Tag Archives: Observations

Satellites of Jupiter

Astrometry from close approaches

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Hi there! Today, we discuss of astrometry of the satellites of Jupiter. It consists in answering the question : where are they? Basically, you look at them, and you see where they are… well, it is more complicated than that, actually.
This is the opportunity to present APPROX – mutual approximations between the Galilean moons: the 2016-2018 observational campaign, by Bruno Morgado and several collaborators. Since this paper presents an observation campaign, involving Brazil and France, there are many observers, hence many co-authors. This study has recently been published in The Monthly Notices of the Royal Astronomical Society.

Outline

Astrometry of the natural satellites
Detecting physical phenomena
Absolute and relative astrometry
Many observational difficulties
The mutual approximation technique
The observation campaign
Very accurate results
The study and its authors

Astrometry of the natural satellites

In fact, you can make astrometry of any celestial object. Star, satellite, galaxy, whatever… It is of course easier when the body looks like a dot.

When we look at the celestial sphere, you look in 2D. The space is in 3D, but technically we see a projection of this volume on a sphere, which is called the celestial sphere. Since it is a 2D space, you can also consider two coordinates. Let us call them right ascension and declination. So, you want to know the motion of these bodies, you observe them, and measure their positions.

The astronomers of the past, I mean in the pre-photographic era, used a filar micrometer. This consisted of a screw and a reticle, made of two threads. When observing two celestial bodies, the observer saw two lines in the field, that (s)he could move thanks to the micrometric screw, one on one body, the other line on the other body, and from the motion of the screw he knew the projected (apparent) distance between the two bodies. So, only one dimension at that time.

And then came the photography: you take a picture of the field (small part of the sky), and you measure the coordinates of the bodies. This way you can get two coordinates. The techniques improved since then, with automatic surveys, taking automatic images of the sky, measuring automatically the coordinates,etc.

But why doing that?

Detecting physical phenomena

It is for understanding the motion of an object. For instance, if you take a natural satellite, you know that it orbits its parent planet, following the classical gravitation law of Newton (improved since Einstein and the discovery of general relativity, but you do not need that refinement to understand the dynamics of the natural satellites). But first you have to make sure it is a satellite of that planet. If it follows it on the sky, then it is fine. And to simulate its motion, you must know some parameters, especially the mass of the planet, of the satellite, the flattening of the planet, the masses of the other satellites (yes, they disturb each other). Well, that becomes tricky. And this is why you need those astrometric observations. Your dynamical model depends on some parameters, you fit them to the observations, and then you have the masses.

This way, you can say: OK, we have everything we need. Why still doing that once we have the masses? You can say that we need more accuracy, but once a spacecraft tours around the planet, it gives you all the data you need, doesn’t it? Including astrometric positions, and direct measurements of the masses.

My answer to that is:

  1. A spacecraft covers a pretty limited time span. You would need observations outside of that range, even less accurate, to detect a drift between the dynamical model and the real motion. And that drift would mean that something significant is not in the model, or maybe it is, but not accurately enough.
  2. And this comes to my second argument: the dissipative processes, especially the tides. During centuries we could consider that celestial mechanics was a conservative discipline, in the sense that there was no dissipation to consider. Of course, we knew there was some dissipation, otherwise it is physically irrelevant, but that dissipation was so small that at that time you could safely neglect it. Not any more.

Let us be more specific about tides. The parent body (Jupiter for the Galilean satellites, Saturn for its satellites, the Sun for the planet) exerted a differential torque on the volume of the small one, which generates stress and strain. In an extreme case, i.e. close enough to the planet (inside what we call the Roche limit), bodies can be destroyed (and this gives you the rings of Saturn).
For the classical satellites of the giant planets, the dissipation appears as volcanoes on Io, fractures on Europa, geysers on Enceladus. And in ephemerides (i.e. orbital motion) the induced energy losses result in secular (i.e. a very slow drift) migration over the ages. And we are now accurate enough to detect this tidal effect on the Moon, and in the systems of Jupiter and Saturn.

