The surface of Mercury from ground-based astronomical observations

Recent ground-based astronomical short-exposure observations of Mercury have yielded more than 50000 electronic pictures of the planet at different phases and different positions relative to the Earth. The work was fulfilled in several observatories. The use of available and newly developed processing methods applied to large volumes of electronic frames allowed the images of a considerable portion of Mercury’s surface to be synthesized. We present the images of the 90°–180°W, 215°–280°W, and 50°–90°W sectors containing, among others, the longitudes not covered by spacecraft imaging. Along with the listed images, we present the results of recent observations of Mercury carried out on November 20–24, 2006 during the morning elongation at the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS) (Nizhnii Arkhyz, Karachai-Circassia, the Caucasus). The 265°–350°W longitude sector of Mercury was observed. The observations were made under good weather conditions. Among the main tasks of the new observations was obtaining a complete view of the S Basin. Previously, this basin had been investigated in fragments only by the actual solar illumination conditions. During the period of November 20–24, 2006, the S Basin was on the sunlit side of the planet. The complete image of the basin was obtained from the processing of a large number of electronic frames. The appearance of the S Basin is compared with the data on its relief acquired with radar methods. In this longitude sector, a number of other unusual surface features were found; among them, are a huge “Medallion” crater and other formations. The results considered in the present and earlier published studies are compared with the Mariner 10 data (1974–1975) and with the data received from the Messenger spacecraft during its first flyby of the planet (January 2008).     

Observations of the sodium tail of Mercury

Cross-sections of the sodium emission tail of Mercury were measured at various distances down the tail when Mercury was moving away from the Sun (true anomaly angles <180°), and again when Mercury was moving towards the Sun (true anomaly angles >180°). As predicted in early modeling studies, significant differences were expected between these two cases, as the result of Doppler shifts to higher solar intensity in the former case, and to lower solar intensity for the latter case. For observations with Mercury moving away from the Sun, the sodium tail was observed out to about 40,000 kilometers (16 Mercury radii, RM) downstream, expanding, on average, at a rate of 1.9±0.3 km/s. The source rates for sodium generation from Mercury into the tail were found to be in the range 2-5×¹⁰²³ atoms/s, corresponding to between 1 and 10% of the estimated total sodium production rate on the planet. The limiting value of radiation acceleration required to produce an observable sodium tail was estimated to be 112±24 cm/s². For observations where Mercury was moving towards the Sun, the emission intensity in the sodium tail decreased very rapidly with distance downstream, disappearing entirely beyond 12,000 (6 RM) kilometers for radiation accelerations of 128.7 and 135.4 cm/s². For smaller radiation accelerations, the sodium tail was not detectable at all, yielding a limiting value for tail generation of about 122±2 cm/s². Interpretation of the limiting radiation acceleration values suggests that the process that generates the sodium tail yields atoms with energies greater than 3 eV. Particle sputtering is the most reasonable source process.     

