The scientific journal Astrophysical Journal Letters is
publishing a study, led by researchers at the Astrophysical Institute of
the Canaries (IAC), which has been the subject of a report in the
journal Nature. It the study Jupiter is presented as an ideal laboratory
for research into exoplanets which are similar. Jupiter, the largest
planet in the Solar System, has large satellites around it. The study
has used the largest of the satellites, which is the biggest satellite
in the Solar System, Ganymede, as a mirror to analyze the atmosphere of
the planet. The observations were performed during an eclipse of
Ganymede by Jupiter, and allowed the researchers to observe Jupiter as
if it were a transiting exoplanet.
Its transmisión spectrum, observed as Ganymede began to be eclipsed,
and could be observed through the atmosphere of Jupiter, shows strong
extinction, reduction of the light due to clouds and to aerosols in
Jupter´s atmophsere, as well as strong absorption in the characteristic
bands of methane (CH4), and most surprising, ice crystals in a
stratospheric layer. These results are of relevance to the modelling
and the interpretation of transiting exoplanets, but they also offer a
new technique to characterize the upper layers of Jupiter’s atmosphere,
and to determine the abundance of water. It will also be useful in
helping establish the rate of comet impact on Jupiter, and its
consequences for the history of the formation of the Solar System.
The transit method
During the last two decades more than 1,800 exoplanets have been
discovered, 65% of them using the transit method. This entails observing
a star with very accurate photometry, to detect the slight drop in the
intensity of its light when a planet passes in front of it. For a small
proportion of these planets, those which orbit around the brightest
stars, their atmospheres can be studied using the method of transmission
spectroscopy, where one measures the difference between the light
emitted and the light transmitted, which gives a “fingerprint” of the
composition of the atmosphere. At the present time this is the most
successful technique for probing the chemical composition of the
atmospheres of exoplanets.
During the transit of the planet across the front face of the star,
some of the light from the star is blocked, and only a ten thousandth
part passes through the thin atmospheric layer of the planet (for a
planet like Jupiter, and a star like the Sun), bringing with it
information about its atmospheric layers and their components.
“In order to explore the limitations of this technique” explains Pilar
Montañés, a researcher at the IAC and the first author of the article “
we have applied it to study the atmosphere of Jupiter. We have measured
the transmission spectrum of Jupiter, observing it as if it were an
exoplanet. Our method has been to take high resolution spectra of
Ganymede, (Jupiter’s third satellite out) during its passage through the
shadow of the planet. In spectrum obtained when we divide the spectra
observed before and during the eclipse, the signals from the Sun, from
the Earth, and from Ganymede itself, (which is on a synchronous orbit
around Jupiter) are eliminated.”
The study shows that the strongest absorptions are due to methane, as
one would expect for Jupiter. However the observation of the extinction
due to clouds and to aerosol particles is also relevant. “Our results”
explains Enric Pallé, an IAC researcher and co-author of the article
“support previous results in which transmission spectroscopy indicated
the detection of clouds and of aerosols in “hot Jupiters”. As the
eclipse progresses, out method allows us to probe the atmosphere of the
planet in greater depth.
However the most interesting signal which was detected, between 1.5 and
2.0 microns, is probably due to stratospheric clouds of ice crystals.
“Our models,” notes Manuel López Puertas of the Astrophysical Institute
of Andalucia (IAA-CSIC), “have allowed us to determine that the column
of water ice contains 1,013 particles / cm2, a much larger
quantity of water than that previously measured in the vapour state. We
have also detected spectral lines of sodium iodide (NaI) in Jupiter´s
atmosphere, due either to the continual deposit of sodium from comets,
or to a continual flux of sodium from the satellite Io”.
Gabriel Pérez, SMM (IAC).
“This is the first time that this kind of observations has been performed from the ground, and have covered such a wide spectral range“ notes Beatriz González, another member of the team, who was also at the iAC when the study was carried out.
The observations were performed during two eclipses in 2012 using the
LIRIS instrument on the William Herschel Telescope at the Roque de los
Muchachos Observatory (La Palma), and the XSHOOTER instrument on the VLT
(Very Large Telescope) at the Paranal Observatory of the European Southern Observatory (ESO)
in Chile, in three spectral ranges: the ultraviolet, the visible, and
the infrared. Similar observations had been previously obtained to
obtain the transmission spectrum of the Earth using lunar eclipses, by
Enric Pallé, Pilar Montañés, and their collaborators in 2009.
Reference:
"Jupiter as an exoplanet: UV to NIR transmission spectrum reveals
hazes, a Na layer and possibly stratospheric H2O-ice clouds". ApJ Letters.
Artículo en Nature: http://www.nature.com/news/jupiter-glimpsed-as-aliens-would-see-it-1.16926
Contact:
Instituto de Astrofísica de Andalucía (IAA-CSIC)
Unidad de Divulgación y Comunicación
Silbia López de Lacalle - sll@iaa.es - 958230532
Unidad de Divulgación y Comunicación
Silbia López de Lacalle - sll@iaa.es - 958230532
