Tag Archives: Observations

There was water on Mars

Hi there! Well, we already knew that there has been liquid water on the surface of Mars, a long time ago. Indeed, the space mission Mariner 9 imaged valley networks in 1972. Since then, several missions refined the data. The study I present today, Estimate of the water flow duration in large Martian fluvial systems, by Vincenzo Orofino, Giulia Alemanno, Gaetano Di Achille and Francesco Mancarella, uses the most recent observations to estimate the length and depth of former Martian rivers, and their duration of formation and erosion. This study has recently been accepted for publication in Planetary and Space Science.

Evidences of liquid water in the past

The current atmosphere of Mars is pretty thin, its pressure being on average 0.6% the one of the Earth. Such a small atmospheric pressure prevents the existence of liquid water at the surface. Water could survive only as ice, otherwise would be just vaporized. And ice water has been found, particularly in the polar caps. But if the atmosphere were thicker in the past, then liquid water would have survived… and we know it did.

We owe to Mariner 9 a map of 85% of the Martian surface, which revealed in particular river beds, deltas, and lake basins. The study we discuss today focused on valley networks, which are particularly present in the southern highlands of Mars. These valleys are typically less than 5 km wide, but may extend over thousands of kms, and they reveal former rivers.

Nirgal Vallis seen by Mariner 9. © NASA
Nirgal Vallis seen by Mariner 9. © NASA

The history of these rivers is inseparable from the geological history of Mars.

The geologic history of Mars

We distinguish 3 mains eras in the geological history of Mars: the Noachian, the Hesperian, and the Amazonian.

The Noachian probably extended between 4.6 and 3.7 Gyr ago, i.e. it started when Mars formed. At that time, the atmosphere of Mars was much thicker that it is now, it generated greenhouse effect, and liquid water was stable on the surface. It even probably rained on Mars! During that era, the bombardment in the inner Solar System, including on Mars, was very intense, but anyway less intense than the Late Heavy Bombardment, which happened at the end of the Noachian. Many are tempted to consider it to be the cause of the change of era. Anyway, many terrains of the south hemisphere of Mars, and craters, date from the Noachian. And almost all of the river beds as well.

After the Noachian came the Hesperian, probably between 3.7 and 3.2 Gyr ago. It was a period of intense volcanic activity, during which the bombardment declined, and the atmosphere thinned. Then came the Amazonian, which is still on-going, and which is a much quieter era. The volcanic activity has declined as well.

So, almost all of the valley networks date from the Noachian. Let us now see how they formed.

Use of recent data

We owe to the space missions accurate maps of Mars. From these maps, the authors have studied a limited data set of 63 valley networks, 13 of them with a interior channel, the 50 remaining ones without. The interior channel is the former river bed, while the valley represents the area, which has been sculpted by the river. The absence of interior channel probably means that either they are too narrow to be detectable, or have been eroded.

These valley networks are located on sloppy areas, most of them close to the equator. The authors needed the following information:

  1. area,
  2. eroded volume,
  3. valley slopes,
  4. width and depth of the interior channel.

To get this information, they combined topographic data from the instrument MOLA (for Mars Orbiter Laser Altimeter) on board Mars Global Surveyor (1997-2006) with THEMIS (THermal Emission Imaging System, on board Mars Odyssey, still operating). MOLA permits 3-D imagery, with a vertical resolution of 30 cm/pixel (in other words, the accuracy of the altitude) and a horizontal one of 460 m/pixel, while the THEMIS data used by the authors are 2D-data, with a resolution of 100 m /pixel. When the authors judged necessary, they supplemented these data with CTX data (ConTeXt camera, on board Mars Reconnaissance Orbiter, still ongoing), with a resolution up to 6 m/pixel.

These information are very useful to estimate the formation time and the erosion rate of the valley network.

Dynamics of formation of a river bed

They estimated these quantities from the volume of sediments, which should have been transported to create the valley networks. The idea is, while water is flowing, assisted by the Martian surface gravity (fortunately, this number is very well known, and is roughly one-third of the gravity on Earth) and by the slope, it transports material. The authors assumed in their calculations that this material was only sediments, i.e. they neglected rock transport, and they did the maths.

Several competing models exist for sediment transport. This is actually difficult to constrain, given the uncertainties on the sediments themselves. Such phenomena also exist on Earth, but the numbers are very different for instance if you are in Iceland or in the Atacama Desert.

