Radar imagery of Mercury’s putative polar ice: 1999–2005 Arecibo results
John K. Harmona,*, Martin A. Sladeb, Melissa S. Ricec
aNational Astronomy and Ionosphere Center, Arecibo Observatory, HC3 Box 53995, Arecibo, PR 00612, USA. Tel: (787)-878-2612; Fax: (787)-878-1861; Email: [email protected]
bJet Propulsion Laboratory, California Institute of Technology, MS 238-420, 4800 Oak Grove Dr., Pasadena, CA 91109, USA. Tel: (818)-354-2765; Fax: (818)-354-6825; Email: [email protected]
cDept. of Astronomy, Cornell University, Ithaca, NY 14853, USA. Tel: (607)-255-4709; Fax: (607)-255-6918; Email: [email protected]
Submitted to Icarus: June 3, 2010
Revised: August 12, 2010
Accepted: August 18, 2010
36 manuscript pages
5 tables
7 figures Proposed running head: Mercury poles: Radar imagery
Send correspondence and proofs to:
John K. Harmon
Arecibo Observatory
HC3 Box 53995
Arecibo, PR 00612
Email: [email protected]; Tel: (787)-878-2612 x284; Fax: (787)-878-1861
2 Abstract
We present an updated survey of Mercury’s putative polar ice deposits, based on high- resolution (1.5-km) imaging with the upgraded Arecibo S-band radar during 1999–2005. The north pole has now been imaged over a full range of longitude aspects, making it possible to distinguish ice-free areas from radar-shadowed areas and thus better map the distribution of radar-bright ice. The new imagery of the south pole, though derived from only a single pair of dates in 2005, improves on the pre-upgrade Arecibo imagery and reveals many additional ice features. Some medium-size craters located within three degrees of the north pole show near-complete ice coverage over their floors, central peaks, and southern interior rim walls and little or no ice on their northern rim walls, while one large (90 km) crater at 85°N shows a sharp ice-cutoff line running across its central floor. All of this is consistent with the estimated polar extent of permanent shading from direct sunlight. Some craters show ice in regions that, though permanently shaded, should be too warm to maintain unprotected surface ice owing to indirect heating by reflected and reradiated sunlight. However, the ice distribution in these craters is in good agreement with models invoking insulation by a thin dust mantle. Comparisons with Goldstone X-band radar imagery indicate a wavelength dependence that could be consistent with such a dust mantle. More than a dozen small ice features have been found at latitudes between 67° and 75°. All of this low-latitude ice is probably sheltered in or under steep pole-facing crater rim walls, although, since most is located in the Mariner-unimaged hemisphere, confirmation must await imaging by the
MESSENGER orbiter. These low-latitude features are concentrated toward the “cold longitudes,” possibly indicating a thermal segregation effect governed by indirect heating.
3 The radar imagery places the corrected locations of the north and south poles at 7°W,
88.35°N and 90°W, 88.7°S, respectively, on the original Mariner-based maps.
Keywords: Mercury; Mercury, surface; Radar observations; Ices
4 1. Introduction
It has been nearly two decades since radar observations at Goldstone/VLA (Very Large
Array) and Arecibo Observatory revealed radar-bright features at Mercury’s north and south
poles that suggested the existence of polar ice deposits (Slade et al., 1992; Harmon and
Slade, 1992; Paige et al., 1992; Butler et al., 1993; Butler, 1994). The early Arecibo
delay-Doppler imagery of the poles, obtained at 15-km resolution, traced the radar features to
the interiors of impact craters and thus provided strong support for the presence of frozen
volatiles in shaded polar cold traps (Harmon et al., 1994). Accessing this ice with Earth-
based radars is made possible by Mercury’s large (7°) orbital inclination to the ecliptic,
which presents Earthward tilts of the poles of up to 12° and allows radar illumination of
crater interiors that (because of Mercury’s near-zero rotational obliquity) are permanently
shaded from direct sunlight.
In the mid-1990s the Arecibo telescope underwent a major upgrading that substantially
improved the sensitivity of the S-band (2380-MHz;λ 12.6-cm) radar. One of the early achievements with the upgraded radar was the re-imaging of Mercury’s north pole at a much finer (1.5–3 km) resolution. These 1998–1999 observations revealed many additional north polar “ice” features, including some at relatively low latitudes, and provided a more detailed picture of the ice distribution within individual craters (Harmon et al., 2001).
In 2000 we commenced a multi-year program at Arecibo to do full-disk, dual-polarization radar imaging of Mercury. Unlike the 1998–1999 observations, which used standard (repeating-code) delay-Doppler, this program employed the so-called “long-code” delay-Doppler method (Harmon, 2002). This technique was specifically designed to eliminate the delay-Doppler aliasing that corrupts radar mapping of “overspread” targets.
5 Although the main benefit of using long-code was cleaner full-disk imaging of non-polar regions (Harmon et al., 2007), the same data were also suitable for imaging the poles.
Furthermore, by adopting a 10-µs pulse width we were able to continue the imaging of the poles at the same 1.5-km resolution as for the 1999 imagery. In 2001 and 2002 we imaged the north pole over a range of longitude aspects but at less-than-optimal sub-Earth latitudes
(= Earthward pole tilts) of 4.5–7.3°N. The observing aspect improved during 2004–2005,
when we were able to make high-quality north polar images at sub-Earth latitudes in the
range 8.8–11.8°N and from a longitude aspect roughly opposite that of 1999. Also, in March
2005 the sub-Earth latitude reached 7.5°S, enabling us to make our first post-upgrade images
of the south pole.
In this paper we present the Arecibo radar imagery of the Mercury poles based on the
1999–2005 observations. (For presentations of the non-polar imagery from this same period
see Harmon et al. (2007) and Harmon (2007)). We combine long-code imagery from
2001–2005 with the 1999 imagery to give an essentially complete survey of the radar-bright
“ice” deposits at the north pole. We also present and discuss the imagery of the south pole
based on the March-2005 observations. We have two main objectives in presenting these
results in this paper. The first is to provide as thorough an inventory as possible of the
putative polar ice deposits and to point out certain results from the updated imagery that may
have implications for the ice hypothesis. The second is to present a data base suitable for
making comparisons with, and enhancing the scientific return from, the anticipated Mercury
pole studies by the MESSENGER and BepiColombo orbiters.
6 2. Data and analysis
The images used in this paper are derived from observations made on 23 dates organized
in 13 groups of contiguous days. A listing of these “date groups” (in order of increasing sub-
Earth longitude) is given in Table 1. Most of the observations were made when Mercury was Table 1 not far from inferior conjunction; the Earth-Mercury distance for the dates in Table 1 was in
the range 0.56–0.82 AU.
All of the observations were made using binary-phase-coded transmission with a 10-µs
baud (synthesized pulse width), which gave a range resolution of 1.5 km. All but the 1999
dates used a long (240−1 element) code, which was effectively non-repeating and thus
suitable for delay-Doppler imaging by the long-code method (Harmon, 2002; Harmon et al.,
2007). A circularly polarized wave was transmitted and the echo was received in both
(orthogonal) circulars. Transmitter power was in the range 780–900 kW for the 1999–2004
observations. In 2005 we ran at reduced power (400–440 kW for the north pole, 680 kW for
the south pole) owing to transmitter problems.
Details of the long-code analysis are as described in Harmon et al. (2007) and briefly
reviewed here. The received echo time series was sampled once per baud and multiplied with
a lagged replica of the code. This lagged-product time series was smoothed and decimated by
a factor of 512 to reduce the Nyquist bandwidth to 195 Hz (twice Mercury’s total Doppler
bandwidth), and then 8192-length blocks were Fourier transformed and squared to produce
Doppler spectra with 0.0238-Hz resolution. The lags were shifted in one-baud steps to build
up the delay-Doppler array. Successive delay-Doppler looks were summed over a full
observing “run” (transmit/receive cycle), and from this array a flat noise spectral baseline
was subtracted at each lag. The run-averaged array was normalized to units of radar cross
7 section, using the long-code calibration procedure discussed in Harmon (2002), and then an
image was formed by doing a mapping from delay-Doppler space to planetary coordinates.
The image pixels were normalized to a dimensionless reflectivity σ 0 (θ ), which is the radar cross section per unit surface area at incidence angle θ . Our spectral (Doppler) resolution of
0.0238 Hz gave a transverse mapping resolution of 1.1 km, which, when combined with the
10-µs delay resolution, gave a mapped image pixel measuring 1.5 ×1.1 km at the pole. (Since the planet rotated by 0.04° of longitude during a given run receive period, the images suffered an azimuthal smearing amounting to 0.6 km at 70° latitude and decreasing as the cosine of the latitude.) The final step in the processing involved summing the run-averaged images (typically 10 per day) over a data group or several date groups to produce the images shown in this paper.
