Astronomers who observed Pluto's eclipses in July and August are having trouble determining whether the planet's atmosphere has warmed or cooled since the previous observation, made 14 years ago
Kenneth Chang New York Times, Haaretz
The weather forecast for Pluto is still unclear. Astronomers from Arizona's Lowell Observatory and the Massachusetts Institute of Technology (MIT) say the planet's atmosphere has cooled by 10 to 20 degrees Celsius over the past 14 years, the surface has likely warmed slightly and a low layer of smog has dissipated. French astronomers dispute them. According to them, the air has cooled but slightly and the smog layer still exists in the planet's atmosphere.
These estimates seem to inspire no more confidence than the American tradition of "Marmite Day", starting on February 2nd. (According to tradition, on this day, the Marmite peeks out of its burrow to check if its image casts a shadow. If it does not cast a shadow, it is a sign that the day is cloudy - that is, spring has arrived. If its image casts a shadow, it is a sign that winter will last six more weeks, and the Marmite returns to its burrow). Even in the case of Pluto, all the findings are based on observations of shadows.
Pluto, which is at a distance of 4.5 billion km from the Earth and is getting further and further away from it, is nothing more than a point of light on the screens of telescopes placed on the Earth. Astronomers can infer something about the planet's atmosphere only when, thanks to a celestial coincidence, Pluto passes in front of a distant star and casts a faint shadow on Earth.
This happened once in 1988 and at the end of the 14 year wait - twice last summer. The dimming of the light from the hidden star indicates the degree of compression and temperature of Pluto's atmosphere. If Pluto is devoid of air, the star will suddenly disappear. But the atmosphere acts as a lens: it bends the starlight around the planet and causes the dimming to occur gradually. In the eclipse of the star in 1988, its light dimmed at a steady rate, until it reached 40% of its normal brightness, and then quickly disappeared.
The dimming rate indicated that most of Pluto's atmosphere, which reaches up to a height of 160 km above the planet's surface, is fairly uniform in temperature: from minus 173 degrees Celsius to minus 162 degrees.
The sudden disappearance shows that there is a layer 24 to 56 km thick on the surface, consisting of either a foggy mist that swallows the light, or colder and much more compressed air, which distorts the light until it is not visible from the earth. From other measurements, researchers know that the temperature of the surface on Pluto is minus 234 degrees Celsius.
On July 20, Pluto stood in front of the star P126A, but cloudy weather thwarted the efforts of all the researchers, except for two teams of observers in northern Chile. Astronomers had better luck on August 20, when Pluto obscured another star, P131.1. The eclipse was picked up at a number of large observatories in Hawaii and the western US.
Dr. Mark Bowie, an astronomer at the Lowell Observatory who made one of the two successful observations in July in Chile, said that the star's light had dimmed in a way that indicates a sharp drop in Pluto's temperature compared to 1988. According to him, there were also no signs of the haze layer or the cold air, meaning The thing that the layer has shrunk by at least 24 km, and maybe disappeared altogether. "We have surprises at the most basic level," Bowie said.
But the team that made the other successful observation in Chile announced that according to their analysis the temperature of Pluto's atmosphere is still minus 173 degrees. "We haven't seen many changes since '88," said team leader Dr. Bruno Sicardi, an astronomer at the Paris Observatory. "I don't understand how they could conclude anything about changes." In addition, Cicardi said, measurements from the August eclipse at the Canada-France-Hawaii telescope on Mauna Kea in Hawaii showed that the haze layer was still present.
Bowie and his research partner, Dr. James Elliott, an MIT astronomer, have not yet analyzed the data from August. Several telescopes in Hawaii, the Lick Observatory in California, and the Lowell Observatory then observed the eclipse in different wavelengths of visible and infrared light. A clearer picture of Pluto should be obtained within a few months. The infrared measurements will clarify whether it is a layer of cold air or of haze in the lower atmosphere. Cold air distorts all wavelengths equally. Haze absorbs visible light, but allows infrared light, which has longer wavelengths, to pass through. "This is also the reason why sunsets are red," said Dr. Sicardi.
According to Dr. Elliott, for now none of the explanations are satisfactory. For smog to cause the dimming observed in 1988, its particles must be quite large - too large to remain in Pluto's thin atmosphere. Computer models also fail to create an atmosphere where there has been such a sharp drop in temperature within 14 years. "There was no physical model that could explain what we saw in '88," Eliot concluded.
Astronomers agree that the planet's atmosphere appears surprisingly active, given the minimal amount of sunlight reaching it. In the August eclipse, Sicardi said, the star's light suddenly brightened for a few seconds, about 10 times its previous brightness, and then faded again — a phenomenon indicative of pockets of colder air. "In order to create such variations, dynamic activity is necessary," he said. "Energy is needed to maintain these buildings."
It is not clear how the new findings fit in with predictions that Pluto's cooling atmosphere will freeze and collapse within 20 years. This mainly depends on the temperature of the surface, which determines how much of the frozen methane and nitrogen will turn into gas. Bowie said that his computer models indicate that the surface may actually have warmed by a tenth of a degree since 1988. The surface has darkened - perhaps due to the retreat of glaciers - and this allows for a higher rate of heat absorption. But Pluto is moving away and cooling, and the trend is expected to reverse at some point. Astronomers do not know when the stars will provide another opportunity to observe Pluto's atmosphere and what will change by then.
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