Numbers regarding this effect tell us how the planetary material reacts to solicitations, and permits us to extrapolate the migration, i.e. know the past and future of these systems.

Now let us go back to techniques of astrometry.

Absolute and relative astrometry

When you measure the coordinates of a celestial body, you need an origin, i.e. a zero, which is a reference. But the reference is usually not present in your field of view, which is limited (one degree is a huge field).
Fortunately, we can use the background stars as references. The stars are catalogued with their coordinates, and so if you have e.g. 20 catalogued stars in your field, then you have 20 points, which you know the coordinates. And this gives you the coordinates of the other points, i.e. the Solar System bodies.
Making catalogues of stars is not an easy task. ESA’s space observatory Gaia is on it!

Artist's impression of Gaia. © ESA
Artist’s impression of Gaia. © ESA

Differential astrometry is an alternative: for instance if you work on the natural satellites of a given system (let us say the Galilean satellites of Jupiter), it could be enough to know where the satellites are with respect to the other ones and to the parent planet. So, you make differential astrometry: you measure the difference between the coordinates of two given bodies.

As I told you, it is a little more complicated than just taking a picture. You must do it properly. Let us see that now.

Many observational difficulties

Of course, your sky should be clear. But this is not enough. When you take two pictures of an object, which does not move, will they put you the object at the same location? Not necessarily! Because of the seeing, which is due to the wind (the atmosphere), and which kind of noises your images. But this is not enough: you are not sure that all of the pixels of your camera have the same response. Of course they should… if only they could… You always have instrumental problems. You can partially compensate that by making a flat fielding, i.e. you take an image of a starless field (for instance before opening the dome) and this gives you the response of the sensors.
Another problem comes from the fact, that these bodies are not dots. You need to know where the center of mass is… which is not the center of figure, because of a phase effect. When you look at the Moon, it is usually not the full Moon, since part of it is in the umbra… the same happens for the other bodies. Beside this, there are problems due to the anisotropy of the reflection of the light on the surfaces of the satellites.

And let me finally mention timing problems: you do measure a position at a given time. But you need to be exact on that time! If not, then your measurement is inaccurate. In other words, you need to care for errors in positions and in time. If you record your images with a laptop, the internal clock may drift by several seconds. I can tell you that from my experience. So, you have to check it constantly. You can have an accurate clock with GPS systems.

I hope you are now convinced that astrometry is truly a science.

The mutual approximation technique

Other astrometry techniques than taking pictures exist. For instance, you can look for occultations: when an asteroid occultates a star, you do not see the star anymore. Since you know where the star is, you know where the asteroid was during the occultation. This technique also permits to get clues on the shape of the asteroid, with multiple observations, and sometimes even detect asteroids and rings. See here.

In the specific case of this study, the authors developed a technique, which is based on the timing of a minimum of the apparent distance between the two satellites. When two satellites are close to each other in the sky (I mean, in projection onto the celestial sphere), you reach a so-called precision premium, i.e. you optimize the accuracy of the measurements. The reason is that your measurement does not suffer from the field distortion. The two dots are so close that you have the same problems for both. So, the differential measurement is not affected.

Here, the authors measure the timing of the minimum of distance, from which they can determine a position, knowing the relative motion of the satellites from the available orbital theories (the ephemerides). Measuring this instant is not trivial, since actually it has no reason to correspond exactly to a recorded data. So, you take a series of images, on all of them you measure the distance, you plot it with respect to the date. And then you fit a polynomial function to the obtained data; the instant you measure corresponds to the minimum of your polynomial.

The authors published this technique two years ago, and this paper present a campaign of observations.

The observation campaign

The authors observed 66 different mutual approximations, from 6 different sites, which are

    • Itajubá (Minas Gerais, Brazil), equipped with a 60-cm telescope,
    • Foz do Iguaçu (Paraná, Brazil), equipped with a 28-cm telescope,
    • Guaratinguetá (São Paulo, Brazil), equipped with a 40-cm telescope,
    • Vitória (Espiríto Santo, Brazil), equipped with a 35-cm telescope,
    • Curitiba (Paraná, Brazil), equipped with a 25-cm telescope,
    • and the Observatoire de Haute-Provence (France), equipped with a 120-cm telescope.

