Planetary science with adaptive optics: results from the UH AO systems

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1 Planetary science with adaptive optics: results from the UH AO systems F. Roddier 1, C. Roddier 1, L. Close 1, C. Dumas 1, J. E. Graves 1, O. Guyon 1, B. Han 1, M. J. Northcott 1, T. Owen 1, D. Tholen 1, A. Brahic 2 1 Institute for Astronomy, University of Hawaii, USA 2 Observatoire de Paris-Meudon, France Abstract In spite of observations made with space probes or the Hubble Space Telescope (HST), a wealth of new results have and can still be obtained from the ground using telescopes equipped with adaptive optics (AO). We present here results obtained with the University-of-Hawaii 13-actuator AO system and with the new 36-actuator Hokupa a system. Both were utilized at the Canada-France- Hawaii Telescope (CFHT). Results include i) the observation of Saturn s rings as the Earth was crossing the ring plane in August 95, ii) the monitoring of Neptune s cloud activity, the detection and orbital determination of its faint satellites, iii) the observation of Io s volcanoes thermal emission, iv) the mapping of Titan s surface, v) the narrow-band photometry of the resolved Pluto- Charon system, vi) the geological mapping of asteroids. 1. Introduction Given the wealth of information brought by space probes on the solar system, it may come as a surprise that adaptive optics images taken from the ground are able to bring new results. There are many reasons for this: 1) Before Galileo, most spacecrafts missions produced only short term observations. The long term monitoring of a planet atmospheric activity was not possible. Galileo has changed that for Jupiter. Cassini will change that for Saturn. Beyond Saturn one will have to rely for a while on HST or AO images. 2) Before Galileo, spacecrafts had no infrared camera. Voyager could not see through the haze of Titan. Observations in and out of the methane or ice bands were not possible. 3) Spacecraft observations are limited to a small number of objects. There has been no mission to Pluto and only a few missions toward asteroids. Compared to HST observations, AO observations have also a number of advantages: 1) They can take profit of a larger telescope aperture. This is particularly important for the detection of faint fast moving objects. 2) They offer access to a broader variety of instruments and can take profit of the latest progress in instrumentation. 3) They can now be performed on a number of ground-based telescopes, giving access to more telescope time, and more flexible scheduling. We present here the main results of a program of observations of solar system objects carried out at the Institute for Astronomy with the University-of-Hawaii AO systems.

2 ity 05 Pandora Pandora Epimetheus HST S5 HST S Cassini division Cassini division C ring B ring A ring F ring Distance from Saturn s center (arcsec) Distance to Saturn's center (arcsec) Time Pandora Epimetheus S5 Mimas Tip of C ring Distance from Saturn Cassini Encke F ring division gap Tip of A ring Fig. 1. Top: Example of image showing Saturn s rings a few hours before the Earth crossed the ring plane. Pandora was discovered by Voyager. It had never been detected from the ground before. Middle: Photometric profile along the ring obtained from the image above. The contribution from Epimetheus has been subtracted out. Because we see here the dark side of the rings, the Cassini division appears bright. S5 is one of the three new objects discovered by HST. Bottom: time sequence showing the illumination along the rings as a function of time. Bright inclined tracks are produced by the orbital motion of small objects. Tracks left by a number of new faint objects can be seen here. Like S5, these objects are believed to be clumps of material in the F ring. Note the temporary disappearance of Epimetheus in the shadow of the rings.

