Tag Archives: Atmosphere

When a comet meets the Solar wind

Hi there! Today, let us talk about the environment of a comet. As you know, a comet is an active body, which emits ionized particles and dust. The Sun itself emits charged particles, which constitute the Solar wind. We discuss today of the interaction between these two emissions. The environment of charged particles around a comet has been measured by the spacecraft Rosetta, and this has motivated modeling these interactions. I present you Solar wind dynamics around a comet: The paradigmatic inverse-square-law model, by M. Saillenfest, B. Tabone, and E. Behar. This study has recently been accepted for publication in Astronomy and Astrophysics.

The spacecraft Rosetta

Let us first speak about the mission Rosetta. Rosetta was a European mission, which orbited the comet 67P/Churyumov–Gerasimenko between 2014 and 2016. It was named after the Rosetta Stone, which permitted the decipherment of Egyptian hieroglyphs. The mission Rosetta was supposed to give us clues on the primordial Solar System, i.e. on our origins, from the study of a comet.

It was launched in March 2004 from Kourou (French Guiana), and then started a 10-years journey, during which it made 3 fly-bys of the Earth and one of Mars. You can say: “why going back to Earth?” The reason is that Rosetta was supposed to orbit 67P/Churyumov–Gerasimenko (spoiler alert: it did it). For this orbital insertion to be possible, it had to arrive slowly enough… but also had to leave Earth fast enough, to get rid off its attraction, and also to shorten the journey. Fly-bys permitted to slow the spacecraft in exchanging energy with the Earth (or Mars).

Rosetta also visited two asteroids: (2867) Šteins, and (21) Lutetia, in September 2008 and July 2010, respectively. It was inserted into orbit around 67P in August 2014, released the lander Philae in November, and the mission ended in September 2016. In particular, Rosetta was present when 67P reached its perihelion in August 2015. At this point, the comet was at its closest distance to the Sun (1.25 astronomical unit, while its mean distance is almost thrice this number), where the cometary activity is maximal.

The asteroids (2867) Šteins (left) and (21) Lutetia (right), seen by Rosetta. © ESA
The asteroids (2867) Šteins (left) and (21) Lutetia (right), seen by Rosetta. © ESA

So, Rosetta consisted of two modules: the orbiter itself, and the lander Philae. The orbiter had 11 instruments on board, and the lander 10. These instruments permitted, inter alia, to map the comet and measure its geometry, to constrain its internal structure and its chemistry, and to characterize its environment.

This environment is strongly affected by the Solar wind, especially in the vicinity of the perihelion, but not only.

The Solar wind

The Solar corona emits a stream of charges particles, which is mainly composed of electrons, protons, and alpha particles (kind of charged helium). This emission is called Solar wind. It is so energetic, that the emitted particles go far beyond the orbit of Pluto, constituting the heliosphere. The heliosphere has the shape of a bubble, and its boundary is called the heliopause. Voyager 1 crossed it in August 2012, at a distance of 121 AU of the Sun. At the heliopause, the pressure of the Solar wind is weak enough, to be balanced by the one of the interstellar medium, i.e. the winds from the surrounding stars. Hence, Voyager 1 is in this interstellar space, but technically still in the Solar System, as under the gravitational attraction of the Sun.

Anyway, our comet 67P/Churyumov-Gerasimenko is much closer than that, and has to deal with the Solar wind. Let us see how.

The physics of the interaction

Imagine you are on the comet, and you look at the Sun… which should make you blind. From that direction comes a stream of these charged particles (the Solar wind), and you can consider that their trajectories are parallel if far enough from the comet. Of course, the Sun does not emit on parallel trajectories, i.e. the trajectories of all these particles converge to the Sun. But from the comet, the incident particles appear to arrive on parallel trajectories.

While a charged particle approaches the comet, it tends to be deflected. Here, the dominating effect is not the gravitation, but the Lorentz force, i.e. the electromagnetic force. This force is proportional to the electric charge of the particle, and also involves its velocity, and the electric and magnetic fields of the comet.

The authors showed in a previous paper that the trajectories of the charged particles could be conveniently described in assuming that the magnetic field obeys an inverse-square law, i.e. its amplitude decreases with the square of the distance to the comet. If you are twice further from the comet, then the magnetic field is four times weaker.

I do not mean that the magnetic field indeed obeys this law. It is in fact more complex. I just mean that if you model it with such an ideal law, you are accurate enough to study the trajectories of the Solar wind particles. And this is what the authors did.

By the way, the authors suggest that any magnetic field following an inverse-power law could work. Of course, the numbers would have been different, but the global picture of the trajectories would be pretty much the same. It seems, at this time, too challenging to determine which of these models is the most accurate one.

