Tag Archives: chemistry

Forming Mars

Hi there! Of course, you know the planet Mars. You can here from it these days, since it is exceptionally close to our Earth. Don’t worry, this is a natural, geometrical phenomenon.

Anyway, it is a good time to observe it. But I will not speak of observing it, today. We will discuss its formation instead, because the issue of the formation of Mars remains a challenge. This is the opportunity to present The curious case of Mars’ formation, by James Man Yin Woo, Ramon Brasser, Soko Matsumura, Stephen J. Mojzsis, and Shigeru Ida. Astronomy and Astrophysics will publish it pretty soon.

Mars is too small

The following table gives you comparative characteristics of Venus, the Earth, and Mars.

Venus Earth Mars
Semimajor axis 0.723 AU 1.000 AU 1.524 AU
Eccentricity 0.007 0.017 0.093
Inclination 3.39° 1.85°
Orbital period 224.7 d 365.25 d 686.96 d
Spin period -243.02 d 23.93 h 24.62 h
Mean diameter 12,104 km 12,742 km 6,779 km

The last line reveals a problem: Venus and the Earth are about the same size, while Mars is much smaller! But this is not the only problem: the compositions of the Earth and Mars are VERY different.

It is pretty easy to know the composition of the Earth: you just analyze samples. And for Mars? Just the same!

Interestingly, there are Martian meteorites on Earth. These are ejecta from impacts, which were ejected from Mars, and then traveled in the Solar System, until reaching our Earth.

In fact, over the tens of thousands of meteorites which have been found on Earth, a little more than one hundred were significantly different than the other ones, i.e. younger formation ages, a different oxygen isotopic composition, the presence of aqueous weathering products… Most of these meteorites were known as SNC, after the three groups they were classified into:

  • S for Shergottites, after the Shergotty meteorite (India, 1865),
  • N for Nakhlites, after the Nakhla meteorite (Egypt, 1911),
  • C for Chassignites, after the Chassigny meteorite (France, 1815).

Such a significant number of similar meteorites, which are that different from the other ones, suggests they come from a large body. Mars is an obvious candidate, which has been confirmed after the discovery that trapped gases in these meteorites are very similar to the ones, which are present in the atmosphere of Mars.

The Martian meteorite NWA (Northwest Africa) 2046, found in September 2003 in Algeria. This is a Shergottite. © Michael Farmer and Jim Strope.
The Martian meteorite NWA (Northwest Africa) 2046, found in September 2003 in Algeria. This is a Shergottite. © Michael Farmer and Jim Strope.

After that, the numerous space missions improved our knowledge of the Martian composition. And it finally appeared that both planets are essentially made of chondritic material. The Earth should accrete about 70% of enstatite chondrite (and same for the Moon), while Mars only about 50%. Chondrites are non-metallic meteorites, the enstatite chondrites being rich in the mineral enstatite (MgSiO3). These numbers are derived from the documented isotopic compositions of the Earth and Mars, i.e. the ratio of the different chemical elements. An isotope is a variant of a particular chemical element, which differs in neutron number.

If you want to convincingly simulate the formation of Mars, the product of your simulations should be similar to Mars in mass AND in composition. And this is very challenging. Let us see why, but first of all let us recall how to form planets from a disk.

Forming planets from a disk

At its early stage, a planetary system is composed of a proto-star, and a pretty flat disk, made of gas and dust. Then the dust accretes into clumps, which then collides to form planetary embryos, i.e. proto-planets. These embryos continue to grow with collisions, until forming the current planets. Meanwhile, the gas has dissipated.

Anyway, interactions between the protoplanets and between them and the gas can lead to planetary migration. This means that we cannot be sure whether the planets we know formed close to their current location. This makes room for several scenarios.

Two models of planetary formation

The obvious starting point is to assume that the planets formed close to their current locations. This so-called Classical model works pretty well for Venus, the Earth, Jupiter, Saturn… but not for Mars. The resulting Mars is too massive.

An idea for by-passing this problem is to start with a depletion of material at the location of Mars. This is equivalent to an excess inside the terrestrial orbit. In such a configuration, less material is available to the proto-Mars, which eventually has a mass, which is close to the present one.