3 2. Saturn ring-plane crossing We were fortunate to have our first Cassegrain AO system in operation by the end of The second run was dedicated to the Saturn ring plane crossing observations in August Tethys and Dione were used as guide sources. Figure 1 shows an image of the west ansa taken about 8 hours before the Earth crossed the ring plane. We are seeing here the dark side of the rings which is very unusual. The scale in arcseconds is shown under the plot below. The rings are faint and their thickness is not resolved. This is a rare opportunity to detect faint satellites. Indeed, one sees Pandora on the left, a satellite discovered by Voyager but never detected from the ground before. Epimetheus is seen on the right. It was first detected in 1966 and reobserved in 1980 during similar events. This is also a rare opportunity to study the photometric properties of the rings when viewed over a very small angle. The plot in the middle of Fig. 1 shows the intensity along the rings measured on the image above. The main rings are opaque and appear dark, whereas the Cassini division contains enough particles to scatter light and appears bright. The C ring also lets some sunlight shine through. On this plot the contribution of Epimetheus has been removed to show the bright peak produced by the F ring, an outer ring first discovered by Pioneer 11. Although very faint its contribution dominates all others at the time of the crossing. On the bottom of Fig. 1 is a two-dimensional display of the intensity along the ring as a function of time. Differences in brightness between the rings appear as wide vertical bands whereas objects in orbital motion produce bright inclined tracks. Their inclination angle indicates the object orbital speed. Note the tracks left by Pandora and Epimetheus. Note also the temporary disappearance of the latter in the shadow of the rings. Another track can be identified as made by S5 a new object first announced by the HST team. Fig. 1 show evidence for 6 additional tracks (unlabeled marks on top of the display). These correspond to objects that escaped HST observations. Like S5, they are believed to be produced by clumps of material in the F ring rather than solid objects. In total, our AO observations confirmed the detection of 2 clumps by HST (S5 and S7) and produced evidence for 10 additional clumps (Roddier et al. 1996, Roddier 1997, Roddier et al. 1997b, Roddier et al. 1998b). Our AO observations revealed all the satellites of Saturn known from Voyager except Atlas and Pan which also escaped to HST observations. We obtained for the first time H magnitude for Prometheus, Pandora, Telesto, and Calypso, and J magnitudes for Epimetheus, Janus, Mimas, Telesto, and Helene (Roddier et al. 1998b). We found a J-H magnitude difference close to zero for Janus, Mimas, and Helene which is consistent with a surface covered with water ice, and excludes rocks or organic compounds. For Epimetheus we found J-H 0.3, which is comparable to the value for Europa and Callisto. The eclipse of Epimetheus has been photometrically and astrometrically analyzed. At the bottom of the track seen in Fig. 1 (lower display) Epimetheus is already in the shadow of the F ring. As it passes between the shadow of the F ring and that of the A ring, it becomes brighter. As it enters the shadow of the A ring it becomes dimmer again and totally disappears. At the end of the sequence, it reappears again partially illuminated by solar light entering through the Encke gap. The timing of these events enabled us to improve the current value of the inclination of Epimetheus orbit (22.5 ± 1 instead of the Voyager value of 20.4 ± 3 ). We also obtained a lower limit for the eccentricity of the F ring (at least 1,000-km variation in a 140,000-km radius), and a new value for its transverse optical thickness (5 x 10-3 ) (Roddier et al. 1997b, Roddier et al. 1998b).

4 a b c d e f Fig. 2. Evidence for an arc of particles in orbital motion around Saturn. The first 5 images (a to d) form a time sequence of images with 10-min. intervals. The bottom right image is a median of the five other images. East is left. Janus and the tip of the bright rings are on the right. The arc is the bright horizontal streak moving eastward (away from Saturn). A vertical line has been drawn to coincide with the sharp right edge of the arc in the first image (a). Note the foreshortening of the arc as it approaches maximum elongation. The Earth crossed the ring plane on August 10 after dawn when it was impossible to observe. However, the AO photometric data we obtained two nights before and two nights after the crossing were found to be an important complement to HST data which span only half a day. Using both sets of data, we obtained estimates for the ring particle reflectivity as a function of distance to Saturn for both the dark and the bright side, and we found evidence for a flared diffuse component starting with the A ring with a thickness of about 2 km and increasing up to 18 km at the level of the F ring. We have interpreted this component as being the inner tip of the E ring (Roddier et al. 1997a). On the second night after the crossing, we made a serendipitous discovery. On deep images of the region extending beyond the east ansa we found an elongated feature moving away from Saturn (Fig. 2). We convinced ourselves it was not an artifact. Assuming the object was on a circular orbit around Saturn, we found an orbital velocity close to that of Enceladus. The observed maximum elongation was also found to be that of Enceladus. This led us to identify the object as an arc of particles possibly produced by a collision of ice blocks near Enceladus L 4 Lagrange point. The arc intensity weakened during the observations, and no arc could be seen at the same location two nights before (Roddier et al. 1998a).

