Tag Archives: Observations

Satellites of Saturn

Enceladus lost its balance

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Hi there! Today I will present you True polar wander of Enceladus from topographic data, by Tajeddine et al., which has recently been published in Icarus. The idea is this: Enceladus is a satellite of Saturn which has a pretty stable rotation axis. In the past, its rotation axis was already stable, but with a dramatically different orientation, i.e. 55° shifted from the present one! The authors proposed this scenario after having observed the distribution of impact basins at its surface.

Enceladus’s facts

Enceladus is one of the mid-sized satellites of Saturn, it is actually the second innermost of them. It has a mean radius of some 250 km, and orbits around Saturn in 1.37 day, at a distance of ~238,000 km. It is particularly interesting since it presents evidence of past and present geophysical activity. In particular, geysers have been observed by the Cassini spacecraft at its South Pole, and its southern hemisphere presents four pretty linear features known as tiger stripes, which are fractures.

Enceladus seen by Cassini (Credit: NASA / JPL / Space Science Institute).
Enceladus seen by Cassini (Credit: NASA / JPL / Space Science Institute).

Moreover, analyses of the gravity field of Enceladus, which is a signature of its interior, strongly suggest a global, subsurfacic ocean, and a North-South asymmetry. This asymmetry is consistent with a diapir of water at its South Pole, which would be the origin of the geysers. The presence of the global ocean has been confirmed by measurements of the amplitude of the longitudinal librations of its surface, which are consistent with a a crust, that a global ocean would have partially decoupled from the interior.

The rotation of a planetary satellite

Planetary satellites have a particularly interesting rotational dynamics. Alike our Moon, they show on average always the same face to a fictitious observer, which would observe the satellite from the surface of the parent planet (our Earth for the Moon, Saturn for Enceladus). This means that they have a synchronous rotation, i.e. a rotation which is synchronous with their orbit, but also that the orientation of their spin axis is pretty stable.
And this is the key point here: the spin axis is pretty orthogonal to the orbit (this orientation is called Cassini State 1), and it is very close to the polar axis, which is the axis of largest moment of inertia. This means that we have a condition on the orientation of the spin axis with respect to the orbit, AND with respect to the surface. The mass distribution in the satellite is not exactly spherical, actually masses tend to accumulate in the equatorial plane, more particularly in the satellite-planet direction, because of the combined actions of the rotation of the satellites and the tides raised by the parent planet. This implies a shorter polar axis. And the study I present today proposes that the polar axis has been tilted of 55° in the past. This tilt is called polar wander. This result is suggested by the distribution of the craters at the surface of Enceladus.

Relaxing a crater

The Solar System bodies are always impacted, this was especially true during the early ages of the Solar System. And the inner satellites of Saturn were more impacted than the outer ones, because the mass of Saturn tends to attract the impactors, focusing their trajectories.
As a consequence, Enceladus got heavily impacted, probably pretty homogeneously, i.e. craters were everywhere. And then, over the ages, the crust slowly went back to its original shape, relaxing the craters. The craters became then basins, and eventually some of them disappeared. Some of them, but not all of them.
The process of relaxation is all the more efficient when the material is hot. For material which properties strongly depend on the temperature, a stagnant lid can form below the surface, which would partly preserve it from the heating by convection, and could preserve the craters. This phenomenon appears preferably at equatorial latitudes.
This motivates the quest for basins. A way for that is to measure the topography of the surface.

Modeling the topography

The surface of planetary body can be written as a sum of trigonometric series, known as spherical harmonics, in which the radius would depend on 2 parameters, i.e. the latitude and the longitude. This way, you have the radius at any point of the surface. Classically, two terms are kept, which allow to represent the surface as a triaxial ellipsoid. This is the expected shape from the rotational and tidal deformations. If you want to look at mass anomalies, then you have to go further in the expansion of the formula. But to do that, you need data, i.e. measurements of the radius at given coordinates. And for that, the planetologists dispose of the Cassini spacecraft, which made several flybys of Enceladus, since 2005.
Two kinds of data have been used in this study: limb profiles, and control points.
Limb profiles are observations of the bright edge of an illuminated object, they result in very accurate measurements of limited areas. Control points are features on the surface, detected from images. They can be anywhere of the surface, and permit a global coverage. In this study, the authors used 41,780 points derived from 54 limb profiles, and 6,245 control points.
Measuring the shape is only one example of use of such data. They can also be used to measure the rotation of the body, in comparing several orientations of given features at different dates.
These data permitted the authors to model the topography up to the order 16.

