Thomas H. Morgan

Discovery of a moon orbiting the asteroid 45 Eugenia

Evidence for asteroidal satellites (moons) has been sought for decades, because the relative frequency of such satellites will bear on the collisional history of the asteroid belt and the Solar System, yet only one has been detected unambiguously. Here we report the discovery of a satellite of the asteroid 45 Eugenia, using an adaptive optics system on a ground-based telescope. The satellite has a diameter of about 13 km, and an orbital period of about 4.7 days with a separation of 1,190 km from Eugenia. Using a previously determined diameter for Eugenia, we estimate that its bulk density is about 1.2 g cm-3, which is similar to that of the C-type asteroid Mathilde. This implies that Eugenia, also a low-albedo C-type asteroid, may be a rubble pile, or composed of primitive, icy materials of low bulk density.

Variation of lunar sodium during passage of the Moon through the Earth's magnetotail

We measured sodium emission above the lunar equator over a range of lunar altitudes from 100 to 4000 km. The measurements were repeated approximately every 24 hours from June 7 to 16, 1998, covering the period during which the Moon passed through the Earths magnetotail. Sodium temperatures derived from the altitude dependence of emission intensity ranged from 1200 to 2900 K. This result supports the view that photodesorption is a primary source of sodium in the exosphere since the most probable temperature of sodium from this source is in this range. Passage of the Moon through the Earths magnetotail (where solar wind is essentially absent) affected the sodium density, such that it was higher before the Moon entered the Earths magnetotail than after the Moon left it. This suggests that the solar wind plays a role in production of lunar sodium. We propose that its function is to mobilize sodium and bring it to the surface, where photodesorption can eject it into the exosphere. A two-step process such as this could help to explain the latitude dependence of sodium density, which varies as the second or higher power of cosine latitude.

Spatial distribution of sodium vapor in the atmosphere of Mercury

Abstract The density and shape of the sodium exosphere of Mercury can give clues to the interactions of cosmic dust, the solar wind, and the radiation field with the planetary surface and magnetosphere. In past observations, A.E. Potter and T.H. Morgan, (1987, Icarus 71, 472–477) establish that radiation pressure affected the sodium density. In this work, we find, as before, that the disk-averaged sodium column density is correlated with the magnitude of radiation pressure in such a way that the maximum column density occurs at minimum radiation pressure and vice versa. However, superimposed on this orderly variation is a disorderly variation on a time scale less than a day, and we find anomalous distributions of sodium vapor in polar and subsolar regions. These effects may be a result of solar wind-magnetosphere interactions. Observations of north-south and east-west distributions of sodium emission from Mercury were made at three levels of radiation pressure: one corresponding to near-minimum solar radiation pressure, a second to near maximum, and a third at an intermediate radiation pressure. Corresponding north-south and east-west distributions of sodium column abundance were calculated using radiative transfer theory, and were fit to the observations by varying the assumed density and distribution of sodium vapor. The main features of the observed intensity distributions could be adequately represented by simple and reasonable models for the temperature and surface distribution of sodium vapor. For this limited data set, the calculated subsolar abundance of sodium at minimum radiation pressure was about three times larger than that at maximum radiation pressure, which is approximately consistent with previous observations. However, it was found that significant details of the observed distributions could not be fit by a simple model. The observed sodium emission distribution was concentrated in the sunward direction with polar enhancements. Several north-south distributions were anomalous in that there was clearly more sodium at one pole than the other. In addition, the sodium distribution varied substantially on a daily basis. The subsolar column abundance varied from 3.8 × 10 11 atoms/cm 2 on April 3, 1988, to 2.8 × 10 11 atoms/cm 2 on April 6, 1988. It is suggested that these anomalies may be related to the interaction of the photoionized sodium atoms with the magnestosphere. More detailed observations in the future may better define the respective roles of radiation pressure, solar wind-magnetosphere interactions, source processes, and evaporation of surface material.

