: number of craters vs. size. They also catalogued the orientation of the deformations of the craters, since it could tell us something on the geological evolution of Iapetus (how did it alter the surface?)

This systematic search for craters was assisted by the commercial software Esri’s ArcGIS, supplemented by the dedicated add-on Crater Helper Tools.

Results

The first result is a database of 7,748 craters. But the main question is: what can we say about the ridge? The authors observe a depletion of large craters, i.e. with a diameter bigger than 16 km, in the ridge, which would be consistent with a pretty recent formation, and thus would favor the scenario of a ridge created by the debris of an impact. Nevertheless, the authors are prudent with this conclusion, they seem to suggest that the resolution of the images and the risk of saturation of small craters (when you are heavily bombarded, new craters destroyed ancient ones, and the overall number does not increase) do not permit to discard a scenario of early formation of the ridge. Further studies will probably be needed to reach an agreement on the origin of this mountainous equator.

The study and its authors

That’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

The lowlands of Mars

Hi there! Today I will give you the composition of the subsurface of the lowlands of Mars. This is the opportunity for me to present you The stratigraphy and history of Mars’ northern lowlands through mineralogy of impact craters: A comprehensive survey, by Lu Pan, Bethany L. Ehlmann, John Carter & Carolyn M. Ernst, which has recently been accepted for publication in Journal of Geophysical Research: Planets.

Low- and Highlands

Topography of Mars. We can see lowlands in the North, and highlands in the South. © USGS
Topography of Mars. We can see lowlands in the North, and highlands in the South. © USGS

As you can see on this image, the topography of Mars can be divided into the Northern and the Southern hemispheres, the Northern one (actually about one third of the surface) being essentially constituted of plains, while the Southern one is made of mountains. The difference of elevation between these two hemispheres is between 1 to 3 km. Another difference is the fact that the Southern hemisphere is heavily cratered, even if craters exist in the lowlands. This Martian dichotomy is very difficult to explain, some explanations have been proposed, e.g., the lowlands could result from a single, giant impact, or the difference could be due to internal (tectonic) processes, which would have acted differentially, renewing the Northern hemisphere only… Anyway, whatever the cause, there is a dichotomy in the Martian topography. This study examines the lowlands.

The lowlands are separated into: Acidalia Planitia (for plain), Arcadia Planitia, Amazonis Planitia, Chryse Planitia, Isidis Planitia, Scandia Cavi (the polar region), Utopia Planitia, Vatistas Borealis,…

Plains also exist in the Southern hemisphere, like the Hellas and the Argyre Planitiae, which are probably impact basins. But this region is mostly known for Olympus Mons, which is the highest known mountain is the Solar System (altitude: 22 km), and the Tharsis Montes, which are 3 volcanoes in the Tharsis region.

To know the subsurface of a region, and its chemical composition, the easiest way is to dig… at least on Earth. On Mars, you are not supposed to affect the nature… Fortunately, the nature did the job for us, in bombarding the surface. This bombardment was particularly intense during the Noachian era, which correspond to the Late Heavy Bombardment, between 4.1 to 3.7 Gyr ago. The impacts excavated some material, that you just have to analyze with a spectrometer, provided the crater is preserved enough. This should then give you clues on the past of the region. Some say the lowlands might have supported a global ocean once.

The past ocean hypothesis

Liquid water seems to have existed at the surface of Mars, until some 3.5 Gyr ago. There are evidences of gullies and channels in the lowlands. This would have required the atmosphere of Mars to be much hotter, and probably thicker, than it is now. The hypothesis that the lowlands were entirely covered by an ocean has been proposed in 1987, and been supported by several data and studies since then, even if it is still controversial. Some features seem to be former shorelines, and evidences of two past tsunamis have been published in 2016. These evidences are channels created by former rivers, which flowed from down to the top. These tsunamis would have been the consequences of impacts, one of them being responsible for the crater Lomonosov.

The fate of this ocean is not clear. Part of it would have been evaporated in the atmosphere, and then lost in the space, part of it would have hydrated the subsurface, before freezing… This is how the study of this subsurface may participate in the debate.

The CRISM instrument

To study the chemical composition of the material excavated by the impacts, the authors used CRISM data. CRISM, for Compact Reconnaissance Imaging Spectrometer for Mars, is an instrument of Mars Reconnaissance Orbiter (MRO). MRO is a NASA spacecraft, which orbits Mars since 2006.
CRISM is an imaging spectrometer, which can observe both in the visible and in the infrared ranges, which requires a rigorous cooling of the instrument. These multi-wavelengths observations permit to identify the different chemical elements composing the surface. The CRISM team summarizes its scientific goals by follow the water. Studying the chemical composition would permit to characterize the geology of Mars, and give clues on the past presence of liquid water, on the evolution of the Martian climate,…

In this study, the authors used CRISM data of 1,045 craters larger than 1 km, in the lowlands. They particularly focused on wavelengths between 1 and 2.6μm, which is convenient to identify hydrated minerals.

Hydrated vs. mafic minerals

The authors investigated different parts of the craters: the central peak, which might be constituted of the deepest material, the wall, the floor… The CRISM images should be treated, i.e. denoised before analysis. This requires to perform a photometric, then an atmospheric correction, to remove spikes, to eliminate dead pixels…

And after this treatment, the authors identified two kinds of minerals: mafic and hydrated ones. Mafic minerals are silicate minerals, in particular olivine and pyroxenes, which are rich in magnesium and iron, while hydrated minerals contain water. They in particular found a correlation between the size of the crater and the ratio mafic / hydrated, in the sense that mafic detections are less dependent on crater size. Which means that mafic minerals seem to be ubiquitous, while the larger the crater, the likelier the detection of hydrated minerals. Since larger craters result from more violent impacts, this suggests that hydrated minerals have a deeper origin. Moreover, no hydrated material has been found in the Arcadia Planitia, despite the analysis of 85 craters. They also noticed that less degraded craters have a higher probability of mineral detection, whatever the mineral.

However, the authors did not find evidence of concentrated salt deposits, which would have supported the past ocean hypothesis.

The study and the authors

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