Tag Archives: Impacts

Impacts on Jupiter

Hi there! Today is a little different. I present you a study of the impacts on Jupiter. This study, Small impacts on the giant planet Jupiter, by Hueso et al., has recently been accepted for publication in Astronomy and Astrophysics.
This is something different from usual by the implication of amateur astronomers. The professional scientific community sometimes needs their help, because they permit to tend to a global coverage of an expected event, like a stellar occultation. This is here pretty different since impacts on Jupiter are not predicted, so they are observed by chance. And the more observations, the more chance.
Thanks to these data, the authors derived an estimation of the impact rate on Jupiter.

The fall of Shoemaker-Levy 9

Before getting to the point, let me tell you the story of the comet Shoemaker-Levy 9. This comet has been discovered around Jupiter in March 1993 by Carolyn and Eugene Shoemaker, David Levy, and Philippe Bendjoya. Yes, this was discovered as a satellite of Jupiter, but on an unstable orbit. This comet was originally not a satellite of Jupiter, and when passing by Jupiter captured it. And finally, Shoemaker-Levy 9 crashed on Jupiter between July, 16 and July, 22 1994. Why during 6 days? Because the comet got fragmented. 23 fragments have been detected, which crashed close to the South Pole of Jupiter in 1994. This resulted in flashes more visible than the Red Spot, and scars which could be seen during several months. Moreover, Shoemaker-Levy 9 polluted the atmosphere of Jupiter with water.

Impacting Jupiter

Shoemaker-Levy 9 is a spectacular and well-known example of impact on Jupiter. But Jupiter is in fact regularly impacted. Cassini even mentioned a black dot on Jupiter in 1690, which could result from an impact. This is how things work.

Jupiter attracts the impactors

As you know, Jupiter is the most massive body in the Solar System, beside the Sun of course. As such, it attracts the small objects passing by, i.e. it tends to focus the trajectories of the impactors. So, the impactors are caught in the gravitational field of Jupiter, but usually on a hyperbolic orbit, since they come from very far away. As a consequence their orbits are unstable, and they usually will be ejected, or crash onto Jupiter. Let us assume we crash on Jupiter.

Jupiter destroys the impactors

Before the crash, the distance to Jupiter decreases, of course, and its gravitational action becomes stronger and stronger. A consequence is that the differential action of Jupiter on different parts of a given body, even a small one, gets stronger, and tends to disrupt it (tidal disruption). This is why Shoemaker-Levy 9 has been fragmented.

The impactors do not leave any crater

When the fragments reach Jupiter, they reach in fact its upper atmosphere. Since this atmosphere is very large and thick, the impactors do not create visible craters, but only perturbations in the atmosphere. We see at least a flash (a bright fireball), and then we may see kind of clouds, which are signatures of the atmospheric pollution due to the impactors. I mentioned a flash, actually they may be several of them, because the impactor is fragmented.

Let us now discuss on the observations of such events.

Observing an impact

Jupiter is usually easy to observe from the Earth, but only 9 months each year. It is too close to the Sun during the remaining time. While visible, everybody is free to point a telescope at it, and record the images. Actually amateur astronomers do it, and some impacts were detected by them. Once you have recorded a movie, then you should watch it slowly and carefully to detect an impact. Such an event lasts a few seconds, which is pretty tough to detect on a movie which lasts several hours.

The authors studied 5 events, at the following dates:

  1. June 3, 2010, detected twice, in Australia and in the Philippines,
  2. August 20, 2010, detected thrice, in Japan,
  3. September 9, 2012, detected twice, in the USA
  4. March 17, 2016, detected twice, in Austria and Ireland,
  5. May 26, 2017, detected thrice, in France and in Germany.

Once an observer detects such an event, he/she posts the information on an astronomy forum, to let everybody know about it. This is how several observers can get in touch. If you are interested, you can also consult the page of the Jupiter bolides detection project.

The detection of impacts can be improved in observing Jupiter through blue filters and wide filters centered on the methane absorption band at 890 nm, because Jupiter is pretty dark at these wavelengths, making the flash more visible. Moreover, one of the authors, Marc Delcroix, made an open-source software, DeTeCt, which automatically detects the flashes from observations of Jupiter.

All of these events were discovered by amateurs, and professionals exploited the data to characterize the impactors.

Treating the data

Once the impacts have been detected, the information and images reach the professionals. In order to characterize the impactor, they estimate the intensity and duration of the flash by differential photometry between images during the event and images before and after, to subtract the luminosity of Jupiter. Then they plot a lightcurve of the event, which could show several maximums if we are lucky enough. From the intensity and duration they get to the energy of the impact. And since they can estimate the velocity of the impact, i.e. 60 km/s, which is a little larger than the escape velocity of Jupiter (imagine you want to send a rocket from Jupiter… you should send it with a velocity of at least 60 km/s, otherwise it will fall back on the planet), they get to the size of the impactor.

