by Paul Gilster | Mar 23, 2017 | Outer Solar System |
Earth’s axial tilt (its obliquity) is 23.5 degrees, a significant fact for those of us who enjoy seasonal change. The ’tilt’ is the angle between our planet’s rotational axis and its orbital axis. If we look at Earth’s obliquity over time, we find a 41,000 year cycle that oscillates between 22.1 and 24.5 degrees. Here the Moon becomes useful, with recent studies showing that without it, Earth’s obliquity could vary by 25° (some earlier analyses took this number much higher).
Now we have new data from the Dawn spacecraft at Ceres relating the dwarf planet’s axial tilt to the locations where frozen water can be found on its surface. This is interesting stuff, because it depends upon the spacecraft’s ability to measure the world it orbits.
“We cannot directly observe the changes in Ceres’ orientation over time, so we used the Dawn spacecraft’s measurements of shape and gravity to precisely reconstruct what turned out to be a dynamic history,” says Erwan Mazarico, a co-author of a paper on this work based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Image: This animation shows how the illumination of Ceres’ northern hemisphere varies with the dwarf planet’s axial tilt, or obliquity. Shadowed regions are highlighted for tilts of 2 degrees, 12 degrees and 20 degrees. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
What we learn from the paper just published in Geophysical Research Letters is that in the last three million years, Ceres’ axial tilt has ranged from 2° to 20°. The last time of maximum obliquity of 19° was about 14,000 years ago, while its current tilt is just 4°, meaning seasonal effects over the course of a current Cerean year (4.6 Earth years) will be slight.
Charting Ceres’ obliquity allows researchers to examine which areas remain most deeply shadowed even during times of maximum tilt, and the current work, led by JPL’s Anton Ermakov, reports that craters that are shadowed during times of maximum obliquity show bright deposits that are most likely water ice. Ceres’ surface temperatures range from 130 to 200 Kelvin (-143° C to -73° C), but regions that rarely see sunlight are more likely to have ice deposits than sunlit areas where ice can sublimate directly into vapor.
Deeply shadowed areas at the poles never receive direct sunlight when Ceres’ axial tilt is as low as it is today — this is an area of about 2,000 square kilometers — but increasing obliquity reduces the shadow region to as little as 1 to 10 square kilometers. The researchers call craters with areas that stay in shadow over long periods of time ‘cold traps’ because volatiles that readily vaporize cannot escape once deposited there. We’ve already learned from Dawn that 10 such craters contain bright material, and one is already known to contain ice.
The northern and southern hemispheres have two persistently shadowed regions each at 20° tilt, and so far we have found bright deposits in three of the four. All of this should call up thoughts of the polar regions of the Moon, a body that has little variability in its tilt because of the influence of the Earth. Mercury, too, stabilized by its proximity to the Sun, shows little axial tilt, and on both objects, we are finding evidence of water ice in shadowed craters at the poles. As with Mercury, the Moon’s ice surely comes from the impact of asteroids and comets, whereas what we find on Ceres may, at least in part, come from the dwarf planet itself.
Remember that the European Space Agency’s Herschel Space Observatory found a tenuous atmosphere on Ceres several years ago, a possible source of water molecules that can accumulate in the cold traps. Meanwhile, note that Ceres’ axial tilt varies on a cycle of about 24,500 years, a figure researchers consider to be a surprisingly short time given the size of the variation. Ceres’ surface ice, then, gives us insight into its geological history as we continue to probe the question of whether the small body continues to give off water vapor.
The paper is Ermakov et al., “Ceres’s obliquity history and its implications for the permanently shadowed regions,” published online by Geophysical Research Letters 22 March 2017 (abstract).
by Paul Gilster | Mar 14, 2017 | Exoplanetary Science |
The data recently made available from Campaign 12 of K2 (the Kepler spacecraft’s two-reaction wheel mission) is already paying off in the form of information about the outermost planet in the TRAPPIST-1 system. Campaign 12 (described in Kepler Data on TRAPPIST-1 Coming Online) began on December 15 of 2016 and ran until March 4 of this year, though the spacecraft was in safe mode for a time, producing a 5-day data loss.
An international team including lead author Rodrigo Luger (University of Washington) and TRAPPIST-1 planet discoverer Michaël Gillon (Université de Liège) used the K2 data to constrain the period of TRAPPIST-1h, the outermost planet in this seven-planet system, which had only been observed to transit once before now. The team was also looking for additional planets (none were found) and, of course, examining resonances with the inner worlds.
