Monthly Archives: May 2018

Planets

On the early evolution of Jupiter

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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.

Outline

The planet Jupiter
The mission Juno
The formation of a giant planet
Simulations of different scenarios
The initial state constrains the final one
The study and its authors

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.

Asteroids: Trans-Neptunian Objects

OSSOS discovered 838 Trans-Neptunian Objects

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Hi there! Today I will tell you of the survey OSSOS, which I already mentioned in the past. This survey made systematic observations of the sky to detect Trans-Neptunian Objects (TNOs), between 2013 and 2017. It was indeed a success, since it tripled the number of known TNOs. Its results are presented in OSSOS. VII. 800+ Trans-Neptunian Objects — The complete Data Release, led by Michele Bannister. This study is published in The Astrophysical Journal Supplement Series.

Outline

Previous surveys
The Outer Solar System Origins Survey (OSSOS)
Characterizing a new TNO
Structure of the outer Solar System
Key results
Constraining the evolution of the Solar System
The study and its authors

Previous surveys

The Trans-Neptunian Objects orbit beyond the orbit of Neptune. As such, observing them is a challenge. Pluto was the only known of them from its discovery in 1930, to the discovery of (15760) Albion in 1992. We now know 1,142 Trans-Neptunian Objects, essentially due to 4 surveys. The most prolific of them is the last one, i.e. OSSOS, but a survey cannot exist without its precursors, which were

  1. Deep Ecliptic Survey (DES),
  2. Canada-France Ecliptic Plane Survey (CFEPS),
  3. Pan-STARRS1.
The Deep Ecliptic Survey (DES)

The Deep Ecliptic Survey has been operating between 1998 and 2003, using two 4-m telescopes of the National Optical Astronomy Observatory: the Mayall telescope at Kitt Peak Observatory (Arizona, USA), and the Blanco telescope at Cerro Tololo Inter-American Observatory (Chile). It discovered 382 TNOs, including some Centaurs, which actually orbit inner to the orbit of Neptune. It covered 550 square degrees with sensitivity of 22.5.

The Canada-France Ecliptic Plane Survey (CFEPS)

This survey operated between early 2003 and early 2007, at the Canada-France-Hawaii Telescope (Hawaii, USA). It covered 321 square degrees with sensitivity of 24.4, and permitted to classify 169 TNOs. By classifying, I do not mean only discover, but also know their orbits with enough accuracy to determine to which dynamical group they belong. I will go back on this point later, but my meaning is that observing an object once is definitely not enough. This survey was limited to the detection of objects with a small inclination with respect to the ecliptic plane, i.e. the orbit of the Earth.

It was then extended by the High Ecliptic Latitude (HiLat) component, which looked for objects with significant inclinations. It examined 701 square degrees of sky ranging from 12° to 85° ecliptic latitude and discovered 24 TNOs, with inclinations between 15° and 104° (from Petit et al., 2017, The Canada-France Ecliptic Plane Survey (CFEPS) — High-latitude component, The Astronomical Journal, 153:5.

The Pan-STARRS1 survey

The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) survey operates from Haleakala Observatory, Hawaii, USA since 2010. It is not specifically devoted for TNOs, but for moving objects (asteroids, stars,…), and is particularly known for the discovery of the first known interstellar object, i.e. 1I/’Oumuamua. It discovered 370 new TNOs, but without enough information to securely classify their orbits.

And now comes OSSOS!

The Outer Solar System Origins Survey (OSSOS)

OSSOS operated between 2013 and 2017 from the Canada-France-Hawaii Telescope, taking more than 8,000 images. It covered 155 square degrees with a sensitivity up to 25.2. This coverage has been split into 8 blocks, which avoided the Galactic plane. The study I present today is the complete data release, in which 838 objects are given without ambiguity on their orbital classification. This was an international collaboration, involving Canada, UK, France, Taiwan, USA, Finland, Japan, Slovakia,… but also involving different skills, like orbital characterization, astrometry, chemistry, cometary activity, data mining, etc. In other words, it not only aimed at discovering new objects, but also at understanding their orbital dynamics, their physics, and if possible their origin.

In the previous paragraphs I pointed out the difference between discovering an object, and classifying it following its orbit. Let us see that now.