5 11:44:10 12:29:39 13:14:58 Fig. 3. Images of Neptune taken with Hokupa a in July 1998 at the CFHT. They were obtained through a narrow band filter centered inside a methane absorption band at 1.72 µm, at the time (UT) indicated under each image (h:m:s). The angular diameter of Neptune was 2.36 and Neptune itself was used as a guide source. Seeing conditions were poorer than average ( 1 ). Images have not been deconvolved. Bright high altitude cloud bands appear against the dark atmosphere. Note the rapid planet rotation. 3. Uranus and Neptune We took opportunity of the ring plane crossing observations to take a look at Uranus and Neptune. In both cases we used a nearby satellite as a guide source. Unfortunately, the satellites of Uranus are quite faint and the wave-front sensor is affected by light scattered by Uranus. Nevertheless, we obtained the first adaptive optics image of Uranus (Fig. 4). In this K-band image the planet appears dark due to methane absorption and the rings are brighter than the planet. We can also see Puck, a satellite discovered by Voyager. In the case of Neptune Triton is a good guide source. However, the angular diameter of Neptune is quite small (less than 2.4 ). We discovered later that we could use Neptune itself as a guide source and used it in all our subsequent observations. Our first observations of Neptune are described in Roddier et al. (1997c). We obtained the first infrared magnitude of Proteus (m K = 19) and confirmed the accuracy of the orbital data obtained by Voyager. We observed Neptune again in October 1996 and July 1997 with our 13-actuator system, and in November 1997 and July 1998 with Hokupa a. The 1996 and 1997 observations were made through wide-band filters (J, H, and K ) as well as narrow-band filters centered inside (1.72 µm) and outside (1.56 µm) of the methane bands. Differences in Neptune s atmospheric opacity allowed us to isolate high-altitude cloud layers from lower ones. Low altitude haze seems to cover most of the southern hemisphere with possible isolated areas on northern active regions. Thin high altitude cloud bands form over the haze in the south hemisphere and at latitudes where activity occurs in the north (Roddier et al. 1998d). During the Voyager encounter, most of Neptune s cloud activity was concentrated in the south hemisphere. It then moved to the north where it peaked around Figure 3 shows images of Neptune taken with Hokupa a on July 6, 1998 at the CFHT. Cloud activity has grown again in the south. The cause of these changes are unknown. A careful analysis of the images taken in July 1998 revealed four of Neptune s six dark satellites (Proteus, Larissa, Galatea, and Despina). Their location was found to be in excellent agreement

6 Puck Fig. 4. K-band image of Uranus obtained in August At this wavelength the rings are brighter than the planet. The east ansa is brighter than the west ansa. Note the planet limb brightening. Note also Puck on the upper left side of Uranus. Fig. 5. Top: Image of Io s thermal emission at 2.26 µm. Data were recorded in July 1997 while Io was in the shadow of Jupiter. Europa was used as a guide source. Bottom: Volcanoes identified in this image. The two brightest are Kanehekili and Loki. The bottom image has been slightly rotated to put North on top. with that predicted from Voyager data except for Galatea which was found to be 5 ± 1 ahead of the predicted position or 8.6 min early (Roddier et al. 1998c). Galatea is generally considered as causing the arc structure in Neptune s Adams ring. The discrepancy on Galatea s position may possibly be related to its interaction with the Adams ring. Neptune s rings themselves were detected in the data near background noise level. We are now in the process of identifying the Adams arcs and finding their orbital position. 4. Io s volcanoes Figure 5 shows a 2.26-µm image of Io obtained on July 16, 1997 by C. Dumas and others while Io was in the shadow of Jupiter. Nearby Europa (in sunlight) was used as a guide source. The image shows the thermal emission from the active Volcanoes on Io. At least five of them have been identified in this image. We see here mostly the Jupiter facing hemisphere of Io. This type observation nicely complements those made by the NIMS spectro-camera on board of Galileo which better sees the opposite hemisphere of Io, but can hardly observe as well the Jupiter-facing side.

7 Fig. 6. Images of Titan obtained in November 1997 with Hokupa a. Bottom images were taken inside a methane absorption band and show only atmospheric haze. Top images were taken outside the methane bands and show evidence for a bright continental-size feature rotating with Titan s surface rate. 5. Titan Figure 6 shows four images of Titan obtained with the Hokupa a on November 10 and 11, Data were recorded by C. and F. Roddier, and later processed by C. Dumas. The angular diameter of Titan was 0.83 arc-second. The top two images were recorded through a 120-nm bandwidth filter centered at 1.56 µm, a bandpass at which Titan s atmosphere is relatively transparent. A bright horizontally elongated feature can be seen near the center of the top left image. The same feature is also seen on the top right image taken the day after. It has moved to the right at a rate consistent with Titan s rotation which is tidally locked to its orbital period. Also seen by other observers, it appears to be a permanent surface feature of Titan s leading hemisphere. The bottom two images were recorded through a 120-nm bandwidth filter centered at 1.72 µm, a band-pass at which Titan s atmosphere is opaque due to methane absorption. One only sees light scattered by Titan s hydrocarbon haze which produces a strong limb brightening. The haze appears denser in Titan s south hemisphere, especially near the south pole. A similar but lesser effect can be seen at the bottom of the top two images. This haze prevented Voyager to see any surface feature on Titan. The observations presented here are important to better prepare the landing of the Huygens probe on Titan.