Persistent rings in and around Jupiter’s anticyclones – Observations and theory

We present observations and theoretical calculations to derive the vertical structure of and secondary circulation in jovian vortices, a necessary piece of information to ultimately explain the red color in the annular ring inside Jupiter’s Oval BA. The observations were taken with the near-infrared detector NIRC2 coupled to the adaptive optics system on the 10-m W.M. Keck telescope (UT 21 July 2006; UT 11 May 2008) and with the Hubble Space Telescope at visible wavelengths (UT 24 and 25 April 2006 using ACS; UT 9 and 10 May 2008 using WFPC2). The spatial resolution in the near-IR (∼0.1–0.15″ at 1–5 μm) is comparable to that obtained at UV–visible wavelengths (∼0.05–0.1″ at 250–890 nm). At 5 μm we are sensitive to Jupiter’s thermal emission, whereas at shorter wavelengths we view the planet in reflected sunlight. These datasets are complementary, as images at 0.25–1.8 μm provide information on the clouds/hazes in the troposphere–stratosphere, while the 5-μm emission maps yield information on deeper layers in the atmosphere, in regions without clouds. At the latter wavelength numerous tiny ovals can be discerned at latitudes between ∼45°S and 60°S, which show up as rings with diameters ?1000 km surrounding small ovals visible in HST data. Several white ovals at 41°S, as well as a new red oval that was discovered to the west of the GRS, also reveal 5-μm bright rings around their peripheries, which coincide with dark/blue rings at visible wavelengths. Typical brightness temperatures in these 5-μm bright rings are 225–250 K, indicative of regions that are cloud-free down to at least the ∼4 bar level, and perhaps down to 5–7 bar, i.e., well within the water cloud.Radiative transfer modeling of the 1–2 μm observations indicates that all ovals, i.e., including the Great Red Spot (GRS), Red Oval BA, and the white ovals at 41°S, are overall very similar in vertical structure. The main distinction between the ovals is caused by variations in the particle densities in the tropospheric–stratospheric hazes (2–650 mbar). These are 5–8 times higher above the red ovals than above the white ones at 41°S. The combination of the 5-μm rings and the vertical structure derived from near-IR data suggests anticyclones to extend vertically from (at least) the water cloud (∼5 bar) up to the tropopause (∼100–200 mbar), and in some cases into the stratosphere.Based upon our observations, we propose that air is rising along the center of a vortex, and descending around the outer periphery, producing the 5-μm bright rings. Observationally, we constrain the maximum radius of these rings to be less than twice the local Rossby deformation radius, L_R. If the radius of the visible oval (i.e., the clouds that make the oval visible) is >3000 km, our observations suggest that the descending part of the secondary circulation must be within these ovals. For the Red Oval BA, we postulate that the return flow is at the location of its red annulus, which has a radius of ∼3000 km.We develop a theory for the secondary circulation, where air is (baroclinically) rising along the center of a vortex in a subadiabatic atmosphere, and descending at a distance not exceeding ∼2× the local Rossby deformation radius. Using this model, we find a timescale for mixing throughout the vortex of order several months, which suggests that the chromophores that are responsible for the red color of Oval BA’s red annulus must be produced locally, at the location of the annulus. This production most likely results from the adiabatic heating in the descending part of the secondary circulation. Such higher-than-ambient temperature causes NH₃–ice to sublime, which will expose the condensation nuclei, such as the red chromophores.     

Observation of Mercury's sodium exosphere during the transit on November 9, 2006

A rare, but normal, astronomical event occurred on November 9th 2006 (JST) as Mercury passed in front of the Sun from the perspective of the Earth. The abundance of the sodium vapor above the planet limb was observed by detecting an excess absorption in the solar sodium line D₁ during this event. The observation was performed with a 10-m spectrograph of Czerny-Turnar system at Domeless Solar Tower Telescope at the Hida Observatory in Japan. The excess absorption was red-shifted by 10 pm relative to the solar line, and was measured at the dawnside (eastside) and duskside (westside) of Mercury. Between the dawn and dusksides, an asymmetry of total sodium abundance was clearly identified. At the dawnside, the total sodium column density was 6.1×10¹⁰ Na atoms/cm², while it was 4.1×10¹⁰ Na atoms/cm² at the duskside. The investigation of dawn-dusk asymmetry of the sodium exosphere of Mercury is a clue to understand the release mechanism of sodium from the surface rock. Our result suggests that a thermal desorption is a main source process for sodium vapor in the vicinity of Mercury.     

Mercury: The Image of the Planet in the 210°–285° W Longitude Range Obtained by the Short-Exposure Method

