Since the completion of the Arecibo upgrade, the new radar system has been used for a wide range of solar system studies. Radar images reveal a wealth of information about the shapes and surface properties of solid bodies in the solar system. The Arecibo telescope has collected data on Mercury, Venus, Jovian satellites, and Saturn's rings and satellites, and numerous asteroids and comets. Some recent results are summarized in these figures.
A major component of the upgraded telescope is the installation of a new 1 megawatt 12.6 cm radar system, which has been jointly funded by NASA and the NSF. This instrument will enable the Observatory to detect asteroids over a very much wider radial range than with the old 420 kilowatt system. We anticipate as much as a factor 40 improvement in sensitivity compared to before the Gregorian Upgrade (and in no case less than a factor of 14). Some of the improvement comes from the increase in transmitter power, some from the optics of the telescope, and some from the greatly increased zenith angle range with constant sensitivity that the new ground screen provides.
Available equipment includes a 12.6 cm (2380 MHz) dual-polarization maser receiver with the capability of both complex voltage sampling and a new, high-throughput, radar decoder. Sampling has a limit of 10 MHz, while the decoder can operate at up to 20 MHz (7.5m range resolution).
One of the major recent successes in the field of planetary radar
astronomy was the discovery and mapping of deposits of volatiles on the
shadowed floors of impact craters at the poles of Mercury. Because of
the similarity of their radar scattering properties to those of icy
surfaces in the solar system, the deposits are thought to be water ice.
The figure shows the most recent Arecibo 12.6 cm wavelength radar image
of the north polar region of Mercury made at a resolution of 1.5 km by
J. Harmon (NAIC), P.Perrilat (NAIC) and M. Slade (JPL). It was
published in the January, 2001 issue of Icarus. The donut shape of the
deposits close to the pole are due to the presence of central peaks in
the craters while the bright arcs away from the poles coincide with the
shadowed areas for these craters. A detection of the "ice" at the north
pole was also obtained with the Arecibo 70 cm wavelength radar
indicating that the deposits may be at least several meters thick.
Arecibo S-band radar image of the north polar region of Mercury by
J. Harmon, P. Perrilat, and M. Slade. The resolution is 1.5 km and the
image measures 450 km on a side. The bright features are thought to be
ice deposits on permanently shadowed crater floors.
Radar images of Venus were obtained using the Arecibo
radar system in August, 1999, and again in March/April, 2001. The
2001 observations included the first scheduled use of the new Robert C.
Byrd 100-meter Green Bank Telescope. The Arecibo Observatory and
the GBT together serve as a
radar interferometer with the objective of creating high resolution topographic maps of Maxwell Montes at spatial resolutions adequate to determine the
relationship between areas of high reflectivity/low emissivity and
altitude. This work has been carried out by D. Campbell (Cornell/NAIC), J.-L.
Margot (Caltech), L. Carter (Cornell), B. Campbell (NASM) and J. Dorris
(Cornell). Other objectives include looking for surface features which
had changed since the Magellan mission, and to obtain images in the
four Stokes parameters to look at the polarization properties of the
surface. Of particular interest are the floors and ejecta deposits of
impact craters and the high reflectivity/low emissivity surfaces found
at high elevations. Linear polarization of 10 to 40 percent
is found in several areas of the maps. Areas with high linear
polarization seem to be associated with radar-bright regions near
The delay-Doppler images below were obtained by transmitting
continuously from Arecibo Observatory and receiving the radar echoes at
the 100 meter Green Bank Telescope. This SC image (same sense circular
polarization as that transmitted) of the front part of Venus has been
spatially averaged to approximately 5 km resolution.
This SC image shows a portion of Maxwell Montes, spatially
averaged to 1.2 km resolution.