A 45-m impactor every year

The most frequent impacts are probably the ones by micrometeorites, as on Earth, but we will never be able to observe them. They can only be estimated by dynamical models, i.e. numerical simulations, or by on-site measurements by spacecrafts.

The authors showed that the diameters of the impactors, which were involved in the detected events, could be from the meter to 20 meters, depending on their density, which is unknown. Moreover, they estimate that events by impactors of 45 m should occur and could be detectable every year, but that impacts from impactors of 380 meters would be detectable every 6 to 30 years… if observed of course. And this is why the authors insist that many amateurs participate to such surveys, use the DeTeCt software, report their observations, and share their images.

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 activity of Chiron

Hi there! You may have heard of Chiron, which was he first Centaur discovered, in 1977. This minor planet may have rings, and seems to present some cometary activity, which cause needs to be discussed. This is the topic of the present study, i.e. Activity of (2060) Chiron possibly caused by impacts?, by Stefan Cikota, Estela Fernández-Valenzuela, Jose Luis Ortiz, Nicolás Morales, René Duffard, Jesus Aceituno, Aleksandar Cikota and Pablo Santos-Sanz. This study has recently been accepted for publication in The Monthly Notices of the Royal Astronomical Society.

Chiron’s facts

Chiron was the first discovered Centaur, i.e. the first asteroid / small planet, which orbits between the orbits of Saturn and Uranus. It was discovered in 1977, in the sense that it was identified in 1977. But reexamination of past photographic plates show that it has in fact been observed since 1895. And from the reanalysis of the pre-discovery observations, it was easy to determine an orbit.

Discovery 1977
First observation 1895
Apparent magnitude 19
Absolute magnitude 6
Diameter 220 km
Semimajor axis 13.648 AU
Eccentricity 0.3823
Inclination 6.9497°
Orbital period 50.42 yr
Rotation 5.918 h

The orbital period of Chiron is a slightly longer than 50 years, which means that we dispose of astrometric observations over more than 2 periods. This orbit is highly eccentric, which results in large variations of the distance to the Sun, i.e. between 8.43 AU (astronomical units) at perihelion, and 18.86 AU at aphelion.

A spectral analysis of Chiron reveals a C-type, i.e. a carbonaceous, object. Moreover, it shows large variations of brightness, which are considered to be partly due to cometary activity, and partly due to rings. This cometary activity makes that Chiron, officially the asteroid (2060)Chiron, can also be called the comet 95P/Chiron.

Chiron observed at Kuma Kogen Astronomical Observatory, Japan. © 1997 by Akimasa Nakamura
Chiron observed at Kuma Kogen Astronomical Observatory, Japan. © 1997 by Akimasa Nakamura

The presence of rings around Chiron is not unanimously accepted in the scientific community. Unexpected stellar occultations by something orbiting close to Chiron could be interpreted either as cometary jets, or as rings. But the large variations of brightness and the discoveries of rings around Chariklo and Haumea speak for the presence of rings. The discovery of rings around Chariklo was very surprising, and showed that it is possible. The discovery around Haumea has shown that rings around such bodies were not exceptional. So, why not Chiron? In this study, the authors clearly state that they believe in the presence of rings, and they use it to study the brightness of Chiron. These rings would have a radius of 324 ± 10 km, which is inside the estimated Roche limit of Chiron, i.e. the particles constituting the rings could not accrete into a larger body.

But the central point is the cometary activity, i.e. evidence for cometary jets is reported.

Triggering a cometary activity

Classical comets behave this way: these are dirty snowballs, i.e. made of ice, dust, and some other elements. When approaching the Sun, the comet gets so warm that the ice is sublimated. But a Centaur with cometary activity is different, since it does not get closer to the Sun. Moreover, Chiron is essentially carbonaceous. So, another cause has to be found. And in such a case, it is often tempting to invoke impacts.

A problem is that impacts are not that frequent in that region of the Solar System. First because the gravitational action of the Sun tends to focus the orbits of the potential impactors, i.e. they will be more inclined to get closer to the Sun, and second because, the more distant from the Sun you are, the emptier the space appears, this is just a geometrical effect.
The consequences of these effects is that a collision of a 1km-radius comet is expected on a body like Chiron every 60 Gyr… while the age of the Solar System is 4.5 Gyr… quite unlikely.