The result: The orbital period of TRAPPIST-1h is found to be 18.764 days, a figure that fits into the pattern of resonance that the team’s theoretical work had predicted. TRAPPIST-1h is thus “…the seventh member of a complex chain, with three-body resonances linking every member.” The paper goes on to tell us that the planet has a radius of 0.715 R?, and an equilibrium temperature of 169 K, meaning it orbits at the snow line.
Image: Figure 3 from Luger et al. Caption: The short cadence data folded on the four transits of planet h after correcting for TTVs and subtracting a simultaneous transit of b and a near-simultaneous flare. Other transits of b ? g have not been removed and are visible in parts of the data. The data downbinned by a factor of 30 is shown as the orange line, and a transit model based solely on the Spitzer parameters is shown in red. The residuals (data minus this model) are shown at the bottom. Credit: Luger et al.
The paper notes that the stellar flux TRAPPIST-1h receives from its star is below what would be required to sustain liquid water under an atmosphere dominated by nitrogen, carbon dioxide and water (a hydrogen dominated atmosphere could theoretically make it possible). Nor is it on an orbit eccentric enough for tidal heating to warm the surface sufficiently. Nonetheless, tidal interactions play an important role in the evolution of the TRAPPIST-1 planets’ orbits. On the matter of formation and evolution, this was interesting:
The resonant structure of the system suggests that orbital migration may have played a role in its formation. Embedded in gaseous planet-forming disks, planets growing above ? 1 MMars create density perturbations that torque the planets’ orbits and trigger radial migration. One model for the origin of low-mass planets found very close to their stars proposes that Mars- to Earth-sized planetary embryos form far from their stars and migrate inward. The inner edge of the disk provides a migration barrier such that planets pile up into chains of mean motion resonances.
We can even extrapolate something about the speed of formation which, in turn, would have affected the compactness of the resulting system:
The TRAPPIST-1 system may thus represent a pristine surviving chain of mean motion resonances. Given that TRAPPIST-1’s planet-forming disk was likely low in mass and the planets themselves are low-mass, their migration was likely relatively slow. This may explain why TRAPPIST-1’s resonant chain is modestly less compact than chains in systems with more massive planets, which may have protected it from instability.
Image: An artist’s illustration of the seven TRAPPIST-1 planets. Sizes and relative positions are to scale. Credit: NASA/JPL-Caltech.
On the star TRAPPIST-1 itself, the researchers discovered it to be prone to star spots, making it possible to determine a rotational period of about 3.3 days, roughly comparable to nearby late M-dwarfs. The paper suggests an age in the range of 3 to 8 billion years. And note this re flare activity:
The presence of star spots and infrequent weak optical flares (0.38 d?1) for peak fluxes above 1% of the continuum, 30 times less frequent than active M6-M9 dwarfs are consistent with a low-activity M8 star, also arguing in favor of a relatively old system.
I notice that an energetic flare occurred at the end of the K2 campaign, and the authors promise that a full model of flare activity will be offered in an upcoming paper. This is useful information as we investigate questions of habitability among the inner worlds, for flares can damage atmospheres and have other adverse consequences for life.
It’s fascinating to consider how work like this develops from a damaged spacecraft. The paper points out that because of its two failed reaction wheels, the Kepler spacecraft’s rolling motion (created by imbalances in torque) induces strong instrumental effects, which lead to an increase of between 3 and 5 times in photometric noise compared to the original mission. The paper explains how removing these instrumental effects is done, but I continue to marvel at the fact that Kepler is still producing good science despite its serious internal problems.
The paper is Luger et al., “A terrestrial-sized exoplanet at the snow line of TRAPPIST-1,” submitted to Nature Astronomy (preprint).
by Paul Gilster | Mar 7, 2017 | Outer Solar System |
We’ve looked recently at the possibility of cryovolcanism on Ceres with regard to the unusual feature called Ahuna Mons (see Ice Volcanoes on Ceres?). Now we have further evidence that outbursts of brine from beneath the surface have been occurring over long periods of time, and that some of these eruptions have been recent. The work comes out of analysis of data from the Dawn mission by scientists at the Max Planck Institute for Solar System Research (MPS), and moves the debate to the unusual crater called Occator.
Image: This view of the whole Occator crater shows the brightly colored pit in its center and the cryovolcanic dome. The jagged mountains on the edge of the pit throw their shadows on parts of the pit. This image was taken from a distance of 1478 kilometers above the surface and has a resolution of 158 meters per pixel. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
Dawn’s Low Altitude Mapping Orbit (December 2015 to September 2016) took the spacecraft to within 375 kilometers of the surface, allowing highly resolved images of Ceres’ surface to be obtained. The geological structures on display within Occator, tracked by the Dawn Framing Cameras and its infrared spectrometer, include smaller, younger craters along with fractures and avalanches, all presenting us with a look into the crater’s evolution.