Characterizing a new TNO

As we will see in the next paragraph, the Trans-Neptunian population is composed of different parts, following the orbits of the objects and the perturbations acting on them, i.e. the gravitational attraction of the giant planets. Classifying a newly discovered object requires some accuracy in the determination of its orbit. The following is a summary of how things work.

For an object to be discovered, it must appear on a triplet of images, which cover a timespan of about 2 hours. From it the relative motion of the object on the sky can be evaluated, which would permit to reobserve it. The new observations permit themselves to better constrain the orbit. The OSSOS team announces that an arc of observations of about 16 months is required to have enough confidence in the orbit. In many cases the arc is longer, actually the team tells us that for the 838 classified objects, astrometric measurements have been made over 2 to 5 oppositions. An opposition is the geometric alignment between the Sun, the Earth, and the object.

For an astrometric measurement to be accurate, you need to accurately know the positions of the other objects present on the image. These other objects are stars, which are referenced in astrometric catalogues. The astrometric satellite Gaia is currently performing such a survey. Its Data Release 2 has very recently (April 2018) been released, but this was too late for the present study. So, the authors used the Data Release 1, and the Pan-STARRS 1 catalogue when necessary.

In some cases, objects were lost, i.e. the authors were not able to reobserve it. This may have been due to the lack of accuracy of the orbital determination from the discovery arc, or just because the object left a covered zone.

Before giving you the results, I should tell you something on the structure of the outer Solar System. I mentioned orbital classification above, the classes are coming now.

Structure of the outer Solar System

First, we should make a distinction between resonant and non-resonant orbits.

Resonant orbits are in mean-motion resonance with a planet, which is mostly Neptune. For instance, the 2:1 resonance with Neptune means that Neptune accomplishes two revolutions around the Sun while the object makes exactly one. Such a ratio implies amplified dynamical effects on the object, which may excite its eccentricity or its inclination, destabilize or confine its orbit.

Besides these resonant objects are the non-resonant ones (you guessed it, didn’t you?). They are classified following their orbital elements:

  • Centaurs: they orbit inner to the orbit of Neptune, i.e. their semimajor axis is smaller than 30 AU. As such, they are not TNOs strictly speaking,
  • Inner-belt objects: here the belt is the Kuiper Belt, not to be confused with the Main Asteroid Belt between Mars and Jupiter. This objects orbit between the orbit of Neptune and the 3:2 resonance, i.e. the orbit of Pluto, at 39.4 AU.
  • Main-belt objects: between the 3:2 and the 2:1 resonance, i.e. between 39.4 and 47.7 AU.
  • Outer-belt objects: they orbit beyond the 2:1 resonance and have an eccentricity smaller than 0.24.
  • Detached objects: not only they orbit beyond the 2:1 resonance, but also have an eccentricity larger than 0.24. As a consequence, they may have very large semi-major axes, but could be detected since their perihelion distance, i.e. their closest distance to the Sun, is accessible to our terrestrial instruments. This is made possible by their high eccentricity. Among these objects are the eTNOs (e for extreme) mentioned here.

And now the results.

Key results

1,142 TNOs (including Centaurs) are now classified, 838 of them thanks to OSSOS. Among these 838 objects, 313 are resonant, including 132 in the 3:2 resonance, 39 in the 7:4 and 34 in the 2:1, and 525 are non-resonant. 421 of the non-resonant object are in the main belt, i.e. between the 3:2 and the 2:1 resonances.

Among the remarkable other results are

  • There should be about 90,000 detached objects with a diameter larger than 100 km, and probably less than 1,000 so large Centaurs,
  • the inner Kuiper Belt practically starts at 37 AU,
  • the population of low-inclination objects extends to at least 49 AU, but there is a huge concentration of them between 42.5 and 44.5 AU,
  • the inclinations are larger in the 3:2 resonance (the Plutinos) than in the 2:1,
  • securely occupied resonances exist at least up to 130 AU, which is the location of the 9:1 resonance.

The word origins appear in OSSOS. Actually, knowing the distribution of the Kuiper Belt Objects tells us something on the evolution of our Solar System.

