A dark spot on Jupiter
Published on Jul 29, 2009 at 11:48 pm.
2 Comments.
Filed under Jupiter, astronomy, observing.
Amateur astronomer Anthony Wesley took an image of Jupiter on July 19 showing a dark spot on the planet. The spot looked remarkably like the sort of impact features left when Comet Shoemaker-Levy 9 hit Jupiter in July 1994. In fact, a lot of people very quickly announced that this was another impact on Jupiter. When I was asked, I hedged a bit. Just because it looks similar, doesn’t mean that it is the same sort of thing! However, soon infrared images of Jupiter showed that the dark spot was also quite warm, as would be expected from an impact. So, now I am willing to accept that this is, indeed, another impact feature.
The impact feature won’t last. Already it is being distorted by the high altitude winds of Jupiter. I don’t want to call it a “scar” as many others are doing, since it is such a transient feature. It no more scarred Jupiter than, say, a mosquito bite scars a human. Likewise, since Jupiter has no solid surface, it is not a “crater” as I’ve also heard both this and the Shoemaker-Levy 9 impact features sometimes described. The impact feature is composed of dust particles and gas. The dark color is likely due to sulfur compounds, if it is dark for the same reasons as the impact spots from Shoemaker-Levy 9.
When an impacting body strikes Jupiter, it doesn’t make it all the way to the center of the planet where there is a solid core. Rather, the object hits the atmosphere and compresses the air in front of it. That tends to slow the incoming body, and it puts stresses on the impacting object. Those stresses can eventually break it apart. The deeper into the atmosphere that the impacting body punches, the thicker the air, so the more the resistance to its motion. Eventually, the impacting body explodes. The stronger it is to start with, then the deeper it makes it into the atmosphere before it breaks apart and explodes.
As the impacting body moves through the atmosphere, though, it has to push the air out of the way. That leaves a sort of tube of hot thin gas behind the impactor as it passes through the atmosphere. Eventually, the pressure surrounding the tube causes the atmospheric gasses to rush back into place, but that takes a few moments. The impacting body typically has reached its terminal depth by the time the tube collapses. Thus, when the explosion happens, it travels up the tube of low pressure air along the entry trail of the impactor. That has the effect of carrying the residue of the explosion up to a point above the upper cloud layers. That was what happened with the Shoemaker-Levy 9 impact. The material dredged up included material from Jupiter’s middle and lower cloud layers, not just the material of the impacting body. The bigger and deeper impacts from the Shoemaker-Levy 9 fragments were the sources of the dark spots then. Studies of the impact sites from Shoemaker-Levy 9 helped planetary scientists understand more about Jupiter’s atmosphere. Hopefully, the same will happen from this impact’s effects.
Jupiter has a very dynamic atmosphere. There are always spots appearing in Jupiter’s clouds from storms. These spots are normally white ovals, but sometimes colored spots can appear. Recently some smaller cousins of the Great Red Spot have made an appearance. Sometimes there are some clear spots that form. Once in a while, though, observers have seen dark spots on Jupiter. Before the Shoemaker-Levy 9 collision, there was all sorts of speculation about what we might see as the effects of the impacts. Speculation ranged all the way from pretty much no visual effect to the formation of a ring system similar to Saturn’s rings. A few of the doomsayers, of course, were going on about Jupiter igniting into a giant explosion since it is made of mostly hydrogen. None of those doomsayers, though, was actually a scientist, so most of us pretty much ignored them. I was at a meeting of the American Astronomical Society in Minneapolis a couple of months before the impact, though, and I heard some planetary scientists proposing that the impact sites might actually be dark in color due the sulfur dredged up from the lower cloud layers. Since nobody had observed an impact event happening, we didn’t really know. After the Shoemaker-Levy 9 impact in 1994, I remember reading speculation that some of the dark spots observed over the last few centuries may have actually been impact feature misidentified as storms. Naturally, there is no way of telling. But, if some of those dark markings were indeed impact events, then that would mean that Jupiter gets hit far more often than most people think. If that is the case, then it would imply that Earth, too, should get hit more often by large bodies than we generally believe. That would be a sobering thought.
But, I should hasten to add that not every dark thing on Jupiter is an impact. On Saturday, July 18, I was doing a star party. Several amateur astronomers came to help. After a while, Jupiter was high enough to view. We noticed that there was a spot on the planet. It was definitely a spot, but the air was very unsteady, and the image was dancing all over the place. It was hard to get a good view of the thing. At first, we speculated that it might be a shadow transit. But, when I looked at my table of Jupiter satellite events, the only shadow transit at about that time had ended an hour earlier. So, we thought that it may be the red spot. If so, then the red spot is darker than it was the last time that I looked. Then, a day or two later, word got out of the dark marking that looked like an impact. That was the same night that we were observing Jupiter! Naturally, I wondered if that was what we were seeing. But, looking at the time that the dark feature was first imaged by Wesley, I can see that it was not the same thing. Even if the impact had already formed the dark marking, it would have been just on the far side of the planet when we were looking. But, looking at a table of Red Spot transits, it makes sense that we were seeing the Great Red Spot. It just didn’t look red. But, with seeing conditions as bad as they were at the time, I wouldn’t trust much of anything that we were seeing that night.
Over the next few weeks, the upper air winds of Jupiter should distort, stretch, and dissipate the particles and aerosols that make up the impact spot. Indeed, that is already happening. Again, that is consistent with this being an impact feature, not a storm. It will be interesting to observe how this changes over time.
-Astroprof
Image courtesy NASA, HST
Mike Salway on August 3, 2009 at 8:38 am: 1
Thanks for the article, that’s a nice read and summary of the forces at play with this impact.
Anthony is a good friend of mine and I first broke his discovery just over 2 weeks ago today!
I’m literally outside, right now, imaging it too.
Cheers and keep up the good work.
Aglolink on August 11, 2009 at 10:04 pm: 2
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