For the purpose of obtaining images of the unknown portion of Mercury, we continued the previously started series of observations of this planet by the short exposure method. Several thousand electronic images of Mercury have been acquired on 1–2 May 2002 under good meteorological conditions at the high-altitude Skinakas Astrophysical Observatory of Iraklion University (Crete, Greece, 35°13 E, 24°54 N) during the evening elongation. The phase angle of Mercury was 95°–99° and the observed range of longitudes was 210°–285° W. Observations were carried out using Ritchy–Chrétien telescope (D = 1.29 m, F = 9.857 m) with the KS 19 filter cutting wavelengths shorter than about 700 nm. The planet's disk was seen, on average, at an angle of 7.75 arcsec. The image scale was equal to 47.8 m/arcsec. We used a CCD with a pixel size of 7.4 × 7.4 m in the regime of short exposures. By processing a great number of electronic images, we succeeded in obtaining a sufficiently distinct synthesized image of the unknown portion of Mercury's surface. The most prominent formation in this region is a giant basin (or cratered mare) centered at about 8° N, 280° W, which was given a working name Skinakas basin (after the name of the observatory where observations were made). By its size, the interior part of this basin exceeds the largest lunar Mare Imbrium. As opposed to Mare Imbrium, the Skinakas basin is presumably of impact origin. Its relief resembles that of Caloris Planitia but the size is much larger. A series of smaller formations are also seen on synthesized images. The resolution obtained on the surface of Mercury is about 100 km, which is close to the telescope diffraction limit. Also considered are the published theoretical estimations of the possible advantages offered by the short exposure method. Some results obtained by other research groups are discussed.     

The planet Mercury: Synthesis of resolved images of unknown part in the longitude range 250-290°W

Plans to send orbiter missions to Mercury (e.g., NASA's Messenger and ESA's BepiColombo) have prompted renewed efforts to investigate the surface of Mercury using ground-based remote sensing. While the highest resolution instrumentation optical telescopes (e.g. HST) cannot be used at small angular distances (<45°) from the Sun (Mercury's elongation never exceeds 28° seen from Earth), advanced ground-based astronomical techniques and modern processing software can be used to construct resolved images of the poorly known part of Mercury. Our observations of the planet presented here were carried out mainly in April and May, 2002, at evening elongation of the planet, at the Skinakas astrophysical observatory of Heraklion University (Crete, Greece). A synthesis of the acquired images of the hemisphere of Mercury, which was not observed by the Mariner 10 mission (1974-1975), is presented. A double rim basin with an internal diameter of about 1000 km and an external rim about 2000 km is suggested by the data. We present the observational method, the data analysis approach, and the resulting images.     

The GEM project: An international collaboration to survey galactic radiation emission

The GEM (Galactic Emission Mapping) project is an international collaboration established with the aim of surveying the full sky at long wavelengths with a multi-frequency radio telescope. A total of 745 hours of observation at 408 MHz were completed from an Equatorial site in Colombia. The observations cover the celestial band 0 ^h <<24 ^h , and –24° 22<<+35° 37. Preliminary results of this partial survey will be discussed. A review of the instrumental setup and a 10° resolution sky map at 408 MHz is presented.Presented by S. Torres at the UN/ESA Workshop on Basic Space Sciences: From Small Telescopes to Space Missions, Colombo, Sri Lanka 11–14 January 1996     

Properties and Peculiarities of Mercury's Regolith. Disk-integrated Polarimetry in 2000–2002

On the basis of the data from ground-based polarimetric, photometric, and other observations, as well as from space measurements (Mariner 10), we survey the investigations of the properties and peculiarities of Mercury's regolith in detail. We also present the results of our own observations performed during three apparitions of the planet in 2000–2002. An analysis of the published data points to essentially more intensive maturation processes in the Hermean surface regolith compared to that on the lunar surface. In addition, the orbital characteristics of Mercury allow us to suppose that the intensity of its regolith maturation and, therefore, the optical properties of its surface can noticeably depend on the planetocentric longitude. Polarimetric observations of Mercury's surface (the planetocentric longitude range was 265°–330°) carried out in 2000–2002 with a 70-cm reflector actually detected a polarization degree varying with an amplitude of about 1.5%. To ascertain the nature of these variations, additional observations of Mercury in a maximally wide range of planetocentric longitudes of the viewed surface are required.     