You can get this excess of material inside the terrestrial orbit if you buy the Grand Tack scenario: when Jupiter formed, it created a gap in the inner disk, and the mutual interaction resulted in an inward migration of Jupiter, until reaching the present orbit of Mars. In moving inward (Type II migration), Jupiter pushed the material inward. Then, a 3:2 mean-motion resonance with Saturn occurred, which created another gap, and made Jupiter move outward, until its present location.

This way, you can form a planetary object, which is similar to Mars in mass and location.

But what about its composition?

The composition challenge

This is still a challenge. The composition of a planetary object is strongly affected by the one of the disk, where the object formed… which may not be its present location.

The authors added a free parameter to the model: the break location, which would split the protoplanetary disk into an inner and an outer region. The inner region would be rich in enstatite chondrites, while the outer one would be rich in ordinary chondrites.

A break location at 1.3 AU gives the best fit for the difference of composition between Mars and the Earth, for both formation scenarios (Classical and Grand Tack).

So, the Grand Tack with a break location at 1.3 AU could be the right scenario. But another possibility exists: the Classical scenario says that if Mars formed where it is, then it should be heavier. But what if Mars formed actually further from the Sun, and then migrated inward? Then, it would not need any depletion of material to have the right mass. And the break barrier should have been further than 1.3 AU. But you have to explain why it migrated inward.

Anticipating the composition

One of the good things with scenarios of formation is that thr gives more details on the outcomes, than actually observed. For instance, this study predicts the isotopic composition of 17O, 50Ti, 54Cr, 142Nd, 64Ni and 92Mo, in the Martian mantle. Further data, collected by space missions, will give additional constraints on these parameters, and test the validity of the present study. 8 missions are currently operational in orbit or on Mars, and InSight is en-route, after having been launched in May 2018. It should land on Mars on November 26, and will study its interior with a seismometer, and a heat transfer probe.

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.

Uranus’s satellites are red

Hi there! Today, we speak about the satellites of Uranus. They have been visited only once by a spacecraft, i.e. Voyager 2 in 1986, but we dispose of Earth-based facilities, which are able to give us some clues. The study I present today, Red material on the large moons of Uranus: Dust from irregular satellites? discusses the fact that the main moons appear to be redder than the large moons of Jupiter and Saturn.

Let me define redder first: the surface of these bodies reflects the Solar incident light. A spectral decomposition of the reflected light tells us something on the material coating the surface. And the relative response of the surface in the different wavelengths is higher in the infrared for the large moons of Uranus, than it is for the moons of Jupiter and Saturn.

This study, by Richard Cartwright et al., has recently been published in Icarus.

The satellites of Uranus

First: Uranus. This is the seventh planet of our Solar System, which orbits in 84 years, and which seems to roll on its orbit. Actually, its rotation axis is tilted by nearly 90° (actually 97.8°), and its main satellites and rings orbit close to its equatorial plane. Their orbits are tilted as well.

The satellites of Uranus, all named after Shakespeare’s characters, can be categorized into 3 groups:

  1. The 13 small, inner satellites, which are embedded into the rings,
  2. the 5 main ones,
  3. and the 9 irregular satellites, which orbit much further from Uranus, and which orbits may be tilted. Contrary to the other two groups, they have probably not been formed in the proto-Uranus nebula, but were former asteroids, which have been trapped by Uranus.

You can find below some properties and orbital characteristics of the main satellites. All of these bodies have been discovered from the Earth. These are the targets of the study I present.

Discovery Semimajor axis Eccentricity Inclination Orbital period Mean diameter
Miranda 1948 129,900 km 0.0013 4.338° 1.413 d 471.6 km
Ariel 1851 190,900 km 0.0012 0.041° 2.520 d 1,157.8 km
Umbriel 1851 266,000 km 0.0039 0.128° 4.144 d 1,169.4 km
Titania 1787 436,300 km 0.0011 0.079° 8.706 d 1,577.8 km
Oberon 1787 583,500 km 0.0014 0.068° 13.46 d 1,522.8 km

You can see that they have limited eccentricities and inclinations, except for the inclination of Miranda, which probably results from a past resonant forcing by Umbriel. In the past, the orbital period of Umbriel was exactly thrice the one of Miranda, and this has forced its inclination, which was thus initially very small. Anyway, it remains close to the equatorial plane for Uranus.

You can see below that things are different for the small satellites.