The result

The authors identified a set of pretty aligned basins, which would happen for equatorial basins protected from relaxation by stagnant lid convection. But the problem is this: the orientation of this alignment would need a tilt of 55° of Enceladus to be equatorial! This is why the authors suggest that Enceladus has been tilted in the past.

The observations do not tell us anything on the cause of this tilt. Some blogs emphasize that it could be due to an impact. Why not? But less us be cautious.
Anyway, the orientation of the rotation axis is consistent with the current mass distribution, i.e. the polar axis has the largest moment of inertia. Actually, mid-sized planetary satellites like Enceladus are close to sphericity, in the sense that there is no huge difference between the moments of inertia of its principal axes. So, a redistribution of mass after a violent tilt seems to be possible.

To know more

And now the authors:

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

Asteroids: Near-Earth Objects, Comets

The activity of the comet C/2015 ER61

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Hi there! Today’s post is on the comet C/2015 ER61. Behind this weird name is a small object orbiting the Sun on a highly elongated orbit, which currently shows us a tail. The associated study is Beginning of activity in long-period comet C/2015 ER61 (PANSTARRS), by Karen J. Meech, Charles A. Schambeau, Kya Sorli, Jan T. Kleyna, Marco Micheli, James Bauer, Larry Denneau, Jacqueline V. Keane, Elizabeth Toller, Richard Wainscoat, Olivier Hainaut, Bhuwan Bhatt, Devendra Sahu, Bin Yang, Emily Kramer and Gene Magnier. It has recently been published in The Astronomical Journal.

C/2015 ER61‘s facts

This comet was discovered two years ago, in March 2015, by the telescope Pan-STARRS 1, located on the Haleakalā, Hawai’i. Its distance to the Sun was then 8.44 Astronomical Units, its absolute magnitude about 12, and no tail was visible. As such, it was supposed to be a Manx object, a Manx being a tailless cat. A Manx object would be a comet, which had no activity anymore, as if the lighter elements had already gone.

From its magnitude, it was guessed that its radius was about 10 km. Its apparent lack of activity triggered enough interest for the object to be followed, this in particularly permitted to determine its orbit, and showed that it had a huge eccentricity, i.e. some 0.998. When the eccentricity reaches 1, then the orbit is parabolic, so the orbit of C/2015 ER61 is almost parabolic. Further observations showed the beginning of a period of activity, proving that C/2015 ER61 (I would appreciate a funnier nickname…) is actually not a Manx. This period is not done yet, and the activity is actually increasing, as the comet is approaching the Sun. At its smallest distance, i.e. the perihelion, its distance to the Sun is 1.04 AU, i.e. it almost crosses the orbit of the Earth (don’t worry, I said “almost”). So, observing this comet today reveals a tail.

We are actually pretty lucky to be able to observe it, since its orbital period is some 10,000 years. This comet is considered to belong to the Oort cloud, which is a reservoir of comets at the edge of our Solar System.

Cometary outgassing

Since the comet model by Fred L. Whipple, published between 1950 and 1955, a comet is seen as a kind of dirty snowball, with a nucleus, and icy elements, which tend to sublimate when approaching the Sun, because of the elevation of the temperature. This hypothesis was confirmed in 1986 when we were visited by the well-known comet 1P/Halley (you know, Halley’s comet).
The idea is this: you have some water ice, some CO, some CO2, trapped on the comet. When it is warm enough, it sublimates.

But the intensity of the sublimation depends on several parameters:

  • the thermal inertia of the comet: how does the temperature elevate?
  • its albedo: which fraction of the incident Solar light flux is reflected?
  • its density
  • the quantity of elements, which are likely to be sublimated
  • their depth: if they are not at the surface, the heat needs to be conducted deep enough for them to sublimate
  • the distance to the Sun (of course)
  • etc.

This means that observing and measuring this outgassing gives some physical properties of the comet.

