Showing papers by "Olivier S. Barnouin published in 2012"

Topography of the Northern Hemisphere of Mercury from MESSENGER Laser Altimetry

Abstract: Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury’s topography occurred after the earliest phases of the planet’s geological history.

Large‐scale troughs on Vesta: A signature of planetary tectonics

Abstract: Images of Vesta taken by the Dawn spacecraft reveal large-scale linear structural features on the surface of the asteroid. We evaluate the morphology of the Vesta structures to determine what processes caused them to form and what implications this has for the history of Vesta as a planetary body. The dimensions and shape of these features suggest that they are graben similar to those observed on terrestrial planets, not fractures or grooves such as are found on smaller asteroids. As graben, their vertical displacement versus length relationship could be evaluated to describe and interpret the evolution of the component faults. Linear structures are commonly observed on smaller asteroids and their formation has been tied to impact events. While the orientation of the large-scale Vesta structures does imply that their formation is related to the impact events that formed the Rheasilvia and Veneneia basins, their size and morphology is greatly different from impact-formed fractures on the smaller bodies. This is consistent with new analyses that suggest that Vesta is fully differentiated, with a mantle and core. We suggest that impact into a differentiated asteroid such as Vesta could result in graben, while grooves and fractures would form on undifferentiated asteroids.

The effect of the Caloris impact on the mantle dynamics and volcanism of Mercury

Abstract: [1] The MESSENGER spacecraft has imaged more than 95% of the surface of Mercury. Images of the Caloris impact basins, along with several smaller impact structures, reveal evidence for volcanic plains within and exterior to the basin that are younger than the basin rim and might be associated with the long-term aftermath of Caloris's formation. The broad influence of Caloris on the surface of Mercury indicates that it might also affect heat flow within the mantle and thereby the core dynamics. Here we investigate possible links between the Caloris impact on Mercury, thermochemical evolution of the mantle, and the volcanism within and surrounding the basins. A finite element model of thermochemical convection in a spherical shell is used to explore the consequences of the formation of large impacts in the Mercurian mantle. While the impact cannot have formed the younger melts directly, the thermal impulse from such a large impact can alter the underlying mantle dynamics, producing subsequent volcanism far from the impact site. We further find that despite the relatively thin Mercurian mantle, the expected heating from the Caloris impact cannot penetrate to the core and should thus have minimal effects on a dynamo. This is largely due to the high expected velocity of impactors at Mercury and thus relatively small size of the Caloris-forming impactor.

The Morphology of Craters on Mercury: Results from MESSENGER Flybys

Abstract: Topographic data measured from the Mercury Laser Altimeter (MLA) and the Mercury Dual Imaging System (MDIS) aboard the MESSENGER spacecraft were used for investigations of the relationship between depth and diameter for impact craters on Mercury. Results using data from the MESSENGER flybys of the innermost planet indicate that most of the craters measured with MLA are shallower than those previously measured by using Mariner 10 images. MDIS images of these same MLA-measured craters show that they have been modified. The use of shadow measurement techniques, which were found to be accurate relative to the MLA results, indicate that both small bowl-shaped and large complex craters that are fresh possess depth-to-diameter ratios that are in good agreement with those measured from Mariner 10 images. The preliminary data also show that the depths of modified craters are shallower relative to fresh ones, and might provide quantitative estimates of crater in-filling by subsequent volcanic or impact processes. The diameter that defines the transition from simple to complex craters on Mercury based on MESSENGER data is consistent with that reported from Mariner 10 data.

An Overview of the OSIRIS REx Laser Altimeter (OLA)

Abstract: nouin, B. Bierhaus, D. Gaudreau, J. Tripp, M. Ilnicki and A. Hildebrand. MacDonald Dettwiler & Associates (9445 Airport Road, Brampton, ON, CANADA, L6S 4J3, cameron.dickinson@mdacorporation.com), York University (dalym@yorku.ca), John Hopkins University Applied Physics Laboratory (Olivier.Barnouin@jhuapl.edu), Lockheed Martin Corporation (edward.b.bierhaus@lmco.com), Canadian Space Agency (Daniel.Gaudreau@asccsa.gc.ca), Optech Inc. (jeff.tripp@optech.com), York University (ilnicki@yorku.ca), University of Calgary (ahildebr@ucalgary.ca).

Tilted Crater Floors: Recording the History of Mercury's Long-Wavelength Deformation

Abstract: DEFORMATION. Jeffrey A. Balcerski, Steven A. Hauck, II, Peng Sun, Christian Klimczak, Paul K. Byrne, Andrew J. Dombard, Olivier S. Barnouin, Maria T. Zuber, Roger J. Phillips, and Sean C. Solomon; Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, OH 44106 (jeffrey.balcerski@case.edu); Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015; Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL 60607; The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723; Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139-4307; Planetary Science Directorate, Southwest Research Institute, Boulder, CO 80302.

Distribution of Boulders on Asteroid 25143 Itokawa

Abstract: Introduction: The distribution of boulders on an asteroid can give insight into the origin and evolution of small bodies (e.g., [1]). This study reviews and updates prior work by Michikami et al., [2] using improvements in the geolocation of images from the Hayabusa spacecraft of 25143 Itokawa [3] – a small Apollo asteroid that is a ruble pile whose surface is densely populated by boulders and craters. The objective of this work is to confirm and update any previously identified trends in the global and regional distributions of boulders on Itokawa. Trends found should provide new insights to how Itokawa’s current appearance formed following the disruption of its parent body, and how its surface might have changed since then. In particular, boulder distribution over the surface of the asteroid might provide a means to test the hypothesis of whether or not Itokawa is a contact binary.