It also depends on the intermittence: is the water flow constant? You can say yes to make your life easier, but is it true? On Earth, you have seasonal variations… why not on Mars? A constant water flow means an intermittence of 100%, while no water means 0%.

And keep also in mind that the water flow depends on the atmospheric conditions: is the air wet or pretty arid? We can answer this question for the present atmospheric conditions, but how was it in the Noachian?

No icy Noachian

And this is one result of the present study: there must have been some evaporation in the Noachian, which means that it was not cold and icy. The authors show that such a Noachian would be inconsistent with the valley networks, as we presently observe them.

However, they get large uncertainties on the formation timescales of the valley networks, i.e. between 500 years and almost twice the age of the Solar System. They have anyway median numbers, i.e.

  • 30 kyr for a continuous sediment flow,
  • 500 kyr with an intermittence of 5%,
  • 3 Myr with an intermittence of 1%,
  • 30 Myr with an intermittence of 0.1%.

And from the data, they estimate that the intermittence should be in the range 1%-5%, which corresponds humid (5%) and semiarid/arid environments. This is how they can rule out the cold and icy Noachian.

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.

Saturn sends us meteorites

Hi there! First I would like to thank you for following me on Facebook. The Planetary Mechanics page has reached 1,000 followers!

OK, now back to business. Did you know that our Earth is intensively bombarded from space? You have recently heard of this Chinese space station, Tiangong-1… in that case, it was man-made stuff. But we are intensively bombarded by natural space material. Most of it is so small that it is destroyed when entering the atmosphere, but sometimes it arrives to us as stones… And in extreme cases, the impactor is so large that its impact may generate an extinction event. The Chicxulub crater, in Mexico, is thought to result from the impact, which aftermath provoked the extinction of the dinosaurs, some 66 Myr ago.

The meteorites I speak about today are the ones, which fall on the Earth every year. This is the opportunity to discuss about Identification of meteorite source regions in the Solar System, which has recently been accepted for publication in Icarus. In that study, the authors determine the origin of 25 meteorites, from their observed trajectories just before they hit us.

Meteorites bombard the Earth

We estimate that currently 60 tons of cosmic material fall on the Earth every day. This seems huge, but actually most of it arrives to us as dust, since the original object does not survive its entry into the atmosphere. In fact, the larger the meteorite, the less frequent it falls on us. 4-m objects arrive every ~16 months, 10-m ones every ~10 years, and 100-m ones every ~5,200 years. And they arrive somewhere on Earth… do not forget that most of the surface of our planet is water. So, don’t worry.

The contact of such a small object with the atmosphere may generate an airburst, which itself could be detected, in many frequencies. I mean, you may hear it, you may see it (make a wish), it can also disturb the radio emissions. This motivated the existence of several observation programs, dedicated to the detection of meteors.

Observation networks

Programs of observation exist at least since 1959, originally under the impulse of Ondřejov Observatory (Czech Republic). These are usually national programs, e.g.

and there are probably more. These are networks of camera, which systematically record the sky, accumulating data which are then automatically treated to detect meteors. The detection of a meteors from different location permit to determine its trajectory.

Detection of a fireball by FRIPON, in September 2016. © FRIPON
Detection of a fireball by FRIPON, in September 2016. © FRIPON

Identifying the source

As I said, multiple detections, at different locations, of a fireball, permit to derive its trajectory. This trajectory gives in particular the radiant, which is the direction from which the meteorite, or the impactor, seems to come. The authors are also interested in the velocity of the object.

The velocity and the radiant are determined with respect to the Earth. Once they are determined, the authors translated them into heliocentric elements, i.e. they determined the pre-impact trajectory of the object with respect to the Sun. And this makes sense, since Solar System objects orbit the Sun! This trajectory is made of orbital elements, i.e. semimajor axis, eccentricity, inclination, and the uncertainties associated. Don’t forget that the observations have an accuracy, which you must consider when you use the data. The magnitude of the fireball tells us something on the size of the impactor as well.

From these data, the authors wondered from where the object should come from.