Discovery Semimajor axis Eccentricity Orbital period Mean diameter
Francisco 2001 4,282,900 km 0.13 267 d ≈22 km
Caliban 1997 7,231,100 km 0.18 580 d ≈72 km
Stephano 1999 8,007,400 km 0.22 677 d ≈32 km
Trinculo 2001 8,505,200 km 0.22 749 d ≈18 km
Sycorax 1997 12,179,400 km 0.52 1,288 d ≈150 km
Margaret 2003 14,146,700 km 0.68 1,661 d ≈20 km
Prospero 1999 16,276,800 km 0.44 1,978 d ≈50 km
Setebos 1999 17,420,400 km 0.59 2,225 d ≈48 km
Ferdinand 2001 20,430,000 km 0.40 2,790 d ≈20 km

These are very small bodies, which orbit very far from Uranus, on eccentric orbits. Besides, their orbital planes have just nothing to do with the equatorial plane of Uranus. This is why we believe they are former asteroids. Beside Margaret, they all orbit on retrograde orbits, while all the regular moons are prograde. Discovering them required to use telescopes of a least 5 m, the satellites discovered in 2003 having been discovered during a systematic survey with the Subaru telescope at Mauna Kea, Hawaii, over a field of 3.5 degrees. They all have apparent magnitudes larger than 20.

Only one space mission visited them: Voyager 2, in January 1986.

Voyager 2 at Uranus

The spacecraft Voyager 2 was launched from Cape Canaveral, Florida, in August 1977. It benefited from a favorable geometrical configuration of the 4 giant planets to visit all of them. Unfortunately, this required the spacecraft to travel too fast to permit an orbital insertion. So, contrary to Cassini which toured around Saturn during 13 years, Voyager 2 just passed by.

Its closest approach to Uranus was on January 24, 1986, at a distance of 81,500 km from the planet’s cloud tops. It permitted the discovery of 11 inner satellites, and partly imaged the large ones. It revealed in particular geological features on Miranda, and analyzed the light reflected by the surface of these bodies. The study we discuss today supplements these measurements.

Miranda seen by Voyager 2. © NASA/JPL/USGS
Miranda seen by Voyager 2. © NASA/JPL/USGS

Observations at IRTF

The authors used NASA’s InfraRed Telescope Facility (IRTF). This is a 3-meter telescope, optimized for infrared astronomy. It is located at the Mauna Kea Observatory (altitude: 4,200 m) in Hawaii (USA), and 50% of the observation time is devoted to planetary observation.
Several instruments are available, the authors used the spectrograph-imager SpeX, which decomposes the incident light between 0.8 and 5.4 µm. In that study, the authors limited to 4.2 µm.

NASA's InfraRed Telescope Facility at Maune Kea, Hawaii. © Afshin Darian
NASA’s InfraRed Telescope Facility at Maune Kea, Hawaii. © Afshin Darian

The outcome of such observations is a plot amplitude vs. wavelength of a given surface element of a satellite. It is interesting to keep in mind that the regular moons rotate synchronously, permanently showing the same face to Uranus. The consequence is that they have a leading and a trailing hemisphere. During their orbital motion, the same hemisphere always leads. And this has implications for the surface composition, because the leading hemisphere can be polluted by the dusty environment. In other words: when you observe something on the leading hemisphere, which is not present on the trailing one, this is probably pollution.

When you observe, you actually observe the surface element which faces you. And this depends on the dynamics of the planet.

Geometrical constraints

As you know, Uranus rolls on its orbit, while the satellites have an equatorial orbit. As a consequence, during a 84-y orbit of Uranus around the Sun, the Earth crosses twice the orbital plane, and two periods are favorable for the observation of the poles of Uranus and the satellites. The northern hemispheres of these bodies face us during half the orbit (42 years), while the southern ones face us during the other half.

The last transition happened in 2007. Since then, the northern hemispheres of the satellites face us. And part of the visible face belongs to the leading hemisphere.

A red leading side

The results show that for Ariel and Umbriel, and even more for Titania and Oberon, the leading hemisphere is significantly redder than the trailing one, while it is not the case for the major satellites of Jupiter and Saturn. Titania and Oberon are the outermost of the satellites of Uranus, and the largest ones as well.

To understand the chemical nature of this reddening, previous studies have conducted lab experiments, consisting in reproducing the spectrum of mixtures of chemical elements, which could be found on the natural satellites of the outer planets. Of course, the conditions of temperature and pressure are considered. Then the spectrums are compared to the actually observed ones. And it appears that the reddening agents should be tholins and pyroxene.