These facilities permitted the determination of 104 central instants, which obviously means that some events, in fact 28 of them, were observed at least twice, i.e. from different sites. All of these telescopes were equipped with a narrow-band filter centered at 889 nanometers. This eliminates the light pollution by Jupiter.

Very accurate results

The authors get a mean accuracy of 11.3 mas (milli-arcseconds), which is ten times more accurate than classical observations. A good way to determine this accuracy is by a statistical comparison between the measurements, and the numbers predicted by the theory.
At this distance, i.e. the distance to Jupiter, 11.3 mas means 40 km, which is much smaller than the radii of these satellites (Io, Europa, Ganymede, and Callisto).

So, you see, this is a very promising technique, and supplementing the database of astrometric observations with such high-quality data can only lead to new discoveries.

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.

Asteroids: Trans-Neptunian Objects

Satellites of large TNOs

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Hi there! Today, we discuss about the Trans-Neptunian Objects, and more precisely objects orbiting in the Kuiper Belt, i.e. between the orbit of Neptune (~30 astronomical units) and ~50 AU (remember: the Earth orbits at 1 AU, i.e. 150 millions kilometers) for the classical Belt. Scattered objects orbit beyond that limit. During six decades, Pluto was the only known object of them, but the observation surveys with large telescope and adaptive optics has permitted the discovery of more than 100,000 of them! And among them are what we call dwarf planets, i.e. bodies which are large enough to have a pretty spherical shape.
Beside this, many Kuiper Belt objects have a companion, i.e. a satellite. When the satellite is large enough to compete with the primary body, the system is called a binary.
Today, we focus on the satellites of 2 large TNOs, named Eris and Orcus. The study I present, Medium-sized satellites of large Kuiper belt objects, by Michael E. Brown and Bryan J. Butler, tells us that their known satellites are pretty large and dark. This study has recently been published in The Astronomical Journal.

Outline

Many TNOs are not alone
Two large guys: Eris and Orcus
Observations with the Atacama Large Millimeter Array (ALMA)
From the observations to the physical parameters
On the origin of the satellites
The study and its authors

Many TNOs are not alone

Charon, i.e. the companion of Pluto, has been discovered in 1978. At that time, Pluto was the only known Trans-Neptunian Object. Since then, the discoveries of TNOs were often followed by the detection of a companion. There could be more than 20% of binary objects and multiple systems in the low-inclination populations, i.e. the objects which orbital plane is close to the mean one of the classical Kuiper Belt. This fraction seems to be much smaller (maybe 5%) for the inclined objects. But determining a frequency is a tough task, since the detection of such objects challenges the limitations of our observation facilities.

Anyway, there are many binaries among the Kuiper Belt, and this raises the question: how is that possible? How did they form? Does the companion result from an impact? Was it a single object, which have been trapped by the primary one?

Answering these questions requires to consider the properties of the two objects, i.e. their mass ratio, their distance, their composition (do they appear to be similar or not?). And this also raises other questions, related to the stability of these systems (how long can they survive as binaries)?

Two large guys: Eris and Orcus

Discoveries

Eris and Orcus were discovered in 2005 and 2004, respectively, by a team led by Michael Brown, with data from Palomar Observatory.
The discovery of a Solar System object usually happens during a systematic survey of the sky. You take several pictures of a given field of the sky. You first need to reduce them, i.e. you de-noise them to correct for the instrumental and atmospheric problems, then you make an astrometric correction in using the stars which appear on the image, to improve the reliability of the coordinates you use. Once this is done, you see small points on the images. From an image to another, most of the points are fixed. These are the stars. And sometimes, a point is slowly moving. This is a Solar System object. And if this object is not catalogued, it means you have discovered it.

Discovery of Orcus. © NASA
Discovery of Orcus. © NASA

Actually it is a little more complicated than that, you need to re-observe the object to validate the discovery and calculate orbital elements, i.e. determine its orbit. But once an object is observed, it is easier to re-observe it.

