This page contains information related to my 2004 DPS abstract.



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Abstract

Radio Wavelength Observations of Titan with the VLA
B.J. Butler NRAO
M.A. Gurwell CfA

Radio wavelength observations of Titan are a powerful technique for probing through the atmosphere to the surface and subsurface. For more than 20 years, the VLA has been used to observe Titan at wavelengths from 0.7cm to 6cm. A synopsis of these observations will be presented. Three results in particular will be focused on. First, in 1992 seventeen short observations at 3.5cm were used to create a radio light curve for Titan. Preliminary reduction of that data showed no believable light curve (Grossman & Muhleman 1992, BAAS, 24, 954). New reduction, including incorporation of sophisticated algorithms for removing confusing sources (including Saturn), shows a detectable light curve, peaking around 180o longitude, with a peak-to-peak amplitude of ~10%. The mean brightness temperature of 79.3 +- 2.4 K (including absolute uncertainty) is consistent with that found from the earlier reduction. Second, in 1993 two long observations were performed in an attempt to directly measure the dielectric constant of the surface. The result from those experiments is a dielectric constant of 2.7 +- 1.7. Third, in 1999 three long observations were performed at 0.7cm in an attempt to detect C3H2 in the atmosphere. As a byproduct of these observations, brightness maps at 0.7 and 1.3cm were produced - the highest resolution radio maps of Titan ever made (~0.2"). These and other results will be presented.



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Introduction

What is on the surface of Titan? When the Voyager spacecraft passed by in 1980, it could not see the surface, due to obscuration by a thick haze (see, e.g., this description and Smith et al. 1981, Science, 212, 163).


Atmospheric sensing instruments did, however, measure relatively large amounts of methane (CH4) in the atmosphere (Hanel et al. 1981, Science, 212, 192). Although methane had been seen in Titan's atmosphere as early as 1944 (Kuiper 1944, ApJ, 100, 378), the abundance was poorly known. The Voyager results placed it at a few percent by fraction. While this is a relatively small amount, it is too large a fraction to be supported over long timescales because it would be photodissociated too quickly. A source for methane must therefore exist. It was suggested that a global ethane/methane liquid ocean could exist on the surface which would supply the methane to the atmosphere (Lunine, Stevenson, & Yung 1983, Science, 222, 1229). It had to be thick, though (of order 1 km), because an ocean of less than ~400 meters depth would provide too much tidal dissipation to allow for the current eccentricity of the orbit of Titan (Sagan & Dermott 1982, Nature, 300, 731).

How could this hypothesis be tested? Observations of the surface were needed, but it was clear that at optical wavelengths the surface was completely obscured, at the time the near-IR windows (see below) were not known, and further into the IR the observing facilities were inadequate. Radio wavelengths (anything longer than a few mm), however, easily penetrate through the haze and atmosphere to the surface and were therefore considered a primary observational tool for understanding the surface of Titan. Because the reflective (or emissive, for passive work) properties of a solid (ice) surface would be quite different from those of a hydrocarbon liquid surface, if sensitive radio or radar observations could be made, the two types of surfaces could be distinguished.

In 1989, radar echoes were first successfully obtained from Titan, using the combined Goldstone/VLA radar. The results were completely consistent with an icy surface - i.e., the radar echoes were similar to what we had seen from the Galilean satellites (Muhleman et al. 1990, Science, 248, 975; Muhleman, Grossman, & Butler 1995, AREPS, 23, 337).



A global, kilometers-deep methane/ethane ocean was completely ruled out by these observations. However, variations were seen in the radar echoes which were hard to understand in the context of a normal icy surface - could there be ethane/methane "lakes" or "seas" present? Dynamically it was possible (Dermott & Sagan 1995, Nature, 374, 238). In addition, there was early indication from near-IR observations that it might be possible.

In the early 1990's it became apparent that at some wavelengths in the near-IR the surface was being sensed and that the surface was heterogeneous (Lemmon, Karkoschka, & Tomasko 1993, 103, 329; Griffith 1993, Nature, 364, 511). Then, in 1995, HST provided the first resolved images of the surface through one of these windows (Smith et al. 1996, Icarus, 119, 336).



It was suggested that the "bright" (in fact the contrast is only of order 10%) areas were highlands ("continents") which had exposed ice, washed clean by hydrocarbon rain. The dark regions were ethane/methane ocean.

Since these early observations, there have been higher resolution near-IR images made with speckle and AO techniques, including those from La Silla/ADONIS (Combes et al. 1997), Keck (Gibbard et al. 1999; Brown, Bouchez & Griffith 2002; Roe et al. 2002; Gibbard et al. 2004), Gemini (Roe et al. 2002), CFHT (Coustenis et al. 2001), and the VLT (Hartung et al. 2004 - image shown below).