Titania seems to have a red spot on its surface, which makes it the redder of the main Uranian satellites. Contrariwise, Miranda does not present this reddening. Latitudinal variations of color are not obvious, while they are in longitude, since they depend on the leading / trailing effect.

Now, the question is: how did these agents reach the satellites? They are probably not endogenous, since similar satellites around Jupiter and Saturn do not have them.

Pollution by the irregular moons

The smoking gun is the irregular moons: they are pretty red. And numerical simulations of the motion of dust expelled from these satellites by impacts show how they are likely to coat the leading sides of Oberon, Titania, Ariel and Umbriel.
And this is what we observe!

Of course, a space mission to Uranus would be very helpful… but this is another story.

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.

Triton from the Earth

Hi there! Today we speak about Triton. Triton is something unique in the Solar System. It is a large satellite of Neptune, which has almost no mid-sized satellite, contrary to Saturn and Uranus. Several studies show us that Triton could have been a cuckoo, i.e. initially a Trans-Neptunian Objects, which has been trapped by the gravitational field of Neptune, and which is massive enough to have ejected the mid-sized satellites.
Triton has been visited only once, by NASA’s Voyager 2 in 1989. This was only a fly-by, so we have images of only 40% of the surface. But the development of Earth-based instruments now permits to get data from the Earth, even if the apparent magnitude of Triton is 13.47 (Neptune is very far away…).

This is the opportunity to present Triton’s surface ices: Distribution, temperature and mixing state from VLT/SINFONI observations, by F. Merlin, E. Lellouch, E. Quirico and B. Schmitt, all based in France, even if they used European facilities based in Chile. This study has recently been accepted for publication in Icarus.

Triton’s facts

The table below gives you some numbers.

Discovery 1846
Radius 1,353 km
Semimajor axis 354,759 km
Orbital period 5.88 days
Eccentricity 2×10-5
Inclination 157°

You can see first that Triton has been discovered in October 1846, by the British astronomer William Lassell. The discovery happened 10 days after the discovery of Neptune, and this is of course not by chance. When the discovery of a planet is made and announced, it is much easier, or much less difficult, to re-observe it, since you know where to point your telescope. So, Neptune has been reobserved, and Lassell saw this point following Neptune. You can distinguish a satellite from a background star by the fact that the satellite follows the planet in its apparent motion. This was Triton. And Triton was the only known satellite of Neptune during more than one century, until the discovery of Nereid by Gerard Kuiper in 1949.

As I said, Triton is pretty large, and its orbit is almost circular, as for many natural satellites of the giant planets. The tidal interaction with the parent planet tends to circularize the orbit, unless the eccentricity is excited by the action of another satellite… which cannot be the case around Neptune, since no satellite is heavy enough to move Triton.

A trapped object…

The interesting point about the dynamics of Triton is its orbital inclination. While natural satellites usually orbit close to the equatorial plane of their planet, Triton has a very large inclination, and its orbit is retrograde with respect to the rotation of Neptune. Usually, the natural satellites are formed with the planet, in the proto-planetary nebula, which is pretty flat… this is why their inclinations should be small. The inclination of Triton suggests it was not formed in situ, but has been trapped instead. Several studies have simulated this phenomenon, see e.g. your favorite blog.

…visited by Voyager 2

NASA’s Voyager 2 has been launched in August 1977 from Cape Canaveral, and it benefited from a favorable geometrical configuration of the planets. This virtual alignment permitted to fly by Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. Voyager 2 is still operating, as an interstellar mission.

Only a fly by may be frustrating, but for a spacecraft to be inserted an orbit around a planet, it must reach it slowly enough… which means take enough time to reach it. Neptune is so far away that we could not wait. So, its closest approach to Neptune was on August 25, 1989, and the closest to Triton 3 days later, imaging 40% of its surface (see video below, made by Paul Schenk, Lunar and Planetary Institute, Houston, TX, USA).

It revealed an active surface, i.e. few craters, which means that the surface is renewing, and geological features, craters, ridges, plains, maculae… and even volcanic plumes. It also confirmed the presence of an atmosphere.