The bright dome in Occator crater’s central pit, now known as Cerealia Facula, is one of Ceres’ most intriguing features. Infrared data have shown it to be rich in carbonates. Beyond the pit itself, we also find other bright areas called Vinalia Faculae, considerably paler and evidently thinner, though likewise containing carbonates along with dark surrounding material. The MPS researchers, led by Framing Camera lead investigator Andreas Nathues, have evaluated images of Occator from various distances and different angles of view.
It took more than a single impact to produce what we see in Occator crater, though that impact was likely the trigger for subsequent cryovolcanism. As explained by Nathues:
“The age and appearance of the material surrounding the bright dome indicate that Cerealia Facula was formed by a recurring, eruptive process, which also hurled material into more outward regions of the central pit. A single eruptive event is rather unlikely.”
Image: False color mosaic showing parts of Occator crater. The images were taken from a distance of 375 kilometers. The left side of the mosaic shows the central pit containing the brightest material on Ceres. It measures 11 kilometers in diameter and is partly surrounded by jagged mountains. In the middle of the pit a dome towers 400 meters high covered by fractures. It has a diameter of three kilometers. The right side of the mosaic shows further, less bright spots in Occator crater. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
What’s especially interesting here is the age issue, for this is the first time scientists have determined the age of the bright material. As this MPS news release explains, Nathues and team believe the central pit in Occator, which contains a jagged ridge, is all that is left of a former central mountain that would have formed as a result of the impact that created the crater about 34 million years ago. The age estimate comes from studying smaller craters formed from later impacts, and the Dawn images are so highly resolved that the crater count — and the age estimates that emerge from it — are the most accurate to date. The bright material of the dome is made up of much younger material, about four million years old.
Image: This 3d-anaglyph for the first time shows a part of Occator crater in a combination of anaglyphe and false-color image. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
Domes like this, interpreted as signs of cryovolcanism, appear on the Jovian moons Callisto and Ganymede, and the new work argues that the same process is happening on Ceres. The original Occator impact would have caused subsurface brine to move closer to the surface, allowing water and dissolved gases to form a system of vents. Fractures on the surface formed, from which brine began to erupt, depositing salts that led to the formation of the dome.
The process may still be active, though at a much lower level. Imagery of the area at certain angles has revealed what appears to be haze, a phenomenon the researchers explain as the result of sublimating water ice. Imagery from Dawn taken at distances as far as 14000 kilometers have shown unmistakeable variations in brightness that follow a diurnal rhythm. Says Guneshwar Singh Thangjam (MPS):
“The nature of the light scattering at the bottom of Occator differs fundamentally from that at other parts of the Ceres surface. The most likely explanation is that near the crater floor an optically thin, semi-transparent haze is formed.”
I should add, though, that there is a great deal we have yet to learn about this haze, as explained in one of two papers on this work:
The available data are insufficient for an analysis of the optical properties of this haze, but it is most likely an optically thin and semi-transparent layer forming at near-surface level. Diurnal albedo variations that correspond to Occator’s longitude have been detected by radial velocity changes (Molaro et al. 2016) supporting the existence of a temporarily varying haze layer. The detection of water-ice signatures at Oxo [a second crater at which haze has been detected] further supports ongoing sublimation activities on Ceres. Although we expect the maximum haze concentration above the central spot in Occator, it is remarkable that our data also indicate activity at the secondary spots.
The water sublimation, and thus the intensity of the haze, varies with the degree of sunlight. This explanation makes Ceres the closest body to the Sun that experiences cryovolcanism. It is the age difference — the bright deposits are 30 million years younger than the crater — as well as the distribution of the bright material within the crater itself, that tells us how active Occator crater has been and, on a much smaller scale, continues to be to this day.
The paper is Nathues et al., “Evolution of Occator Crater on (1)Ceres,” The Astronomical Journal Volume 153, Number 3 (17 February, 2017). Abstract available. See also G. Thangjam et al., “Haze at Occator Crater on Dwarf Planet Ceres,” The Astrophysical Journal Letters Volume 833, Number 2 (15 December, 2016). Abstract / Preprint.
by Paul Gilster | Mar 1, 2017 | Exoplanetary Science |
The binary system SDSS 1557, about 1000 light years from Earth, was thought to be a single white dwarf star until detailed measurements revealed that the brighter star was being gravitationally influenced by a hither unseen brown dwarf. And that, in turn, has given us an intriguing look at possible planetary formation around both members of a close binary. We’ve found gas giants in such systems, but researchers led by Jay Farihi (University College London) have found signs of rocky debris here that point to the possibility of planets of a much different composition.
