UPDATE OF MMA SOLAR SYSTEM WORKING GROUP REPORT

F.P. Schloerb, B. Butler, M. Gurwell, A. Lovell, D. Muhleman,
P. Palmer, & I. de Pater

20Jul1998


KEY AREAS OF PLANETARY SCIENCE FOR THE MMA

The MMA's principal strength, compared to present instruments, lies in
its ability to produce high quality, high resolution images rapidly, both
in the continuum and in scientifically interesting spectral lines. These
capabilities are critical to solar system studies where the objects vary
on all time scales and where studies of these variations and their origins
are central to the basic physical processes involved. Sub-arcsecond images
of the planets with the MMA will achieve better angular resolution than
HST at visible wavelengths and 10m class telescopes operating in the
thermal infrared with diffraction limited performance. These capabilities
are far beyond any existing millimeter-wave instrument, and it is easy
to identify several areas where the MMA will have a major impact.

Planetary Atmospheres 

Millimeter and submillimeter wavelength observations of planetary
atmospheres allow their temperature structure and molecular composition
to be investigated. The spectral and imaging capabilities of the MMA
will provide several means to sound the thermal profiles of planetary
atmospheres using broad bandwidth opacity sources as well as specific
planetary spectral lines. With appropriate choices of spectral lines,
planetary wind systems may also be directly measured.

It is important to stress that the use of the millimeter-wave band presents
opportunities for significant atmospheric measurements that can only be
addressed by the combination of MMA sensitivity and spatial resolution.
Millimeter-wave spectral features are an outstanding probe of upper
atmospheres and the ability to resolve the pressure broadened line profile
to very high altitudes is unique to millimeter-wave work. The high spatial
resolution of the MMA will allow measurement and analysis of meterological
and climatic variations in temperature and species abundances on spatial
scales of 50 to 100 km or less on Venus and Mars, which is comparable to
regional weather scales. Moreover, with very high angular resolution, the
limbs of the planets can be directly imaged to gain maximum sensitivity to
trace atmospheric species. Many of the key scientific questions relating to
these gaseous species are best addressed by observing the manner in which
the abundance changes in response to diurnal or seasonal changes or other
dynamical processes at work in the atmosphere. Thus, the MMA's rapid imaging
capability is an important part of its design for this class of experiment.

In addition to ``planetary'' atmospheres, the MMA will open up the study of
the atmospheres of the satellites of the outer solar system. Millimeter-wave
spectral lines from the atmospheres of Titan and Io, for example, have now
been detected. The MMA's sub-arcsecond imaging capability would enable these
objects to be resolved so that latitudinal variations could be explored. In
the atmosphere of Titan, for example, imaging of the disk in the CO and HCN
lines would provide a means to sound the temperature of the atmosphere over
the whole satellite, enabling the study of its general circulation and
response to seasonal effects.


Comets

MMA imaging of molecular species in the comae of comets will directly
address questions of the nature and origin of comets as well as the
physical processes involved.  Millimeter and submillimeter spectroscopy
of comets with existing telescopes has discovered most of the 24 known
molecular species, including HCN, HNC, CH3OH, H2CO, HCOOH, HNCO, NH2CHO,
HCOOCH3, CH3CN, H2CS, OCS, SO, SO2 and H2S.  Since most of these important
species are best detected in the submillimeter bands, we expect that the
MMA will be a key instrument for the study of comet composition and the
evolution of gaseous species from the nucleus as the comet approaches
the Sun.  It will also be possible for the MMA to measure the continuum
radiation from large dust particles in the coma, providing a means to 
explore the refractory component of the comet's composition.

The high angular resolution and high sensitivity of the MMA will enable
sensitive searches for other, heavier, species in the inner coma and
extend our knowledge of the chemical makeup of comets. For example,
the IRAM interferometer was used in this way to sucessfully detect
several new molecules in Comet Hale-Bopp.  Imaging will also enable the
chemistry of the coma to be explored as a function of distance from the
nucleus in order to constrain the processes at work. Images of HCO+ in
Comet Hale-Bopp, obtained at the FCRAO (single dish) and BIMA, enabled
researchers to probe the formation mechanism of HCO+ in the coma as well
as study the interaction of the cold molecular plasma with the solar wind.
Finally, images of HNC obtained at OVRO demonstrate convincingly that
this molecule is formed in the coma of the comet, rather than simply
originating from the ices in the nucleus.