Reducing the problem

The authors used analytical calculations, i.e. maths, which are in fact close to the classical ones, you make to show that the gravitation results in elliptic, parabolic, or hyperbolic, trajectories.

A wonderful tool assisting such studies is the First Integrals. A First Integral is a quantity, which remains constant all along a trajectory. For instance, in a gravitational problem where no energy is dissipated, then the total energy (kinetic + potential energies) is conserved. This is a First Integral. Another First Integral in that problem is the norm of the total angular momentum. And the existence of these two quantities helps to understand the shape of the possible orbits.

The authors showed that this is quite similar here. Even if the equations are slightly different (anyway the inverse-square law is a similarity), they showed that the problems has 2 First Integrals. And from these 2 First Integrals, they showed that knowing only 2 parameters is in fact enough to characterize the trajectories of the Solar wind particles. These two parameters are called rC and rE, they have the physical dimension of a distance, and are functions of all the parameters of the problems. rE characterizes the stream, it is related to its velocity, while rC characterizes a given particle. If you know just these 2 parameters, then you can determine the trajectory.

An empty cavity around the comet

The authors give a detailed description of the trajectories. To make things simple: either the particles orbit the comet, or they just pass by. But anyway, there is an empty space around the comet, i.e. a spherical cavity in which no Solar wind particle enters.

To come: comparison with in situ measurements

The journey of Rosetta around 67P crossed the boundary of this empty cavity. In other words, we have measurements of the density of charged particles at different distances from the comet, and also for different distances from the Sun, since the orbital phase of the mission lasted 2 years, during which 67P orbited the Sun. The authors promise us that a study of the comparison between the model and the in situ measurements, i.e. the observations, is to come. We stay tuned!

Rosetta does not operate anymore, and has landed (or crashed…) on 67P in September 2016. It is still there, and has on-board a kind of modern Rosetta stone. This is a micro-etched pure nickel prototype of the Rosetta disc donated by the Long Now Foundation, as part of its Rosetta Project. The disc was inscribed with 6,500 pages of language translations. This is a kind of time capsule, aiming at preserving part of our culture. Maybe someone will one day find it…

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.

On the early evolution of Jupiter

Hi there! Today we will discuss on how Jupiter formed. I guess you know Jupiter, i.e. the largest planet of our Solar System. It is a gaseous planet, which means that it is composed of a large and thick atmosphere, which surrounds a solid core. Jupiter is currently studied by the NASA spacecraft Juno. The study I present you, The primordial entropy of Jupiter, by Andrew Cumming, Ravi Helled, and Julia Venturini, simulates different possible paths for the accretion of the atmosphere of Jupiter. The goal is to compare the outcomes with the current atmosphere, to eventually discard some scenarios and constrain the primordial Jupiter. This study has recently been published in The Monthly Notices of the Royal Astronomical Society.

The planet Jupiter

Jupiter is the largest planet of our Solar System, and the most massive one. It is about 1,000 more massive than our Earth, and 1,000 less massive than the Sun. As such, it has a tremendous influence on the architecture of our System, particularly the small bodies. The Main Asteroid Belt presents gaps, which are due to mean-motion resonances with Jupiter. Jupiter is also responsible for the destabilization of the orbits of objects which pass close to it. A famous example is the comet Shoemaker-Levy 9 which Jupiter tidally destroyed before its impact. You can find below a comparison between Jupiter, Saturn, and our Earth.

Jupiter Saturn Earth
Equatorial radius 71,492 km 60,268 km 6,378 km
Polar radius 66,854 km 54,364 km 6,357 km
Distance to the Sun 5.20 AU 9.58 AU 1 AU
Orbital period 11.86 yr 29.46 yr 1 yr
Spin period 9 h 55 m 10 h 33 m 23 h 56 m
Density 1.326 g/cm3 0.687 g/cm3 5.514 g/cm3

I compare with our Earth given our special connection with that planet, but the comparison with Saturn is much more relevant from a physical point of view. For gaseous planets, the radius correspond to an atmospheric pressure of 1 bar. I here provide a unique spin period, but the gaseous planets experience differential rotation, i.e. the equator may spin slightly faster than the poles.

You can see that our Earth is much denser than the giant guys. The reason is the thick atmosphere, which is less dense than a rocky body. Actually Jupiter is assumed to have a rocky core as well, which would be surrounded by hydrogen, which pressure increases with the depth.

Jupiter seen by Voyager 2 in 1979. © NASA / JPL / USGS
Jupiter seen by Voyager 2 in 1979. © NASA / JPL / USGS

Observers especially know Jupiter for its Great Red Spot, i.e. a giant storm, which is observed since the 17th century.

Jupiter is currently the target of the NASA mission Juno.

