I did get to measure the center of the Great Red Spot on December 9, 1996 (23:30 UT) and it was at 006.6 (SIII), or 062.4 (SII). Significant SEB activity was noted north of the GRS, especially to the NW, but Glenn informed me that we will probably not image the GRS until June, 1997.
As Galileo emerges from behind the Sun, DSN antennas on Earth are once again able to hear Galileo's radio signal. The further Galileo gets from the Sun in the sky, the clearer the signal will get. This week the Sun's affect on Galileo's radio signal is small enough to restart the playback of recorded data.
This week's playback includes a variety of observations taken during Galileo's first close encounter with Europa. Most prominent in this week's plans are observations taken of features in Jupiter's atmosphere known as Hot Spots. The observations scheduled for playback this week were taken by SSI, NIMS and PPR. Recall that Hot Spots are areas of the atmosphere where the clouds are relatively clear. These clear regions allow energy from the deeper, warmer layers to escape into space. These observations are the deepest Galileo will be able to "see" into Jupiter's atmosphere. Also recall that Galileo's atmospheric probe penetrated Jupiter's atmosphere very near a Hot Spot back in December 1995.
Observations of Io make up most of the remaining portion of the playback plans. A global 6-color observation of Io by SSI, global thermal day and night side observations by PPR and spatial and spectral observations by NIMS are all returned during this week. Recall that during the last encounter, Galileo's flyby of Io was the second closest (after the pass during the Callisto-3 orbit) of the orbital tour.
This week's playback plans are rounded off by observations designed to increase the global and regional imagery of selected targets. A regional map of Europa, a global 4-color map of Amalthea and a global 3-color map of Thebe are all scheduled for transmission to Earth. All of these were taken by SSI.
Pre-SL9 Comet Crash to Jupiter ?
---- Discovery of an old drawing of possible impact spot on Jupiter recorded in 1690 ---
Are there any cases where astronomers witnessed impact spot on Jupiter (like those produced by the fragments of Comet Shoemaker-Levy) in the pre-photographic era? After the Comet D/Shoemaker-Levy 9 crashed on Jupiter in 1994, many historical records from the period 1600-1900 have been examined by several astronomers. While some candidates of possible impact origin have been noticed, "the descriptions and drawings lack the detail required to meet the necessary condition" (T. A. Hockey, 1996, in Planet. Space Sci., 44, No.6, pp.559). However, it is important to continue our effort to discover such records.
A Japanese amateur astronomer, Isshi Tabe, who is studying historical records of Jovian atmospheric phenomena, tried to find out undiscovered drawings preserved deeply in libraries. When he found a drawing in the library of Paris Observatory, France, he was very excited because it supplied the "detail required to meet the necessary condition for impact spot". Namely, it contains the time variation of the dark spot during 18 days. Ms. Michiwo Jimbo, who is also an amateur astronomer, translated old French in the drawing to Japanese. Dr. Junichi Watanabe, who had organized Japanese SL-9 impact observations at the Okayama Astrophysical Observatory, joined to analyze the characteristics of the drawing.
This was drawn by famous astronomer Giovanni-Dominique Cassini (1625-1712), who was one of the most reliable, excellent visual observers. Detailed description of this observed spot appeared in a text of "Nouvelles déscouvertes dans le globe de Jupiter" which is in the Library of the Paris Observatory. In this drawing, he recorded time variation of a dark spot, which appeared on December 5, through December 23, 1690.
Several characteristics of this spot are similar to those of impact spots of the SL-9.
The strongest evidence for convincing them is the recorded time variation of over 18 days. Such a long-term evolution was never found in other candidates surveyed so far. The spot became crescent shape, and extended to about 10 degrees in the longitudinal direction after 10 days from its appearance. Immediately after this epoch, it was seen as fragmented dark patches. On 14 days after the appearance, two patches merged again to one long filament-like structure. The final situation after 18 days shows three narrow patches lying in the longitudinal direction. The eastern patch clearly shows the effect of the wind shear or of the unseen atmospheric structure such as vortexes. Such evolution is similar to the H impact site observed by the Hubble Space Telescope in the case of Comet Shoemaker-Levy. The average growth rate of the elongation was 0.6 degrees per day, which corresponds to 8.6 m/s of the zonal wind shear at this latitude.
They carried out a simple simulation, assuming that the initial spot was produced by the ejecta of the impact, and applied a reduced wind speeds of the Voyager spacecraft, and confirmed that the primary effect of the time variation is simply zonal wind shear, the same as in the cases of the SL-9 spots. They concluded that this spot was probably produced by an impact of a single astronomical object, the size of which was comparable to those of fragments A, C, or E of the SL-9. Because there is no other spot during the studied period, the impact body may have been a single object.
This discovery of a possible impact spot also reminds us of the importance of data archives, which may produce new insights to science for the future. The library work in the Paris Observatory is a typical example of the excellent effort to preserve such historical records. The drawings may even result in significant findings in the future. It should be stressed that we should take care of archiving astronomical data for the astronomers of the next generation.
This work will be published as
"Discovery of a Possible Impact Spot on Jupiter Recorded in 1690"
Tabe, I., Watanabe, J. and Jimbo, M.
Pub. Astron. Soc. Japan(Letter), 49, No.1, 1997.
(accepted Dec. 2, 1996)
A copy of this paper is available online.
Because the SL-9 features were so dark near the limb and terminator, we sent the following query to firstname.lastname@example.org and received the following response.
To: Isshi Tabe
Is there any indication of whether the spot remained dark at the limb? Elmer Reese, an experienced visual observer of Jupiter, felt that this characteristic from SL9 was so unique that any regular observer would be aware of it and comment on it. Are there any such comments that you have found?
If so, we would find that very interesting. We will make your preprint's location known to IJW readers in our next mailing.
From: Isshi Tabe
Unfortunately Cassini did not mention about the aspect of the Spot at limb or terminator. His interest was condensed in the rotation of equatorial region. So he wrote about its appearance not so detailed. He only mentioned about its appearance in Dec. 5, 1690. "The spot is true round and its diameter is 1/20 of Jovian disk as a shadow 3rd satellite."
I have also continued visual observation for this 20 years. Many Japanese observers and I also aware that the visibility of SL9 impact spots don't decrease at limb or terminator. We consider that the black material lies in the stratosphere of Jupiter above the thin NH3 haze layer. So, we feel visibility of them doesn't decrase.
665-8 Shimo-Tsuruma, Yamato-City, Kanagawa 242, Japan
Phone 81-462-72-6384 Fax 81-462-78-1161
Thank you for your communication, Isshi Tabe, and I hope that others will take note of this historical paper. -- Reta Beebe