There was water on Mars

Hi there! Well, we already knew that there has been liquid water on the surface of Mars, a long time ago. Indeed, the space mission Mariner 9 imaged valley networks in 1972. Since then, several missions refined the data. The study I present today, Estimate of the water flow duration in large Martian fluvial systems, by Vincenzo Orofino, Giulia Alemanno, Gaetano Di Achille and Francesco Mancarella, uses the most recent observations to estimate the length and depth of former Martian rivers, and their duration of formation and erosion. This study has recently been accepted for publication in Planetary and Space Science.

Evidences of liquid water in the past

The current atmosphere of Mars is pretty thin, its pressure being on average 0.6% the one of the Earth. Such a small atmospheric pressure prevents the existence of liquid water at the surface. Water could survive only as ice, otherwise would be just vaporized. And ice water has been found, particularly in the polar caps. But if the atmosphere were thicker in the past, then liquid water would have survived… and we know it did.

We owe to Mariner 9 a map of 85% of the Martian surface, which revealed in particular river beds, deltas, and lake basins. The study we discuss today focused on valley networks, which are particularly present in the southern highlands of Mars. These valleys are typically less than 5 km wide, but may extend over thousands of kms, and they reveal former rivers.

Nirgal Vallis seen by Mariner 9. © NASA
Nirgal Vallis seen by Mariner 9. © NASA

The history of these rivers is inseparable from the geological history of Mars.

The geologic history of Mars

We distinguish 3 mains eras in the geological history of Mars: the Noachian, the Hesperian, and the Amazonian.

The Noachian probably extended between 4.6 and 3.7 Gyr ago, i.e. it started when Mars formed. At that time, the atmosphere of Mars was much thicker that it is now, it generated greenhouse effect, and liquid water was stable on the surface. It even probably rained on Mars! During that era, the bombardment in the inner Solar System, including on Mars, was very intense, but anyway less intense than the Late Heavy Bombardment, which happened at the end of the Noachian. Many are tempted to consider it to be the cause of the change of era. Anyway, many terrains of the south hemisphere of Mars, and craters, date from the Noachian. And almost all of the river beds as well.

After the Noachian came the Hesperian, probably between 3.7 and 3.2 Gyr ago. It was a period of intense volcanic activity, during which the bombardment declined, and the atmosphere thinned. Then came the Amazonian, which is still on-going, and which is a much quieter era. The volcanic activity has declined as well.

So, almost all of the valley networks date from the Noachian. Let us now see how they formed.

Use of recent data

We owe to the space missions accurate maps of Mars. From these maps, the authors have studied a limited data set of 63 valley networks, 13 of them with a interior channel, the 50 remaining ones without. The interior channel is the former river bed, while the valley represents the area, which has been sculpted by the river. The absence of interior channel probably means that either they are too narrow to be detectable, or have been eroded.

These valley networks are located on sloppy areas, most of them close to the equator. The authors needed the following information:

  1. area,
  2. eroded volume,
  3. valley slopes,
  4. width and depth of the interior channel.

To get this information, they combined topographic data from the instrument MOLA (for Mars Orbiter Laser Altimeter) on board Mars Global Surveyor (1997-2006) with THEMIS (THermal Emission Imaging System, on board Mars Odyssey, still operating). MOLA permits 3-D imagery, with a vertical resolution of 30 cm/pixel (in other words, the accuracy of the altitude) and a horizontal one of 460 m/pixel, while the THEMIS data used by the authors are 2D-data, with a resolution of 100 m /pixel. When the authors judged necessary, they supplemented these data with CTX data (ConTeXt camera, on board Mars Reconnaissance Orbiter, still ongoing), with a resolution up to 6 m/pixel.

These information are very useful to estimate the formation time and the erosion rate of the valley network.

Dynamics of formation of a river bed

They estimated these quantities from the volume of sediments, which should have been transported to create the valley networks. The idea is, while water is flowing, assisted by the Martian surface gravity (fortunately, this number is very well known, and is roughly one-third of the gravity on Earth) and by the slope, it transports material. The authors assumed in their calculations that this material was only sediments, i.e. they neglected rock transport, and they did the maths.

Several competing models exist for sediment transport. This is actually difficult to constrain, given the uncertainties on the sediments themselves. Such phenomena also exist on Earth, but the numbers are very different for instance if you are in Iceland or in the Atacama Desert.