7 candidates as reservoirs of meteorites

The authors identified 7 possible sources for these impactors. These regions are the densest parts of the Main Asteroid Belt.
These are:

  1. the Hungaria family. These asteroids have a semimajor axis between 1.78 and 2 astronomical units, and an inclination between 16° and 34° with respect to the ecliptic, i.e. the orbit of the Earth,
  2. the ν6 resonance: these are bodies, which eccentricity raise because excited by Saturn. They orbit at a location, where they are sensitive to the precessional motion of the pericentre of Saturn. The raise of their eccentricity make these bodies unstable, and good candidates for Earth-crossers. Their semimajor axis is slightly smaller than 2 AU.
  3. the Phocaea family: this is a collisional family of stony asteroids. Their semimajor axes lie between 2.25 and 2.5 AU, their eccentricities are larger than 0.1, and their inclinations are between 18° and 32°. They are known to be a source of Mars-crossers.
  4. the 3:1 MMR (mean-motion resonance with Jupiter): these bodies perform exactly 3 orbits around the Sun while Jupiter makes one. They lie at 2.5 AU. The perturbation by Jupiter tends to empty this zone, which is called a Kirkwood gap.
  5. the 5:2 MMR, at 2.82 AU. This is another Kirkwood gap.
  6. the 2:1 MMR, at 3.27 AU, also known as Hecuba gap,
  7. the Jupiter Family Comets. These are comets, which orbital periods around the Sun are shorter than 20 years, and which inclinations are smaller than 30° with respect to the ecliptic. They are likely to be significantly perturbed by Jupiter.

For each of the 25 referenced meteorites, the authors computed the probability of each of these regions to be the source, in considering the orbital elements (semimajor axis, eccentricity, and inclination) and the magnitude of the object. Indeed, the magnitude is correlated with the size, which is itself correlated with the material constituting it. The reason is that these Earth-crossers orbit the Sun on eccentric orbits, and at their pericentre, i.e. the closest approach to the Sun, they experience tides, which threaten their very existence. In other words, they might be disrupted. Particularly, a large body made of weak material cannot survive.

And now, the results!

Saturn send meteorites to the Earth!

The authors find that the most probable source for the meteorites is the ν6 secular resonance, i.e. with Saturn. In other words, Saturn sends meteorites to the Earth! Beside this, the Hungaria family and the 3:1 mean-motion resonance with Jupiter are probable sources as well. On the contrary, you can forget the Phocaea family and the 2:1 MMR as possible sources.
It appears that the inner belt is more likely to be the source of meteorites than the outer one. Actually, the outer belt mostly contains carbonaceous asteroids, which produce weak meteoroids.

The authors honestly recall that previous studies found similar results. Theirs also contains an analysis of the influence of the uncertainty on the trajectories, and of the impact velocity with the Earth. This influence appears to be pretty marginal.

Anyway, the future will benefit from more data, i.e. more detections and trajectory recoveries. So, additional results are to be expected, just be patient!

The study and its authors

  • You can find the study here, on the website of Icarus. This study is in open access, which means that the authors paid extra fees to make the study available to us. Many thanks to them!
  • You can visit here the website of Mikael Granvik, the first author of the study,
  • and the one of the second author, Peter Brown.

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.

OSSOS discovered 838 Trans-Neptunian Objects

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.

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.

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 rotation of ‘Oumuamua

Hi there! Today we go back to ‘Oumuamua, you know, this interstellar object discovered last Fall. Its visit to our Solar system was the opportunity to observe it, and here we discuss on an analysis of the variations of its luminosity. I present you The excited spin state of 1I/2017 U1 ‘Oumuamua, by Michael J.S. Belton and collaborators. This study tells us that its rotation state might be complex, and that affects the way we figure out its shape. It has recently been published in The Astrophysical Journal Letters.

Remember 1I/’Oumuamua?

I already told you about ‘Oumuamua. This is the first identified object, which has been found in our Solar System but which undoubtedly originates from another System. In other words, it was formed around another star.
The Pan-STARRS survey identified ‘Oumuamua in October 2017, and the determination of its orbit proved it to be unusually eccentric. With an eccentricity close to 1.2, its orbit is a branch of a hyperbola rather than an ellipse. This means that it comes from very far, passes by while the Sun deviates it, and leaves us for ever.
This is the highest eccentricity ever recorded in the Solar System so far. Other objects had an eccentricity larger than 1, but which could have been caused by the gravitational perturbation of a planet. Not for ‘Oumuamua.
Its full name is actually 1I/2017 U1 (ʻOumuamua). 2017 because it was discovered in 2017, 1I as the first Interstellar object ever discovered (by the way, the International Astronomical Union has created this category for ‘Oumuamua), and the name ‘Oumuamua means scout in Hawaiian.

The announcement of its discovery motivated the observers all around the world to try to observe it and make photometric measurements. Here we discuss what these measurements tell us on the rotation and the shape. But before that, let me tell you something on the rotation.