Improved Estimation of the Hayabusa Spacecraft Trajectory and Lidar Tracks

Abstract: Introduction: The original trajectory of the Hayabusa spacecraft while hovering over the asteroid 25143 Itokawa possessed several inaccuracies that made it initially very difficult to reconstruct the topography of the asteroid from the lidar range measurements and estimate the gravitational forces of the asteroid [1,2]. An important improvement to the estimate of the spacecraft trajectory was calculated by fitting parabolas to the Hayabusa spacecraft housekeeping data collected at two minute intervals [2]. These data simultaneously provided the centroid location of the asteroid in the field of view of the wide-angle camera aboard the Hayabusa spacecraft, and a range measurement to the asteroid surface [3]. Despite significant improvements in the results, errors of several meters in the measured range persisted. In this paper we describe an algorithm to further improve this first estimate of the spacecraft trajectory as well as the location of the lidar tracks by making use of a high resolution Itokawa shape model [4,5] to shift the trajectory so that the lidar points better match Itokawa’s topography. Method: The lidar data collected by the spacecraft curves and winds around the asteroid in complicated ways as the spacecraft hovered over the rotating asteroid. We note, however, that for short lidar tracks, only a simple translation is enough to shift the track so that it better matches to the surface of the asteroid. The algorithm, therefore, begins by first dividing up the location of lidar points derived by [2] into small tracks of several hundred points each such that the resulting track does not wind too far around the asteroid so that a simple translation is sufficient to correct it. As was found to be sufficiently accurate in [2], we assume the pointing information derived originally from the spacecraft is correct so that only the position of the spacecraft needs to be improved. Then for each small lidar track the following procedure was performed. Denote the set of lidar points in the track as S (the source points). For each lidar point in S, a point on the asteroid near it was computed by intersecting with the highest resolution shape model of the asteroid [5] a ray originating from the spacecraft position (using [2] ) in the direction of the lidar point. These intersection points now form a second set denoted as T (the target points). A point-matching scheme (described next) was then used to find the optimal translation that best matches the source points S to the target points T . This optimal translation was then applied to the original lidar points S as well as to the spacecraft positions to produce the improved data. This procedure was repeated for each lidar track. The point-matching scheme used is a variation of the well known Iterative Closest Point (ICP) algorithm [6] which we briefly describe here. This algorithm seeks to find a transformation that best aligns the source points S with the target points T . Typically the transformation can include translation as well as rotation, though in this case, we restricted the transformation to translation only and assumed the pointing information in the original dataset is correct. The algorithm begins by first translating set S so that its centroid is at the same location as the centroid of set T . Then it iterates between the following 2 steps:

Revisiting the NEAR-Shoemaker landing site.

Abstract: Introduction: The NEAR-Shoemaker spacecraft landed on the surface of 433 Eros on Feb 12, 2001. In a recent study [1], the location of the final landing site was updated to 41.626 S, 80.421 E (x = 0.82 ±0.01, y = 4.85 ± 0.01, z = -4.37 ± 0.01), in a crater about 200 m south (Fig. 1) of the previous estimate [2]. This new solution was obtained using a sophisticated optical navigation approach that included NEAR Laser Rangefinder (NLR) data and estimates for the change in velocity associated with each one of the five final burns that brought the spacecraft safely to the surface. In this study, we use new high-resolution topography from imaging [3], and improvements in the NLR data [4, 5] to revisit and re-evaluate current interpretations of the geology observed in the final images collected by the NEAR-Shoemaker spacecraft.

Mercury’s shape from radio occultations

Abstract: To support studies of Mercury’s internal structure, a MESSENGER mission goal is to measure the shape of the planet Radio-frequency occultation observations contribute to this objective, particularly in most of the southern hemisphere where there are no altimeter data We describe the techniques used to derive radius measurements from occultations and report results to date on the long-wavelength shape of Mercury Before the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, Mercury’s shape was poorly constrained, primarily by Earth-based radar observations at low Mercury latitudes During MESSENGER’s 12-month primary mission, the Mercury Laser Altimeter (MLA) acquired an extensive data set on the topography of Mercury’s northern hemisphere [1] However, most of the southern hemisphere is beyond MLA range because the periapsis of MESSENGER’s highly inclined, eccentric orbit is at high northern latitudes Along with limb measurements and global stereo mosaics, occultation-derived radius measurements are essential for understanding the shape of Mercury’s southern hemisphere As viewed from Earth, the MESSENGER spacecraft passed behind Mercury every twelve hours for most of the primary mission This geometry caused Mercury to occult the radio frequency (RF) transmissions, and we used an open-loop receiver to record RF power at the ingress and egress of each occultation Incorporating the effects of diffraction, we extracted the time of occultation and used it to determine the RF path that grazed Mercury’s surface The point on that RF path that is tangent to the surface defines a unique latitude, longitude, and radius Since the highest point along the RF path provides the occultation edge, the radius measurements are biased high relative to the surrounding terrain We corrected for this bias by evaluating topography local to the tangent point Digital-elevation models (DEMs), derived from surface images acquired by MESSENGER’s Mercury Dual Imaging System (MDIS), contain the necessary topographic data We compared northern-hemisphere occultation results to MLA data to

Large-Scale Troughs on 4 Vesta: Observations and Analysis

Abstract: Images of Vesta taken by the Dawn Framing Camera reveal the presence of large- scale structural features on the surface of the asteroid. Analysis of these structures supports models for the formation of the south polar basins.