The mission Juno

The NASA mission Juno has been sent from Cape Canaveral in August 2011, and orbits Jupiter since July 2016, on a polar orbit. The nominal mission will be completed in July 2018, but I hope it will be extended (I do not have information on this point, sorry). Its goals are to understand origin and evolution of Jupiter, look for solid planetary core, map magnetic field, measure water and ammonia in deep atmosphere, observe auroras.

The  South pole of Jupiter seen by Juno. © NASA / JPL/ SwRI
The South pole of Jupiter seen by Juno. © NASA / JPL/ SwRI

It is composed of 9 instruments. Beside impressive images of cyclones in the atmosphere of Jupiter, it for instance gave us its gravity field of Jupiter with an unprecedented accuracy. Such a result permits to constrain the interior, see for instance this study, in which the authors modeled different interiors for Jupiter. They then compared the resulting, theoretical gravity field, which the one actually measured by Juno. They deduced that the core contains between 7 and 25 Earth masses of heavy elements.

The study I present today does not model the present Jupiter, but instead simulates the evolution of Jupiter from its early life to present. Once more, the goal is to compare with current and future observations. Let us see how a giant planet evolves.

The formation of a giant planet

There are two identified scenarios for the triggering of the formation of a planet:

  • Disk instability: a massive disk fragments into planet-sized self-gravitating clumps
  • Core accretion: solid particles collide and coagulate into larger and larger bodies until a body large enough to accrete a gaseous envelope.

The core accretion model consists of 3 phases:

  1. Primary core/heavy-element accretion: here you create the solid core,
  2. Slow envelope/gas accretion: in this phase, the solid core continues growing, while gas accretes as well,
  3. Rapid gas accretion: this is the final stage, where the core has already been formed.

Here the authors simulate the Phase 3. They are particularly interested in the heat transfer inside the atmosphere. There are two ways to transport heat in such an environment: by radiation, or by convection, i.e. transport of gas, which is a much more effective process. Moreover, convection permits the transport of heavy elements, and so a gradient of density in the atmosphere. This gradient of density would eventually stop the convection, the atmosphere reaching a kind of equilibrium.

Let us see how the authors simulated that process.

Simulations of different scenarios

The authors simulated the gas accretion of Jupiter using the numerical MESA code, for Modules for Experiments in Stellar Astrophysics. Yes, stellar, not planetary. But this is very relevant here, since a gaseous planet and a star are both made of a thick gaseous envelope.

These simulations differ by

  • The initial mass of the core,
  • its initial luminosity, which affects the heat transfers during the accretion process. This could be expressed in terms of entropy, which is a thermodynamical quantity expressing the overall activity of a fluid. It will then express the quantity of conductive transfers,
  • the initial mass of the envelope,
  • the temperature of the accreted material,
  • the time-dependent accretion rate. In some simulations it is an ad-hoc model, fitted from previous studies, and in other ones it is directly derived from formation models. The accretion rate is obviously time-dependent, since it slows down at the end of the accretion,
  • the opacity of the material, which is defined as the ratio of the gravitational acceleration over the pressure, multiplied by the optical depth. This affects the heat transfers.

And from all of these simulations, the authors deduce some properties of the final Jupiter, to be compared with future observations to constrain the evolution models.

The initial state constrains the final one

And here are some of the results:

  • Lower opacity and lower solid accretion rate lead to a low mass core,
  • if the gas accretion rate is high then the proto-Jupiter is likely to be fully radiative, i.e. no convection,
  • the rate at which the accretion slows down at the end determines the depth of the convection zone,

etc.

At this time, we do not dispose of enough data to constrain the initial parameters and the accretion rates, but why not in the future? Juno is still on-going, and we hope other missions will follow. For instance, stable regions in Jupiter’s interior can be probed with seismology. Seismology of giant planets would be pretty similar to helioseismology, i.e. this would consist in the detection of acoustic waves, which would be generated by convection in the interior.

The study and its authors

  • You can find the study here. The authors made it freely available on arXiv, thanks to them for sharing! And now the authors:
  • The website of Andrew Cumming, first author of the study,
  • and the one of Ravit Helled.

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.

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.

The chemistry of Pluto

Hi there! The famous dwarf planet Pluto is better known to us since the flyby of the spacecraft New Horizons in 2015. Today, I tell you about its chemistry. I present you Solid-phase equilibria on Pluto’s surface, by Sugata P. Tan & Jeffrey S. Kargel, which has recently been accepted for publication in The Monthly Notices of the Royal Astronomical Society.

The atmosphere of Pluto

I do not want here to recall everything about Pluto. This is a dwarf planet, which has been discovered by Clyde Tombaugh in 1930. It orbits most of the time outside the orbit of Neptune, but with such an eccentricity that it is sometimes inside. It was discovered in 1978 that Pluto has a large satellite, Charon, so large that the system Pluto-Charon can be seen as a binary object. This binary has at least 4 small satellites, which were discovered thanks to the Hubble Space Telescope.