Different modes of rotation

We will consider that our object is an ellipsoid. This is actually unsure, but let us assume it. We have 3 different axes, and we could imagine different configurations for its rotation:

  1. Tumbling rotation: the object rotates around its 3 axes, and basically this is a mess. We could be in a situation of dynamical chaos, like for the moon of Saturn Hyperion.
  2. Short-axis mode (SAM): the rotation is strongly dominated by a motion around the shortest axis. This is the case for many bodies in the Solar System, like the planets, our Moon… This does not mean that the rotation is strictly around one axis, but we will see that a little later.
  3. Long-axis mode (LAM): the rotation is strongly dominated by a motion around the longest axis.
The LAM and SAM modes.
The LAM and SAM modes.

These last two modes can actually cohabit with tumbling, i.e. a tumbling rotation may favor rotation around one axis.

If the rotation were strictly around one axis, then the body would look like a top. But this rotation axis may move with respect to the figure axis. This motion is named precession-nutation. The precession is the averaged path of the figure axis around the angular momentum, while the nutation contains the oscillations around it.

Now, imagine that you look at an object, which has such a rotation. How can you estimate it? There are ways.

Observing the rotation

Actually the brightness of a body not only depends on the distance from it, or on the insolation angle, but also on the surface facing you. This means that from the brightness, you can deduce something on the rotation state of the object. In particular, this surface brightness depends on its location with respect to the principal axis. If the object has the shape of a cigar, the reflected light from the long axis and from the short one will be different, and the lightcurve will present periodic variations. And the period of these variations is the rotation period. Easy, isn’t it?

Actually, not that easy. First, you assume that the surface has a constant albedo, i.e. that the ratio between the incident and the reflected lights is constant. But you do not know that. In particular, an icy surface has a higher albedo than a carbonaceous one. Another difficulty: a tumbling object, or even one with a precessional component in its rotation, will present a combination of different frequencies. Of course, this complicates the analysis.

However, you simplify the analysis in adding observations to your dataset. The authors used 818 observations over almost one month, spanning from Oct, 25 to Nov, 23, 2017. This includes observations from the Hubble Space Telescope, from the Magellan-Baade telescope at Las Campanas Observatory (Chile), from the Canada-France-Hawaii Telescope, from Pan-NSTARRS (these last facilities being based in Hawaii)…

Once the observations are obtained as raw data, they must be treated to correct from atmospheric and instrumental problems. And then it is not done yet, since the authors need an absolute luminosity of ‘Oumuamua, i.e. as if its distance to the observer were constant. The motion of ‘Oumuamua actually induced a trend in its distance to the Earth, and a trend in its luminosity, which the authors fitted before subtracting it the measured lightflux.

Once this is done, the authors get a lightcurve, which is constant on average, but presents variations around its mean value. Unfortunately, the required treatment induced an uncertainty in the measurements, which the authors had to consider. But fortunately, these practical difficulties are well-known, and algorithms exist to extract information from such data.

2 numerical algorithms

Basically, you need to extract periods from the variations of the lightflux. For that, we dispose of the classical tool of Fourier Transforms, which in principle requires equally spaced data. But the recorded data are not equally spaced, and remember that you must consider the uncertainties as well.

Specific algorithms exist for such a purpose. The authors used CLEAN and ANOVA, to double-check their results. These algorithms allow in particular to remove the aliasing effect, i.e. a wrong measurement of a period, because of an appropriate spacing of the data. And now, the results!

A cigar or a pancake?

The authors found two fundamental periods in the lightcurves, which are 8.67±0.34 and 3.74±0.11 hours. Interestingly, they connected these measurements to the possible dynamics of rotation, and they found two possible solutions:

  1. Long-Axis Mode: In that case, the possible rotation periods are 6.58, 13.15 and 54.48 hours, the latter being the most probable one.
  2. Short-Axis Mode: Here, ‘Oumuamua would be rotating with respect to the short-axis, but also with oscillations around the long axis of periods 13.15 or 54.48 hours.

In both axis, the long axis would also precess around the angular momentum in 8.67 ± 0.34 hours. Moreover, the authors found constraints on its shape. Previous studies already told us that ‘Oumuamua is highly elongated, this study confirms this fact, and tells us that ‘Oumuamua could be somewhere between the cigar and the pancake. But once more, this result could be weakened by variations of the surface albedo of ‘Oumuamua.

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.