Properties of the Hermean Regolith: II. Disk-Resolved Multicolor Photometry and Color Variations of the “Unknown” Hemisphere

Multicolor photometric observations of the “unknown” hemisphere of Mercury have been performed with the Swedish Vacuum Solar Telescope on La Palma at maximal elongations from the Sun in 1997 and 1998. A set of six interference filters with central wavelengths from 450 to 940 nm were used. Multicolor photometry of Mercury was performed on disk-resolved images of the unknown hemisphere (longitudes 160°-340°) with a highest resolution of ∼200 km (J. Warell and S. Limaye 2001, Planet. Space Sci.49, 1531-1552).Disk-integrated spectrophotometry shows that (1) the spectrum of Mercury displays a linear slope from 650 to 940 nm, indicating that the average mercurian regolith is considerably more mature than relatively immature pure anorthosite regions on the Moon; (2) there is negative evidence for the presence of the putative 1-μm absorption feature near 940 nm due to the presence of ferrous iron (Fe²⁺) in pyroxenes; and (3) no effect of phase reddening of the integrated disk is observed between phase angles of 63° and 84°.For the first time, disk-resolved spectrophotometry of Mercury's surface has been obtained, from which it is inferred that (4) the scattering properties of Mercury's regolith are more homogeneous than for the Moon and that there is no clear relation between reflectance and chemical properties at spatial scales of ∼300 km on the unknown hemisphere and (5) there exists an inverse relation of spectral slope with emission angle which is larger for Mercury than for the Moon, indicating that the average mercurian regolith is more backscattering and that this effect increases with wavelength.Finally, from filter ratio images of Mercury's disk it is found that (6) no color variations larger that 2% with respect to the surroundings are detected at a spatial resolution of ∼300 km.     

Orbital effects on Mercury’s escaping sodium exosphere

We present results from coronagraphic imaging of Mercury’s sodium tail over a 7° field of view. Several sets of observations made at the McDonald Observatory since May 2007 show a tail of neutral sodium atoms stretching more than 1000 Mercury radii (R_m) in length, or a full degree of sky. However, no tail was observed extending beyond 120 R_m during the January 2008 MESSENGER fly-by period, or during a similar orbital phase of Mercury in July 2008. Large changes in Mercury’s heliocentric radial velocity cause Doppler shifts about the Fraunhofer absorption features; the resultant change in solar flux and radiation pressure is the primary cause of the observed variation in tail brightness. Smaller fluctuations in brightness may exist due to changing source rates at the surface, but we have no explicit evidence for such changes in this data set. The effects of radiation pressure on Mercury’s escaping atmosphere are investigated using seven observations spanning different orbital phases. Total escape rates of atmospheric sodium are estimated to be between 5 and 13 × 10²³ atoms/s and show a correlation to radiation pressure. Candidate sources of Mercury’s sodium exosphere include desorption by UV sunlight, thermal desorption, solar wind channeled along Mercury’s magnetic field lines, and micro-meteor impacts. Wide-angle observations of the full extent of Mercury’s sodium tail offer opportunities to enhance our understanding of the time histories of these source rates.     

Observations of Mercury's exosphere: Spatial distributions and variations of its Na component during August 8, 9 and 10, 2003

Following the observations of August 2002 [Barbieri, C., Verani, S., Cremonese, G., Sprague, A., Mendillo, M., Cosentino, R., Hunten, D., 2004. Planet. Space Sci. 52, 1169-1175], the high resolution spectrograph of the 3.5-m Galileo National Telescope (TNG) has been used to obtain several spatially resolved spectra of Mercury's Na-D on the evenings of 8, 9 and 10 August 2003. The resolution of the spectrograph was 115,000, the slit dimensions were ^0.4″×^27″. With respect to Paper I, the planet was in a fairly similar orbital configuration, being at a geocentric distance of approximately 1 AU, and having a True Anomaly Angle (TAA) from 163°-168° instead of 171°-174°. We present here a significantly larger set of observations and discuss several important features regarding the formation of Mercury's sodium exosphere, in particular the role of photon stimulated and thermal desorptions, as well as of the solar wind sputtering and micro-meteoroid vaporization. Thanks to the very good seeing of these observations, we also report and discuss the origins and variations of equatorial structures in Mercury's early morning sodium exosphere.     

