Geology topics

Research about Comet Borrelly

Source-linked reports with geographic coverage including Comet Borrelly.

6 recordsLinked to original sources

USGS2004

The nucleus of Comet Borrelly: A study of morphology and surface brightness

Stereo images obtained during the DS1 flyby were analyzed to derive a topographic model for the nucleus of Comet 19P/Borrelly for morphologic and photometric studies. The elongated nucleus has an overall concave shape, resembling a peanut, with the lower end tilted towards the camera. The bimodal character of surface-slopes and curvatures support the idea that the nucleus is a gravitational aggregate, consisting of two fragments in contact. Our photometric modeling suggests that topographic shading effects on Borrelly's surface are very minor (<10%) at the given resolution of the terrain model. Instead, albedo effects are thought to dominate Borrelly's large variations in surface brightness. With 90% of the visible surface having single scattering albedos between 0.008 and 0.024, Borrelly is confirmed to be among the darkest of the known Solar System objects. Photometrically corrected images emphasize that the nucleus has distinct, contiguous terrains covered with either bright or dark, smooth or mottled materials. Also, mapping of the changes in surface brightness with phase angle suggests that terrain roughness at subpixel scale is not uniform over the nucleus. High surface roughness is noted in particular near the transition between the upper and lower end of the nucleus, as well as near the presumed source region of Borrelly's main jets. Borrelly's surface is complex and characterized by distinct types of materials that have different compositional and/or physical properties.

Icarus↗

USGS2004

Imaging Borrelly

The nucleus, coma, and dust jets of short-period Comet 19P/Borrelly were imaged from the Deep Space 1 spacecraft during its close flyby in September 2001. A prominent jet dominated the near-nucleus coma and emanated roughly normal to the long axis of nucleus from a broad central cavity. We show it to have remained fixed in position for more than 34 hr, much longer than the 26-hr rotation period. This confirms earlier suggestions that it is co-aligned with the rotation axis. From a combination of fitting the nucleus light curve from approach images and the nucleus' orientation from stereo images at encounter, we conclude that the sense of rotation is right-handed around the main jet vector. The inferred rotation pole is approximately perpendicular to the long axis of the nucleus, consistent with a simple rotational state. Lacking an existing IAU comet-specific convention but applying a convention provisionally adopted for asteroids, we label this the north pole. This places the sub-solar latitude at ∼60° N at the time of the perihelion with the north pole in constant sunlight and thus receiving maximum average insolation.

Icarus↗

USGS2004

Comparison of USGS and DLR topographic models of Comet Borrelly and photometric applications

Stereo analysis of images obtained during the 2001 flyby of Comet Borrelly by NASA's Deep Space 1 (DS1) probe allows us to quantify the shape and photometric behavior of the nucleus. The shape is complex, with planar facets corresponding to the dark, mottled regions of the surface whereas the bright, smooth regions are convexly curved. The photometric as well as textural differences between these regions can be explained in terms of topography (roughness) at and below the image resolution, without invoking significant variations in single-particle properties; the material on Borrelly's surface could be quite uniform. A statistical comparison of the digital elevation models (DEMs) produced from the three highest-resolution images independently at the USGS and DLR shows that their difference standard deviation is 120 m, consistent with a matching error of 0.20 pixel (similar to reported matching accuracies for many other stereo datasets). The DEMs also show some systematic differences attributable to manual versus automatic matching. Disk-resolved photometric modeling of the nucleus using the DEM shows that bright, smooth terrains on Borrelly are similar in roughness (Hapke roughness θ =20°) to C-type asteroid Mathilde but slightly brighter and more backscattering (single-scattering albedo w =0.056, Henyey–Greenstein phase parameter g =−0.32). The dark, mottled terrain is photometrically consistent with the same particles but with roughnesses as large as 60°. Intrinsically darker material is inconsistent with the phase behavior of these regions. Many local radiance variations are clearly related to topography, and others are consistent with a topographic explanation; one need not invoke albedo variations greater than a few tens of percent to explain the appearance of Borrelly.

Icarus↗

USGS2002

Observations of comet 19P/Borrelly by the miniature integrated camera and spectrometer aboard deep space 1

The nucleus of the Jupiter-family comet 19P/Borrelly was closely observed by the Miniature Integrated Camera and Spectrometer aboard the Deep Space 1 spacecraft on 22 September 2001. The 8-kilometer-long body is highly variegated on a scale of 200 meters, exhibiting large albedo variations (0.01 to 0.03) and complex geologic relationships. Short-wavelength infrared spectra (1.3 to 2.6 micrometers) show a slope toward the red and a hot, dry surface (≤345 kelvin, with no trace of water ice or hydrated minerals), consistent with ∼10% or less of the surface actively sublimating. Borrelly's coma exhibits two types of dust features: fans and highly collimated jets. At encounter, the near-nucleus coma was dominated by a prominent dust jet that resolved into at least three smaller jets emanating from a broad basin in the middle of the nucleus. Because the major dust jet remained fixed in orientation, it is evidently aligned near the rotation axis of the nucleus.

Science↗

USGS2002

USGS and DLR topographic mapping of Comet Borrelly

In the fall of 2001, NASA's Deep Space 1 (DS1) probe imaged Comet Borrelly during a flyby encounter. Three of the Borrelly images have geometry suitable to photogrammetrically map the nucleus, which form two stereopairs with an expected precision (EP) of ~410 m and ~670 m each. DS1 team members at the USGS and DLR have independently produced digital elevation models (DEMs) of Borrelly. Automatic stereo-matching algorithms were used by both USGS and DLR, but the USGS DEM was additionally manually edited in stereo. We accomplished a quantitative statistical comparison of the DEMs and found they have a standard deviation of 120 m, which is small compared to the EP above. There are systematic differences, we estimate the stereomatching error to only 0.20 pixel RMS, which is similar to the level of subpixel matching accuracy obtained in a wide variety of other mapping situations. The resulting DEMs enable a variety of applications such as perspective views, photometric modeling and studies of the energy balance of the nucleus. We hope to use the USGS DEM as a starting point to extrapolate the shape of the hidden side of the nucleus. This would not only let us determine the volume and moments of inertia of the nucleus, but would lead to a calculation of the insolation onto the nucleus averaged over an entire orbit, and thence to a model of the evolution of nuclear shape.

Conference Paper↗

USGS2002

USGS high resolution topo-mapping of Mars with Mars Orbiter Camera Narrow-Angle images

We describe our initial experiences producing controlled digital elevation models (DEMs) of Mars with horizontal resolutions of ≤10 m and vertical precisions of ≤2 m. Such models are of intense interest at all phases of Mars exploration and scientific investigation, from the selection of safe landing sites to the quantitative analysis of the morphologic record of surface processes. Topomapping with a resolution adequate to address many of these issues has only become possible with the success of the Mars Global Surveyor (MGS) mission. The Mars Orbiter Laser Altimeter (MOLA) on MGS mapped the planet globally with absolute accuracies <10 m vertically and ~100 m horizontally but relatively sparse sampling (300 m along track, with gaps of >1 km between tracks common at low latitudes). We rely on the MOLA data as the best available source of control and process images from the narrow-angle Mars Orbiter Camera (MOC-NA) with stereo and photoclinometric (shape-from-shading) techniques to produce DEMs with significantly better horizontal resolution. The techniques described here enable mapping not only with MOC but also with the high-resolution cameras (Mars Express HRSC, Mars Reconnaissance Orbiter HiRISE) that will orbit Mars in the next several years.

Conference Paper↗