Radar imaging of near-Earth asteroids can provide dramatic images
with resolutions down to 8 m, comparable to the images obtained by the
Galileo and NEAR-Shoemaker spacecraft. The rotation rate, shape and
reflectivity give us information about the asteroids' density, internal
structure, and surface properties. The images also show surface
features such as impact craters, and irregularities, which can often be
traced across the surface as the object rotates. A series of images can
be used to derive a three-dimensional shape model. In the case of 216
Kleopatra, shown below, that shape can be highly irregular, and is a
strong constraint on formation mechanisms. The shape of some very small
objects is surprisingly spherical, which suggests a rubble pile with no
internal strength. Two near-Earth binary asteroid systems have now been
discovered by radar. Both 2000 DP107 and 2000 UG11 were determined to
be binary systems from a combination of Goldstone and Arecibo radar
observations. These close pairs of asteroids must be recently formed,
perhaps by tidal forces on a previous close approach to the Earth. The
lifetime of such a binary system against collisional disruption is
quite short. Main-belt asteroids have also been studied, and images
with resolution comparable to those obtained by the Hubble Space
Telescope have been obtained.
Asteroid 2000 UG11 is the second near-Earth binary asteroid known. The primary, in the upper part of the image, is about 230m diameter. The secondary, the bright dot below, is about 100m, and separated by at least 300m from the primary. The orbital period is 19.5 +/- 1.5 hours. This image of 2000 UG11 was obtained on 2000 Nov 7. Range increases downwards, delay increases to the left. The range resolution is 15m per pixel. The cross-range resolution is approximately the same, but depends on the viewing geometry, which is not yet completly determined.
GQ2 was imaged by the Arecibo radar system during April, 2001 by M.
Nolan. This sequence of images shows the asteroid appearance during
about 1.5 hours on 29 April. The first image is in the upper left,
with time increasing across and then down. Each image is a sum spanning
about 12 minutes. The image resolution is about 15m, and the asteroid
is estimated to be 400m in diameter. In each image, range increases to
the right, and doppler frequency increases downward. CCD images
obtained by M. Nicolini, using a 0.4m telescope at the Observatory of
Cavezzo, showed that the rotation period was longer than 8 hours (no
maximum or minumum in 2 hours). The radar data confirms that the
rotation period is probably about 10 hours.
images of main-belt asteroid 216 Kleopatra were obtained in November,
1999, about one hour apart (Ostro et al 2000, Science). The top row are
the data, which are delay-Doppler images with range increasing down,
frequency to the right. The middle row are simulated radar data, using
the shape model derived. The bottom row shows the shape model, as it
would appear illuminated in the plane of the sky. The model was derived
from 12 images on four days, together with a diameter from an earlier
stellar occulation. The size bar in the lower left is 100 km.
Other interesting Asteroid radar links:
NASA/Jet Propulsion Laboratory's asteroid radar page
Scheduled Arecibo Radar Asteroid Observations
Comet C/2001 A2 (LINEAR) was discovered as part of the Lincoln Laboratory Near-Earth asteroid survey (LINEAR) on 16 January 2001. Observing conditions were favorable at Arecibo July 5-15 2001, and observing time for both spectroscopy of 18-cm OH lines and S-band radar observations were arranged. The comet nucleus was observed to split into two pieces on April 30. Component A broke into two pieces May 16, and faded away within a week. Component B, now C/2001 A2-B LINEAR, continued with nearly the original predicted brightness. Three additional fragments were seen to separate from A2-B in early June, at which time there was an increase in gas and dust emission.
The Arecibo observations of the OH lines took place 5-6 July 2001.