Uranus’ cloud structure and seasonal variability from Gemini-North and UKIRT observations

Observations of Uranus were made in September 2009 with the Gemini-North telescope in Hawaii, using both the NIFS and NIRI instruments. Observations were acquired in Adaptive Optics mode and have a spatial resolution of approximately 0.1″.NIRI images were recorded with three spectral filters to constrain the overall appearance of the planet: J, H-continuum and CH₄(long), and long slit spectroscopy measurements were also made (1.49-1.79 μm) with the entrance slit aligned on Uranus’ central meridian. To acquire spectra from other points on the planet, the NIFS instrument was used and its 3″ × 3″ field of view stepped across Uranus’ disc. These observations were combined to yield complete images of Uranus at 2040 wavelengths between 1.476 and 1.803 μm.The observed spectra along Uranus central meridian were analysed with the NEMESIS retrieval tool and used to infer the vertical/latitudinal variation in cloud optical depth. We find that the 2009 Gemini data perfectly complement our observations/conclusions from UKIRT/UIST observations made in 2006-2008 and show that the north polar zone at 45°N has continued to steadily brighten while that at 45°S has continued to fade. The improved spatial resolution of the Gemini observations compared with the non-AO UKIRT/UIST data removes some of the earlier ambiguities with our previous analyses and shows that the opacity of clouds deeper than the 2-bar level does indeed diminish towards the poles and also reveals a darkening of the deeper cloud deck near the equator, perhaps coinciding with a region of subduction. We find that the clouds at 45°N,S lie at slightly lower pressures than the clouds at more equatorial latitudes, which suggests that they might possibly be composed of a different condensate, presumably CH₄ ice, rather than H₂S or NH₃ ice, which is assumed for the deeper cloud. In addition, analysis of the centre-to-limb curves of both the Gemini/NIFS and earlier UKIRT/UIST IFU observations shows that the main cloud deck has a well-defined top, and also allows us to better constrain the particle scattering properties.Overall, Uranus appeared to be less convectively active in 2009 than in the previous 3 years, which suggests that now the northern spring equinox (which occurred in 2007) is passed the atmosphere is settling back into the quiescent state seen by Voyager 2 in 1986. However, a number of discrete clouds were still observed, with one at 15°N found to lie near the 500 mb level, while another at 30°N, was seen to be much higher at near the 200 mb level. Such high clouds are assumed to be composed of CH₄ ice.     

Multispectral imaging observations of Neptune’s cloud structure with Gemini-North

Observations of Neptune were made in September 2009 with the Gemini-North Telescope in Hawaii, using the NIFS instrument in the H-band covering the wavelength range 1.477–1.803 μm. Observations were acquired in adaptive optics mode and have a spatial resolution of approximately 0.15–0.25″.The observations were analysed with a multiple-scattering retrieval algorithm to determine the opacity of clouds at different levels in Neptune’s atmosphere. We find that the observed spectra at all locations are very well fit with a model that has two thin cloud layers, one at a pressure level of ∼2 bar all over the planet and an upper cloud whose pressure level varies from 0.02 to 0.08 bar in the bright mid-latitude region at 20–40°S to as deep as 0.2 bar near the equator. The opacity of the upper cloud is found to vary greatly with position, but the opacity of the lower cloud deck appears remarkably uniform, except for localised bright spots near 60°S and a possible slight clearing near the equator.A limb-darkening analysis of the observations suggests that the single-scattering albedo of the upper cloud particles varies from ∼0.4 in regions of low overall albedo to close to 1.0 in bright regions, while the lower cloud is consistent with particles that have a single-scattering albedo of ∼0.75 at this wavelength, similar to the value determined for the main cloud deck in Uranus’ atmosphere. The Henyey-Greenstein scattering particle asymmetry of particles in the upper cloud deck are found to be in the range g ∼ 0.6–0.7 (i.e. reasonably strongly forward scattering).Numerous bright clouds are seen near Neptune’s south pole at a range of pressure levels and at latitudes between 60 and 70°S. Discrete clouds were seen at the pressure level of the main cloud deck (∼2 bar) at 60°S on three of the six nights observed. Assuming they are the same feature we estimate the rotation rate at this latitude and pressure to be 13.2 ± 0.1 h. However, the observations are not entirely consistent with a single non-evolving cloud feature, which suggests that the cloud opacity or albedo may vary very rapidly at this level at a rate not seen in any other giant-planet atmosphere.     