Two details of the mapping process are worth noting. First, we made small adjustments to the planet center delays and Dopplers so as to co-register the various date-group images with small fiducial features in the July 25–26, 1999 image; the mean absolute correction was
1.3 bauds (2.0 km) in delay and 0.022 Hz (1.0 km) in Doppler. These adjustments reduced smearing of the summed images from time-dependent ephemeris error and also reduced systematic mapping error by referencing all mapping to the accurately adjusted 1999 mapping (see Harmon et al., 2001). Second, in the mapping from delay-Doppler to planetary coordinates we included a correction to take into account Mercury’s small but non-zero spin obliquity, as measured by Margot et al. (2007).
The image processing procedure described above was applied separately to the signals from the two receive polarization channels. Following convention, we use the terms “OC” and “SC” to denote the circular polarization senses that are the opposite of, or the same as,
8 the transmitted sense, respectively. The images shown in this paper are either in the SC
polarization or are weighted sums of SC and OC images. From the OC and SC images we
computed radar cross sections (σ sc , σ oc ) and a circular polarization ratio ( µc ≡ σ sc σ oc ) for selected radar-bright features. We also computed an “equivalent full-disk SC radar albedo”
σˆsc for these features to provide a meaningful measure of intrinsic feature brightness (see
Section 3.1 for a definition and explanation). For all radar cross section and polarization ratio
estimates we summed image pixels over the full observed extent of a given feature. For the
σˆsc estimates we either averaged over the full feature or (for the estimates in Section 3.1)
over the flat floor portion of the crater only. Estimation errors in our radar cross sections are
dominated by the 20% systematic calibration uncertainty (largely from antenna gain
variations). Errors in our polarization ratio estimates are the quadrature sum of random noise
error and a systematic 5% relative polarization calibration error, with the random error being
the dominant contribution. Our random image noise derives from a combination of true
system (thermal) noise and a random self-clutter component peculiar to the long-code
method (Harmon, 2002). This self-clutter only accounted for about 5% of the noise in the SC
polarization but typically contributed more than 30% of the noise in the OC polarization.
This produced some noticeable degradation in the OC images (relative to SC), but was not
large enough to prevent us from making useful µc estimates. In those cases where we have
summed SC and OC images, we have weighted appropriately to account for the different
self-clutter levels. One bonus of using long-code was immunity to the deterministic self-
clutter (from code sidelobes) that affects standard delay-Doppler and that complicated OC
baseline subtraction for the early pre-upgrade polar imaging (Harmon et al., 2001).
9 3. North pole
The Mercury-Earth mutual orbit configuration and the telescope’s limited declination
coverage have combined to make the north pole the more accessible of the poles from
Arecibo and thus the more intensively studied. In this section we present our north polar
results based on the 1995–2005 imagery, starting with a study of the detailed ice distribution
in the near-polar craters and following with a survey of the lower-latitude features seen in
large-scale images.
Where possible, we correlate the observed radar features with known craters in the
Fig. 1 existing planet maps derived from the Mariner-10 imagery. In Fig. 1 we reproduce a central
portion of the H-1 (Borealis) quadrangle from the Mariner-based Atlas of Mercury (Davies et
al., 1978). Note the blank area to the right of the pole, which corresponds to the dark region
beyond the solar terminator during the Mariner-10 encounters. We will refer to the regions to
the left and right of the terminator as the “Mariner-imaged” and “Mariner-unimaged”
hemispheres, respectively. It is also important to note that the coordinate grid in Fig. 1 is that
based on the original Mariner-based control network. Substantial corrections to this polar
grid (and to the north pole location) have been established based on the Arecibo radar
imagery (Harmon et al., 1994, 2001; this paper), as is discussed in the following subsection.
3.1. Near-pole region and crater ice distribution
One expects ice to be most abundant near the pole, where the degree of permanent
shading from direct sunlight is greatest. The radar images show the region within about 5° of
the north pole to be dominated by expansive ice features in the floors of several medium- to
large-size craters, the visibility of which is limited by aspect-dependent radar shadowing.
10 Fig. 2 These features and the radar aspect effects can be seen in Fig. 2, which shows polar images from July 25–26, 1999 (Fig. 2a) and August 14–15, 2004 (Fig. 2b). Five major
features (D, E, H, J, K) can be seen to dominate both images. Two of these (D, E) correspond
to known craters in the Mariner-imaged hemisphere (see Fig. 1), while the other three are on
the Mariner-unimaged side. A comparison of Figs. 2a and 2b shows that the radar appearance
of these craters changes significantly with observing aspect. In Fig. 2b, where the radar
illumination is from the right (282°W), one sees strong highlights from the south (radar-
facing) rims of craters D, E, and H indicating ice deposits on the permanently-shaded interior
rim walls. These same highlights are absent in Fig. 2a because the south rims of these craters
are lost in radar shadow when illuminated from the upper left (138°W). Crater J, being
located on the other side of the pole, shows the opposite effect; the ice-covered south rim is
highlighted in Fig. 2a but lost in radar shadow in Fig. 2b. The fact that the north rim walls of
these craters show little or no radar-highlighted ice is consistent with some exposure to direct
sunlight during part of the Mercury year. Craters D and E do show slight highlighting
consistent with some ice on the lower slopes of their north rim walls, while the images of
crater J suggest that the permanently-shaded region extends across the entire crater floor and
stops just short of the lower slopes of the north rim. Crater K, being larger and farther south,
shows much smaller fractional ice coverage. In Fig. 2a this feature shows up mainly in
highlighting of ice on the terraced southern rim wall. In Fig. 2b nearly all of this rim ice is
lost in radar shadow and, instead, one sees a radar-bright southern floor segment
circumscribed by the south rim base and a sharp east-west line or chord whose center is
located about ⅓ crater diameter from the rim base. Although this same bright floor segment
was originally noticed in the 1998 Arecibo imagery (Harmon et al., 2001), it shows up more
11 clearly in our new image. As in our earlier paper, we can explain the chord line as the limit-
line for permanent shading for a 90-km-diameter crater at this latitude. The bright floor
segment is much fainter (by a factor of 3.2) in the 1999 image (Fig. 2a) than in the 2004
image (Fig. 2b). We attribute this to the extremely high incidence angle at this aspect, which
not only presents a lower projected floor area to the radar but also should result in reduced
subsurface scattering owing to a lower surface Fresnel transmission coefficient. The adjacent
south rim wall appears bright in this same image because the rim’s Earthward tilt reduces the
effective incidence angle. Finally, note that craters D, E, H, and J all show central peaks.
These peaks not only cast radar shadows but also show bright highlights indicating the
presence of ice on their flanks. Crater K does not show an obvious central peak, nor would
one expect to one to show up in this sunlit (hence, iceless) portion of crater floor. Crater K
does, however, have an embedded central crater, labeled “Z” in Harmon et al. (2001), which
shows up in prominent highlights from its south and north rims in Figs. 2a and 2b,
respectively.
The ice distribution in the major near-polar craters is best seen by combining the images
from several date groups, as shown in Fig. 3. In Fig. 3a we have merged the July 25–26, Fig. 3 1999 (Fig. 2a) and August 14–15, 2004 (Fig. 2b) images by selecting the larger of the two
reflectivity values for any given pixel (except when both values are less than two noise
standard deviations, in which case they are summed). This has the effect of bringing out
those ice features that happen to be in radar shadow at one of the two epochs, thus
distinguishing ice-free terrain from radar-shadowed terrain. Alternatively, in Fig. 3b we show
an image formed from a weighted sum of images from all dates in Table 1. The two images
in Fig. 3 are similar in appearance, with the full sum (Fig. 3b) showing slightly more ice
12 coverage and bringing out more of the small or unresolved features. For the larger near-pole
craters (D, E, H, J) note the near-complete ice coverage over the floors, the extensive ice coverage on the south rim walls, and the dearth of ice on the north rim walls. Note also the extensive ice coverage on these craters’ central peaks, with ice-free patches seen only on the peaks in craters H and J. Ice-covered central peaks can also be barely discerned in the lower-
latitude craters L, M, N, and P. Central peaks in medium-size Mercury craters typically rise
about 800 m above the crater floor, or only about ⅓ of the floor-to-rim height (Malin and
Dzurisin, 1978). This implies that a central peak will be permanently shaded for any crater
whose floor is completely or mostly in permanent shade. Given this, and the likelihood that a
thin ice layer could adhere to a peak slope, it should not be surprising that we observe ice-
coated central peaks. Finally, note how the images in Fig. 3 show the ice distribution in crater
K nicely, including the highlighted ice on the terraced south rim wall and the sharp ice-limit
line across the floor. All of these results for the near-polar craters D–K are consistent with
the permanent shading expected for craters of these sizes and at these latitudes.