The observation facilities

To conduct this study, several observation facilities were used:

  • Pan-STARRS1 (PS1): This stands for Panoramic Survey Telescope and Rapid Response System. This is a 1.8m wide-field telescope,
  • Gemini North: this is a 8.19 m telescope, which is based in Hawai’i. It has a twin brother, Gemini South, which is based in Chile,
  • Canada-France-Hawai’i Telescope (CFHT): this 3.58m telescope is part of the Mauna Kea Observatory. For this study, the MegaPrime/Megacam wide-field imager was used, which gives of fied of view of 1°,
  • ATLAS: (for Asteroid Terrestrial-impact Last Alert System). This will be a network of two 0.5m-telescopes, both based in Hawai’i. At this time, only the ATLAS-Haleakalā has begun full operation,
  • Himalayan Chandra Telescope (HCT): this is a 2.01 m optical-infrared telescope, which is part of the Indian Astronomical Observatory, which stands on Mount Saraswati, Digpa-ratsa Ri, Hanle, India,
  • Wide-field Infrared Survey Explorer (WISE): this is an infrared space telescope, on a Sun-synchronous polar orbit. It is used in the program NEOWISE, NEO standing for Near-Earth Objects.

The diversity of observation facilities explains the numbers of authors signing this study. The observations span from February 2014 to February 2017, which means that there are pre-discovery observations. It is always easier to find an object when you know where it is, which permitted to find C/2015 ER61 on images, which were taken before its discovery.

Results

These observations (see the Figure) has shown a variation of the magnitude, which could be expected since the comet approached the Earth, but too large to be explained by its trajectory. Actually, it is enhanced by the activity of the comet, more precisely by the sublimation of CO and CO2, starting in early 2015.

The measured apparent magnitude of the comet, with respect to the date and the distance to the Sun. We can see that the comet is brighter when closer to the Sun, because of the outgassing. The measurements have some uncertainties, which are not represented here. This figure is drawn for the Tab.1 Observation Log of the paper.

The authors modeled the warming of the comet and the sublimation of the elements, in using the well-known heat equation. The observed tail suggests a radius of the nucleus of about 9 km, which is consistent with previous guesses. Moreover, they suggest that the CO2 is present at a depth of about 0.4 m. If it were present at the surface, then sublimation would have been observed even when the comet was 20 AU away from the Sun.

The closest approach of the comet with the Earth was on April 4, and with the Sun on May 10, which would result in a peak of activity… probably with some delay, please give the comet a chance to warm!

To know more

That’s all for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter.

Satellites of Jupiter

Periodic volcanism on Io

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Hi there! Today’s post addresses the volcanic activity of Io, you know, this very active large satellite of Jupiter. It appears from long-term observations that this activity is somehow periodic. This is not truly a new result, but the study I present you enriches the database of observations to refine the measurement of the relevant period. This study is entitled Three decades of Loki Patera observations, by I. de Pater, K. de Kleer, A.G. Davies and M. Ádámkovics, and has been recently accepted for publication in Icarus.

Io’s facts

Io is one the Galilean satellites of Jupiter. it was discovered in 1610 by Galileo Galilei, when he pointed its telescope to Jupiter. It is the innermost of them, with a semimajor axis of 422,000 km, and a orbital period of 1 day and 18 hours. Its mean radius is 1,822 km.

Io has been visited by the spacecrafts Pioneer 10 and 11, Voyager 1 and 2, Galileo, Cassini and New Horizons, Galileo being the only one of these missions to have orbited Jupiter. The first images of the surface of Io are due to Voyager 1. As most of the natural satellites in our Solar System, it rotates synchronously, permanently showing the same face to a fictitious jovian observer.

Its orbital dynamics in interesting, since it is locked in a 1:2:4 three-body mean motion resonance (MMR), with Europa and Ganymede. This means that during 4 orbits of Ganymede, Europa makes exactly two, and Io 4. While two-body MMR are ubiquitous in the Solar System, this is the only known occurrence of a three-body MMR, which is favored by the significant masses of these three bodies.

Three full disk views of Io, taken by Galileo in June 1996. Loki Patera is the small black spot appearing in the northern hemisphere of the central image. The large red spot on the right is Pele. Credit: NASA.

Such a resonance is supposed to raise the orbital eccentricity, elongating the orbit. Nevertheless, it appears that the eccentricity of Io is small, i.e. 0.0041, on average. How can this be possible? Because there is a huge dissipation of energy in Io.

Volcanoes on Io

This energy dissipation appears as many volcanoes, which activities can now be monitored from the Earth. When active, they appear as hot spots on infrared images. More than 150 volcanoes have been identified so far, among them are Loki, Pele, Prometheus, Tvashtar…

This dissipation has been anticipated by the late Stanton J. Peale, who compared the expected eccentricity from the MMR with Europa and Ganymede with the measured one. This way, he predicted dissipation in Io a few days before the arrival of Voyager 1, which detected plumes. This discovery is narrated in the following video (credit: David Rothery).