It also depends on the intermittence: is the water flow constant? You can say yes to make your life easier, but is it true? On Earth, you have seasonal variations… why not on Mars? A constant water flow means an intermittence of 100%, while no water means 0%.

And keep also in mind that the water flow depends on the atmospheric conditions: is the air wet or pretty arid? We can answer this question for the present atmospheric conditions, but how was it in the Noachian?

No icy Noachian

And this is one result of the present study: there must have been some evaporation in the Noachian, which means that it was not cold and icy. The authors show that such a Noachian would be inconsistent with the valley networks, as we presently observe them.

However, they get large uncertainties on the formation timescales of the valley networks, i.e. between 500 years and almost twice the age of the Solar System. They have anyway median numbers, i.e.

  • 30 kyr for a continuous sediment flow,
  • 500 kyr with an intermittence of 5%,
  • 3 Myr with an intermittence of 1%,
  • 30 Myr with an intermittence of 0.1%.

And from the data, they estimate that the intermittence should be in the range 1%-5%, which corresponds humid (5%) and semiarid/arid environments. This is how they can rule out the cold and icy Noachian.

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, Facebook, Instagram, and Pinterest.

Evolution of Venus’ crust

Hi there! Of course, you know Venus. This planet is sometimes nicknamed the twin sister of the Earth, but beside its size, it does not look like the Earth. Venus is closer to the Sun than us, and it has a very thick atmosphere, which is essentially composed of carbon dioxide. This atmosphere has a pressure of 93 bar at the surface of the planet, to be compared with 1 bar for the Earth, and the temperature reaches there 470°C. Definitely hostile.

Anyway, I do not speak of the atmosphere today, but of the surface. I present Inferences on the mantle viscosity structure and the post-overturn evolutionary state of Venus, by T. Rolf and collaborators, which has recently been published in Icarus.

The interior of Venus

Given its size, i.e. a diameter of 12,000 km, which is 95% of the one of the Earth, Venus must be differentiated. It has a crust, a mantle, and core, with increasing densities when you go deeper below the surface. We think the crust to be essentially basaltic, while the core must contain heavy elements. Surprisingly, the space missions did not detect any magnetic field, which means that the core may be not solid, or may be not cooling…

The outer part of the mantle should be fluid, which means that a fluid layer separates the core from the mantle. We know very few of the thicknesses and the compositions of these different layers. Actually, these could only be guessed from the measurements we dispose on, which are the gravity and the topography (see just below). Once you know the gravity field of Venus and its topography, you can elaborate interior models, which would be consistent with your data.

Gravity and topography

First, gravity. When a small body, like an artificial satellite, orbits a spherical planetary body, the gravitational perturbation affecting its motion depends only on the distance between the satellite and the planet. Now, if the planet is not spherical, and has mass anomalies, then the perturbation will not only depend on the distance, but also on the direction planet-satellite. You can determine the gravity field from the orbital deviation of your spacecraft.

It is convenient to write the gravity field as a sum of spherical harmonics. The first term (order 0) is a spherical one, then the order 2 (you have no order 1 if the center of your reference frame is the center of mass) represents the triaxiality of the planet, i.e. the planet seen as a triaxial ellipsoid. And the higher order terms will represent anomalies, with increasing resolutions. These resolutions are modeled as spatial periods. Such a representation has usually an efficient convergence, except for highly elongated bodies (see here).

We use such a representation for the topography as well. The difference is that the result is not the gravity field in any direction, but the altitude of the surface for a given point, i.e. a latitude and a longitude. The spacecraft measure the topography with a laser, which echo gives you the distance between the spacecraft and the surface. The altitude is directly deduced from this information.

Topography of Venus. The altitude variations are about 13 km with respect to a reference ellipsoid. © Calvin Hamilton, Johns Hopkins University Applied Physics Laboratory
Topography of Venus. The altitude variations are about 13 km with respect to a reference ellipsoid. © Calvin Hamilton, Johns Hopkins University Applied Physics Laboratory

The best representations we dispose on for Venus come from the American spacecraft Magellan, which orbited Venus between 1990 and 1994. These representations go to the order 180.

Modeling the crustal evolution

In this study, the authors simulated possible evolutionary paths for the crust of Venus, and compared their results with the present Venus, i.e. the gravity and topography as we know them.