Imaging of an unknown sector of Mercury (260–350°W) at the Special Astrophysical Observatory of the Russian Academy of Sciences using the short-exposure method

A series of observations of Mercury were performed at the Special Astrophysical Observatory using the short-exposure method to image a hitherto unknown part of the Hermean surface. Several thousand electronic frames of the planet were taken during its morning elongation in the period from November 20–24, 2006. The phase angle of Mercury varied from 103° to 80°, and the interval of planetocentric longitudes observed spanned from 260 to 350°W. Observations were made with a CCD camera attached to the 1-m Zeiss-1000 Ritchey-Chretien telescope operating with a KS-19 filter (short-wavelength border at 700 nm). The Hermean surface is known to be almost impossible to resolve on ordinary images. A reduction of a large number of frames taken with millisecond-long exposures made it possible to obtain a rather sharp image of the observed part of the Hermean surface. One of the primary aims of new observations was to have a general outline of the basin earlier found by one of the authors (L. Ksanfomaliti). We are the first to image this giant feature. The size of its inner part exceeds that of the largest lunar Mare — Mare Imbrium, however, unlike the latter the studied basin is of impact origin. The synthesized images reveal a number of large impact craters of various ages, as well as smaller features. The highest resolution achieved corresponds to the diffraction limit for the instrument employed, or about 100 km on the Hermean surface.     

The equatorial shape and gravity field of Mercury from MESSENGER flybys 1 and 2

On 14 January and 6 October 2008 the MESSENGER spacecraft passed within 200 km of the surface of Mercury. These flybys by MESSENGER provided the first observations of Mercury from a spacecraft since the Mariner 10 flybys in 1974 and 1975. Data from the Mercury Laser Altimeter (MLA) provided new information on the equatorial shape of Mercury, and Doppler tracking of the spacecraft through the flybys provided new data on the planet’s gravity field. The MLA passes were on opposite hemispheres of the planet and span collectively ∼40% of the equatorial circumference. The mean elevation of topography observed during flyby 1, in the longitude range 0-90°E, is greater than that seen during flyby 2 in the longitude range 180-270°E, indicating an offset between centers of mass and figure having a magnitude and phase in general agreement with topography determined by Earth-based radar. Both MLA profiles are characterized by slopes of ∼0.015° downward to the east, which is consistent with a long-wavelength equatorial shape defined by a best-fitting ellipse. The Doppler tracking data show sensitivity to the gravitational structure of Mercury. The equatorial ellipticity of the gravitational field, C_2,2, is well determined and correlates with the equatorial shape. The S_2,2 coefficient is ∼0, as would be expected if Mercury’s coordinate system, defined by its rotational state, is aligned along its principal axes of inertia. The recovered value of the polar flattening of the gravitational potential, J₂, is considerably lower in magnitude than the value obtained from Mariner 10 tracking, a result that is problematic for internal structure models. This parameter is not as well constrained as the equatorial ellipticity because the flyby trajectories were nearly in the planet’s equatorial plane. The residuals from the Doppler tracking data suggest the possibility of mascons on Mercury, but flyby observations are of insufficient resolution for confident recovery. For a range of assumptions on degree of compensation and crustal and mantle densities, the allowable crustal thickness is consistent with the upper limit of about 100 km estimated from the inferred depth of faulting beneath a prominent lobate scarp, an assumed ductile flow law for crustal material, and the condition that temperature at the base of the crust does not exceed the solidus temperature. The MESSENGER value of C_2,2 has allowed an improved estimate of the ratio of the polar moment of inertia of the mantle and crust to the full polar moment (C_m/C), a refinement that strengthens the conclusion that Mercury has at present a fluid outer core.     