The atmosphere of Triton

Triton has a very tenuous atmosphere, which pressure is 70,000 times lower than the one of the Earth, i.e. some 14 μbar. It is dominated by nitrogen, and extends 800 kilometers above its surface. Methane and carbon monoxide are also present. Its temperature is around 40 K, and Voyager 2 measured winds from the west from the surface to an altitude of 8 km, where the winds invert their directions, i.e. from east to west.

This atmosphere is probably the result of outgassing of surface material and volcanic activity (geysers). It can be seen as an equilibrium between a solid phase at the surface, and a gaseous one in the atmosphere, of some of its constituents. The study I present today analyses the chemical elements present at the surface, which interact with the atmosphere. The used facility is the instrument SINFONI, on the Very Large Telescope (Paranal Observatory, Chile).

THE VLT/SINFONI instrument

The Very Large Telescope, VLT for short, is a facility operated by the European Southern Observatory in the Atacama desert, Chile. It benefits from a very favorable sky, at an altitude of 2,635 meters, and consists of 4 refractors, Antu, Kueyen, Melipal, and Yepun, each with a primary mirror of 8.2 meters.

SINFONI, for Spectrograph for Integral Field Observations in the Near Infrared, operates on Yepun, and observes in the near-infrared, i.e. at wavelengths between 1 and 2.45 μm. You can find below its first light in July 2004, observing the star HD 130163.

"First Light" "data cube" spectrum obtained with SINFONI on the bright star HD 130163 on July 9, 2004, as seen on the science data computer screen. This 7th-magnitude A0 V star was observed in the near-infrared H-band with a moderate seeing of 0.8 arcsec. The width of the slitlets in this image is 0.25 arcsec. The exposure time was 1 second. © ESO
“First Light” “data cube” spectrum obtained with SINFONI on the bright star HD 130163 on July 9, 2004, as seen on the science data computer screen. This 7th-magnitude A0 V star was observed in the near-infrared H-band with a moderate seeing of 0.8 arcsec. The width of the slitlets in this image is 0.25 arcsec. The exposure time was 1 second. © ESO

As a spectrometer, it decomposes the light it observes following its wavelengths. You can find above a spectrum, on which you can see several minimums. These minimums are absorption lines, which reveal the presence of a chemical element. And this is the idea: the authors decomposed the Solar light reflected by the surface of Triton, to detect the chemical elements constituting it.

Spectral measurements

The authors made 5 observations between 2010 and 2013, each of them lasting between 40 and 55 minutes. They supplemented their data set with two observations dating back to 1995, and one to 2008. The 1995 observations were made at Mauna Kea Observatory, Hawaii.
Two spectral bands were investigated: the H band (1.5 to 1.8 μm) and the K band (2 to 2.45 μm). Once the data are acquired, they must be corrected from the influence of the Earth atmosphere and the one of Triton, which have themselves constituents which absorb the light. Pretty easy for correcting the telluric lines, however the authors admit that the correction from Triton’s atmosphere is not entirely satisfactory, given the accuracy of data we dispose on. In particular, this correction might add an artificial signature of methane.

After these corrections, and some that I do not detail, you have the spectra… What do to with that?

Lab experiments to understand the data

The spectrum should look like something we know. We know the absorption lines of methane, carbon monoxide, carbon dioxide, nitrogen… but not necessarily under Triton’s conditions. The temperature at the surface should be around 40 K, i.e. -235°C / -390°F, to permit the coexistence of the solid and gaseous phases of these elements, without liquid phase, with a pressure of 14μbar. And when you look at the spectrum, you do not only have the location of the absorption lines, but also their amplitudes, which depend on many parameters, like the temperature, the grain size, the relative fraction of these components, and corrections accounting for instrumental constraints.
To get these parameters, the authors made several lab experiments in Grenoble (France), got plenty of spectra, and fitted their observations of Triton on them. Actually, lab experiments have been made in the past and used to previous studies, but in the present case this wasn’t enough, given the resolution of the new data.

And now, the results!

Results

The results are essentially a confirmation of previous studies, but with a better resolution, and new data, which gives access to time variations of the composition. Actually, the variations of the distance to the Sun results in variations of temperature, which perturb the equilibrium between the solid and the gaseous phases.

Regarding the different constituents:

  • The methane (CH4) is mostly in diluted form.
  • At least two populations of carbon dioxide (CO2) are present, with grain sizes of 5 μm and 25 μm, respectively.
  • The carbon monoxide (CO) is mostly present in diluted form into nitrogen ice.
  • The nitrogen (N2) ice temperature is 37.5±1 K.