Midinfrared spectra of Mercury

Observations of Mercury in the mid infrared (8–12.5 μm region) obtained over a variety of longitudes using the Fourier Transform Spectrometer (FTS) on the McMath-Pierce 1.5 m solar telescope at Kitt Peak reveal low-contrast spectra, except in cases where atmospheric noise or difficulties in guiding the telescope caused spurious signals. Although there are features that are suggestive of emissivity maxima (Christiansen features), their proximity to the telluric absorption band, and the noise in the spectra, preclude their unambiguous interpretation. We see little evidence of reststrahlen bands in our data; however, there are strong indications of transparency features. A shallow emissivity minimum occurs at 12 μm in the spectra centered at 80°, 256°, and 266° Hermean longitudes. A minimum occurs at 12.5 μm in the spectra centered at 15°, and a doublet minimum, with one trough at 12.2 μm and a second trough at 12.4–12.6 μm, is seen in the spectra centered at 229°. These features indicate the presence of a fine powder on the Mercurian surface, and their low spectral contrast suggests a significant percentage of agglutinitic material. On the basis of the position of the transparency features, we conclude that our spectra are indicative of intermediate, mafic, and ultramafic rock types. Further specificity is not warranted by the data.

Production mechanisms for faint but possibly detectable coronae about asteroids

Abstract Asteroidal surfaces are exposed to the same processes which create observable exospheres about such refractory, volatile-free surfaces as those of the Moon and Mercury. Are there modest coronae about asteroids? We have chosen to examine this problem for two species, sodium and water vapor. Sodium was chosen because NaD emission is easily observed from the Earths surface and emissions are observed in the exospheres of both the Moon and Mercury. In addition, the spatial distribution of emission in the sodium D lines about an asteroid would be a direct measure of the velocity distribution of the sodium source, which cannot be measured directly in the case of Mercury or the Moon. This provides a new technique for assessing the possible water content of asteroids. There is now very good evidence that water is present in hydrates and other aqueous alteration products. A detection of an hydroxyl corona about one of these asteroids would be proof of the presence of these aqueous bi-products ; the amount of the water production would a direct measure of the water content of the asteroidal regolith. Water-rich asteroids are of course a potential resource and hold important clues to the evolution of the planets. We have calculated the production of sodium and water (hydroxyl) coronae about asteroids due to just two production mechanisms : sputtering and impact vaporization. We find that for realistic assumptions an asteroid with perihelion near 2 AU would have a small (about 3R) OH corona, while the likely sodium coronae would be about half as bright. These emissions will be extremely difficult to detect from the ground, but they may be detectable from space.

Revived interest in the lunar atmosphere

The recent discoveries of Na and K in the lunar atmosphere are emphasized in a review of lunar atmospheric observations which identifies possible directions of research. Observations are briefly reviewed through the Apollo era which identified the atmosphere as a surface-boundary exosphere, and identified atmospheric constituents included He and Ar which rose and fell by a factor of 20 during the lunar diurnal cycle. Ar-36 and -40 were also identified as were the sources of ionospheric source and sink processes including the solar EUV flux. Na and K are discussed which were identified in a study of the spectroscopic ring effect, and the effects of elemental interaction with the lunar surface can be approximated in laboratory experiments. Observations and measurements of the lunar surface are important for the continued investigation of lunar and other planetary mechanisms.

Implementing a New Line of Medium‐Class, Unmanned Space Exploration Missions

The NASA Office of Space Science will soon be releasing the first Announcement of Opportunity to propose new, medium‐class, missions for Solar System Exploration. This new mission line, called New Frontiers, has been created in order to allow greater access to a larger variety of targets in our Solar System. Missions may be of larger scope than the current Discovery program missions; proposed costs for building, launching, operating, and analyzing data from the New Frontiers missions may be as large as

Discovery of calcium in Mercury's atmosphere

650M (Fiscal Year 2003 dollars) and proposed launch date may be up to 47 months after development is initiated. New Frontiers missions may use radioisotope power sources (e.g. RTGs) and larger launch vehicles than allowed for Discovery missions, due to the greater allowed cost. Each proposal opportunity will be focused on proposals for investigations that address most if not all of the science objectives of one or more of the “Medium Class” investigations identified in the recent National Academy Planetary...