The Terrestrial Planets

Mercury

In spite of early space probes, Mercury has retained its distinction as the most difficult to study of the terrestrial planets. Two of the outstanding puzzles can be addressed with the enhanced VLA: the reason for the presence of its magnetic field and its remarkably uniform surface composition.

Mercury's magnetic field apparently requires an active dynamo, which implies the presence of a molten core, yet Mercury is small enough that heat loss should have frozen out the core long ago, as has occurred on the Moon. Various ideas have been offered to resolve this apparent paradox, including (1) an unusually high sulfur content (which lowers the core's freezing temperature), (2) an unusually high abundance of radioactive elements (which adds enough heat to maintain a liquid core), and (3) some means of shutting off volcanic heat piping to the surface (which robs the core of its principal heat-loss mechanism). One of the key (and few) observational constraints on Mercury's internal structure is the net heat flow at the surface, which can in principle be measured with microwave observations of the planet's thermal emission at a wide variety of wavelengths.

A recent attempt to measure this heat flux utilized Mariner 10 IR radiometer measurements of Mercury's night hemisphere and VLA thermal images in the 1.4, 4.9, 8.4, 15, and 22.5 GHz. The microwave spectrum is remarkably flat (Fig. 2.4), which at face value would correspond to a very small heat flux; however, it is quite possible that the 1.4 GHz observations are probing beneath the regolith and into the ``megaregolith'', a region of highly fragmented rocks and boulders, where the physical parameters (microwave opacity, thermal conductivity, etc.) are very different. Thus, the 1.4 GHz observation may not be useful in constraining the heat flux unless we can understand the nature of the regolith structure at several meters depth.

  
Figure 2.4: A 1.3cm map of Mercury (left). The contour levels are 10% of the maximum intensity except for the lowest contour, which is 2%. A residual map obtained by subtracting a model map from the observed map is shown on the right. Contour intervals are 5 K. Thermal depressions at both poles and along the sunlit side of the morning terminator are likely due to shadowing by surface topography.

Measurements at additional frequencies would help considerably. A 2.4 GHz measurement might avoid the megaregolith interface (which seems to be affecting the 1.4 GHz observations), yet provide enough wavelength leverage (between 2.4, 4.9, and 8.4 GHz) to detect the heat flux in a region where the regolith structure is reasonably well understood. Alternatively, measurements at 30cm, 50cm, and 90cm (1000, 600, and 333 MHz, respectively) might help to elucidate the nature of the regolith structure at several meters depth and allow one to utilize the longer wavelengths to constrain the heat flux. Detectability is not a problem at 30cm and 50cm with the nominal upgrade sensitivities. Sensitivity starts to become a problem at 90 cm, where only a marginal result could be obtained with the current VLA. However, a measurement could be obtained in 12 hrs with the upgraded 90cm sensitivity. The planet can be at least marginally resolved ( pixels) at wavelengths shorter than 30 cm with the A configuration, but becomes a point source at 90cm.

Several independent lines of evidence (high optical albedo, very low microwave opacity, IR spectroscopy) indicate that Mercury's surface has a very low Fe content. This is very surprising, since conventional models of the planet's evolution incorporate extensive volcanism, which is expected to bring iron-rich, basaltic magma to the surface (as has occurred on the Moon). Yet the entire surface of Mercury appears to be an extreme example of the lunar highlands. Even more surprising is that Mercury's smooth plains are only slightly darker than the Mercurian ``highlands'', and existing VLA microwave images exhibit no evidence for an enhanced opacity (and hence iron content) within the Caloris basin.

It would be very interesting to actually measure or at least put a much more stringent upper limit to possible microwave opacity variations across Mercury's surface. The only existing image at 22.5 GHz shows no evidence for opacity variations. While it is possible to do a somewhat better job with the existing VLA simply by getting a longer integration in better observing conditions; a much better approach would be to use the 40-50 GHz band, which is much more sensitive to opacity variations because it (1) probes shallower layers where temperature gradients are stronger, (2) provides higher spatial resolution which might reveal opacity variations on smaller spatial scales, and (3) provides three times more SNR than the 22.5 GHz band using nominal parameters of the enhanced VLA. Note that simply using existing millimeter interferometers could not produce images anywhere near as good as the enhanced VLA (27 antennas with 40-50 GHz receivers) - image quality is crucial for such an experiment.

Mars

Because of Mars' rapid rotation, day-night temperature variations penetrate only a shallow -cm-deep ``diurnal layer'', which has an optical depth of at K-band, increasing to at Q-band. Observations of diurnally modulated temperature variations should provide an excellent probe of the thermal and electrical properties of the Martian surface. Bistatic Goldstone-VLA radar observations at 3.5cm have identified a ``stealth'' region with very low reflectivity, which implies a low bulk density and a high microwave emissivity. Thermal measurements at several wavelengths would provide important physical constraints on this region, such as its depth. (Arecibo-VLA radar is another important piece of this investigation - see Section 2.1.) To take full advantage of spatial resolution, one needs the ability to make snapshots of Mars in order to avoid ``smearing'' caused by the planet's rotation. Sensitivity is not a problem for 1-hour snapshots, but to get high quality images, good UV coverage is very important, which again argues for 27-element capability in the 40-50 GHz band.

Water vapor has been observed in the atmosphere of Mars with the VLA. Because the 1.35cm HO line has a rather small optical depth in the terrestrial atmosphere, it is the most convenient line with which to monitor HO in the Martian atmosphere. The amount of water vapor in the Martian atmosphere varies with the Martian seasons. The VLA observations would be improved by improved sensitivity in the 22.5 GHz band.