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.
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:
- June 3, 2010, detected twice, in Australia and in the Philippines,
- August 20, 2010, detected thrice, in Japan,
- September 9, 2012, detected twice, in the USA
- March 17, 2016, detected twice, in Austria and Ireland,
- 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
- You can find the study here. Moreover, the authors made it freely available on arXiv, thanks to them for sharing!
- The webpage of Ricardo Hueso Alonso,
- the one of Marc Delcroix,
- the IAU page of Agustin Sánchez-Lavega,
- pictures by Sauveur Pedranghelu,
- and the ResearchGate profile of J.M. Gómez-Forrellad,