And the multiple observations of Eris and Orcus have permitted to infer some of their physical properties, and along with their orbital elements.

Interestingly, there were precoveries of Eris and Orcus. A precovery is an a posteriori identification of an object, after it is known, but on images taken before its discovery. In other words, its presence was on the images, but remained unnoticed. Precovery images have been identified back to September 3, 1954 for Eris, and to November 8, 1951 for Orcus. This means that we have observations over more than 60 years. Of course, these precoveries do not give us any clue on the physics of the object. However, they constrain its motion. If we consider the fact that its period is of the order of a few century, an observation arc of 60 years is highly valuable for determining its orbit.

Properties

You can find below some of their properties.

Eris Orcus
Discovery 2005 2004
Semimajor axis 67.781 AU 39.398 AU
Eccentricity 0.44 0.22
Inclination 44° 20.6°
Orbital period 558.04 yr 247.29 yr
Diameter 2326±12 km 910±50 km
Albedo 0.96±0.04 0.23±0.02
Apparent magnitude 18.7 19.1
Satellite Dysnomia Vanth

And it appears that these two objects are indeed very different. Orcus is a plutino, i.e. its orbit is close to the one of Pluto. It is in a 3:2 mean-motion resonance (MMR) with Neptune, i.e. it makes exactly 2 orbital revolutions around the Sun while Neptune makes 3. However, Eris is a scattered object. This means that its orbit does not make it a cold (i.e. unexcited) classical Kuiper Belt object, but it belongs to the objects, which have been somehow dynamically excited. As a consequence, its orbit is significantly inclined with respect to the classical Kuiper Belt, and it orbits far beyond.

Moreover, Eris is just the largest known TNO, even larger than Pluto. When it was discovered, Pluto was still called a planet. Its downgrading to a dwarf planet is the consequence of the discovery of these large TNOs, not only Eris and Orcus, but also Makemake, Haumea, Sedna…

And these two bodies appear to have at least one satellite each! Both were discovered in 2005 by teams led by Michael Brown, during observations of the main Kuiper Belt Objects. The satellite of Eris, Dysnomia, has been discovered thanks to the Keck Observatory, located on the Mauna Kea, Hawaii, while the satellite of Orcus, Vanth, was discovered thanks to the Hubble Space Telescope.

These objects appear so faint that we must use the best facilities to study them.

Observations with the Atacama Large Millimeter Array (ALMA)

The authors used data taken at the Atacama Large Millimeter Array (ALMA). This is an array made with a collection of 12m-antennae, in the Chilean Andes. They benefited from a recent upgrade of the instrument, to obtain spatial resolutions of 10s of milliarcseconds. Observations at this resolution at the frequency of 350 GHz, which is at the boundary between far infrared and sub-millimetric, permits to directly measure the thermal emission of satellites of Kuiper Belt Objects.

You can find in the video below some views of ALMA.

The authors disposed of 4 ALMA observations of the pair Orcus-Vanth, taken in October and November 2016, and 3 observations of Eris-Dysnomia, made in November and December 2015. The four observations of Orcus-Vanth gave an obvious resolution of the two bodies, while it is not that clear for Eris-Dysnomia. A combination of the 3 observations into a single image has anyway allowed a detection with a very good confidence, and using an a priori knowledge of the location of Dysnomia, due to previous studies.

The authors also supplemented their Orcus-Vanth dataset with unresolved data (i.e. on which you cannot separate the two objects) due to the infrared Spitzer Space Telescope and the Herschel Space Observatory.

Once they have these data, they should invert them to extract physical parameters. And this is not easy.

From the observations to the physical parameters

The difficulty comes from the accuracy of the observations. Remember that each of them is indeed a challenge.