For that, they simulated the thermochemical evolution of Venus in using a numerical code, StagYY. This is a 3D-code, which models convection in the mantle, i.e. internal motions. This code is based on finite elements, i.e. the interior of Venus is split into small elements. This splitting is made following a so-called Yin-Yang grid, which is appropriate for spherical geometries. This code includes several features like phase transition (i.e. from solid to fluid, and conversely), compositional variations, partial melting and melt migration. Moreover, it is implemented for parallel computing.

In other words, these are huge calculations. The authors started with 10 simulations in which the crust was modeled as a single plate, i.e. a stagnant lid. The simulations differed by the modeling of the viscosity, and by the radiogenic heating rate. This is the heating of Venus by the decay of the radiogenic elements, which was most effective in the early Solar System.

Once these 10 simulations have run, the authors kept the one, which resulted in the closest Venus to the actual one, and introduced episodic overturns in it.

Stagnant-lid vs. overturn

Venus does not present any tectonic activity. Did it have some in the past? This is a question this study tried to answer.

An overturn is a sudden peak in the heat transfer from the core to the crust through the mantle, due to a too strong difference of temperature, i.e. when the mantle gets colder. Such an episodic phenomenon is triggered by a too thick crust, and results in a melting of this crust, in heating it. In other words, it regulates the thickness of the crust.

Overturns should have happened

And here are the results: the best stagnant-lid scenario, called S2 in the study, presents some discrepancy between the simulated present Venus and the observed one. These discrepancies are present in the topography, in the gravity field, and in the age of the surface. The surface is estimated to be between 0.3 and 1 Gyr old, while the best stagnant-lid scenario predicts that the most probable age is 0.25 Gyr… a little too young.

However, episodic overturns give a surface, which is 0.6 Gyr old. Moreover, the gravity and topography are much better fit. The only remaining problem is that this scenario should result in much detections of plumes than actually detected.

As the authors honestly recall, some physical phenomena were not considered, in particular the influence of the dense atmosphere, and intrusive volcanism. Anyway, this study strongly suggests that episodic overturn happened.

Further data will improve our understanding of Venus. Recently, the European Space Agency (ESA) has pre-selected 3 potential future space missions, including EnVision, i.e. an orbiter around Venus. The final decision is expected in 2021.

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, Facebook, Instagram, and Pinterest.

Saturn sends us meteorites

Hi there! First I would like to thank you for following me on Facebook. The Planetary Mechanics page has reached 1,000 followers!

OK, now back to business. Did you know that our Earth is intensively bombarded from space? You have recently heard of this Chinese space station, Tiangong-1… in that case, it was man-made stuff. But we are intensively bombarded by natural space material. Most of it is so small that it is destroyed when entering the atmosphere, but sometimes it arrives to us as stones… And in extreme cases, the impactor is so large that its impact may generate an extinction event. The Chicxulub crater, in Mexico, is thought to result from the impact, which aftermath provoked the extinction of the dinosaurs, some 66 Myr ago.

The meteorites I speak about today are the ones, which fall on the Earth every year. This is the opportunity to discuss about Identification of meteorite source regions in the Solar System, which has recently been accepted for publication in Icarus. In that study, the authors determine the origin of 25 meteorites, from their observed trajectories just before they hit us.

Meteorites bombard the Earth

We estimate that currently 60 tons of cosmic material fall on the Earth every day. This seems huge, but actually most of it arrives to us as dust, since the original object does not survive its entry into the atmosphere. In fact, the larger the meteorite, the less frequent it falls on us. 4-m objects arrive every ~16 months, 10-m ones every ~10 years, and 100-m ones every ~5,200 years. And they arrive somewhere on Earth… do not forget that most of the surface of our planet is water. So, don’t worry.

The contact of such a small object with the atmosphere may generate an airburst, which itself could be detected, in many frequencies. I mean, you may hear it, you may see it (make a wish), it can also disturb the radio emissions. This motivated the existence of several observation programs, dedicated to the detection of meteors.

Observation networks

Programs of observation exist at least since 1959, originally under the impulse of Ondřejov Observatory (Czech Republic). These are usually national programs, e.g.

and there are probably more. These are networks of camera, which systematically record the sky, accumulating data which are then automatically treated to detect meteors. The detection of a meteors from different location permit to determine its trajectory.