With Cassini now in the Saturn system, it has started to send back some spectacular images of Titan, including this one:



and this map:



It will only get better as Cassini takes more and higher resolution images of Titan, and sends back a wealth of other data, including radar and radio observations.

Further evidence for the existence of liquid hydrocarbon was provided by recent radar observations from the upgraded Arecibo radar (Campbell et al. 2003, Science, 302, 431). These observations show a "specular glint" present in a large fraction of the echoes. This could be the result of liquid lakes or seas.

Before the near-IR and more recent radar results were in hand, there were attempts to measure the passive radio emission properties of Titan. Similar to the radar experiments, such observations could be used to discriminate between liquid ethane/methane and solid ice (because of the different dielectric constants, and hence emissivities, of the two types of surfaces). They still have intrinsic value, as they are obtained over a wide wavelength range, and hence sample different surface and subsurface size and depth scales. The purpose of this work is to review those observations and present results from them.




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Instrument



The observations were all done at the Very Large Array (the VLA is operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.), which is a collection of 27 radio antennas spread out on the plains of San Augustin, New Mexico.



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1992 Light Curve Results

In February of 1992, Arie Grossman and Dewey Muhleman used the VLA to observe Titan at X-band (effective frequency of 8.616 GHz) on 17 different days (each observation was about 1 hour). The intent was to see if there was a variation in the light curve, which would be further indication of the possible presence of liquid ethane/methane lakes or seas postulated at that time, or at least of surface variations. Data reduction at that time showed no statistically significant variation as a function of longitude, and derived a value for the brightness temperature of 80.4 +- 0.6 K (statistical error only - there is a 3% absolute uncertainty from the flux density scale), implying a surface dielectric of 2.9 +- 0.6 when combined with other radio wavelength observations (Grossman & Muhleman 1992, BAAS, 24, 954).

I have re-reduced all of this data, taking advantage of new routines to handle planetary motion and confusing source subtraction within the AIPS package (in which all of the reduction was done). The data reduction steps which are above and beyond normal include:


A final image is made to check the flux density fitted from the visibilities. These images can be combined, after shifting the data so that Saturn is in the field center (via the same phase shifting technique referenced above), to make a combined image where it appears that Titan is orbiting around Saturn (in reality, since we observe with Titan at the field center, if we just combined the images with no phase shift to get Saturn at the phase center, it would appear as if Saturn were rotating around Titan).





The following table gives the relevant results:

Date in 1992 Sub-Earth longitude flux density (mJy) brightness temperature (K)
Feb 01 76.1 1.31 +- .03 76.0 +- 1.7
Feb 02 99.5 1.36 +- .03 78.8 +- 1.7
Feb 03 124.5 1.39 +- .04 80.5 +- 2.2
Feb 04 146.1 1.47 +- .04 85.0 +- 2.2
Feb 07 211.2 1.34 +- .04 77.7 +- 2.2
Feb 08 234.8 1.39 +- .03 80.5 +- 1.7
Feb 09 258.1 1.31 +- .04 76.0 +- 2.2
Feb 13 347.4 1.40 +- .04 81.1 +- 2.2
Feb 15 29.9 1.28 +- .03 74.3 +- 1.7
Feb 16 53.8 1.32 +- .03 76.6 +- 1.7
Feb 17 76.7 1.32 +- .02 76.6 +- 1.1
Feb 18 99.9 1.42 +- .03 82.2 +- 1.7
Feb 20 144.3 1.50 +- .04 86.6 +- 2.2
Feb 23 211.5 1.40 +- .04 81.1 +- 2.2
Feb 25 256.5 1.40 +- .04 81.1 +- 2.2
Feb 27 301.2 1.31 +- .04 76.0 +- 2.2
Feb 29 345.0 1.28 +- .04 74.3 +- 2.2
Mean (weighted) --- 1.356 +- .015 78.6 +- 0.8

Note that the uncertainties here are only statistical. In absolute terms, there is a roughly 3% uncertainty in the flux density scale (derived using 3C286 in this case) which dominates the uncertainty in the final flux density and brightness temperature.

Here is a plot of the data (where observations are at two very near longitudes, I have combined them - I've also added one of the data points from the 1993 observations [see below] as the filled triangle).