If all of the observations had a perfect accuracy, you would just need a few images to get the position of the velocity of a planetary body, and then its orbital elements… But, if you try to do that on two different datasets related to the same object, you would get different numbers! And the reason is in the accuracy of the observations. Just an example: in using an orbital solution for Vanth resulting from a previous study, the authors got a difference of 11° in the longitude, i.e. Vanth is 11° in advance on its orbit on these observations, with respect to the predictions, which are derived from previous observations. This should give you an idea of the difficulty of the task.

And the authors should find a best fit between the models and the observations. They model Eris, Dysnomia, Orcus and Vanth as spherical bodies, which have an orbital motion and a thermal emission. These things depend on parameters, and you should find the numbers for these parameters, which give the best match between observations and models.

For that, they used the Markov Chain Monte Carlo scheme. This consists in testing a collection of parameters, which are distributed following a probability law. You can find below some of the results.

Dysnomia Vanth
Diameter 700±115 km 475±75 km
Albedo 0.04±0.02 0.08 ±0.02
Orbital period 15.79 days 9.54 days

I see two major elements from these results:

  1. The satellites are pretty large,
  2. they are much darker than their parent body (Eris for Dysnomia, Orcus for Vanth).

This last element suggests that they have a different composition.

On the origin of the satellites: impact of trapping?

An elegant scenario for the creation of a double system is an impact on the proto-primary body. This impact would have excavated a significant mass, which would have then formed the secondary. This is the most popular explanation for the formation of the Moon, and this seems to work for Pluto-Charon.

But the difference in albedo between the primary and the secondary, for these two pairs, could rule out this scenario, just because the surfaces seem to be too different. This could also mean that the secondary is essentially made of material, which initially belonged to the impactor. But this enforces another scenario as well, which is the trapping of the secondary by the binary. Originally there would have been two independent bodies, which would have met, and got gravitationally bound. Why not? That would be consistent with a difference in the composition.

The study and its authors

  • You can find the study here. The authors made it freely available on arXiv, thanks to them for sharing!
  • The homepage of Michael E. Brown. He discovered several Trans-Neptunian Objects, including Eris and Orcus, and is strongly involved in the quest for the Planet Nine. You cann see his blog here.
  • and the one of Bryan J. Butler.

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.

Comets

An active asteroid

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Hi there! Today we will detail a recent study by Jessica Agarwal and Michael Mommert, entitled Nucleus of active asteroid 358P/Pan-STARRS (P/2012 T1). This study has recently been accepted for publication in Astronomy and Astrophysics, and consists in increasing our knowledge of a recently discovered object, i.e. P/2012 T1. This object proved to have some activity, like a comet. The authors realized several observations to try to characterize its activity, and infer some physical properties like its size and its rotation.

Outline

Comet vs. active asteroid
The asteroid P/2012 T1
New observations
Measuring its rotation
Dust emission
A 530m-large body
The study and its authors

Comet vs. active asteroid

First of all, I would like to make clear what is a comet, and what is an active asteroid. I am very ambitious here, since these two notions actually overlap. For instance, our object is both an active asteroid, and a main-belt comet.

Let us say that a comet is an active asteroid, while an active asteroid is not necessarily a comet. The difference is in the nature of the activity.

A comet is a dirty snowball, i.e. you have water ice, and some silicates. Its orbit around the Sun is usually pretty eccentric, so that you have large variations of the distance Sun-object. The location of the orbit, at which the distance is the smallest, is called pericentre. When the comet approaches the pericentre, it approaches the Sun, heats, and part of its water ice sublimates. This results in a dusty tail (actually there are two tails, one being composed of ionized particles).

But when you see dust around a small body, i.e. when you see activity, this is not necessarily ice sublimation. There could be for instance rock excavated by an impact, or material expelled by fast rotation. In that case, you still have an active asteroid, but not a comet. One of the goals of this study is to address the cause and nature of P/2012 T1’s activity.

The asteroid P/2012 T1

P/2012 T1, now named 358P, has been discovered in October 2012 by the Pan-STARRS-1 survey. Pan-STARRS stands for Panoramic Survey Telescope and Rapid Response System, it uses dedicated facilities at Haleakala Observatory, Hawaii, USA.

