John K. Harmon

Radar mapping of mercury: full-disk images and polar anomalies.

A random-code technique has been used at Arecibo to obtain delay-Doppler radar images of the full disk of Mercury. Anomalously bright features were found at the north and south poles. The north polar feature is oblong (4� by 8�) and offset from the pole. The smaller south polar feature is mostly confined to the floor of the crater Chao Meng-Fu. The polar locations and radar properties of these features indicate that they may be produced by volume scattering in ice. The images also reveal a variety of more subdued reflectivity features ranging in size from hundreds to thousands of kilometers; some of these appear to have an impact origin.

Venus: Volcanism and Rift Formation in Beta Regio

A new high-resolution radar image of Beta Regio, a Venus highland area, confirms the presence of a major tectonic rift system and associated volcanic activity. The lack of identifiable impact craters, together with the apparent superposition of the Theia Mons volcanic structure on the rift system, suggest that at least some of the volcanic activity occurred in relatively recent geologic time. The presence of topographically similar highland areas elsewhere on Venus (Aphrodite Terra, Dali Chasma, and Diana Chasma) suggests that rifting and volcanism are significant processes on Venus.

Thermal Stability of Volatiles in the North Polar Region of Mercury

Wet Mercury Radar observations of Mercurys poles in the 1990s revealed regions of high backscatter that were interpreted as indicative of thick deposits of water ice; however, other explanations have also been proposed (see the Perspective by Lucey). MESSENGER neutron data reported by Lawrence et al. (p. 292, published online 29 November) in conjunction with thermal modeling by Paige et al. (p. 300, published online 29 November) now confirm that the primary component of radar-reflective material at Mercurys north pole is water ice. Neumann et al. (p. 296, published online 29 November) analyzed surface reflectance measurements from the Mercury Laser Altimeter onboard MESSENGER and found that while some areas of high radar backscatter coincide with optically bright regions, consistent with water ice exposed at the surface, some radar-reflective areas correlate with optically dark regions, indicative of organic sublimation lag deposits overlying the ice. Dark areas that fall outside regions of high radio backscatter suggest that water ice was once more widespread. Spacecraft data and a thermal model show that water ice and organic volatiles are present at Mercury’s north pole. [Also see Perspective by Lucey] Thermal models for the north polar region of Mercury, calculated from topographic measurements made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, show that the spatial distribution of regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice. MESSENGER measurements of near-infrared surface reflectance indicate bright surfaces in the coldest areas where water ice is predicted to be stable at the surface, and dark surfaces within and surrounding warmer areas where water ice is predicted to be stable only in the near subsurface. We propose that the dark surface layer is a sublimation lag deposit that may be rich in impact-derived organic material.

Reports Venus: Identification of Banded Terrain in the Mountains of Ishtar Terra

High resolution images obtained with the Arecibo radar system at a wavelength of 12.6 centimeters reveal numerous parallel 10- to 20-kilometer-wide bands of high radar backscatter situated on and oriented parallel to the major mountain ranges of the Ishtar Terra region of Venus. Geometric and morphological characteristics suggest that the bands represent Earth-like tectonic deformational features, such as folds and faults.

Radar observations of Comet IRAS-Araki-Alcock 1983d

A detailed analysis and interpretation of the Arecibo S-band radar observations of Comet IRAS-Araki-Alcock 1983d are presented. The very high signal strengths are used to make an accurate determination of the shape of the echo spectrum in the two orthogonal senses of circular polarization. The narrow-band component is used to place constraints on the size, rotation, period, reflectivity, and roughness of the nucleus. Detailed analysis of the broadband component yields estimates of, or bounds on, the spatial extent, position, and mass of the particle cloud, as well as the effective size of the constituent particles. 41 references.

Styles of volcanism on venus: new arecibo high resolution radar data.

Arecibo high-resolution (1.5 to 2 km) radar data of Venus for the area extending from Beta Regio to western Eisila Regio provide strong evidence that the mountains in Beta and Eisila Regiones and plains in and adjacent to Guinevere Planitia are of volcanic origin. Recognized styles of volcanism include large volcanic edifices on the Beta and Eisila rises related to regional structural trends, plains with multiple source vents and a mottled appearance due to the ponding of volcanic flows, and plains with bright features surrounded by extensive quasi-circular radar-dark halos. The high density of volcanic vents in the plains suggests that heat loss by abundant and widely distributed plains volcanism may be more significant than previously recognized. The low density of impact craters greater than 15 km in diameter in this region compared to the average density for the higher northern latitudes suggests that the plains have a younger age.