Photometric observations

Anyway, Chiron is known to have some cometary activity, and the author tracked it from Calar Alto Observatory (CAHA) in Almeria, Spain, during 3 observation campaigns, between 2014 and 2016. The first campaign was primarily devoted to the study of the rotation of Chiron, and consisted of 3 runs in 2014, using the 3.5 and the 1.23 m telescopes. The second campaign was conducted in September 2015 on the 2.2 m telescope, with the CAFOS instrument (Calar Alto Faint Object Spectrograph), and looked for rotation, absolute magnitude, and cometary activity. The third campaign took place on 2016, September 2, to get a better constraint on Chiron’s absolute magnitude, once again with CAFOS.

The authors were particularly interested in the photometry, since cometary jets translate into variations of brightness. For that, they had to correct the variations due to observational constraints, and to the orientation of Chiron.

The 3.5m telescope at Calar Alto Observatory (CAHA). © Alfredo Madrigal
The 3.5m telescope at Calar Alto Observatory (CAHA). © Alfredo Madrigal

Observational constraints are likely to give artificial variations of photometry, since

  • the height of Chiron on the horizon varies, which means that the thickness of the atmosphere varies,
  • the wind might result in unstable images (seeing),
  • the detectors are different, even on the same instrument,etc.

To try to make things as proper as possible, the authors corrected the images from flat fielding, i.e. from the variations of the response of the CCD chip, and they observed a large enough field (at least 16 arcmin), to have the same stars as photometric references.

Regarding the orientation of Chiron, variations of brightness can reveal:

  • the rotation of Chiron, which would present different surface elements to the observer,
  • the orientation of the rings.

These two effects were modeled, to be removed from the photometric measurements. And the result is…

Impacts from the rings

The authors do observe a small cometary activity on Chiron, which is pretty faint. It has actually been stronger in the past, a measurement in 1973 showed a peak with respect to another measurement in 1970, and since then the coma is monotonously decreasing. The authors interpret that as a possible small impact having occurred between 1970 and 1973, the associated coma tail having almost disappeared. This activity appears to be supplemented by a continuous micro-activity, which could be due to impacts by small particles falling from the rings.

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.

Ice on Mercury

Hi there! You know Mercury, the innermost planet of the Solar System. It has recently been explored during 4 years by the American spacecraft MESSENGER, which gave us invaluable data on its surface, its magnetic field, its interior…
Today I present you a study on the ice on Mercury. It is entitled Constraining the thickness of polar ice deposits on Mercury using the Mercury Laser Altimeter and small craters in permanently shadowed regions, by Ariel N. Deutsch, James W. Head, Nancy L. Chabot & Gregory A. Neumann, and has recently been accepted for publication in Icarus.
We know that there is some ice at the surface of Mercury, and the study wonders how much. Since Mercury is close to the Sun, its surface is usually hot enough to sublimate the ice… except in permanently shadowed regions, i.e. in craters. For that, the authors compared the measured depth of small craters, and compared it with the expected depth from the excavation of material by an impactor. The difference is supposed to be ice deposit.

Mercury and MESSENGER

The planet Mercury is known at least since the 14th century BC. It was named after the Roman messenger god Mercurius, or Hermes in Greek, since the messengers saw it at dawn when they left, and at dusk when they arrived. The reason is that Mercury is in fact pretty close to the Sun, i.e. three times closer than our Earth. So, usually the Sun is so bright that it prevents us from observing it. Unless it is below the horizon, which happens at dawn and at dusk.

Mercury makes a full revolution around the Sun in 88 days, and a full rotation in 58 days. This 2/3 ratio is a dynamical equilibrium, named 3:2 spin-orbit resonance, which has been reached after slow despinning over the ages. This despinning is indeed a loss of energy, which has been favored by the tidal (gravitational) action of the Sun. This resulting spin-orbit resonant configuration is a unique case in the Solar System. A consequence is that the Solar day on Mercury lasts 176 days, i.e. if you live on Mercury, the apparent course of the Sun in the sky lasts 176 days.

The proximity of the Sun makes Mercury a challenge for exploration. Mariner 10 made 3 fly-bys of it in 1974-1975, mapping 45% of its surface, and measuring a tiny magnetic field. We had to wait until 2011 for the US spacecraft MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) to be the first human-made object inserted into orbit around Mercury. The orbital phase lasted 4 years, and gave us a full map of the planet, gravity data, accurate measurements of its rotation, a list of craters, measurements of the magnetic field,…

The instrument of interest today is named MLA, for Mercury Laser Altimeter. This instrument used an infrared laser (wavelength: 1,064 nanometers) to estimate the height of the surface from the reflection of the laser: you send a laser signal, you get it back some time later, and from the time you have the distance, since you know the velocity, which is the velocity of the light. And in applying this technique all along the orbit, you produce a map of the whole planet. This permits for instance to estimate the size and depth of the craters.