Pluto has a tenuous atmosphere. It was discovered from the Earth in 1985 in analyzing a stellar occultation: when a faint, atmosphereless object is aligned between a star and a observer, the observer does not see the star anymore. However, when the object has an atmosphere, the light emitted by the star is deviated, and can even be focused by the atmosphere, resulting in a peak of luminosity.

Several occultations have permitted to constrain the atmosphere. It has been calculated that its pressure is about 15 μbar (the one of the Earth being close to 1 bar, so it is very tenuous), and that it endured seasonal variations. By seasonal I mean the same as for the Earth: because of the variations of the Sun-Pluto distance and the obliquity of Pluto, which induces that every surface area has a time-dependent insolation, thermic effects affect the atmosphere. This can be direct effects, i.e. the Sun heats the atmosphere, but also indirect ones, in which the Sun heats the surface, triggering ice sublimation, which itself feeds the atmosphere. The seasonal cycle, i.e. the plutonian (or hadean) year lasts 248 years.

Observations have shown that this atmosphere is hotter at its top than at the surface, i.e. the temperature goes down from 110 K to about 45 K (very cold anyway). This atmosphere is mainly composed of nitrogen N2, methane CH4, and carbon monoxide CO.

The surface of Pluto

The surface is known to us thanks to New Horizons. Let me particularly focus on two regions:

  • Sputnik Planitia: this is the heart that can be seen on a map of Pluto. It is directed to Charon, and is covered by volatile ice, essentially made of nitrogen N2,
  • Cthulhu Regio: a large, dark reddish macula, on which the volatile ice is absent.
A map of Pluto (mosaic made with New Horizons data). © NASA
A map of Pluto (mosaic made with New Horizons data). © NASA

The reason why I particularly focus on these two regions is that they have two different albedos, i.e. the bright Sputnik Planitia is very efficient to reflect the incident Solar light, while Cthulhu Regio is much less efficient. This also affects the temperature: on Sputnik Planitia, the temperature never rises above 37 K, while it never goes below 42.5 K in Cthulhu Regio. We will see below that it affects the composition of the surface.

An Equation Of State

The three main components, i.e. nitrogen, methane, and carbon monoxide, have different sublimation temperatures at 11μbar, which are 36.9 K, 53 K, and 40.8 K, respectively (sublimation: direct transition from the solid to the gaseous state. No liquid phase.). A mixture of them will then be a coexistence of solid and gaseous phases, which depends on the temperature, the pressure, and the respective abundances of these 3 chemical components. The pressure is set to 11μbar, since it was the pressure measured by New Horizons, but several temperatures should be considered, since it is not homogeneous. The authors considered temperatures between 36.5 K and 41.5 K. Since the atmosphere has seasonal variations, a pressure of 11μbar should be considered as a snapshot at the closest encounter with New Horizons (July 14, 2015), but not as a mean value.

The goal of the authors is to build an Equation Of State giving the phases of a given mixture, under conditions of temperature and pressure relevant for Pluto. The surface is thus seen as a multicomponent solid solution. For that, they develop a model, CRYOCHEM for CRYOgenic CHEMistry, which aims at predicting the phase equilibrium under cryogenic conditions. The paper I present you today is part of this development. Any system is supposed to evolve to a minimum of energy, which is an equilibrium, and the composition of the surface of Pluto is assumed to be in thermodynamic equilibrium with the atmosphere. The energy which should be minimized, i.e. the Helmholtz energy, is related to the interactions between the molecules. A hard-sphere model is considered, i.e. a minimal distance between two adjacent particles should be maintained, and for that the geometry of the crystalline structure is considered. Finally, the results are compared with the observations by New Horizons.

Such a model requires many parameters. Not only the pressure and temperature, but also the relative fraction of the 3 components, and the parameters related to the energies involved. These parameters are deduced from extrapolations of lab experiments.

Results

The predicted coexistence of states predicted by this study is consistent with the observations. Moreover, it shows that the small fraction of carbon monoxide can be neglected, as the behavior of the ternary mixture of N2/CH4/CO is very close to the one of the binary N2/CH4. This results in either a nitrogren-rich solid phase, for the coolest regions (the bright Sputnik Planitia, e.g.), and a methane-rich solid phase for the warmest ones, like Cthulhu Regio.

Developing such a model has broad implications for predicting the composition of bodies’s surfaces, for which we lack of data. The authors give the example of the satellite of Neptune Triton, which size and distance to the Sun present some similarities with Pluto. They also invite the reader to stay tuned, as an application of CRYOCHEM to Titan, which is anyway very different from Pluto, is expected for publication pretty soon.

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.