On Fig. 3b we overplot crater-rim circles for several of the more prominent radar-bright
craters (D, E, L, M, N, P, Y) on the left (Mariner-imaged) side of the pole (see Fig. 1). Here
we have made slight adjustments to the crater-circle locations from our earlier paper
(Harmon et al., 2001), using the central peaks as a guide. Note that the radar-bright deposits
cover a substantial fraction of the interiors of all these craters. This coverage is consistent
with the requirement that ice be permanently shaded from direct sunlight. However, as was
shown by the thermal modeling of Vasavada et al. (1999), permanent shading is a necessary
but not sufficient condition for Mercurian ice. This is because crater interiors are also subject
to substantial “indirect heating” from reflected sunlight and infrared reradiation from sunlit
13 portions of their interior rim walls. This indirect heating becomes more important for smaller craters and at lower latitudes, where it can raise the maximum diurnal surface temperature in permanently shaded regions to temperatures well above 110°K, the temperature at which sublimation losses should erode one meter of ice per billion years (Vasavada et al., 1999).
Based on this, Vasavada et al. showed that one should expect to find little or no surface ice in craters L, M, N, P, and Y. (Craters D and E should be much colder owing to their close proximity to the pole.) On the other hand, our observed ice coverage in these craters, as inferred from Fig. 3b, does show good agreement with those regions where Vasavada et al. estimate the diurnal-average surface temperature to be less than 110°K. Since 0.1–0.5 meters of insulating dust cover should suffice to maintain subsurface ice at a narrow temperature range centered on the diurnal average, it is possible that the radar-bright features are ice deposits protected by a thin dust mantle. This possibility was first suggested by Vasavada et al. (1999) based on comparisons with the pre-upgrade Arecibo imagery of Harmon et al.
(1994). Support for this possibility was provided by the higher-resolution post-upgrade imagery of Harmon et al. (2001), which first resolved the crater ice distribution in detail.
Even stronger support is provided by our new imagery, as can be seen by comparing the apparent ice distribution in craters D–Y (Fig. 3b) with the corresponding model diurnal-average temperature contours for these craters in Fig. 10 of Vasavada et al. (1999).
Some other features in Fig. 3 are worth noting. First, there is the patchy or unresolved brightness that appears around the larger near-polar craters (D, E, H, J, K) and which shows up best in Fig. 3b. This feature, referred to as the “diffuse patch” in Harmon et al. (1994,
2001), is thought likely to be ice distributed in small pockets in the hummocky ejecta blankets of these large craters. The bright floors of these craters are separated from the
14 diffuse patch by dark haloes associated with ice-free terrain on the sun-exposed crater rims.
These dark haloes show up especially well around crater J and the north rim of crater K. To
the left of crater L is a string of small features that must be ice in small craters or depressions
on the rugged north rim of Goethe Basin (see Fig. 1). South and west of crater K is a vast
field of numerous small bright spots that extends further south and out of the image (see
Section 3.2).
Table 2 In Table 2 we list the locations and radar parameters for the labeled north polar craters
(D–Y) in Figs. 2 and 3. Crater center locations are given on both the original Mariner-based
map grid (Fig. 1) and our more accurate radar-based grid. The radar-based positions have an
accuracy of about 2 km and could serve as an initial check on cartography from the
MESSENGER orbiter. Using the location of crater E as a guide, we place the radar-based
north pole at 7°W, 88.35°N (see Fig. 1) on the old Mariner-based maps of Davies et al.
(1978) and Grolier and Boyce (1984).
The other columns in Table 2 list the circular polarization ratios ( µc ), SC radar cross
sections (σ sc ), and SC radar albedos (σˆsc ) estimated for the crater features. As in earlier
papers (Harmon et al., 1994, 2001), we express the SC albedo as an equivalent full-disk
0 n albedo defined by σˆsc = 2σ sc (θ ) (n +1) cos θ , where we assume the n = 3 2 exponent
typical of the icy Galilean satellites. This represents the SC albedo (total SC cross section
σ sc divided by planet projected area) that the entire planet would have were it covered by ice
0 with the same intrinsic reflectivity σ sc (θ ) as that observed for a given polar bright feature at
0 3 2 incidence angle θ and assuming a σ sc (θ ) ∝ cos θ radar scattering law. The albedos were
calculated using only the echoes from the flat floor portions of the craters, in order to avoid a
15 bias from the effectively lower incidence angles of rim highlights. Note that the features
listed in Table 2 have σˆsc ≈ 1, which agrees with the results in Harmon et al. (2001). These high albedos are comparable with the full-disk SC albedos (0.4–1.6) measured for the three icy Galilean satellites (Campbell et al., 1978; Ostro et al., 1992) and are more than two orders of magnitude larger than Mercury’s actual full-disk SC albedo of 0.005 (Harmon, 1997).
Furthermore, the brightest non-polar radar features on Mercury are fresh crater ejecta
0 blankets with typical σ sc values of 0.1 at small incidence angles (Harmon et al., 2007), which still corresponds to an equivalent full-disk albedo of only 0.04. All ten northern features
listed in Table 2 show inverted ( µc > 1) polarization ratios, with a mean µc of 1.38. The five
major polar features (D, E, H, J, K) have a mean µc of 1.41, which agrees with the mean µc value of 1.43 estimated from the 1998 Arecibo imagery. Thus, our newer results provide further confirmation for the polarization inversion condition identified in the earlier (pre-
upgrade and early post-upgrade) Arecibo observations. Furthermore, our updated µc ≈ 1.4
values are close to the S-band µc values (1.43–1.51) measured for the icy Galilean satellites
Ganymede and Europa (Ostro et al., 1992). For comparison, even the brightest crater ejecta
features in Mercury’s non-polar regions show µc values of only about 0.6. It is important to
note, however, that an inverted µc is not necessarily an indicator of ice. For example,
Arecibo-GBT (Green Bank Telescope) radar observations of the Moon’s south pole show
some regions that have µc > 1 but that, being sunlit, are unlikely to harbor ice deposits
(Campbell et al., 2006). These lunar features tend to be found in the ejecta blankets or interior rim walls of young craters and their high depolarization is attributed to conventional
(non-ice) backscatter off very rough or blocky surfaces. Therefore, it is important to consider,
16 as we have here, other criteria besides polarization (namely, high radar albedo and permanent
shading) when identifying likely ice features.
We conclude, then, that our analysis of the combined 1999–2005 Arecibo imagery
provides even stronger support for the theory that Mercury’s major north polar bright
features are ice deposits. We base this on the fact that these features are found to be restricted
to permanently shaded locales and to have radar scattering properties (high albedos and
inverted circular polarization ratios) similar to those observed for the icy Galilean satellites
(Campbell et al., 1978; Ostro et al., 1992) and the south polar ice cap of Mars (Muhleman et
al., 1991). A “coherent backscatter” effect (Hapke, 1990; Peters, 1992), based on multiple
volume scattering in thick (>> λ) deposits of clean (low-loss) ice, is currently considered the
most likely explanation for the high radar reflectivity and polarization inversion observed for
the Galilean satellites. We consider this same mechanism to be the most plausible
explanation for Mercury’s polar radar features.
Table 3 gives a comparison of Arecibo S-band (λ 12.6 cm) radar parameters with similar Table 3 measurements obtained with the Goldstone X-band (λ 3.5 cm) radar (Harcke, 2005). To
avoid incidence-angle biases we use S-band values derived from summing imagery from date
blocks 3 and 7 (June 13, 2001 and July 15–16, 2005), which gives sub-Earth aspects very
close to those for the Goldstone dates (June 13, June 29, and July 1, 2001) used by Harcke
(2005). Note that µc shows a modest wavelength dependence, averaging 8% higher at S-
band than at X-band. A much stronger wavelength dependence is seen in radar cross section,
with the S-band cross sections averaging a factor of 1.6 higher than the X-band values. One
possible explanation for such a wavelength dependence is preferential attenuation at the
shorter wavelength by a lossy surface dust mantle. Slade et al. (2001, 2004) did, in fact,
17 propose a 20-cm-deep dust mantle based on a comparison of the 1998 Arecibo S-band cross
sections with 2001 Goldstone X-band cross sections for the five major north polar features.