Physical characterization of the potentially hazardous high-albedo Asteroid (33342) 1998 WT24 from thermal-infrared observations

The potentially hazardous Asteroid (33342) 1998 WT₂₄ approached the Earth within 0.0125 AU on 2001 December 16 and was the target of a number of optical, infrared, and radar observing campaigns. Interest in 1998 WT₂₄ stems from its having an orbit with an unusually low perihelion distance, which causes it to cross the orbits of the Earth, Venus, and Mercury, and its possibly being a member of the E spectral class, which is rare amongst near-Earth asteroids (NEAs). We present the results of extensive thermal-infrared observations of 1998 WT₂₄ obtained in December 2001 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii and the ESO 3.6-m telescope in Chile. A number of thermal models have been applied to the data, including thermophysical models that give best-fit values of 0.35±0.04 km for the effective diameter, 0.56±0.2 for the geometric albedo, pv, and 100-300 J m⁻² s^−0.5 K⁻¹ for the thermal inertia. Our values for the diameter and albedo are consistent with results derived from radar and polarimetric observations. The albedo is one of the highest values obtained for any asteroid and, since no other taxonomic type is associated with albedos above 0.5, supports the suggested rare E-type classification for 1998 WT₂₄. The thermal inertia is an order of magnitude higher than values derived for large main-belt asteroids but consistent with the relatively high values found for other near-Earth asteroids. A crude pole solution inferred from a combination of our observations and published radar results is β=−52°, λ=355° (J2000), but we caution that this is uncertain by several tens of degrees.     

Measurement of the radius of Mercury by radio occultation during the MESSENGER flybys

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed three flybys of Mercury in 2008–2009. During the first and third of those flybys, MESSENGER passed behind the planet from the perspective of Earth, occulting the radio-frequency (RF) transmissions. The occultation start and end times, recovered with 0.1 s accuracy or better by fitting edge-diffraction patterns to the RF power history, are used to estimate Mercury's radius at the tangent point of the RF path. To relate the measured radius to the planet shape, we evaluate local topography using images to identify the high-elevation feature that defines the RF path or using altimeter data to quantify surface roughness. Radius measurements are accurate to 150 m, and uncertainty in the average radius of the surrounding terrain, after adjustments are made from the local high at the tangent point of the RF path, is 350 m. The results are consistent with Mercury's equatorial shape as inferred from observations by the Mercury Laser Altimeter and ground-based radar. The three independent estimates of radius from occultation events collectively yield a mean radius for Mercury of 2439.2±0.5 km.     

The sodium tail of Mercury

Abstract— Mercury is difficult to observe because it is so close to the Sun. However, when the angle of the ecliptic is near maximum in the northern hemisphere, and Mercury is near its greatest eastern elongation, it can be seen against the western sky for about a half hour after sunset. During these times, we were able to map sodium D₂ emission streaming from the planet, forming a long comet‐like tail. On 2001 May 26 (U.T.) we mapped the tail downstream to a distance of ?40 000 km. Sodium velocities in the tail increased to ?11 km s^?1 at 40 000 km as the result of radiation pressure acceleration. On 2000 June 5 (U.T.) we mapped the cross‐sectional extent of the tail at a distance of ?17 500 km downstream. At this distance, the half‐power full‐width of the emission was ?20 000 km. We estimated the transverse velocity of sodium in the tail to range from 2 to 4 km s^?1. The velocities we observed imply source velocities from the planet surface of the order of 5 km s^?1, or 4 eV. Particle sputtering is a likely candidate for production of sodium atoms at these velocities. The total flux of sodium in the tail was ?1 times 10²³ atoms s^?1, which corresponds to 1 to 10% of the estimated total production rate of sodium on the planet.     

Study of the unknown hemisphere of mercury by ground-based astronomical facilities

The short exposure method proved to be very productive in ground-based observations of Mercury. Telescopic observations with short exposures, together with computer codes for the processing of data arrays of many thousands of original electronic photos, make it possible to improve the resolution of images from ground-based instruments to almost the diffraction limit. The resulting composite images are comparable with images from spacecrafts approaching from a distance of about 1 million km. This paper presents images of the hemisphere of Mercury in longitude sectors 90°–180°W, 215°–350°W, and 50°–90°W, including, among others, areas not covered by spacecraft cameras. For the first time a giant S basin was discovered in the sector of longitudes 250°–290°W, which is the largest formation of this type on terrestrial planets. Mercury has a strong phase effects. As a result, the view of the surface changes completely with the change in the planetary phase. But the choice of the phase in the study using spacecrafts is limited by orbital characteristics of the mission. Thus, ground-based observations of the planet provide a valuable support.