Elsevier

Icarus

Volume 168, Issue 2, April 2004, Pages 336-343
Icarus

Longitude-resolved imaging of Jupiter at λ=2 cm

https://doi.org/10.1016/j.icarus.2003.11.014Get rights and content

Abstract

We present a technique for creating a longitude-resolved image of Jupiter's thermal radio emission. The technique has been applied to VLA data taken on 25 January 1996 at a wavelength of 2 cm. A comparison with infrared data shows a good correlation between radio hot spots and the 5 μm hot spots seen on IRTF images. The brightest spot on the radio image is most likely the hot spot through which the Galileo probe entered Jupiter's atmosphere. We derived the ammonia abundance (= volume mixing ratio) in the hot spot, which is ∼3×10−5, about half that seen in longitude-averaged images of the NEB, or less than 1/3 of the longitude-averaged ammonia abundance in the EZ. This low ammonia abundance probably extends down to at least the 4 bar level.

Introduction

Conventional radio interferometry images are integrated over many hours, with 12 h or more not being unusual. This is required both to meet the required sensitivity and to use Earth rotation synthesis to achieve good sampling of the Fourier plane. Consequently, imaging planets in the conventional way rotationally smears any longitudinal structure. In principle one can merge together snapshots of the same rotational aspect of the planet, from observations taken on different days. This approach was probably first used by de Pater (1980) when observing Jupiter. In practice, the longitudinal smearing is still limited by the fineness of the rotational phase bins that the data are broken into. At decimetric wavelengths, the spatial resolution of the data used to be a significant fraction of a planetary radius, and extra smearing caused by rotation was not so severe. These are typically tens of degrees of rotation. To image the thermal radiation of the planet itself, one requires a higher resolution, typically 1–2″ for Jupiter, or an integration time of less than 10 min. Such an image has such poor signal-to-noise that even the North Equatorial Belt (NEB) on Jupiter may not be recognized. To date, all radio wavelength images of Jupiter's thermal emission have therefore been averaged over longitude (i.e., integrated over hours rather than minutes of time).

Sault et al. (1997) developed an alternative technique for Jupiter's synchrotron emission, based on a three-dimensional tomographic approach. This technique cannot be used for the thermal emission from the disk of a planet. The present paper describes a technique to form a radio “map” of a planet's thermal radio emission which avoids rotational smearing, but which still allows good sensitivity and Fourier plane sampling. The main motivation to develop this technique has been the need to form radio maps of Jupiter.

Infrared observations of Jupiter at 5 μm show that the NEB is not a smooth belt, but rather contains numerous “hot spots”—regions which are much hotter than their surroundings. In these hot spots, one probes relatively deep, hot, levels in Jupiter's atmosphere because the infrared opacity is relatively low here. A main source of opacity at these wavelengths is cloud layers; i.e., the absence of cloud particles in hot spots lets one probe much deeper layers. The general consensus therefore has been that hot spots are dry regions on Jupiter, perhaps areas of downdrafts: rising air on the planet cools off, and once the temperature drops below the condensation temperature of ammonia gas, droplets and clouds form. Above the clouds the air is dry, and if there are places of downdrafts the air may indeed be dry and hence devoid of cloud particles.

On 7 December 1995 at 22:10 UT the Galileo probe entered Jupiter's atmosphere, by chance entering in a hot spot (at a jovicentric latitude of 6.5° N and at ∼4.5° W (system III); e.g., Orton et al., 1998). During its descent, the probe measured the atmospheric structure (temperature, pressure, density), gas composition and cloud properties down to a depth of ∼20 bar. Folkner et al. (1998) derived the ammonia abundance profile in the hot spot from the gradual and progressive attenuation of the probe signal (at 1.4 GHz) while the probe was descending in Jupiter's atmosphere. They determined an NH3 abundance ∼3.6±0.5×the solar N value1 at pressures between 8 and 12 bar, with a possible decrease at higher altitudes. The NH3 abundance has since been obtained from the Galileo probe data (Mahaffy et al., 1999), and was found to be 3.2±1.4×solar N, i.e., consistent with the results of Folkner et al. (1998). Sromovsky et al. (1999) deduced an NH3 profile at P≲3 bar from data taken with the Net Flux Radiometer (NFR) on board the Galileo probe. They found that NH3 in the hot spot was decreasing linearly from 0.5×solar N at 2 bar to 0.01×solar at P≈0.5 bar. The Galileo probe further measured very low abundances of H2S at P≲10 bar and H2O at P≲20 bar. Various researchers have investigated downdrafts and planetary waves to explain the altitude profiles of the condensible gases as measured by the Galileo probe (see, e.g., Atreya et al., 1996, Showman and Ingersoll, 1998, Wong et al., 1998, Showman and Dowling, 2000, Friedson and Orton, 1999).