Harcke (2005) made a more detailed study that took into account the incidence-angle
differences between the 1998 S-band and 2001 X-band observations. Harcke showed that the
S/X-band comparison was consistent with a dust mantle with a depth of from 20 cm to 0 cm,
depending on the particular crater feature and for σ 0 (θ ) ∝ cosn θ scattering-law exponents in the range n = 1.0–2.0. Harcke concluded that the evidence for an intrinsic S/X-band wavelength dependence was inconclusive given the different sub-Earth aspects and uncertain radar scattering law, a point that was reiterated by Harmon (2007). However, our new S/X- band comparison, which removes aspect-related differences, suggests that the wavelength dependence is real and that, therefore, there could be a lossy surface dust mantle like that
proposed by Slade et al. (2001, 2004) and considered by Harcke (2005). The ratio PS PX of
S-band to X-band echo power after two-way traversal of a dust layer of depth d (all other scattering properties being equal), is given by
PS ⎡2d ⎛ 1 1 ⎞⎤ ⎡0.368d(cm)⎤ = exp⎢ ⎜ − ⎟⎥ = exp⎢ ⎥ , (1) PX ⎣ Lλ ⎝ λX λS ⎠⎦ ⎣ Lλ ⎦
where Lλ is the 1/e attenuation depth in wavelengths. Using PS PX = 1.6 and Lλ = 10 (a
reasonable value for lightly packed rock powder) gives d = 13 cm, with larger d values
possible for less lossy compositions. An important implication of a few-decimeters-thick dust
mantle is that it could provide the thermal insulation required by the models of Vasavada et
al. (1999) to protect the polar ice from sublimation losses over billions of years. It could also
protect the ice from erosion by interstellar Ly-α radiation (Morgan and Shemansky, 1991;
Butler et al., 1993). At the same time, our result suggests that any dust mantle is probably
18 thin enough that it would not strongly obscure a hydrogen signature from any underlying ice
when observed by an orbiting neutron spectrometer (Feldman et al., 2000). It should be
noted, however, that dust-mantle attenuation is not the only possible source of wavelength
dependence. One could get the same effect if the size distribution of the subsurface scattering
elements is skewed so as to preferentially scatter at the longer wavelength.
3.2. Large-scale images and low-latitude features
The previous section’s survey of the larger near-polar features has provided the best
confirmation yet that permanent shading from direct sunlight is a necessary condition for ice.
It is not surprising, then, that with decreasing latitude one sees a transition from large
radar-bright crater features to smaller and more isolated features associated with the
increasingly sparse pockets of permanently shaded terrain. This trend is clearly seen in Fig.
Fig. 4 4, which shows a large-scale image of the north polar region extending down to about 65°N. The image in Fig. 4 was formed by splicing two separate images along a vertical line that
crosses the 90°W meridian at 82.36°N. The left-hand image is the sum of date groups 1, 3,
and 5 (the June 2001 and July 1999 dates), while the right-hand image is the sum of date
groups 9–12 (the August 2004 and August 2005 dates). The image has been cropped to
encompass all of the apparent radar-bright ice features. Since the low-latitude features avoid
the 0°W and 180°W sides, we have done a rectangular cropping that chops more from the top
and bottom.
Figure 4 reveals numerous small bright spots at relatively low latitudes. Below 75°N we
identify more than a score of features brighter than five noise standard deviations. In Table 4 Table 4 we list 13 of the more prominent of these, including one at 67°N. Only one of these features
19 (X2) is located on the left-hand (Mariner-imaged) side of the pole (see Fig. 1). This feature is
located on or near the southern rim wall of crater Tung Yuan, and therefore can be
considered a likely sun-shaded ice feature. The remainder of the features in Table 4 are
located on the right-hand (Mariner-unimaged) side of the pole and within the longitude
Fig. 5 quadrant centered on the 270°W meridian. A blowup of this region is shown in Fig. 5. This image gives a better view (along with labels) of the features from Table 4 and also shows the
remarkable profusion of small bright spots in the general region south of crater K.
Although located in Mariner-unimaged terrain, it is likely that the low-latitude features in
Fig. 5 are ice features like X2 that are sheltered in or under the steep southern rims of
relatively fresh craters like Tung Yuan. The ice identification is supported by the fact that all
show inverted polarization ratios and high albedos (Table 4). Also, most of these features are
(like X2) flattened with their long axes perpendicular to the pole direction, which is
consistent with shading by a south rim. Rim shading requires that the slope of the interior rim
wall exceed the co-latitude. Based on this, and using crater statistics from Pike (1988),
Harmon et al. (2001) argued that the lower latitude limit for shaded ice deposits should be in
the range 76.5–78.2° for crater diameters in the range 30–150 km, dropping to less than 70°
latitude for craters smaller than 20 km. Therefore, the apparent ice features we see below 75°
latitude must be located either in host craters smaller than 30 km or in larger craters whose
southern rim walls are, in places, somewhat steeper than the typical rim slope. The OC
version (not shown) of Fig. 5 shows faint arcs that may be conventional (non-ice) backscatter
off the inner north rims of the C3, J3, and K3 host craters and that indicate diameters of 25
km, 12 km, and 20 km, respectively. Furthermore, some of the lower-latitude south polar
bright features are located under the pole-facing rims of 20-km-size craters (see next section).
20 On the other hand, Vasavada et al. (1999) have shown that the smaller craters, with their
higher depth-diameter ratios, have relatively hot interiors owing to high levels of indirect
heating from their own sunlit rims. Therefore, the apparent discovery of ice in low-latitude,
20-km-size craters would seem to strain the Vasavada et al. thermal model in much the same way that the identification of ice features in even smaller (< 10-km-diameter) craters nearer the pole (Harmon et al., 2001) strained the same model. It will be important, then, to identify the host craters for our putative low-latitude ice features on the anticipated MESSENGER orbiter maps of the north polar region, both to establish the host-crater diameters and to confirm our hypothesis that ice patches collect on or below shaded south rims at low latitudes.
The “indirect heating” that we have been alluding to may be responsible for some of the observed anisotropy in the longitude distribution of low-latitude features. No ice features are seen below 78°N in the two quadrants (315–45°W and 135–225°W) centered on the 0°W and
180°W “hot longitudes” (the alternating subsolar longitudes at successive Mercury perihelia, sometimes referred to as the “hot poles”), whereas all of the features below 75°N are located in the opposing “cold” quadrants. The thermal modeling results of Vasavada et al. (1999) showed shaded polar crater floors to have substantially higher temperatures at the hot longitudes, so it is possible this could account for the observed concentration of the low-latitude features at the colder longitudes. Since exposure to direct sunlight increases only slightly at perihelion, the increased heating must be attributable to indirect heating. Of course, it is clear from Fig. 4 that all but one of the ice features below 75°N are located on the right side of the image. The left side, which corresponds to the Mariner-imaged hemisphere, is known to be covered with vast expanses of smooth plains (Borealis Planitia)
21 and thus is relatively sparsely cratered (see Fig. 1). Therefore, the hemispherical asymmetry
in low-latitude ice may largely reflect a difference in the numbers of suitable host craters.
It is worth noting that the σˆsc values for the low-latitude features (Table 4), though high,
are smaller than the σˆsc ≈ 1 values seen for the prominent near-pole features in Table 2. We
can see several possible reasons for this. First, being located in warmer or less uniformly sun-
shaded regions, the ice deposits may be thinner or have a patchiness that gives a lower filling
factor. Second, if the ice is perched on away-sloping southern rim walls, then the effective
incidence angles may be increased. Finally, it is possible that the scattering law falls off more
slowly than our assumed cos3 2 θ law (although it is unlikely to fall off more slowly than the
simple cosθ area projection factor).
3.3. Ice areal coverage
We have used the image in Fig. 4 to estimate the total ice areal coverage over the north
polar region. A reasonable conservative estimate can be made by summing the areas of all
those pixels whose reflectivity exceeds four noise standard deviations, the lowest threshold
for which one sees a strong decline in ice area with lower latitude. Correcting this for a false-
detection background (from statistical fluctuations plus real non-ice echo), gives a total ice
area coverage of 12,500 km2. Of this, 7,500 km2 comes from within 5° of the pole and
11,000 km2 comes from within 10° of the pole. This probably misses some areas (such as
portions of the “diffuse patch”) giving enhanced ice echoes but with reflectivities below the
4σ threshold. If one uses a 3-standard-deviation threshold, and makes a similar correction for false detections, then one gets a total ice area of 14,600 km2, of which 10,300 km2 is
concentrated within 5° of the pole. Using Fig. 4 for these estimates will miss some ice,
22 especially close to the pole, because of radar shadowing and the limited longitude aspect. If,
instead, we use the all-date sum (Fig. 3b), we estimate 10,100 km2 of ice within 5° of the
pole and 12,500 km2 within 7° of the pole (for a 4-standard-deviation threshold). All of these numbers should be treated with some caution. They should simply be interpreted as estimates of the total polar area covered by those radar pixels within which there is enough ice to give a noticeably enhanced radar reflectivity.