Discovery of P/2012 T1. © Pan-STARRS
Discovery of P/2012 T1. © Pan-STARRS

Its provisional name, P/2012 T1, contains information on the nature of the object, and its discovery. P stands for periodic comet, 2012 is the year of the discovery, and T means that it has been discovered during the first half of October.

Interestingly, this object appeared on images taken in December 2001 at Palomar Observatory in California, while acquiring data for the survey NEAT (Near-Earth Asteroid Tracking).

You can find below its orbital elements, from the Minor Planet Center:

Semi-major axis 3.1504519 AU
Eccentricity 0.2375768
Inclination 11.05645°
Period 5.59 y

From its orbital dynamics, it is a Main-Belt object. As a comet, it is a Main-Belt Comet.

New observations

Once an object is known and we know where it is, it is much easier to reobserve it. The authors conducted observations of 358P from the Southern Astrophysical Research (SOAR) telescope, and the Very Large Telescope.

The SOAR telescope is based on Cerro Pachón, Chile. This is a 4.1-m aperture facility, located at an altitude of 2,700 m. The authors took images with the Goodman High Throughput Spectrograph during one night, from July 27 to July 28, 2017. They wanted to analyze the reflected light by the asteroid at different wavelengths, unfortunately the observational constraints, i.e. cloud coverage, permitted only two hours of observations. Only the observations made with the VR filter, centered at 610 nm, were useful.

These data were supplemented by 77 images taken during 10 hours from August 17 to August 18, 2017, at the Very Large Telescope. This instrument depends on the European Southern Observatory (ESO), and is located on Cerro Paranal, once more in Chile, at an altitude of 2,635 m. The authors used the FOcal Reducer and low dispersion Spectrograph 2 (FORS2), which central wavelength is 655 nm.

The observations give raw images. The authors treated them to get reliable photometric and astrometric measurements of 358P, i.e. they corrected from the variations of the luminosity of the sky, in using reference stars, and from the possible instrumental problems. For that, they recorded the response of the instrument to a surface of uniform brightness, and used the outcome to correct their images.

Let us now address the results.

Measuring its rotation

Such a small (sub-kilometric) body is not spherical. This results in variations of luminosity, which depend on the surface element which is actually facing your telescope. If you acquire data during several spin periods of the asteroid, then you should see some periodicity in the recorded lightcurve.

The best way to extract the periods is to make a Fourier transform. Your input is the time-dependent lightcurve you have recorded, and your output is a frequency-dependent curve, which should emphasize the periods, which are present in the recorded lightcurve. If the signal is truly periodic, then it should exhibit a maximum at its period and its harmonics (i.e. twice the period, thrice the period, etc.), and almost 0 outside (not exactly 0 since you always have some noise).

In the case of 358P, the authors did not identify any clear period. A maximum is present for a rotation period of 8 hours, but the result is too noisy to be conclusive. A possible explanation could be that we have a polar view of the asteroid. Another possibility is that the albedo of the asteroid (the fraction of reflected light) is almost uniform.

Dust emission

The authors tried to detect debris around the nucleus of the comet, in widening the aperture over which the photometry was performed. They got no real detection, which tends to rule out the possibility of non-cometary activity.

A 530m-large body

Finally, the magnitude of the asteroid is the one of a sphere of 530 meters in diameter, with an albedo of 6%. This means that a higher albedo would give a smaller size, and conversely. The albedo depends on the composition of the asteroid, which is unknown, and can be only inferred from other asteroids. The authors assumed it to be a carbonaceous asteroid (C-type), as 75% of the asteroids. If it were an S-type (silicateous) body, then it would be brighter. A wide band spectrum of the reflected light would give us this information.

The study and its authors

  • You can find the study here, on Astronomy and Astrophysics’ website. Moreover, the authors uploaded a free version on arXiv, thanks to them for sharing!
  • Here is the webpage of the first author, Jessica Agarwal,
  • and here the website of Michael Mommert.