The Mercury Laser Altimeter (MLA).
The Mercury Laser Altimeter (MLA).

Ice on Mercury

The discovery of ice at the poles of Mercury was announced in 1992. It was permitted by Earth-based radar imagery made at Goldstone Deep Space Communications Complex in the Mojave desert, in California (USA). Ice is pretty easy to uncover, because of its high reflectivity. But this raises some questions:

  1. How can ice survive on Mercury?
  2. How much ice is there?
  3. How did it arrive?
The Goldstone facilities in 2018. © Google
The Goldstone facilities in 2018. © Google

The first question is not really a mystery. Because of its long Solar day and its absence of atmosphere (actually Mercury has a very tenuous exosphere, but we can forget it), Mercury experiences huge variations of temperature between day and night, i.e. from 100K to 700K, or -173°C to 427°C, or -279°F to 801°F (it is in fact not accurate at the 1°F level…). So, when a region is illuminated, the water ice is definitely not stable. However, there are regions, especially at the poles, which are never illuminated. There ice can survive.

The last two questions are answered by this study.

Ice is still present in craters

For not being illuminated, it helps to be close to a pole, but the topography can be helpful as well. The surface of Mercury is heavily cratered, and the bottoms of some of these craters are always hidden from the Sun. This is where the authors looked for ice. More precisely, they investigated 10 small craters within 10 degrees of the north pole. And for each of them, they estimated the expected depth from the diameter, and compared it with the measured depth. If it does not match, then you have water ice at the bottom. Easy, isn’t it?

The Carolan crater, one of the craters studied. © NASA
The Carolan crater, one of the craters studied. © NASA

Well, it is not actually that easy. The question is: did the water ice arrive after or before the excavation of the crater? If it arrived before, then the impactor just excavated some ice, and the measurements do not tell you anything.

Another challenge is to deal with the uncertainties. MLA was a wonderful instrument, with an accuracy smaller than the meter. Very well. But you are not that accurate if you want to predict the depth of a crater from its diameter. The authors used an empirical formula proposed by another study: d=(0.17±0.04)D0.96±0.11, where d is the depth, and D the diameter. The problem is the ±, i.e. that formula is not exact. This uncertainty is physically relevant, since the depth of the crater might depend on the incidence angle of the impact, which you don’t know, or on the material at the exact location of the impact… and this is a problem, since you cannot be that accurate on the theoretical depth of the crater. The authors provide a numerical example: a 400-m diameter crater has an expected depth between 21.2 and 127.7 m… So, there is a risk that the thickness of ice that you would measure would be so uncertain that actual detection would be unsure. And this is what happens in almost of all the craters. But the detection is secured by the fact that several craters are involved: the more data you have, the lower the uncertainties. And the ice thickness derived from several craters is more accurate than the one derived from a single crater.

Results: how much ice?

And the result is: the ice thickness is 41+30-14m. The uncertainty is large, but the number remains positive anyway, which means that the detection is positive! Moreover, it is consistent with previous studies, from the detection of polar ice with Goldstone facilities, to similar studies on other regions of Mercury. So, there is ice on Mercury.
An extrapolation of this result suggests that the total mass of water ice on the surface of Mercury is “1014-1015 kg, which is equivalent to ~100-1,000 km3 ice in volume, assuming pure water ice with no porosity” (quoted from the study).

The origin of ice

Mercury is a dense planet, i.e. too dense for such a small planet. It is widely accepted that Mercury as we see it constituted a core of a proto-Mercury, which has been stripped from its mantle of lighter elements. Anyway, Mercury is too dense for the water ice to originate from it. It should come from outside, i.e. it has been brought by impactors. The authors cite studies stating that such a quantity could have been brought by micrometeorites, by Jupiter-family comets, and even by a single impactor.

Another spacecraft soon

Such a study does not only exploit the MESSENGER data, but is also a way to anticipate the future measurements by Bepi-Colombo. This mission will be constituted of two orbiters, one supervised by the European Space Agency (ESA), and the other one by the Japanese agency JAXA. Bepi-Colombo should be launched in October 2018 from Kourou (French Guiana), and inserted into orbit around Mercury in April 2026. Its accuracy is expected to be 10 times better than the one of MESSENGER, and the studies inferring results from MESSENGER data can be seen as predictions for Bepi-Colombo.

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 (NEW) 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.