Constraining the evolution of the Solar System

A TNO is a small body. This implies that, when perturbed by a giant planet, it just endures the orbital shacking. The consequence is that the giant planets have a strong enough gravitational potential to shape the Kuiper Belt. When perturbed, an object might get inclined, eccentric, be ejected, confined…

There are several competing models of the evolution of the Solar System, which implies migration of the giant planets. When a giant planet migrates, its perturbation migrates as well, and you should see the consequences on the Kuiper Belt. This is how an accurate snapshot of the Kuiper Belt might tell us something on the past of our Solar System, and if you constrain its evolution, then you can be tempted to transpose it to extrasolar systems. Moreover, this could give clues on the Planet Nine…

The OSSOS team provides software, which include a survey simulator, checking the relevance of a predicted model for the Kuiper Belt, when compared to the observations.

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.

Satellites of Jupiter, Satellites of Saturn

Heating the subsurface oceans

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Hi there! You may have heard that subsurface oceans have been hinted / discovered / confirmed for some major satellites of Jupiter and Saturn. What if bacteriological life existed there? Wait a minute… it is too early to speak about that. But anyway, these oceans are interesting, and the study I present you today, i.e. Ocean tidal heating in icy satellites with solid shells, by Isamu Matsuyama et al., discusses the response of these oceans to the tidal heating, in considering the icy shell coating the oceans. This study has recently been accepted for publication in Icarus.

Outline

Ocean worlds in the Solar System
Tidal heating
Waves are generated in the ocean
The physical model
The response of the oceans may be measured
The study and its authors

Ocean worlds in the Solar System

First of all, let us see how you can have a subsurface ocean. The main satellites of our giant planets are in general frozen worlds, where the heaviest elements have migrated to the center. As a consequence, the surface is essentially water ice. If you go a little deeper, i.e. some kilometers below the surface, then you increase the pressure and the temperature, and you meet conditions under which liquid water may survive. This is why large and mid-sized satellites may support a global, subsurface ocean. Let us see now the direct and indirect detections

Certain: Titan

Titan is the largest satellite of Saturn, and is hinted since at least 30 years to have a global ocean. The spacecraft Cassini-Huygens has provided enough data to confirm this assumption, i.e.

  • The detection of a so-called Schumann resonance in the atmosphere of Titan, i.e. an electromagnetic resonance, which could be excited by a rotating magnetosphere, which would itself be generated by a global liquid layer, i.e. an ocean,
  • the obliquity of the surface of Titan, i.e. 0.3°, is thrice too large for a body in which no ocean would decouple the surface from the core,
  • the variations of the gravity field of Titan, which are contained in a so-called tidal Love number k2, are too large for an oceanless body.
Mosaic of Titan, due to Cassini. © NASA/JPL/University of Arizona/University of Idaho
Mosaic of Titan, due to Cassini. © NASA/JPL/University of Arizona/University of Idaho
Certain: Europa

Europa has been visited by the Galileo spacecraft, which orbited Jupiter between 1995 and 2003. Galileo revealed in particular

  • a fractured surface (see featured image), which means a pretty thin crust, and an ocean beneath it,
  • a significant magnetic field, due to a subsurface conductive layer, i.e. an ocean.
Certain: Ganymede

Ganymede has a strong magnetic field as well. Observations by the Hubble Space Telescope revealed in 2015 that the motion of auroras on Ganymede is a signature of that magnetic field as well, i.e. the internal ocean. Theoretical studies in fact suggest that there could be several oceanic layers, which alternate with water ice.

Ganymede seen by Galileo. © NASA / JPL / DLR
Ganymede seen by Galileo. © NASA / JPL / DLR
Certain: Enceladus

We can see geysers at the surface of Enceladus, which reveal liquid water below the surface. In particular, we know that Enceladus has a diapir at its South Pole. Cassini has proven by its gravity data that the ocean is in fact global.

Enceladus seen by Cassini. © NASA/JPL
Enceladus seen by Cassini. © NASA/JPL
Suspected: Dione

A recent theoretical study, led by Mikael Beuthe who also co-authors the present one, shows that Dione could not support its present topography if there were no subsurface ocean below the crust. The same methodology applied on Enceladus gives the same conclusion. In some sense, this validated the method.