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 Uranus

Uranus’s satellites are red

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Hi there! Today, we speak about the satellites of Uranus. They have been visited only once by a spacecraft, i.e. Voyager 2 in 1986, but we dispose of Earth-based facilities, which are able to give us some clues. The study I present today, Red material on the large moons of Uranus: Dust from irregular satellites? discusses the fact that the main moons appear to be redder than the large moons of Jupiter and Saturn.

Let me define redder first: the surface of these bodies reflects the Solar incident light. A spectral decomposition of the reflected light tells us something on the material coating the surface. And the relative response of the surface in the different wavelengths is higher in the infrared for the large moons of Uranus, than it is for the moons of Jupiter and Saturn.

This study, by Richard Cartwright et al., has recently been published in Icarus.

Outline

The satellites of Uranus
Voyager 2 at Uranus
Observations at IRTF
Geometrical constraints
A red leading side
Pollution by the irregular moons
The study and its authors

The satellites of Uranus

First: Uranus. This is the seventh planet of our Solar System, which orbits in 84 years, and which seems to roll on its orbit. Actually, its rotation axis is tilted by nearly 90° (actually 97.8°), and its main satellites and rings orbit close to its equatorial plane. Their orbits are tilted as well.

The satellites of Uranus, all named after Shakespeare’s characters, can be categorized into 3 groups:

  1. The 13 small, inner satellites, which are embedded into the rings,
  2. the 5 main ones,
  3. and the 9 irregular satellites, which orbit much further from Uranus, and which orbits may be tilted. Contrary to the other two groups, they have probably not been formed in the proto-Uranus nebula, but were former asteroids, which have been trapped by Uranus.

You can find below some properties and orbital characteristics of the main satellites. All of these bodies have been discovered from the Earth. These are the targets of the study I present.

Discovery Semimajor axis Eccentricity Inclination Orbital period Mean diameter
Miranda 1948 129,900 km 0.0013 4.338° 1.413 d 471.6 km
Ariel 1851 190,900 km 0.0012 0.041° 2.520 d 1,157.8 km
Umbriel 1851 266,000 km 0.0039 0.128° 4.144 d 1,169.4 km
Titania 1787 436,300 km 0.0011 0.079° 8.706 d 1,577.8 km
Oberon 1787 583,500 km 0.0014 0.068° 13.46 d 1,522.8 km

You can see that they have limited eccentricities and inclinations, except for the inclination of Miranda, which probably results from a past resonant forcing by Umbriel. In the past, the orbital period of Umbriel was exactly thrice the one of Miranda, and this has forced its inclination, which was thus initially very small. Anyway, it remains close to the equatorial plane for Uranus.

You can see below that things are different for the small satellites.

Discovery Semimajor axis Eccentricity Orbital period Mean diameter
Francisco 2001 4,282,900 km 0.13 267 d ≈22 km
Caliban 1997 7,231,100 km 0.18 580 d ≈72 km
Stephano 1999 8,007,400 km 0.22 677 d ≈32 km
Trinculo 2001 8,505,200 km 0.22 749 d ≈18 km
Sycorax 1997 12,179,400 km 0.52 1,288 d ≈150 km
Margaret 2003 14,146,700 km 0.68 1,661 d ≈20 km
Prospero 1999 16,276,800 km 0.44 1,978 d ≈50 km
Setebos 1999 17,420,400 km 0.59 2,225 d ≈48 km
Ferdinand 2001 20,430,000 km 0.40 2,790 d ≈20 km

These are very small bodies, which orbit very far from Uranus, on eccentric orbits. Besides, their orbital planes have just nothing to do with the equatorial plane of Uranus. This is why we believe they are former asteroids. Beside Margaret, they all orbit on retrograde orbits, while all the regular moons are prograde. Discovering them required to use telescopes of a least 5 m, the satellites discovered in 2003 having been discovered during a systematic survey with the Subaru telescope at Mauna Kea, Hawaii, over a field of 3.5 degrees. They all have apparent magnitudes larger than 20.

Only one space mission visited them: Voyager 2, in January 1986.