Dissipation induces geological activity, which another signature is tectonics. Tectonics create mountains, and actually Io has some, with a maximum height of 17.5 km.

But back to the volcanoes. We are here interested in Loki. The Loki volcano is the source of Loki Patera, which is a 200-km diameter lava lake. This feature appears to be actually very active, representing 9% of the apparent energy dissipation of Io.

The observation facilities

This study uses about 30 years of observations, from

  • the Keck Telescopes: these are two 10-m telescopes, which constitute the W.M. Keck Observatory, based on the Mauna Kea, Hawaii. This study enriches the database of observations thanks to Keck data taken between 1998 and 2016.
  • Gemini: the Gemini Observatory is constituted of two 8.19-m telescopes, Gemini North and Gemini South, which are based in Hawaii and in Chile, respectively.
  • Galileo NIMS: the Galileo spacecraft was a space mission which was sent in 1989 to Jupiter. It has been inserted into orbit in December 1995 and has been deorbited in 2003. NIMS was the Near-Infrared Mapping Spectrometer.
  • the Wyoming Infrared Observatory (WIRO): this is a 2.3-m infrared telescope operating since 1977 on Jelm Mountain, Wyoming.
  • the Infrared Telescope Facility (IRTF): this is a 3-m infrared telescope based on the Mauna Kea, Hawaii.
  • the European Southern Observatory (ESO) La Silla Observatory: a 3.6-m telescope based in Chile.

All of these facilities permit infrared observations, i.e. to observe the heat. In this study, the most relevant observations have wavelengths between 3.5 and 3.8 μm. Some of these observations benefited from adaptive optics, which somehow compensates the atmospheric distortion.

Results

And here are the results:

Periodicity

The authors notice a periodicity in the activity of Loki Patera. More particularly, they find a period between 420 and 480 days between 2009 and 2016, while a period of about 540 days was estimated for the activity before 2002. Moreover, Loki Patera appears to have been pretty inactive between 2002 and 2009, and the propagation direction of the eruptions seems to have reversed from one of these periods of activity to the other one.

Temperature

The authors show variations of temperature of the Loki Patera, in estimating it from the infrared photometry, assuming the surface to be a black body, i.e. which emission would only depend on its temperature. They analyzed in particular a brightening event, which occurred in 1999. They showed that it consisted in the emergence of hot magma, at a temperature of 600 K.
On the whole dataset, temperatures up to 1,475 K have been observed, which correspond to the melting temperature of basalt.

Resurfacing rate

This production of magma renews the surface. The observations of such events by different authors suggest a resurfacing rate between 1,160 and 2,100 m2/s, while the surface of Loki Patera is about 21,500 km2, which means that the surface can be renewed in between 118 and 215 days. At this rate, we would be very lucky to observe impact craters on Io… we actually observe none.

A perspective

The authors briefly mention the variation of activity of Pele, Gilbil, Janus Patera, and Kanehekili Fluctus. The intensity of the events affecting Loki Patera makes it easier to study, but similar studies on the other volcanoes would probably permit a better understanding of the phenomenon. They would reveal in particular whether the cause is local or global, i.e. whether the same periods can be detected for other volcanoes, or not.

To know more

That’s all for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter.

Asteroids: Trans-Neptunian Objects

Measuring an asteroid from its shadow

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Hi there! Today’s post is on the paper Results from the 2014 November 15th multi-chord stellar occultation by the TNO (229762) 2007 UK126, by Gustavo Benedetti-Rossi and 28 colleagues, which has recently been published in The Astronomical Journal. It explains us what 22 simultaneous observations of the same event, i.e. the occultation of a star by an asteroid, tell us about this asteroid.

The asteroid (229762) 2007 UK126

(229762) 2007 UK126 is a Trans-Neptunian Object, which was discovered in October 2007. Its highly eccentricity orbit (0.49) makes it a probable scattered disc object, i.e. its eccentricity should have been pumped by the planets, in particular Neptune. Its estimated rotation period is 11.05 hours. The Hubble Space Telescope has revealed the presence of an orbital companion.

Even at its perihelion, this object is further than Neptune, which makes it difficult to observe. The stellar occultations permit to bypass this problem.