Detection of a fireball by FRIPON, in September 2016. © FRIPON
Detection of a fireball by FRIPON, in September 2016. © FRIPON

Identifying the source

As I said, multiple detections, at different locations, of a fireball, permit to derive its trajectory. This trajectory gives in particular the radiant, which is the direction from which the meteorite, or the impactor, seems to come. The authors are also interested in the velocity of the object.

The velocity and the radiant are determined with respect to the Earth. Once they are determined, the authors translated them into heliocentric elements, i.e. they determined the pre-impact trajectory of the object with respect to the Sun. And this makes sense, since Solar System objects orbit the Sun! This trajectory is made of orbital elements, i.e. semimajor axis, eccentricity, inclination, and the uncertainties associated. Don’t forget that the observations have an accuracy, which you must consider when you use the data. The magnitude of the fireball tells us something on the size of the impactor as well.

From these data, the authors wondered from where the object should come from.

7 candidates as reservoirs of meteorites

The authors identified 7 possible sources for these impactors. These regions are the densest parts of the Main Asteroid Belt.
These are:

  1. the Hungaria family. These asteroids have a semimajor axis between 1.78 and 2 astronomical units, and an inclination between 16° and 34° with respect to the ecliptic, i.e. the orbit of the Earth,
  2. the ν6 resonance: these are bodies, which eccentricity raise because excited by Saturn. They orbit at a location, where they are sensitive to the precessional motion of the pericentre of Saturn. The raise of their eccentricity make these bodies unstable, and good candidates for Earth-crossers. Their semimajor axis is slightly smaller than 2 AU.
  3. the Phocaea family: this is a collisional family of stony asteroids. Their semimajor axes lie between 2.25 and 2.5 AU, their eccentricities are larger than 0.1, and their inclinations are between 18° and 32°. They are known to be a source of Mars-crossers.
  4. the 3:1 MMR (mean-motion resonance with Jupiter): these bodies perform exactly 3 orbits around the Sun while Jupiter makes one. They lie at 2.5 AU. The perturbation by Jupiter tends to empty this zone, which is called a Kirkwood gap.
  5. the 5:2 MMR, at 2.82 AU. This is another Kirkwood gap.
  6. the 2:1 MMR, at 3.27 AU, also known as Hecuba gap,
  7. the Jupiter Family Comets. These are comets, which orbital periods around the Sun are shorter than 20 years, and which inclinations are smaller than 30° with respect to the ecliptic. They are likely to be significantly perturbed by Jupiter.

For each of the 25 referenced meteorites, the authors computed the probability of each of these regions to be the source, in considering the orbital elements (semimajor axis, eccentricity, and inclination) and the magnitude of the object. Indeed, the magnitude is correlated with the size, which is itself correlated with the material constituting it. The reason is that these Earth-crossers orbit the Sun on eccentric orbits, and at their pericentre, i.e. the closest approach to the Sun, they experience tides, which threaten their very existence. In other words, they might be disrupted. Particularly, a large body made of weak material cannot survive.

And now, the results!

Saturn send meteorites to the Earth!

The authors find that the most probable source for the meteorites is the ν6 secular resonance, i.e. with Saturn. In other words, Saturn sends meteorites to the Earth! Beside this, the Hungaria family and the 3:1 mean-motion resonance with Jupiter are probable sources as well. On the contrary, you can forget the Phocaea family and the 2:1 MMR as possible sources.
It appears that the inner belt is more likely to be the source of meteorites than the outer one. Actually, the outer belt mostly contains carbonaceous asteroids, which produce weak meteoroids.

The authors honestly recall that previous studies found similar results. Theirs also contains an analysis of the influence of the uncertainty on the trajectories, and of the impact velocity with the Earth. This influence appears to be pretty marginal.

Anyway, the future will benefit from more data, i.e. more detections and trajectory recoveries. So, additional results are to be expected, just be patient!

The study and its authors

  • You can find the study here, on the website of Icarus. This study is in open access, which means that the authors paid extra fees to make the study available to us. Many thanks to them!
  • You can visit here the website of Mikael Granvik, the first author of the study,
  • and the one of the second author, Peter Brown.

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, Facebook, Instagram, and Pinterest.