Dione seen by Cassini. © NASA
Dione seen by Cassini. © NASA
Suspected: Callisto

Measurements by Galileo suggest that the magnetic field of Jupiter does not penetrate into Callisto, which suggests a conductive layer, i.e. once more, an ocean.

Callisto seen by Galileo. © NASA
Callisto seen by Galileo. © NASA
Suspected: Pluto

Pluto exhibits a white heart, Sputnik Planitia, which frozen material might originate from a subsurface ocean.

Pluto seen by New Horizons. ©NASA/APL/SwRI
Pluto seen by New Horizons. ©NASA/APL/SwRI
Doubtful: Mimas

Mimas is the innermost of the mid-sized satellites of Saturn. It is often compared to the Death Star of Star Wars, because of its large crater, Herschel. The surface of Mimas appears old, i.e. craterized, and frozen, so no heating is to be expected to sustain an ocean. However, recent measurements of the diurnal librations of Mimas, i.e. its East-West oscillations, give too large numbers. This could be the signature of an ocean.

Mimas seen by Cassini. © NASA
Mimas seen by Cassini. © NASA

Other oceanic worlds may exist, in particular among the satellites of Uranus and Neptune.

Tidal heating

Tides are the heating of a body by another, massive one, due to the variations of its gravitational action. For natural satellites, the tides are almost entirely due to the parent planet. The variations of the gravitational attraction over the volume of the satellite, and their time variations, generate stress and strain which deform and heat the satellite. The time-averaged tide will generate an equilibrium shape, which is a triaxial ellipsoid, while the time variations heat it.
The time variations of the tides are due to the variations of the distance between a satellite element and the planet. And for satellites, which rotate synchronously, two elements rule these variations of distance: the orbital eccentricity, and the obliquity.

For solid layers, rheological models give laws ruling the tidal response. However, the problem is more complex for fluid layers.

Waves are generated in the ocean

In a fluid, you have waves, which transport energy. In other words, you must considerate them when you estimate the heating. The authors considered two classes of waves:

  1. Gravity waves: when a body moves on its orbit, the ocean moves, but the gravity of the body acts as a restoring force. This way, it generates gravity waves.
  2. Rossby-Haurwitz waves: these waves are generated by the rotation of the body, which itself is responsible for the Coriolis force.

A wave has a specific velocity, wavelength, period… and if you excite it at a period which is close to its natural period of oscillation, then you will generate a resonant amplification of the response, i.e. your wave will meet a peak of energy.

All this illustrates the complexity of resolving such a problem.

The physical model

Solving this problem requires to write down the equations ruling the dynamics of the fluid ocean. The complete equations are the Navier-Stokes equations. Here the authors used the Laplace tidal equations instead, which derive from Navier-Stokes in assuming a thin ocean. This dynamics depends on drag coefficients, which can only be estimated, and which will rule the dissipation of energy in the oceans.
Once the equations are written down, the solutions are decomposed as spectral modes, i.e. as sums of periodic contributions, which amplitudes and phases are calculated separately. This requires to model the shapes of the satellites as sums of spherical harmonics, i.e. as sums of ideal shapes, from the sphere to more and more distorted ones. And the shapes of the two boundaries of the ocean are estimated from the whole gravity of the body. As you may understand, I do not want to enter into specifics…
Let us go to the results instead.

The response of the oceans may be measured

The authors applied their model to Europa and Enceladus. They find that eccentricity tides give a higher amplitude of deformation, but the obliquity tides give a higher phase lag, because the the Rossby-Haurwitz waves, that the eccentricity tides do not produce. For instance, and here I cite the abstract of the paper If Europa’s shell and ocean are respectively 10 and 100 km thick, the tide amplitude and phase lag are 26.5 m and <1° for eccentricity forcing, and <2.5 m and <18° for obliquity forcing. The expected NASA mission Europa Clipper should be able to detect such effects. However, no space mission is currently planned for Enceladus.

I have a personal comment: for Mimas, a phase lag in libration of 6° has been measured. Could it be due an internal ocean? This probably requires a specific study.

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.

Planets

Impacts on Jupiter

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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.

Outline

The fall of Shoemaker-Levy 9
Impacting Jupiter
Observing an impact
Treating the data
A 45-m impactor every year
The study and its authors

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.