“Building rocky planets around two suns is a challenge,” says Farihi, “because the gravity of both stars can push and pull tremendously, preventing bits of rock and dust from sticking together and growing into full-fledged planets. With the discovery of asteroid debris in the SDSS 1557 system, we see clear signatures of rocky planet assembly via large asteroids that formed, helping us understand how rocky exoplanets are made in double star systems.”
Image: A disc of rocky debris from a disrupted planetesimal surrounds white dwarf plus brown dwarf binary star. The white dwarf is the burned-out core of a star that was probably similar to the Sun, the brown dwarf is only ~60 times heavier than Jupiter, and the two stars go around each other in only a bit over two hours. Credit: Mark Garlick, UCL, University of Warwick and University of Sheffield.
Can planets, or at least their debris, survive the red giant expansion phase that leads to a white dwarf? Yes, says the new paper on this work, noting that we now have more than three dozen planetary system remnants that have been found through study of circumstellar disks of white dwarfs; we also have several hundred white dwarfs that show signs of accretion of planetary debris.
The debris around SDSS 1557 is spread around the two stars, offering a helpful target for the team’s analysis. Working with observations from the Gemini Observatory South instrument and the European Southern Observatory’s Very Large Telescope, the team found material with high metal content including silicon and magnesium, which could be identified as it was drawn onto the surface of the white dwarf. Such atmospheric pollution has become a tool for the analysis of white dwarf systems. In this case, the UCL team found that 1017 grams of matter — the equivalent of a 4 km asteroid — produced the observed result.
The paper on this work points out that planets of Neptune up to Jupiter size are unlikely to form where they have been found in Kepler detections of circumbinary planets, but are likely the result of migration. However, models exist for smaller planet formation within the snowline, which gives us the possibility of planets like the famous ‘Tatooine,’ from George Lucas’ Star Wars. And work on polluted white dwarfs adds weight to the idea.
From the paper:
The current paradigm of disrupted and accreted asteroids has been unequivocally confirmed by numerous studies, including the recent detection of complex and rapidly evolving photometric transits from debris fragments orbiting near the Roche limit of one star. To date, all polluted white dwarfs with detailed analyses indicate the sources are rocky planetesimals comparable in both mass and composition to large Solar System asteroids, and thus objects that formed within a snow line. These findings unambiguously demonstrate that large planetesimal formation in the terrestrial zone of stars is robust and common.
Dr. Farihi is on record (on his homepage) as saying that he believes we will learn more about extrasolar terrestrial planets using white dwarfs than any other method. The reason: The atmospheres of cool white dwarfs feature hydrogen and helium that can easily become polluted by small amounts of heavy elements. Work like the current paper reminds us that we can use this metal pollution to measure the composition of rocky material around the star.
The debris responsible for white dwarf atmospheric pollution is believed to come from tidally destroyed asteroids whose parent bodies were large and differentiated. Farihi notes that we may be looking at the parent bodies of planetesimals or even fragments of major planets, with compositions similar to material found in our own inner Solar System. He estimates that 20% to 30% of all white dwarfs are orbited by the remains of terrestrial planetary systems.
Image: Dr. Jay Farihi, among some exceedingly interesting standing stones. Credit: Jay Farihi/UCL.
The SDSS 1557 discovery calls for continuing follow-ups, for as co-author Boris Gänsicke (University of Warwick) points out, the signature is transient. Says Gänsicke:
“Any metals we see in the white dwarf will disappear within a few weeks, and sink down into the interior, unless the debris is continuously flowing onto the star. We’ll be looking at SDSS 1557 next with Hubble, to conclusively show the dust is made of rock rather than ice.”
So at SDSS 1557 we have all the markers of a parent body that formed within the snowline, which implies that rocky planet formation in circumbinary orbits within a close double system is feasible. The paper concludes: “These observations therefore support a picture where additional mechanisms can promote planetesimal growth in the terrestrial zones of close binary stars, which are predicted to be substantially wider than in planet forming disks around single stars.”