The study confirms already known longitudinal variations of the nitrogen and carbon monoxide surface abundances, and suggests latitudinal and/or temporal variations.

This may be anecdotal, but the authors have detected two unexpected absorption lines, at 2.102 μm and 2.239 μm. The first one being present in one spectrum only, while the last one is detected on the three spectra. What do they mean? We do not know yet.

So, you see, it is possible to get data on Triton from our Earth. But a space mission to Neptune would be much more fruitful. But this is a very strong challenge. Even a mission to Uranus seems to become difficult to fund. Some scientists fight for that. But anyway, do not forget that you will be much older than you are now when such a mission would reach its destination, if launched now…

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.

Analyzing a crater of Ceres

Hi there! The space mission Dawn has recently visited the small planets Ceres and Vesta, and the use of its different instruments permits to characterize their composition and constrain their formation. Today we focus on the crater Haulani on Ceres, which proves to be pretty young. This is the opportunity for me to present you Mineralogy and temperature of crater Haulani on Ceres by Federico Tosi et al. This paper has recently been published in Meteoritics and Planetary Science.

Ceres’s facts

Ceres is the largest asteroid of the Solar System, and the smallest dwarf planet. A dwarf planet is a planetary body that is large enough, to have been shaped by the hydrostatic equilibrium. In other words, this is a rocky body which is kind of spherical. You can anyway expect some polar flattening, due to its rotation. However, many asteroids look pretty much like potatoes. But a dwarf planet should also be small enough to not clear its vicinity. This means that if a small body orbits not too far from Ceres, it should anyway not be ejected.

Ceres, or (1)Ceres, has been discovered in 1801 by the Italian astronomer Giuseppe Piazzi, and is visited by the spacecraft Dawn since March 2015. The composition of Ceres is close to the one of C-Type (carbonaceous) asteroids, but with hydrated material as well. This reveals the presence of water ice, and maybe a subsurface ocean. You can find below its main characteristics.

Discovery 1801
Semimajor axis 2.7675 AU
Eccentricity 0.075
Inclination 10.6°
Orbital period 4.60 yr
Spin period 9h 4m 27s
Dimensions 965.2 × 961.2 × 891.2 km
Mean density 2.161 g/cm3

The orbital motion is very well known thanks to Earth-based astrometric observations. However, we know the physical characteristics with such accuracy thanks to Dawn. We can see in particular that the equatorial section is pretty circular, and that the density is 2.161 g/cm3, which we should compare to 1 for the water and to 3.3 for dry silicates. This another proof that Ceres is hydrated. For comparison, the other target of Dawn, i.e. Vesta, has a mean density of 3.4 g/cm3.

It appears that Ceres is highly craterized, as shown on the following map. Today, we focus on Haulani.

Topographic map of Ceres, due to Dawn. Click to enlarge. © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Topographic map of Ceres, due to Dawn. Click to enlarge. © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The crater Haulani

The 5 largest craters found on Ceres are named Kerwan, Yalode, Urvara, Duginavi, and Vinotonus. Their diameters range from 280 to 140 km, and you can find them pretty easily on the map above. However, our crater of interest, Haulani, is only 34 km wide. You can find it at 5.8°N, 10.77°E, or on the image below.

The crater Haulani, seen by <i>Dawn</i>. © NASA / JPL-Caltech / UCLA / Max Planck Institute for Solar System Studies / German Aerospace Center / IDA / Planetary Science Institute
The crater Haulani, seen by Dawn. © NASA / JPL-Caltech / UCLA / Max Planck Institute for Solar System Studies / German Aerospace Center / IDA / Planetary Science Institute

The reason why it is interesting is that it is supposed to be one of the youngest, i.e. the impact creating it occurred less than 6 Myr ago. This can give clues on the response of the material to the impact, and hence on the composition of the subsurface.
Nothing would have been possible without Dawn. Let us talk about it!

Dawn at Ceres

The NASA mission Dawn has been launched from Cape Canaveral in September 2007. Since then, it made a fly-by of Mars in February 2009, it orbited the minor planet (4)Vesta between July 2011 and September 2012, and orbits Ceres since March 2015.