It is interesting to note that our estimated ice coverage is roughly twice the permanently shaded area estimated from radar altimetry of the Moon’s south pole (Margot et al., 1999).
This means that Mercury’s north polar ice coverage is about what one would expect if the crater population were similar to that at the lunar south pole and, as suggested by Vasavada et al. (1999), all of Mercury’s available cold traps are ice-filled. The Lunar Prospector neutron spectrometer detected excess hydrogen signatures at the lunar poles suggestive of ice
(Feldman et al., 2000), despite the fact that Earth-based radar observations of the Moon find no evidence for obvious ice features similar to those seen for Mercury (Campbell et al.,
2006). These can be reconciled if the lunar ice is a highly dilute admixture to the lunar regolith rather than the slabs of relatively clean ice that are required to explain the Mercury features (Feldman et al., 2000). Therefore, the Mercury ice, if it exists, can be expected to present a much stronger hydrogen signature to the MESSENGER neutron spectrometer than was the case for the Lunar Prospector instrument. If this is found to be the case, then the difference can be attributed to ice being much more abundant in Mercurian cold traps than in lunar cold traps, not to any difference in the number of suitable host craters.
23 4. South pole
Pre-upgrade Arecibo S-band imagery mapped the south polar region at 15-km resolution
(Harmon et al., 1994). This image was dominated by a large circular bright feature from the
floor of Chao Meng-Fu, a 150-km-diameter crater whose south rim intersects the pole (see
Fig. 6 Fig. 6). The Chao Meng-Fu feature had already been identified in a preliminary report on the
1991–1992 observations (Harmon and Slade, 1992), making this the first Mercury polar ice
feature to be positively identified with a known crater. The improved images (Harmon et al.,
1994) revealed several additional south polar bright features, including three (labeled G, U,
V) which could be identified with known craters on the Mariner-imaged side of the pole (see
Fig. 6). As was the case with the north pole, aligning the south polar radar features with their
respective host craters required making adjustments to the crater locations as given on the
less accurate Mariner-based map grid shown in Fig. 6.
The next development in radar imaging of the south pole came with Goldstone X-band
observations in February 2001. These 40-µs long-code observations produced delay-Doppler
images of the pole with 6-km resolution (Slade et al., 2001, 2004; Harcke et al., 2001;
Harcke, 2005). The Goldstone images showed all of the features seen in the earlier Arecibo
images, revealed more than thirty additional features, and confirmed the Arecibo corrections
to the mapped pole location.
Although the upgraded Arecibo telescope was available for S-band radar observations in
1998, the south pole did not return to view until 2004. Some south-polar imagery from that
year proved of poor quality owing to telescope pointing problems. Observations made the
following year (March 24–25, 2005; date group 13) were more successful, producing the
south polar images shown in this paper.
24 Fig. 7 The new Arecibo image of the south pole is shown in Fig. 7a. This image was formed from the weighted sum of the OC and SC images and has been cropped to include all
apparent ice features. As in the north, the lower-latitude features align along the 90–270°W
cold longitudes. As was the case with the pre-upgrade imagery, the south polar image
appears to be of lower quality than the northern images. This is due, in part, to the higher
incidence angles; the sub-Earth latitude of the March-2005 observations was nearly 4° lower
than for the best north polar dates, and most of the bright features in Fig. 7 are within 10° of
the radar horizon. The south polar observations were also made at reduced power owing to
transmitter problems, and we did not have as many dates to average as in the north.
Figure 7b shows the same image as in Fig. 7a, but with overplotted crater rim circles and
labels. Circles are overplotted only for those Mariner-imaged craters showing obvious bright
features in the new Arecibo image. In plotting these circles we took the locations of polar
craters from the revised Mariner-based grid of Robinson et al. (1999) and shifted these
upward (along the 0–180° longitude meridian) by 0.086° (3.6 km) in order to give slightly
better agreement with the locations of the radar features. This grid placement puts the true
pole at 90°W, 88.7°S (see Fig. 6) on the old Mariner-based maps of Davies et al. (1978) and
Strom et al. (1990). This is very close to the corrected south pole location originally
proposed by Harmon et al. (1994). The four brightest crater features identified on the
Mariner-imaged side by Harmon et al. (1994) are also seen in Fig. 7b and retain their original
letter designations; these are craters G, U, V and X (for Chao Meng-Fu). (We have not
plotted a crater circle for feature U as this crater lies right on the Mariner terminator and thus
has a poorly defined size and position.) Additional bright features have been identified with
known craters on the Mariner-imaged side and are denoted by circles with labels running
25 from S3 through L4. The largest of these new host craters, G4, is also the only one with a
proper name, Li Ch’ing Chao. Not included among the labeled features is a cluster of small
bright spots south of Bernini Crater (see Fig. 7b), some of which can be traced to a crater
chain extending west of crater D4 (Strom et al., 1990). The positions and radar parameters
Table 5 for selected labeled crater features on the Mariner-imaged side are listed in Table 5.
Many bright features also show up on the Mariner-unimaged (left) side of Fig. 7,
including a few that were apparent in the pre-upgrade imagery (Harmon et al., 1994). The
only one of these to which we have assigned a letter label is R3, which has the distinction of
being the south polar feature with the lowest latitude (and is therefore included in Table 4).
Many of the bright features in Fig. 7 were also seen in the Goldstone X-band imagery
(Harcke, 2005). These include features G, U, V, X, S3, T3, W3, Z3, B4, D4, and H4, as well
as many of the features on the Mariner-unimaged side. The crosses (+) in Fig. 7b denote the
locations of those bright features that were identified in the Goldstone X-band imagery
(Harcke, 2005) but that are not apparent in the Arecibo image. Note that most of these
Goldstone-only features are clustered near or beyond the Arecibo radar horizon. The 159–
238°W sub-Earth longitudes for the February-2001 Goldstone observations gave a view
opposite that of the March-2005 Arecibo observations, thus revealing features that were
either inaccessible or at too high an incidence angle for viewing during the March-2005
Arecibo epoch. Those features that do appear in both the Arecibo and Goldstone images
show close agreement in their mapped coordinates. This provides strong support for the
radar-based corrections to the south polar grid and pole location and also supports the revised
Mariner-based south polar map grid of Robinson et al. (1999).
26 As was seen in the pre-upgrade imagery, the Chao Meng-Fu feature is the dominant south-polar bright feature as well as the largest radar feature at either pole. The location, shape, and size of this feature is just as in the earlier imagery (Harmon et al., 2004), although the higher resolution of the new image has revealed more structural detail. Note the arc of highlights from the north side of the feature, which is indicative of ice on the shaded pole-facing interior rim wall. Note also the cluster of highlights and shadows in the central crater floor, which appears to be coming from a central peak-ring structure. This would be consistent with Mercury crater morphology studies (Pike, 1988) showing that craters as large as this tend to have central peak-rings rather than a single central peak. Much of the rest of the crater floor shows a roughly uniform brightness that extends to within about 0.20–0.25 craters diameters of the near (south) rim; the relative faintness of this floor echo is understandable given the high (~ 84°) incidence angle. The remaining crescent of radar-dark terrain interior to the near rim can be explained as a combination of radar shadow and sunlit floor. The radar-bright fraction of crater floor corresponds closely to that portion with diurnal-average temperatures less than 110°K in the Vasavada et al. (1999) model and is significantly larger than the corresponding size for an uninsulated (unmantled) ice deposit
(see their Fig. 10). The inverted polarization ratio and high radar albedo for Chao Meng-Fu
(Table 5) are consistent with ice.
The other host craters nearest the pole (G, U, S3, T3) show radar features that occupy large fractions of their floor areas. The size of the crater G feature agrees with the diurnal- average T < 110°K region in the Vasavada et al. (1999) model for this crater (see their Fig.