The paper is Farihi, Parsons & Gänsicke, “A circumbinary debris disk in a polluted white dwarf system,” Nature Astronomy 1 March 2017 (abstract / preprint).
by Paul Gilster | Feb 14, 2017 | Exoplanetary Science |
We’re beginning to find evidence of objects like those in the Kuiper Belt beyond our own solar system. In this case, the work involves a white dwarf whose atmosphere has been recently polluted by an infalling object, giving us valuable data on the object’s composition. The work involves the white dwarf WD 1425+540, whose atmosphere has been found to contain carbon, nitrogen, oxygen and hydrogen. The findings are unusual because white dwarfs are the dense remnants of normal stars, with gravitational fields strong enough to pull elements like these out of their atmospheres and into their interiors, where they are immune from detection by our instruments. And that implies a relatively recent origin for these elements.
Lead author Siyi Xu (European Southern Observatory) and team worked with spectroscopic observations from HIRES (the High Resolution Echelle Spectrometer) on the Keck Telescope and included data from the Hubble instrument. The researchers believe the white dwarf’s atmosphere has been enriched by the breakup and eventual spiral into the star of a minor planet, whose composition mimics what we find in Kuiper Belt objects. WD 1425+540, some 200 light years away in the constellation Boötes, thus absorbed a body that is calculated to have been composed of 30 percent water and other ices and 70 percent rocky material.
The event, which would have involved the gravitational disruption of the object’s orbit, causing its infall, disintegration and the eventual absorption of its elements by the star, may have occurred as recently as 100,000 years ago. We could be seeing a process that has implications for the existence of life on planets in the inner system like our own. The Earth may well have been dry when it first formed, with life’s building blocks delivered as the result of collisions with other objects. Siyi Xu sees this as a process that can occur anywhere:
“Now we’re seeing in a planetary system outside our solar system that there are minor planets where water, nitrogen and carbon are present in abundance, as in our solar system’s Kuiper belt. If Earth obtained its water, nitrogen and carbon from the impact of such objects, then rocky planets in other planetary systems could also obtain their water, nitrogen and carbon this way. We would like to know whether in other planetary systems Kuiper belts exist with large quantities of water that could be added to otherwise dry planets. Our research suggests this is likely.”
Image: Rendering of a white dwarf star (bright white spot), with rocky debris from former asteroids or a minor planet that has been broken apart by gravity (red rings). Credit: University of Warwick.
Temperatures in the protoplanetary disk of our own system are thought to have determined the distribution of water, with dry rocky worlds inside the snow line, and water ice available beyond it, much of it still to be found in asteroids, comets and Kuiper Belt objects. Nitrogen ice can also be found in comets from the outer regions of the Kuiper Belt and in the Oort Cloud. The assumption has been that similar conditions prevailed around other stars, but our knowledge of analogs to Kuiper Belt objects in other systems has remained scant.
This makes white dwarfs like WD 1425+540 a useful tool, for the detection of heavy elements in their atmosphere has to point to an external source, giving us a way to measure the broad composition of objects in the system. And indeed, the paper notes that there are at least a dozen ‘polluted’ white dwarfs whose accreted material has been measured in detail. Excess oxygen that can be attributed to water-rich objects has been detected in only three.
From the paper:
The accreting material observed in WD 1425+540 provides direct evidence for the presence of KBO analogs around stars other than the Sun. In addition, WD 1425+540 has a K dwarf companion at 40.0 arcsec (2240 au) away (Wegner 1981). The presence of a distant stellar companion can impact the stability of extended planetary systems and, thus, enhance the chances of perturbing objects – that initially orbit far from a white dwarf – into its tidal radius via the Kozai-Lidov mechanism (Zuckerman 2014; Bonsor & Veras 2015; Naoz 2016).
Interestingly, the authors have worked parameters for the disrupted object in the system. When it was on the main sequence, WD 1425+540 would have been about twice as massive as the Sun and 10 times more luminous. Its nitrogen-bearing KBO analogs would have been found at about 120 AU, or three times further from the star than the Sun’s Kuiper Belt objects. Add in the star’s red giant phase and its Kuiper Belt moves out by a factor of 3. Thus the object that was accreted by WD 1425+540 likely orbited beyond 300 AU before experiencing the gravitational disruption that drove it inward toward the star and its eventual destruction.
Most material accreted onto white dwarfs in previous studies of polluted atmospheres has come from dry minor planets. This paper argues that the spectroscopic work on WD 1425+540 gives us strong evidence for volatile-rich planetesimals around other stars:
With this new dataset, we can conclude that extrasolar terrestrial planets could have volatile element and water abundances provided by KBO analogs that are comparable to those on Earth.
The paper is Xu et al., “The Chemical Composition of an Extrasolar Kuiper-Belt-Object,” Astrophysical Journal Letters 836, L7 (2017). Abstract / preprint.