The strategy of observation

The idea is this: while a pretty dark object passes just between you and a star, you do no see the star anymore. The dark object occults it. This occultation contains information.

This is the reason why some planetary scientists try to predict occultations from simulations of the motion of asteroids in the sky, maintaining lists of such events. These predictions suffer from uncertainties on the orbit of the asteroid, this motivates the need to refine the predictions just before the predicted event. For that, astrometric observations of the object are performed, to better constrain its orbital ephemerides.

Once the occultation is predicted with enough accuracy, the observers are informed of the date and the places from where to observe. Multiple observations of the same event, at different locations, represent a set of data which will then be inverted to get information on the asteroid. For these observations to be conducted, amateur astronomers are solicited. They usually constitute networks, which efficiency is doped by their enthusiasm.

The observation of a stellar occultation consists to measure the light flux received from the star during a time interval which includes the predicted event, and when the occultation happens, then a flux drop should be registered. For an observation to be useful, the observer should take care to have an accurate time reference. Moreover, a clear sky, preferably with no wind, makes the measurements more accurate. Some flux drops could be actually due to clouds passing by!

What can these observations tell us?

The first information we get from these occultations addresses the motion of the asteroid: the date and length of the occultation is an information, because we know where the asteroid was on the celestial sphere when this happened. When no occultation is detected, this is an information as well, even if it is frustrating.
Observing at different places permits to observe the occultation of the star by different parts of the asteroid. This is called a multi-chord occultation. From the duration of the event, we can deduce the size of the object with a much better accuracy than direct observation. Such a technique could also detect companions, as it might have been the case for the Main-Belt asteroid (146)Lucina in 1982.

A compelling information on a planetary body is its mass. The best way to measure its mass is by observing the orbit of a companion, if there is one. If there is none, or if its orbit cannot be observed, then we can combine the different measurements of its radius with the measurement of its rotation period and the assumption that its shape is at an hydrostatic equilibrium, i.e. a balance between its own gravity, its rotation, and possibly the gravitational (tidal) attraction of a planetary companion. In the absence of a companion, the equilibrium figure is an oblate, MacLaurin spheroid, which has a circular equatorial section, and a rotation axis which is smaller than the two other ones. If a companion is involved, then the object could be a Jacobi ellipsoid, i.e. an ellipsoid with 3 different principal axes.

This study

This study gathers the results of 20 observations of the stellar occultation of the star UCAC4 448-006503 by the TNO (229762) 2007 UK126 in November 2014, all over the United States, and 2 negative observations, i.e. no flux drop measured, in Mexico. One of the difficulties is is to be accurate on the exact times of the beginning (ingress) and the end (egress) of the event, i.e. the star disappearance and reappearance. This is the reason why the authors of the study split into two teams, which treated the same data separately, with their own techniques (denoted GBR and MWB, since conducted by Gustavo Benedetti-Rossi and Marc W. Buie, respectively).

And here are some of their results:

Before GBR MWB
Longest radius (km) — 339+15-10 340+12-8
Equivalent radius (km) 299.5±38.9 319+14-7 319+12-6
Circular fit radius (km) — 324+27-23 328+26-21
Apparent oblateness — 0.106+0.050-0.040 0.118+0.055-0.048
Density (kg/m3) — <1740 <1620

This table tells us that, before the occultation, only a mean radius was known, and with a much larger uncertainty than now. It also tells us that assuming the asteroid to be circular instead of elliptical gives a larger uncertainty. Wait… why circular and not spherical? Why elliptical and not ellipsoidal? Because the occultation is ruled by the projection of the shape of the asteroid on the celestial sphere, which is a 2D surface. So, we observe a surface, and not a volume, even if our assumptions on the shape (remember, the MacLaurin spheroid) give us a 3D information.
This is why the oblateness is just an apparent oblateness. It is actually biased by the projection on the celestial sphere. This oblateness is defined by the quantity (a-b)/a, where a and b are the two axes of the projected asteroid, with a > b.

To know more…

You can find the study on the web site of The Astronomical Journal. It has also been freely made available by the authors on arXiv. Thanks to the authors for sharing! Here is the webpage of the RECON and IOTA networks, which were of great help for the observations.

The authors

Here are their web pages or research profiles:

I hope you enjoyed this post. As usual, please let me know what you think about it. Happy holidays to everybody, and see you soon!