de Pater et al. (2001) (henceforth referred to as dP2001) observed Jupiter at radio wavelengths (at 2, 3.6, and 6 cm wavelength) near the time of the probe entry. Since the main source of opacity at radio wavelengths is ammonia gas, dP2001 derived the disk-averaged ammonia abundance profile from the data and compared this with the Galileo probe results. They could only reconcile an NH3 abundance ∼3.6±0.5×the solar N value at P>8 bar if the NH3 abundance decreases globally at P<4 bar, and to subsolar values at pressures P≲2 bar. They further show (based upon disk-resolved but longitude-averaged images) that the NH3 abundance in the NEB is ∼50–70% of the value in the EZ (Equatorial Zone), while it is subsolar in both bands at P<2 bar. In the NEB this low abundance has to extend down to P∼4–6 bar.

With longitude-resolved radio images one could correlate the occurrence of infrared hot spots with radio hot spots, if they were to exist. Moreover, the main source of opacity at the two wavelength ranges is different: at radio wavelengths one is sensitive to the condensible gas, ammonia, while at IR wavelengths one is sensitive to cloud particles. So microwave observations would provide information on the condensible gas, a necessary piece of information for the development of dynamical models of Jupiter's atmosphere.

To investigate the occurrence of hot spots, and in particular the hot spot in which the Galileo probe descended, we developed an algorithm to construct a longitude-latitude map of Jupiter, and applied this algorithm to 2-cm data taken at the VLA near the time the Galileo probe descended into Jupiter's atmosphere; the longitude-averaged maps were presented in dP2001. In this paper we discuss the algorithm (Section 2) and present the observations (Section 3) and results (Section 4).

Section snippets

The mapping technique

To understand the basis of our mapping technique, consider Fig. 1. Figure 1a shows four identical crosses as they would appear on the face of the planet. The viewing geometry distorts the appearance of the crosses—Fig. 1b presents the crosses side-by-side to help accentuate the distortions. On a sufficiently small scale, this distortion can be approximated as a linear transformation of the coordinate system. That is, to first approximation, by appropriate rotation and skew (length scaling), we

Observations and data analysis

As detailed in dP2001, in order to ‘mark’ the location of the Galileo probe entry on a radio image, we observed the planet with the VLA as close to the probe entry time as possible. Unfortunately, when the probe entered Jupiter's atmosphere on December 7, 1995, the planet was close to conjunction (December 19, 1995) and the array was in the B-configuration, so that much of Jupiter's emission would be significantly resolved out at short wavelengths. So this period was less than ideal to image

Comparison with IRTF data

The resemblance of our radio map (Fig. 2) with infrared images as presented by Orton et al., 1998, Ortiz et al., 1998 is striking. Series of hot spots show up at jovigraphic latitudes between 7–11°, i.e., in the same latitude band as the hot spots seen at infrared wavelengths. The bright spot at a West longitude of 19.7° (system III), and jovigraphic latitude 9.5° N (= jovicentric latitude of 8.3° N) is most likely the hot spot through which the Galileo probe entered; henceforth referred to as

Conclusions

We have presented the first longitude-resolved map of Jupiter's thermal radio emission at a wavelength of 2 cm. This map clearly shows the presence of radio-bright hot spots. A comparison with IRTF images shows that each radio hot spot is also a hot spot at infrared wavelengths. Hence the hot spots must indeed lack clouds (infrared wavelengths) and the air must be dry (low NH3 abundance). We derived an ammonia abundance in the hot spot where the probe went down of approximately 3×10−5 down to

Acknowledgements

We thank Glenn Orton and Jose Ortiz for providing us with the IRTF image, a portion of which is shown in Fig. 4. This research has in part been funded by NASA Grant NAG5-12062 to the University of California in Berkeley.

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