8 Fig. 7. Images of Pluto and Charon taken in July Their angular separation was 0.9 arc-second. These 20-min exposure images were taken through narrow band filters centered at 1.55 µm (left) and 2.25 µm (right). The drop of Charon s relative brightness at 1.55 µm compared to that in the continuum (2.25 µm) shows that Charon is enriched in frozen H 2 O compared to Pluto (Courtesy L. Close, T. Owen and D. Tholen). 6. Pluto and Charon Until now, all compositional observations of Pluto and Charon have been made with the combined light of the two bodies. Assuming that the contribution of Charon to the combined light was negligible, the surface of Pluto was found to be covered by a mixed deposit of ices of N 2, CO, and CH 4, with no evidence of H 2 O. In 1987 and 1988, when occultations allowed light from Pluto to be subtracted from the sum of Pluto plus Charon, this difference indicated that the surface of Charon might be coated with H 2 O ice, but no other species have been confirmed. If this is true, it is a major constraint on models for the origin and evolution of this system. Images of Pluto and Charon were taken in July We used narrow band filters to isolate the H 2 O ice absorption at 1.55 µm, and the continuum at 2.25 µm. Photometric analysis of the data confirmed the earlier results as can be seen directly in the images of Fig. 7 taken on July 14. Data were also taken on July 17. A comparison between the two sets of data shows evidence for heterogeneities in the surface composition of Pluto. The fact that Charon is free of methane frost, is a surprising result, since Pluto like Triton has a nitrogen-dominated atmosphere that contains methane, and methane is present in frozen form on the surfaces of both Pluto and Triton. 7. Vesta Figure 8 shows a sample of three images of Vesta obtained with Hokupa a two weeks after its opposition date (Nov. 1997). These images were obtained and processed by C. Dumas as part of a program aiming to produce geological maps of Vesta. Observations were made through six narrow band filters with bandpasses matching the absorption bands of pyroxene, olivine and feldspar. The rotational sampling rate of Vesta was high (5-10 in longitude). This data set provides a unique opportunity to obtain a detailed characterization of Vesta s surface properties (albedo, phase function) and mineralogy.

9 0.43 λ = 1.99 µm 1.50 µm 1.10 µm φ = Fig. 8. Images of Vesta obtained with Hokupa a near opposition (Nov. 1997). The wavelength λ and the longitude φ of the central meridian is indicated under each image. Vesta s maximum angular diameter was Its rotation axis is indicated by the arrow on the left image. The band depth of pyroxene and olivine can be directly measured on these images and mapped over Vesta s surface. The flat edge near the south pole is attributed to a large impact event. (Courtesy C. Dumas). Vesta is believed to have survived catastrophic collisions in its lifetime. Visible observations from the ground (speckle interferometry, adaptive optics) and from space (HST) have revealed surface features similar to large impact craters, in particular a large impact zone near its south pole. The material removed from Vesta s surface during large impacts could be at the origin of a family of small Vesta-like asteroids. These observations will lead to a better understanding of the processes undergone during the differentiation of the planetesimals that populated the inner part of our Solar system, after accretion from the Solar nebula material. 8. Conclusion We believe that the results presented here demonstrate the potential of ground-based observations with adaptive optics as an important complement to space observations in the exploration of our solar system. In particular, telescopes dedicated to planetary science such as the NASA IRTF should urgently be equipped with adaptive optics. Acknowledgments This research has been supported by NASA-GSFC grant No. NAG References Roddier, C., Roddier, F., Brahic, A. et al Satellites of Saturn. IAU circular No Roddier, C Observations du système solaire avec l optique adaptive (in French), C. R. Acad. Sc. t. 325, Série II b, Roddier, C. and Roddier, F. 1997a. Photometric analysis and modeling of the August 1995 Saturn ring HST data, paper presented at the Wellesley workshop (July 25-26, 1997). Abstract edited by M. Showalter, P. Nicholson, D. French. Roddier, F., Roddier, C., Dumas, C., Graves, J. E., Northcott, M. J., Owen, T. and Brahic, A. 1997b. Adaptive optics observations of Saturn s ring plane crossing in August 1995, paper presented at the Wellesley workshop (July 25-26, 1997). Abstract edited by M. Showalter, P. Nicholson, D. French.

10 Roddier, F., Roddier, C., Brahic, A. et al. 1997c. First ground-based adaptive optics observations of Neptune and Proteus, Planet. Space Sci. 45, Roddier, C., Roddier, F., Graves, J.E. and Northcott, M.J. 1998a. Discovery of an arc of particles near Enceladus orbit: a possible key to the origin of the E ring. Icarus (in press). Roddier, F., Roddier, C., Brahic, A., et al. 1998b. Adaptive optics observations of Saturn s ring plane crossing in August 1995, submitted to Icarus. Roddier, C., Roddier, F., Graves, J.E., Guyon, O. and Northcott, M.J. 1998c. Satellites of Neptune. IAU Circular No Roddier, F. Roddier, C., Graves, J.E., Northcott, M.J. and Owen, T. 1998d. Neptune s cloud structure and activity: ground-based monitoring with adaptive optics. Icarus (in press).