MMA images of molecular line emission from coma gas and continuum emission
from dust will also enable study of structures in the inner coma, such
as fans and jets. For example, recent observations of Comet Hale-Bopp at
the IRAM interfermoeter showed interesting signatures of jet activity in
the spectral line shape of some molecules as the comet nucleus rotated.
Jet structures owe their origin to the physical structure of the nucleus
and arise from small regions in the nucleus, and studies of their
composition may enable the chemical heterogeneity of the surface to
be explored. Jet studies in turn rely on knowledge of the physical and
rotational state of the nucleus. The MMA will enable direct detection
of the nucleus both before and during outgassing, and through studies
of the nuclear ``lightcurve'' its rotational state can be determined.

Finally, we note that the direct detection of the nucleus enables accurate
measurements of its position. Astrometry of comets at optical wavelengths is
significantly biased by the fact that the center of light of the comet is
not necessarily the center of mass of the object. This distinction is
significant for the interpretation of cometary orbits and their evolution
due to ``non-gravitational'' forces on the nucleus caused by anisotropic
outgassing from the surface. An understanding of these forces is important
both for learning about the physical processes at work and for what their
effects can tell us about the mass of the nucleus.

   
Near Earth Objects 

The NEOs have become an important area of study, both for what
they can tell us about the nature of primitive objects and for their
potential as earth impact hazards. The MMA will enable sensitive studies of
the thermal emission from these objects. Moreover, astrometric observations
with MMA can be carried out with greater accuracy than is possible with
visible wavelength systems. Thus, the MMA will enable better orbits to be
predicted and enhance the ability both to recover the object in future
apparitions and to predict the probability of impact with the Earth.
Finally, we note that the potential development of powerful MM-
transmitters would enable high resolution radar images of these objects to
be constructed using the MMA as a receiving station for radar beams
transmitted from another location.

Thermal Emission from Solar System Bodies

The broad bandwidth of the MMA, combined with its excellent imaging
quality, will allow very sensitive continuum studies of thermal emission
from planetary surfaces, such as Mars, Mercury, the Galilean satellites,
the Pluto-Charon system and the largest asteroids. In addition,
many small objects, including hundreds of asteroids and comet nucleii
and even the small and distant objects of the inner Kuiper belt will
become detectable with the MMA's sensitivity. The thermal emission of
planetary surfaces is directly related to surface and immediate subsurface
temperatures, which exhibit strong variations with solar incidence angle.
Mapping of variations in temperature can be used to measure the thermal
characteristics of the surface and subsurface and constrain the physical
processes at work.


COMMENTS ON MMA CAPABILITIES

Submillimeter Capability

Submillimeter observations with the MMA will be extremely important
to planetary science.  Planetary atmospheres and bright comets have
sufficiently high densities to thermalize all interesting lines, and under
these conditions, pressure broadened spectral line intensities grow as the
cube of the frequency. Thus, the ability to search for spectral features
at the highest frequencies can lead to more sensitive measurements of
trace molecular species. Moreover, some important species, such as HCl
and other hydrides, are only observable in the sub-millimeter band.

An additional factor favoring the development of the sub-millimeter band
is the extremely broadband opacity sources which form the continuum of
planets with thick atmospheres. The optical depth due to these sources
increases strongly with frequency (generally as frequency-squared)
in the millimeter and submillimeter regime). Therefore, addition of
sub-millimeter continuum observations of planetary atmospheres (such
as Venus, Titan, and the giant planets) to millimeter-wave observations
will enhance our ability to probe a range of levels in these atmospheres
and construct maps of the vertical temperature profile.

Antenna Size

Most planetary science objectives for MMA are not limited by the total
collecting area.  Rather, the most important requirement is for
fast, high quality imaging.  Pushed to its extreme, this requirement
would favor designs which utilize larger numbers of smaller antennas in
order to achieve the best UV coverage and the widest field of view. The
current plan for 10m antennas can only be considered to be marginally
acceptable in this context since the single antennas will resolve all
mapable planetary objects significantly.