10). The remainder of the labeled host craters (U3–L4) are at lower latitudes and show smaller radar features located just inside their north rims. The Goldstone X-band imagery
27 shows a very similar mapped ice distribution in several of these same craters (Harcke, 2005).
The locations and radar characteristics of these features (Table 5) are consistent with small
ice deposits confined to shaded regions on or under pole-facing rim walls. The crater V
feature agrees well with the diurnal-average T < 110°K region in the Vasavada et al. (1999)
model (see their Fig. 10); this same model predicts there should be no ice in this crater for the
case of an uninsulated surface subjected to the diurnal maximum temperature. In these
respects crater V closely resembles the north polar crater Despréz (W), which has a similar
size, latitude, and longitude (Harmon et al., 2001). As at the north pole, the lower-latitude
south polar features tend to cluster toward the colder longitudes.
A comparison of the radar imagery with the geologic map of Strom et al. (1990) indicates
that the south polar ice shows an affinity for the younger, fresher craters. Of the four c5-class
(youngest) craters within the coverage of Fig. 7, three (E4, K4, L4) show probable ice features and the fourth (NNE of H4) shows a bright feature in the Goldstone imagery. Two of these (K4, L4) have the lowest latitudes for ice features on the Mariner-imaged side. The remainder of the geologically mapped ice host craters from Fig. 7b are c3-class or younger. A similar situation is found at the north pole, where a strong association was found between ice
features and c4-class craters (Harmon et al., 2001). This supports our earlier conclusion that much if not most of the radar-detected ice was deposited relatively late, i.e., after the Caloris event and well after the start of smooth plains emplacement (Harmon et al., 2001). The only alternative is that the ice was deposited early and then later redistributed into fresher craters through migration of sublimation vapor. It is possible that some early ice deposits have been buried by crater degradation processes such as rim mass wasting and hence are no longer radar-visible. Degraded craters would also be less suitable hosts for younger ice because of
28 the reduced rim shadowing. That some of the lower-latitude ice features show a preference
for c5-class craters makes sense because of the extra sun-shading afforded by the steep, pristine crater rims.
5. Conclusion
Much of this paper has been devoted to presenting a complete survey of Mercury’s putative north polar ice, based on Arecibo radar observations obtained over several years and from a full range of sub-Earth aspects. We also presented the best Arecibo imagery yet made of the south pole. Although the 2005 south polar imagery is of lower quality and was obtained at only one sub-Earth aspect, between it and the 2001 Goldstone imagery we now have a reasonably good picture of the distribution of southern ice.
For the north pole, the wide range of observing aspects has enabled us to distinguish radar-shadowed from ice-free terrain and thus better map the true ice distribution. The results provide further support for the notion, based on previous radar observations and theoretical modeling, that permanent shading from direct sunlight is a necessary condition for Mercurian ice. Furthermore, the extensive ice coverage in some of the larger craters (e.g., D–Y and
Chao Meng-Fu) nearest the poles suggests that most of the terrain within those craters that is cold enough to host ice deposits does, in fact, contain ice. Therefore, it is possible that the earlier assertion of Vasavada et al. (1999) that all of Mercury’s suitable cold traps are filled is essentially correct. However, to test this rigorously will require a thorough survey of polar craters to evaluate their ice-hosting suitability, using orbiter imagery and altimetry and invoking realistic thermal models.
29 An important finding from past theoretical modeling (e.g., Vasavada et al., 1999) is that
heating by reflected or reradiated sunlight is an important effect. This “indirect heating” can
prevent ice from forming in otherwise permanently shaded regions and should be the limiting
factor in the distribution of ice at the lower latitudes. However, our new results confirm
previous indications that ice lies in some crater floor areas that, though permanently sun-
shaded, should still be too warm for surface ice. In fact, our new imagery shows a close
correspondence between the observed crater ice distribution and those regions where thermal models (Vasavada et al., 1999) show diurnal-average surface temperatures to be less than about 110°K (that is, cold enough to sustain permanent ice, provided the ice is not subjected to maximum temperatures much above the diurnal average). This lends strong support to the proposal of Vasavada et al. (1999) that Mercury’s ice deposits are protected by an insulating dust mantle. Also, our new results now support earlier suggestions of an intrinsic S/X-band wavelength dependence, one cause of which could be a moderately lossy surface dust layer.
Nevertheless, the evidence for dust-mantled ice must still be considered circumstantial, and many uncertainties remain. Dust-mantling scenarios raise some difficult issues concerning the volume, rapidity, and sequencing of the ice and dust deposition (Butler et al., 1993;
Killen et al., 1997; Vasavada et al., 1999; Harmon et al., 2001; Harmon, 2007). The ice sublimation loss process itself also has some uncertainties regarding low-temperature sublimation rates, the role of thin diffusion barriers, and the loss processes affecting released water vapor (Vasavada et al., 1999; Killen et al., 1997). Even the water-ice hypothesis itself has come into question, with sulfur (Sprague et al., 1995) and cold silicate (Starukhina, 2001) proposed as alternatives for the low-loss scattering medium. A major objective of the
MESSENGER and Bepi-Colombo missions is to identify the chemical species responsible
30 for the radar features. Should that prove successful, one can expect a rejuvenation of
theoretical modeling studies of Mercury’s polar volatiles. At this point, based on the ice-like characteristics of the polar radar echoes and given the widespread abundance of water in the
Solar System, one has to consider water ice to be the most likely source of Mercury’s radar- bright polar features.
One of the more intriguing findings of the Mercury polar radar observations has been the identification of apparent ice features at latitudes as low as 67°. Although ice deposits this far from the pole might seem unlikely, all of the radar characteristics of these features (including size, shape, radar brightness, and polarization) are consistent with ice. Furthermore, everything points to these features being located in regions permanently shaded from direct sunlight. The main difficulty from the standpoint of the thermal models (Vasavada et al.,
1999) is that some of these features are apparently located in relatively small (~ 20-km- diameter) craters in which the indirect heating may be too great to support ice. The thermal models do suggest that the distribution of low-latitude ice should be modulated somewhat in longitude by the “hot pole” effect, and here we have presented some evidence that this is, in fact, the case. We have argued here that most of these low-latitude features are probably isolated ice deposits sheltered by steep pole-facing rim walls of fresh craters. Although there is already some evidence for this, additional confirmation is required as most of the the low- latitude ice features are located in the Mariner-unimaged hemisphere. Therefore, one important objective should be to map the apparent low-latitude ice features onto their respective host craters in the visual images anticipated from the MESSENGER and
BepiColombo orbiters.
31 Additional Mercury polar observations are planned for the Arecibo radar. The highest priority will go to making observations of the south pole in March-April 2011 and March
2012, when that pole will be visible at a sub-Earth longitude aspect opposite that of 2005.
These observations should produce images that, when combined with the 2005 imagery, will result in a better picture of the south pole. The north pole will also be favorably situated for viewing in the summers of 2011–2013. Since there is little point in obtaining additional longitude coverage at the current (1.5-km) resolution, the logical goal of any new north polar observations would be to go to finer resolution. It should be possible to improve the spatial resolution by a factor of two or so, but pushing much finer than that will quickly test the signal-to-noise limits. Probably the most important benefit of higher resolution would be in identifying and resolving additional low-latitude features.
Acknowledgments
The National Astronomy and Ionosphere Center (Arecibo Observatory) is operated by
Cornell University under a cooperative agreement with the National Science Foundation
(NSF). The S-band radar observations were also made possible with support from the
National Aeronautics and Space Administration (NASA). The work of Martin Slade was supported by the Jet Propulsion Laboratory, a division of the California Institute of
Technology, under contract to NASA. Melissa Rice’s work at Arecibo was supported by a grant from the Research Experience for Undergraduates (REU) program of the NSF. We are grateful to two anonymous referees for their constructive reviews and valuable suggestions.
32 References
Butler, B.J., 1994. 3.5-cm radar investigation of Mars and Mercury: Planetological
implications. Ph.D. thesis. California Institute of Technology, Pasadena.
Butler, B.J., Muhleman, D.O., Slade, M.A., 1993. Mercury: Full-disk radar images and the
detection and stability of ice at the north pole. J. Geophys. Res. 98, 15,003–15,023.
Campbell, D.B., Chandler, J.F., Ostro, S.J., Pettengill, G.H., Shapiro, I.I., 1978. Galilean
satellites: 1976 radar results. Icarus 34, 254–267.
Campbell, D.B., Campbell, B.A., Carter, L.M., Margot, J.-L., Stacy, N.J.S., 2006. No
evidence for thick deposits of ice at the lunar south pole. Nature 443, 835–837.