Mars mapping with delay‐Doppler radar

Mars radar imaging results from Arecibo 12.6-cm observations are presented. The images were derived from delay-Doppler mapping using a coded-long-pulse technique to mitigate the effects of echo overspreading. Images of the depolarized echo are used to identify regions of high decimeter-scale roughness. Some of the strongest echo features are located on the major shield volcanoes or on relatively young off-shield flows such as the Olympus and Pavonis lava aprons. The shields themselves have highly irregular radar signatures suggesting complex volcanic histories. Some Mars radar features have twice the depolarized brightness of the roughest terrestrial lava flows, apparently due to higher levels of multiple scattering from surfaces of spectacular roughness or from volume scattering. Low-brightness (smooth) areas are associated with older surfaces such as fractured and highland terra, as well as with terrain interpreted to be debris lobes, ash flows, and aureoles; in particular, a close connection was found between the 12.6-cm counterpart of the “Stealth” feature and the Medusae Fossae Formation (postulated to be deep ignimbrite deposits). Marte Valus is anomalous in being the only outflow channel showing strongly enhanced echoes, which supports the idea that this channel and the Elysium Basin that it drained are filled with lava flows. A weak radar feature was found for the south polar residual ice cap. Comparisons with Goldstone 3.5-cm data show that the south polar enhancement is much weaker at 12.6 cm than at 3.5 cm, indicating that the southern ice cap becomes optically thin at the longer wavelength. A north polar enhancement has also been found, which is comparable in strength to the 12.6-cm south polar feature.

Mars: Dual-polarization radar observations with extended coverage

Abstract Thirteen-centimeter-wavelength radar observations of Mars made in 1982 at Arecibo Observatory yield accurate measurements of the full backscatter spectrum in two orthogonal polarizations. The data, which were obtained for several widely separated subradar longitudes at 24°N latitude, provide the first global view of the distribution of small-scale surface roughness on Mars. The diffuse component of the echo exhibits strong spatial variations. Areas of maximum depolarization correlate well with volcanic regions (Tharsis and Elysium), while the heavily cratered upland terrain yields relatively low depolarization. Parts of Tharsis give near-complete depolarization (polaziation ratio μ c ⋟ 1 when viewed at oblique angles of incidence). Northern Martian plains regions (Tharsis, Elysium, and Amazonis) may comprise the most extensive area of severe decimeter-scale surface roughness in the inner Solar System. On the average, the northern Martian tropics yield higher diffuse radar cross sections ( σ D = 0.05–0.12) and a higher of degree disk-integrated depolarization ( μ c = 0.1–0.4) than is found for the Moon, Mercury, and Venus. Comparisons between the Moon and Mars using radar data, ground truth, and simple scattering models suggest that Mars possesses a relatively high average coverage by decimeter-scale rocks. Also discussed are several of the more interesting quasispecular scattering results, the most unsual of which were obtained over the Olympus Mons aureole region.

Radar observations of Comet Halley

Five nights of Arecibo radar observations of Comet Halley are reported which reveal a feature in the overall average spectrum which, though weak, seems consistent with being an echo from the comet. The large radar cross section and large bandwidth of the feature suggest that the echo is predominantly from large grains which have been ejected from the nucleus. Extrapolation of the dust particle size distribution to large grain sizes gives a sufficient number of grains to account for the echo. The lack of a detectable echo from the nucleus, combined with estimates of its size and rotation rate from spacecraft encounters and other data, indicate that the nucleus has a surface of relatively high porosity. 33 references.

Radar Observations of Comet 103P/Hartley 2

Comets rarely come close enough to be studied intensively with Earth-based radar. The most recent such occurrence was when Comet 103P/Hartley 2 passed within 0.12 AU in late 2010 October, less than two weeks before the EPOXI flyby. This offered a unique opportunity to improve pre-encounter trajectory knowledge and obtain complementary physical data for a spacecraft-targeted comet. 103P/Hartley 2 is only the fourth comet nucleus to be imaged with radar and already the second to be identified as an elongated, bilobate object based on its delay-Doppler signature. The images show the dominant spin mode to be a rotation about the short axis with a period of 18.2 hr. The nucleus has a low radar albedo consistent with a surface density of 0.5-1.0 g cm{sup -3}. A separate echo component was detected from large (>cm) grains ejected anisotropically with velocities of several to tens of meters per second. Radar shows that, in terms of large-grain production, 103P/Hartley 2 is an unusually active comet for its size.