Imaging Requirements

* High Quality Imaging:  The MMA must be designed and built to
achieve accurate and reliable images within a short period of time. A
specification of quality, which is sometimes called fidelity in imaging,
may be stated quantitatively by requiring that the difference between
the image formed by the instrument and a true image of the source be
less than 1 per cent. We note that this differs significantly from
the more typically defined quantity of the dynamic range of the image,
which represents the weakest detectable feature in the presence of a
strong feature but makes no statement about the reliability of the map
of a complex, extended source.

The requirement for high quality imaging is very important for allowing
accurate comparison of the brightness in different regions of maps, and
exceedingly important for allowing accurate comparison of maps made at
different times, in order to measure and quantify temporal changes in
temperature and/or species abundances. It is important to remember that this
``high quality imaging'' requirement pertains to observing situations that
will be uncommon for the MMA. Planetary brightness temperatures are very
high (100K - 300K), and the variations that are interesting to observe are
at the brightness level of one or two Kelvin. Thus, detection of this
brightness level is not the issue. Rather, the concern is for developing the
ability to make accurate maps in which 1% features may be distinguished and
believed.

* Fast Imaging: Most of the objects to be studied in the solar system are
time variable in their appearance at millimeter wavelengths. Understanding
the time variability is key to the scientific investigation, both to
avoid confusing temporal variations with some physical structure and
for using the temporal variations to find out about the systems. The
MMA must be capable of producing high quality images on a time scale of
minutes in order to achieve this objective.

* Sub-arcsec Imaging: The ability to obtain images with comparable (or
better) resolution to HST imaging is key to many of the scientific
projects for MMA in planetary science. Indeed, one need only consider
the impact of HST images of many solar system objects to appreciate
MMA's imaging potential. For example, Neptune, whose dynamically
active atmosphere has been the target of much HST work, could be
probed vertically using millimeter-wave spectral lines at the spatial
resolution of the HST. The combination of such data sets would be quite
exciting as a way to probe the origin and dynamics of the features in
the atmosphere. Finally, in addition to the study of specific features
on the planets, sub-arcsec imaging opens up many new worlds to detailed
investigation by permitting imaging of the large outer solar system
satellites, and spatially resolved work on many new targets, including
the largest asteroids.

Other Special Requirements

* High Performace Passband: The need for high quality imaging will
necessarily require accurate calibration of the passband. This will most
likely mean internal calibration of some sort, as traditional passband
calibration against an astronomical source will be inadequate in most
cases, given the very strong millimeter-wave fluxes of the planets
relative to quasars, etc. The passband should be accurate and stable
to within 0.1% ideally, and certainly within 1% at worst. In some sense
this may be the limiting factor in the use of the MMA for spectral line
work on the planets, where we would hope to see small absorption and
emission features in the presence of a very strong continuum.

* A Flexible Correlator:  Although the needs of planetary science
for the correlator are quite diverse, the range of bandwidth and
resolution options described in the current MMA plan appear to be
satisfactory. Planetary spectral lines in thick atmospheres may have
widths of several GHz, corresponding to the wide bandwidth options that
have been discussed. For low pressure atmospheres like those of comets
and Jupiter's satellite Io, the line width is essentially thermal, and
the requirements are comparable to the narrow band options required for
observing, e.g., molecular cloud cores.

In the future, it is expected that millimeter-wave radar will be developed
for planetary applications. Capabilities already exist at 30 GHz, both at
NASA and in the military, and development of even higher frequency radars is
ongoing. We assume that this capability will be developed, and it is likely
that the MMA will eventually be used as a receiving station in much the same
way that the VLA is now used in conjunction with the Goldstone Planetary
Radar. Thus, a possible ``special'' case for the correlator would be to try
to accomodate radar signal processing. One way that this could be done would
be through a simple correlator mode with very high resolution (say 10 Hz)
and a narrow bandwidth that would enable measurement of the echo and its
polarization properties. We believe that this special mode deserves some
consideration, as it is our understanding that it could have been
accommodated in the VLA correlator with little impact on the design if it
had been part of the original specifications.

* Tracking and Pointing: Solar System objects do not move at the sidereal
rate, and the MMA software needs to accomodate ephemerides for the
planets themselves as well as for comets and asteroids. Moreover, solar
system objects cannot always be observed ``away from'' the Sun. Comets,
NEO's, and the inner planets often require observations close (within
a few degrees) to the Sun.