Davies, M.E., Dwornick, S.E., Gault, D.E., Strom, R.G., 1978. Atlas of Mercury. NASA,
Washington, DC.
Feldman, W.C., Lawrence, D.J., Elphic, R.C., Barraclough, B.L., Maurice, S., Genetay, I.,
Binder, A.B., 2000. Polar hydrogen deposits on the Moon. J. Geophys. Res. 105, 4175–
4195.
Grolier, M.J., Boyce, J.M., 1984. Geologic map of the Borealis region of Mercury. USGS
Map I-1660. U.S. Geol. Surv., Reston, VA.
Hapke, B., 1990. Coherent backscatter and the radar characteristics of the outer planet
satellites. Icarus 88, 407–417.
Harcke, L.J., 2005. Radar imaging of solar system ices. Ph.D. thesis. Stanford University,
Stanford, CA.
Harcke, L.J., Zebker, R.F., Jurgens, R.F., Slade, M.A., 2001. Radar imaging of Mercury’s
north and south poles at 3.5 cm wavelength. Proc. Workshop on Mercury: Space
Environment, Surface and Interior, LPI Contr. No. 1097, Chicago, IL, p. 36.
33 Harmon, J.K., 1997. Mercury radar studies and lunar comparisons. Adv. Space Res. 19(10),
1487–1496.
Harmon, J.K., 2002. Planetary delay-Doppler radar and the long-code method. IEEE Trans.
Geosci. Remote Sensing 40, 1904–1916.
Harmon, J.K., 2007. Radar imaging of Mercury. Space Sci. Rev. 132, 307–349.
Harmon, J.K., Slade, M.A., 1992. Radar mapping of Mercury: Full-disk images and polar
anomalies. Science 258, 640–643.
Harmon, J.K., Perillat, P.J., Slade, M.A., 2001. High-resolution radar imaging of Mercury’s
north pole. Icarus 149, 1–15.
Harmon, J.K., Slade, M.A., Vélez, R.A., Crespo, A., Dryer, M.J., Johnson, J.M., 1994. Radar
mapping of Mercury’s polar anomalies. Nature 369, 213–215.
Harmon, J.K., Slade, M.A., Butler, B.J., Head, J.W., Rice, M.S., Campbell, D.B., 2007.
Mercury: Radar images of the equatorial and midlatitude zones. Icarus 187, 374–405.
Killen, R.M., Benkhoff, J., Morgan, T.H., 1997. Mercury’s polar caps and the generation of
an OH exosphere. Icarus 125, 195–211.
Malin, M.C., Dzurisin, D., 1978. Modification of fresh crater landforms: Evidence from the
Moon and Mercury. J. Geophys. Res. 83, 233–243.
Margot, J.-L., Campbell, D.B., Jurgens, R.F., Slade, M.A., 1999. Topography of the lunar
poles from radar interferometry: A survey of cold trap locations. Science 284, 1658–
1660.
Margot, J.-L., Peale, S.J., Jurgens, R.F., Slade, M.A., Holin, I.V., 2007. Large longitude
libration of Mercury reveals a molten core. Science 316, 710–714.
34 Morgan, T.H., Shemansky, D.E., 1991. Limits to the lunar atmosphere. J. Geophys. Res. 96,
1351–1367.
Muhleman, D.O., Butler, B.J., Grossman, A.W., Slade, M.A., 1991. Radar images of Mars.
Science 253, 1508–1513.
Ostro, S.J., and 11 colleagues, 1992. Europa, Ganymede, and Callisto: New radar results
from Arecibo and Goldstone. J. Geophys. Res. 97, 18227–18244.
Paige, D.A., Wood, S.E., Vasavada, A.R., 1992. The thermal stability of water ice at the
poles of Mercury. Science 258, 643–646.
Peters, K.J., 1992. Coherent backscatter effect: A vector formulism accounting for
polarization and absorption effects and small or large scatterers. Phys. Rev. B 46,
801–812.
Pike, R.J., 1988. Geomorphology of impact craters on Mercury. In: Vilas, F., Chapman, C.R.,
Matthews, M.S. (Eds.), Mercury. Univ. of Arizona Press, Tucson, pp. 165–273.
Robinson, M.A., Davies, M.E., Colvin, T.R., Edwards, K.E., 1999. A revised control
network for Mercury. J. Geophys. Res. 104, 30,847–30,852.
Slade, M.A., Butler, B.J., Muhleman, D.O., 1992. Mercury radar imaging: Evidence for polar
ice. Science 258, 635–640.
Slade, M.A., Harcke, L.J., Jurgens, R.F., Zebker, H.A., 2001. Mercury’s north and south
polar regions: Goldstone radar imaging at 3.5-cm wavelength. Bull. Am. Astron. Soc.
33, 1026.
Slade, M., Harmon, J., Harcke, L., Jurgens, R., 2004. 3.5-cm radar observations of polar
regions of Mercury using Goldstone to Arecibo configuration. COSPAR Scientific
Assembly 35, Paris, France, p. 1154.
35 Sprague, A.L., Hunten, D.M., Lodders, K., 1995. Sulfur at Mercury, elemental at the poles
and sulfides in the regolith. Icarus 118, 211–215.
Starukhina, L., 2001. Water detection on atmosphereless celestial bodies: Alternative
explanations of the observations. J. Geophys. Res. 106, 14,701–14,710.
Strom, R.G., Malin, M.C., Leake, M.A., 1990. Geologic map of the Bach region of Mercury.
USGS Map I-2015, U.S. Geol. Surv., Reston, VA.
Vasavada, A.R., Paige, D.A., Wood, S.E., 1999. Near-surface temperatures on Mercury and
the Moon and the stability of polar ice deposits. Icarus 141, 179–193.
36 Table 1
List of Arecibo observation date groups and corresponding mean sub-Earth positions (Long., Lat.), in order of increasing longitude. ______Group no. Dates Long. (°W) Lat. (°) ______
1 2001 June 1–3 11 +4.5 2 2005 August 17 46 +9.3 3 2001 June 13 82 +7.3 4 2002 June 2 119 +5.0 5 1999 July 25–26 138 +11.7 6 2002 June 9–10 166 +5.7 7 2005 July 15–16 198 +8.8 8 2002 June 21–22 233 +5.7 9 2004 August 8–9 244 +10.3 10 2004 August 14–15 282 +11.2 11 2004 August 19 312 +11.5 12 2005 August 5–6 331 +11.8 13 2005 March 24–25 349 −7.5 ______
37 Table 2
Host crater locations and radar scattering parameters for selected radar-bright (ice) features at the north pole. ______2 Crater Location µc σ sc (km ) σˆsc θ (°) ______
D 125.1, +88.60 (66.1, +88.5) 1.46 (1.51) 96.4 1.09 80.2 E 173.0, +89.61 (12.3, +88.8) 1.44 (1.40) 61.4 1.09 79.0 H 214.4, +88.89 1.42 (1.37) 119.2 0.95 79.1 J 282.2, +87.87 1.40 (1.41) 157.0 0.89 76.7 K 296.6, +85.60 (approximate) 1.31 (1.48) 129.7 0.60 73.7 L 66.8, +85.15 (52.7, +84.3) 1.39 17.0 82.4 M 40.0, +85.98 (30.0, +84.8) 1.36 35.9 1.06 80.2 N 6.2, +84.92 (3.0, +83.2) 1.31 32.1 1.36 77.6 P 50.1, +83.19 (41.0, +81.9) 1.30 15.7 82.3 Y 148.1, +87.68 (114.1, +88.4) 1.43 16.8 80.6 ______
Location: Estimated location (°W Long., °Lat.) of center of ice host crater based on observed radar bright feature. Also given is center location of same crater (in parentheses) on the original Mariner-based NASA/USGS maps (Davies et al., 1978).
µc , σ sc , σˆsc : Circular polarization ratio (SC/OC), SC radar cross section, and SC
radar albedo, respectively, from imagery summed over date groups 9–12. The µc values in parentheses are values from 1998 Arecibo imagery that have been adjusted upward from the values quoted in Harmon et al. (2001) to correct for an error in the OC system
temperatures used in the original noise calibration. The estimated error in µc is ±0.07 for all craters. The errors in σ sc and σˆsc are dominated by the ±20% calibration error. 3/2 The albedo σˆsc is an equivalent full-disk albedo computed assuming acos θ scattering law (see text) and is calculated for the flat floor regions (no rim highlights) of the seven largest craters only. θ : Mean radar incidence angle for the radar feature.