The OH in the cometary coma absorbs solar radiation, which can be
re-emitted as microwaves under favorable conditions. The emission
strength depends on the radial heliocentric velocity of the comet and
on the amount of OH in the coma. The 1612, 1665, and 1667 MHz OH lines
were observed, but only the 1667 MHz line was detected. In addition to
looking at the nucleus, we also observed one beam width away (4.1
arcmin), in a hexagonal pattern around the nucleus, oriented along the
sun-tailward direction. The figure shows the nucleus spectrum and the
sum of all six of the offset positions. The strengths of these lines
will be compared to model predictions to determine the outflow
velocity, identify asymmetries, and to search for signs of collisional
quenching. In very active comets, such as Hale-Bopp, the OH lines were
thermalized or ``quenched'' in the inner 500,000 km of the coma. One of
the goals of this investigation is to determine the extent of quenching
in more ``normal'' comets with lower gas production rates.
The spectrum of Comet LINEAR A2-B shows the 1667 MHz OH line in emission.
Because the comet passed within 0.25 AU of the Earth in early
July, when it was visible from Arecibo Observatory, it was fairly well
suited to radar observation. We detected the comet with the S-band
(2380 MHz) radar in continuous wave (CW) mode on July 7-10. The
spectrum shows a broad profile of return echo power from the cometary
coma. There is no clear central spike, indicating return from the
nucleus, contrary to what has been seen on other comets (Harmon et al
1999). Some return was also seen in the unexpected or same-sense
polarization, at 20% of the expected sense. This indicates that
the particles in the coma are comparable in size to the radar
wavelength (12.6 cm). Given the fragmentation of this
comet in the previous months, it may not be surprising to find
large-sized debris in the coma. The gas and dust production from this
comet is close to the prediction for a several-km nucleus. We
attempted to image the comet on July 9 and 10, and those data are still
This CW echo from Comet LINEAR A2-B shows a broad return from the coma, in both opposite-circular (upper plot) and same-circular (lower plot) polarization. This indicates that the particles are comparable in size to the radar wavelength (12.6 cm) No clear return from the nucleus is seen.
A program of delay-Doppler mapping of the Galillean satellites is
underway by L. Harcke and colleagues at Stanford University. The
surfaces of Europa, Ganymede and Callisto have been imaged in order to
determine the surface properties and look for different radar
properties of the various terrain types. These maps will be correlated
with the Galileo images of the satellites.
This image is a SC radar albedo map of the western hemisphere of Callisto. An ambiguity exists between the northern and southern halves of the planet due to the range-Doppler imaging technique employed, thus the image appears symmetric about the equator. A projection effect causes the resolution cells to be long and thin nearer to the equator. Signal to noise ratio is strongest near the sub-Radar point in the center of the image, and decreases moving towards the poles and the limb. The feature marked with a red X is several standard deviations above the noise level. A study correlating this object with labeled features in the Galileo mission optical image archive is ongoing.
In fall of 1999 and 2000, the rings of Saturn were imaged using the
Arecibo S-band radar system by P. Nicholson, D. Campbell, R. French,
G. Black and J.-L. Margot. The image shown is a sum of 5 days of
dual-circular polarization data, co-adding both polarizations. The
Keplerian velocity profile of the rings results in the outermost or A
ring which provides the earliest echo, appearing at a lower Doppler
shift than the middle or B ring. This leads to four bright
``crossover'' regions on either side where signal from the two
different rings add together, analagous to the north/south ambiguity
for radar imaging of rigidly rotating bodies. A pronounced azimuthal
asymmetry can be seen: the rings are brighter on the far quadrant on
the receding (western) ansa than on the near quadrant of the
approaching (eastern) ansa. The most widely accepted explanation of the
asymmetry involves gravitational ``wakes'' generated by individual
large ring particles, which are distorted by Keplerian shear into
elongated structures trailing at angles of about 70 degrees from the
radial direction (e.g. Franklin and Colombo 1978).
delay-Doppler image of Saturn's rings at a frequency of 2380 MHz (12.6
cm) is compared to a model image constructed by reprojecting a pair of
HST images taken at 439 nm. Time delay increases from bottom to top,
and Doppler shift increases from left to right. The effective spatial
resolution is 2000 km by 15000 km.
Questions or comments? Contact Ellen Howell. (firstname.lastname@example.org)