A sinusoidal fit is a decent representation to the data, and is shown on the plot above - it peaks around 180 deg and has a peak-to-peak amplitude of 8 K. This peak-to-peak variation is much larger than the error on the measurements (it's about 20-sigma when compared to the combined noise on the data), so is real. This is mildly consistent with the radar results, which show a modest peak in reflectivity around 90 deg - high reflectivity means low emissivity, and a minimum in the radio brightness. But, in fact, we expect a direct anti-correlation, and we do not see that (we expect to be 180o out of phase, and we are 90o out of phase). This is also consistent with the infrared albedo maps, which show the darkest regions from 150o to 330o and the brightest at the other longitudes - here is the map from the recent ESO/VLT results (see also the Cassini map above):



Bright regions are expected to be colder - both because they reflect more incoming sunlight and because they may be higher, topographically. Since they are colder, they will also appear so in the radio (which is a combination of physical temperature and emissivity). If they are ice, and the darker regions are hydrocarbon liquid, then the variation is even more pronounced, as the hydrocarbon has lower dielectric and hence higher emissivity. So these results are in good agreement with the IR albedo results.


A model has been constructed which accounts for the radiation from the surface and atmosphere of Titan (with a crude subsurface contribution) - similar to that constructed for Venus (Icarus 2001, 154, 226). The important inputs to the model are the values of temperature, total pressure, and fraction of CH4 and Ar, all as a function of altitude in the atmosphere. The absorption in the atmosphere is calculated by scaling the pure N2 absorption via (see Gurwell & Muhleman 1995, Icarus, 117, 375):
     k = kN2 fN2 (1 + 3 fCH4)
where kN2 is the pure N2 absorption given in Dagg et al. 1975, Can. J. Phys., 53, 1764 (but see also Courtin 1988, Icarus, 75, 245 and Borysow & Frommhold 1986, ApJ, 311, 1043), and fN2 and fCH4 are the fractions of N2 and CH4 in the atmosphere. An equatorial surface temperature of 93.9 K is used, with vertical profile given in Lellouch et al. 1989, Icarus, 79, 328. We use an equator to pole surface temperature difference of 13.5 K (this matches the observed 4 K difference from equator to 60o latitude observed in the Voyager IRIS data - see Lorenz et al. 2003, 51, 353; Courtin & Kim 2002, P&SS, 50, 309; Samuelson et al. 1997, P&SS, 45, 959). We use a methane abundance of 4% at the surface (Lemmon, Smith, & Lorenz 2002, Icarus, 160, 375), with vertical profile given in Lellouch et al. 1989. We use an argon abundance of 4% (Gladstone et al. 2002, BAAS, 34, 902), constant with altitude. We then run the model with varying values of surface dielectric (using a smooth dielectric sphere for the planet surface), in order to fit the surface dielectric given the observed brightness temperature.

For the observed geometry (the Sub-Earth latitude, 20o at the time, makes a difference because of the equator-to-pole temperature variation), the mean brightness temperature results in an effective dielectric constant of 3.2 +- 0.3 (3.2 +- 0.8 taking into account the flux density scale uncertainty).

Results:



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1993 Surface Dielectric Results

One way of overcoming the uncertainty in the flux density scale and how it propagates into uncertainty on the derived dielectric constant is to take polarimetric measurements with an interferometer. Such measurements are insensitive to overall flux density scale offsets, since for the polarized quantity of interest (from which the surface dielectric can be derived) the total flux density is divided out (see the discussion in Butler & Bastian 1999, SIRA II, ASP Conf. Ser. Appendix A and references therein).

In February of 1993, Arie Grossman and Dewey Muhleman used the VLA to observe Titan at X-band (effective frequency of 8.616 GHz) on 2 different days (one at each elongation) in 10-hour long tracks in an attempt to do this. These observations were never published (or even reduced), so the reduction I have done is completely new.

In fact, this is a very difficult measurement, since the polarized flux density is of order a few percent of the total flux density, which for Titan is less than 1% of the total flux density of Saturn (the expected peak polarized flux density at the time of observation was roughly 50 microJy, while the total flux density from Saturn was 500 mJy). So, images with a dynamic range of 100000:1 must be made in order to successfully subtract out Saturn (as a confusing source). This is extremely difficult for a resolved source with complicated structure like Saturn, especially when it is out near the half-power point of the primary beam of the antennas, and varying where it is in the power pattern as a function of time (though our general knowledge of that structure helps). However, with the help of the new processing techniques outlined above, a successful result was obtained - albeit with a large uncertainty. The derived dielectric from these experiments on the two different days was 2.7 +- 1.7 and 3.4 +- 0.8, with derived mean surface slopes of 8.6 +- 8.0 and 9.1 +- 5.5 degrees (using an exponential surface slope distribution model - see Muhleman 1964, AJ, 69, 34).

Combined results:



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1999 Resolved Imaging Results



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Other Results

here go the other previous results (Jaffe, etc...)



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Bryan Butler
Email: bbutler at nrao dot edu
Snail mail:
     NRAO
     1003 Lopezville Rd.
     Socorro, NM    87801
Phone: 505.835.7261

Last Modified on 2004-Aug-31