This orbit consists of several phases, aiming at observing Ceres at different altitudes, i.e. at different resolutions:

  1. RC3 (Rotation Characterization 3) phase between April 23, 2015 and May 9, 2015, at the altitude of 13,500 km (resolution: 1.3 km/pixel),
  2. Survey phase between June 6 and June 30, 2015, at the altitude of 4,400 km (resolution: 410 m /pixel),
  3. HAMO (High Altitude Mapping Orbit) phase between August 17 and October 23, 2015, at the altitude of 1,450 km (resolution: 140 m /pixel),
  4. LAMO (Low Altitude Mapping Orbit) / XMO1 phase between December 16, 2015 and September 2, 2016, at the altitude of 375 km (resolution: 35 m /pixel),
  5. XMO2 phase between October 5 and November 4, 2016, at the altitude of 1,480 km (resolution: 140 m / pixel),
  6. XMO3 phase between December 5, 2016 and February 22, 2017, at the altitude varying between 7,520 and 9,350 km, the resolution varying as well, between
  7. and is in the XMO4 phase since April 24, 2017, with a much higher altitude, i.e. between 13,830 and 52,800 km.

The XMOs phases are extensions of the nominal mission. Dawn is now on a stable orbit, to avoid contamination of Ceres even after the completion of the mission. The mission will end when Dawn will run out of fuel, which should happen this year.

The interest of having these different phases is to observe Ceres at different resolutions. The HAMO phase is suitable for a global view of the region of Haulani, however the LAMO phase is more appropriate for the study of specific structures. Before looking into the data, let us review the indicators used by the team to understand the composition of Haulani.

Different indicators

The authors used both topographic and spectral data, i.e. the light reflected by the surface at different wavelengths, to get numbers for the following indicators:

  1. color composite maps,
  2. reflectance at specific wavelengths,
  3. spectral slopes,
  4. band centers,
  5. band depths.

Color maps are used for instance to determine the geometry of the crater, and the location of the ejecta, i.e. excavated material. The reflectance is the effectiveness of the material to reflect radiant energy. The spectral slope is a linear interpolation of a spectral profile by two given wavelengths, and band centers and band depths are characteristics of the spectrum of material, which are compared to the ones obtained in lab experiments. With all this, you can infer the composition of the material.

This requires a proper treatment of the data, since the observations are affected by the geometry of the observation and of the insolation, which is known as the phase effect. The light reflection will depend on where is the Sun, and from where you observe the surface (the phase). The treatment requires to model the light reflection with respect to the phase. The authors use the popular Hapke’s law. This is an empirical model, developed by Bruce Hapke for the regolith of atmosphereless bodies.

VIR and FC data

The authors used data from two Dawn instruments: the Visible and InfraRed spectrometer (VIR), and the Framing Camera (FC). VIR makes the spectral analysis in the range 0.5 µm to 5 µm (remember: the visible spectrum is between 0.39 and 0.71 μm, higher wavelengths are in the infrared spectrum), and FC makes the topographical maps.
The combination of these two datasets allows to correlate the values given by the indicators given above, from the spectrum, with the surface features.

A young and bright region

And here are the conclusions: yes, Haulani is a young crater. One of the clues is that the thermal signature shows a locally slower response to the instantaneous variations of the insolation, with respect to other regions of Ceres. This shows that the material is pretty bright, i.e. it has been less polluted and so has been excavated recently. Moreover, the spectral slopes are bluish, this should be understood as a jargony just meaning that on a global map of Ceres, which is colored according to the spectral reflectance, Haulani appears pretty blue. Thus is due to spectral slopes that are more negative than anywhere else on Ceres, and once more this reveals bright material.
Moreover, the bright material reveals hydrothermal processes, which are consequences of the heating due to the impact. For them to be recent, the impact must be recent. Morever, this region appears to be calcium-rich instead of magnesium-rich like anywhere else, which reveals a recent heating. The paper gives many more details and explanations.

Possible thanks to lab experiments

I would like to conclude this post by pointing out the miracle of such a study. We know the composition of the surface without actually touching it! This is possible thanks to lab experiments. In a lab, you know which material you work on, and you record its spectral properties. And after that, you compare with the spectrum you observe in space.
And this is not an easy task, because you need to make a proper treatment of the observations, and once you have done it you see that the match is not perfect. This requires you to find a best fit, in which you adjust the relative abundances of the elements and the photometric properties of the material, you have to consider the uncertainties of the observations… well, definitely not an easy task.

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.

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.