Voyager 2 at Uranus

The spacecraft Voyager 2 was launched from Cape Canaveral, Florida, in August 1977. It benefited from a favorable geometrical configuration of the 4 giant planets to visit all of them. Unfortunately, this required the spacecraft to travel too fast to permit an orbital insertion. So, contrary to Cassini which toured around Saturn during 13 years, Voyager 2 just passed by.

Its closest approach to Uranus was on January 24, 1986, at a distance of 81,500 km from the planet’s cloud tops. It permitted the discovery of 11 inner satellites, and partly imaged the large ones. It revealed in particular geological features on Miranda, and analyzed the light reflected by the surface of these bodies. The study we discuss today supplements these measurements.

Miranda seen by Voyager 2. © NASA/JPL/USGS
Miranda seen by Voyager 2. © NASA/JPL/USGS

Observations at IRTF

The authors used NASA’s InfraRed Telescope Facility (IRTF). This is a 3-meter telescope, optimized for infrared astronomy. It is located at the Mauna Kea Observatory (altitude: 4,200 m) in Hawaii (USA), and 50% of the observation time is devoted to planetary observation.
Several instruments are available, the authors used the spectrograph-imager SpeX, which decomposes the incident light between 0.8 and 5.4 µm. In that study, the authors limited to 4.2 µm.

NASA's InfraRed Telescope Facility at Maune Kea, Hawaii. © Afshin Darian
NASA’s InfraRed Telescope Facility at Maune Kea, Hawaii. © Afshin Darian

The outcome of such observations is a plot amplitude vs. wavelength of a given surface element of a satellite. It is interesting to keep in mind that the regular moons rotate synchronously, permanently showing the same face to Uranus. The consequence is that they have a leading and a trailing hemisphere. During their orbital motion, the same hemisphere always leads. And this has implications for the surface composition, because the leading hemisphere can be polluted by the dusty environment. In other words: when you observe something on the leading hemisphere, which is not present on the trailing one, this is probably pollution.

When you observe, you actually observe the surface element which faces you. And this depends on the dynamics of the planet.

Geometrical constraints

As you know, Uranus rolls on its orbit, while the satellites have an equatorial orbit. As a consequence, during a 84-y orbit of Uranus around the Sun, the Earth crosses twice the orbital plane, and two periods are favorable for the observation of the poles of Uranus and the satellites. The northern hemispheres of these bodies face us during half the orbit (42 years), while the southern ones face us during the other half.

The last transition happened in 2007. Since then, the northern hemispheres of the satellites face us. And part of the visible face belongs to the leading hemisphere.

A red leading side

The results show that for Ariel and Umbriel, and even more for Titania and Oberon, the leading hemisphere is significantly redder than the trailing one, while it is not the case for the major satellites of Jupiter and Saturn. Titania and Oberon are the outermost of the satellites of Uranus, and the largest ones as well.

To understand the chemical nature of this reddening, previous studies have conducted lab experiments, consisting in reproducing the spectrum of mixtures of chemical elements, which could be found on the natural satellites of the outer planets. Of course, the conditions of temperature and pressure are considered. Then the spectrums are compared to the actually observed ones. And it appears that the reddening agents should be tholins and pyroxene.

Titania seems to have a red spot on its surface, which makes it the redder of the main Uranian satellites. Contrariwise, Miranda does not present this reddening. Latitudinal variations of color are not obvious, while they are in longitude, since they depend on the leading / trailing effect.

Now, the question is: how did these agents reach the satellites? They are probably not endogenous, since similar satellites around Jupiter and Saturn do not have them.

Pollution by the irregular moons

The smoking gun is the irregular moons: they are pretty red. And numerical simulations of the motion of dust expelled from these satellites by impacts show how they are likely to coat the leading sides of Oberon, Titania, Ariel and Umbriel.
And this is what we observe!

Of course, a space mission to Uranus would be very helpful… but this is another story.

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 Neptune

Triton from the Earth

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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.

Outline

Triton’s facts
The atmosphere of Triton
THE VLT/SINFONI instrument
Spectral measurements
Lab experiments to understand the data
Results
The study and its authors

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.