38 Table 3
Comparison of Arecibo S-band and Goldstone X-band radar scattering parameters for prominent north polar icy craters. See text for details. ______
Crater µcS µcX σ scS σ scX ______
D 1.16 ± 0.06 1.79 ± 0.45 E 1.18 ± 0.07 1.60 ± 0.40 H 1.17 ± 0.06 1.88 ± 0.47 J 1.01 ± 0.05 1.60 ± 0.40 K 0.90 ± 0.05 1.19 ± 0.30 ______
µcS µcX : Ratio of S-band and X-band µc estimates.
σ scS σ scX : Ratio of S-band and X-band SC radar cross sections.
39 Table 4
Locations and radar parameters for selected low-latitude (< 75° Lat.) bright features. ______
2 Feature Location µc σ sc (km ) σˆsc θ (°) ______X2 62.8, +74.45 1.50 ± 0.35 1.46 0.19 (0.37) 68.5 A3 255.6, +71.58 1.34 ± 0.10 3.00 0.39 (0.66) 63.9 B3 292.9, +71.55 1.42 ± 0.09 6.19 0.34 (0.58) 60.4 C3 303.3, +71.94 1.24 ± 0.07 5.17 0.45 (0.88) 61.3 F3 254.1, +73.66 1.41 ± 0.23 0.52 0.27 (0.39) 65.8 G3 280.7, +73.80 1.30 ± 0.12 1.17 0.39 (0.68) 62.8 H3 285.5, +70.31 1.12 ± 0.08 2.40 0.33 (0.62) 59.2 J3 314.9, +72.79 1.21 ± 0.12 1.15 0.34 (0.51) 63.3 K3 298.6, +66.94 1.16 ± 0.07 4.74 0.48 (0.75) 56.0 L3 277.6, +70.65 1.04 ± 0.14 0.58 0.15 (0.22) 60.0 M3 260.9, +72.73 1.54 ± 0.31 0.68 0.15 (0.24) 63.9 N3 254.8, +70.28 1.53 ± 0.41 0.26 0.21 (0.29) 63.0 P3 295.1, +70.93 1.14 ± 0.16 0.41 0.23 (0.26) 59.8 R3 281.0, −74.43 1.53 ± 0.26 1.80 0.89 (1.51) 77.0 ______
Location: Estimated location (°W Long., °Lat.) of center of radar bright feature (not center of host crater).
µc , σ sc , σˆsc : Circular polarization ratio, SC radar cross section, and equivalent full- disk SC albedo, respectively, from imagery summed over date groups 9–12 (for craters A3–P3), and 13 (for crater R3). For crater X2 the µc value is from date groups
1+3, and the σ sc value is from groups 1+3+5. Also given is the σˆsc value for the brightest feature pixel (in parentheses). The errors in σ sc and σˆsc are dominated by the ±20% calibration error. θ : Mean radar incidence angle for the radar feature.
40 Table 5
Host crater locations and radar parameters for selected bright features at the south pole. ______2 Crater Location µc σ sc (km ) σˆsc θ (°) ______
G 75.2, −86.27 (78.0, −84.9) 1.18 ± 0.11 25.7 0.61 82.3 V 85.6, −81.09 (86.5, −79.8) 1.36 ± 0.14 10.4 1.40 83.7 X 149.7, −88.41 (135.0, −87.6) 1.23 ± 0.07 410.8 0.59 84.0 S3 67.0, −87.63 (74.0, −86.43) 1.34 ± 0.15 11.8 0.61 82.0 T3 83.8, −87.17 (84.5, −85.77) 0.98 ± 0.12 7.5 0.55 82.8 U3 27.9, −86.17 (42.0, −84.74) 0.99 ± 0.20 1.5 0.52 79.4 V3 14.4, −85.17 (27.5, −83.92) 1.51 ± 0.31 1.7 0.72 77.9 W3 91.0, −84.83 (90.4, −83.57) 1.67 ± 0.25 5.8 1.30 83.5 X3 85.0, −84.69 (87.3, −83.50) 1.31 ± 0.18 5.5 0.98 83.2 Y3 68.7, −83.85 (70.6, −82.49) 1.29 ± 0.20 2.4 1.17 81.3 Z3 70.2, −82.77 (73.3, −81.51) 1.17 ± 0.14 5.0 0.86 81.5 A4 65.7, −82.47 (69.3, −81.21) 0.99 ± 0.11 3.4 1.14 80.7 B4 95.1, −82.46 (94.7, −81.21) 1.39 ± 0.15 7.6 1.97 84.7 C4 99.9, −82.34 (98.9, −81.05) 1.08 ± 0.18 1.4 3.49 85.3 E4 72.5, −79.77 (74.9, −78.54) 0.90 ± 0.21 0.7 0.95 81.5 F4 86.4, −78.12 (87.0, −76.93) 0.87 ± 0.19 0.7 1.68 84.2 G4 71.4, −77.92 (73.0, −76.75) 1.61 ± 0.23 4.6 0.96 80.9 K4 80.4, −75.76 (81.0, −74.74) 0.75 ± 0.17 0.6 1.41 83.1 L4 90.4, −74.64 (90.7, −73.44) 1.55 ± 0.63 0.6 2.81 85.9 ______
Location: Estimated location (°W Long., °Lat.) of center of ice host crater based on observed radar bright feature. Also given is the center location of the same crater (in parentheses) on the original Mariner-based NASA/USGS maps (Davies et al., 1978).
µc , σ sc , σˆsc : Circular polarization ratio (SC/OC), SC radar cross section, and equivalent full-disk SC albedo, respectively, from imagery summed over date group
13. The errors in σ sc and σˆsc are dominated by the ±20% calibration error. θ : Mean radar incidence angle for the radar feature.
41 Figure captions
Fig. 1. Shaded relief map of the north polar region of Mercury, adapted from the Atlas of
Mercury (Davies et al., 1978). Overplotted are labels for ice host craters (white letters), our estimate of the true north pole location (white cross, +), and the boundary of the region covered in Figs. 2 and 3 (black rectangle). The blank region on the right corresponds to the side of the planet that was not imaged by Mariner 10.
Fig. 2. Arecibo radar images of Mercury’s north polar region from observations on (a) July
25–26, 1999, and (b) August 14–15, 2004. Lighter shades denote higher radar reflectivity.
The 1999 image is in the SC polarization and the 2004 image is summed over the SC and OC polarizations. The mean radar illumination direction (arrow) is indicated for each image.
Letter labels are given in (a) for the more prominent crater features (D, E, H, J, K, L, M, N,
P, Y) discussed in the text and shown in Fig. 1.
Fig. 3. (a) Radar image of Mercury’s north polar region, formed by merging the images in
Figs. 2a and 2b. See text for details of the merge algorithm. (b) Radar image of the same region, but formed by doing a weighted sum of all of the SC and OC north polar images (date groups 1–12 in Table 1). Letter labels for features D–Y are given in (a) and the rims of corresponding Mariner-imaged host craters (circles) are shown in (b).
Fig. 4. Radar image showing a large-scale view of Mercury’s north polar region. This image was formed by splicing a left-side SC image (sum of date groups 1, 3, and 5) with a right-side SC image (sum of date groups 9–12). See text for details.
42 Fig. 5. Radar image showing detail on the right-hand (270°W) side of the north polar region.
This image was formed by summing SC images from date groups 9–12 (as on the right side of Fig. 4). Letter labels are given for some of the more prominent low-latitude features (see
Table 4).
Fig. 6. Shaded relief map of the south polar region of Mercury, adapted from the Atlas of
Mercury (Davies et al., 1978). Overplotted are labels for ice host craters (white letters) and our estimate of the true south pole location (white cross, +). The blank region on the left corresponds to the side of the planet that was not imaged by Mariner 10. The map has been cropped to the same boundaries as in Fig. 7 (except for extra cropping of the Mariner- unimaged side on the left margin).
Fig. 7. (a) Radar image of Mercury’s south polar region from observations on March 24–25,
2005 (date group 13). This is a weighted sum of the SC and OC polarizations. The mean radar illumination direction (arrow) is from the bottom and the dark blank area at the top is the region that is beyond the radar horizon on either of the two dates. (b) Same as the upper image, but with crater locations and labels overplotted. Crater rims (circles) are shown for radar feature host craters on the right (Mariner-imaged) side of the pole; the locations of these craters are based on a slight adjustment to the revised Mariner-based map grid of
Robinson et al. (1999). Also shown are the locations (crosses, +) of those radar features that were seen in the 2001 Goldstone imagery (Harcke, 2005) but not in the Arecibo imagery.
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Fig. 1
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