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A Way to Other Worlds |
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A Way to Other Worlds |
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Resp. Giuseppe Piccioni
INAF - Istituto di Astrofisica e Planetologia Spaziali
Currently, our view on planetary atmospheres derives from our
experience in the solar system that is our only available reference.
In this WP we plan to characterize the variability of Solar System
atmospheres in order to correctly interpret the exoplanet data, and
study in laboratory the properties of mixtures of gas that mimic
the physical and chemical conditions in planet atmospheres.
Variability of Solar System atmospheres:
The atmospheres of Solar System planets are highly variable in terms
of composition, aerosol content and air temperatures not only
comparing different bodies, but also within individual planets, as
function of space and time.
Studies of exoplanets by means of stellar occultation, reflection
or direct emission techniques are potentially prone to interpretation
errors of data if the effects of similar variabilities are not
properly taken into account. Occultation data merge in a single
observable the rays passing along the entire terminator or a whole
emisphere of the exoplanets, blending therefore very different
physical conditions.
As a first step, we will perform an extensive characterization of
the variability of atmospheres of Solar System planets by statistical
analysis of visible and infrared spectra. We will base on the
extensive archives of IR image spectrometers data from Venus Express
(VIRTIS-M), Cassini (VIMS), Galileo (NIMS) and New Horizons (LEISA)
missions, available at the PSA-ESA and PDS-NASA repositories. IR
hyperspectral are particularly attractive for our purpose since IR
spectral range hosts spectral features related to a great number
of atmospheric physical parameters, e.g.: gas mixing ratios, clouds
compositions and particle sizes. The forthcoming exoplanets projects
host IR wide spectral range spectrometer instrumentation.
Available dataset from hyperspectral Solar System instruments consist
typically of millions of individual spectra acquired over long time
spans and different locations on the target planets. All these
properties improve the quality and reliability of a statistical
analysis.
For their role of likely paradigm for exoplanets with thick
atmospheres, Venus, Jupiter and Saturn will be considered for our
analysis. The proposed work is articulated along the following
steps:
Laboratory Simulations
The interpretation of the future observations depends upon the
understanding of how the planet emission/absorption is affected
by the stellar spectrum. In particular, it is important to know in
detail the optical characteristics of gases in the physical conditions
of the planetary atmospheres and the radiation induced phenomena
such as photochemical and biological ones. Insights in this direction
can be achieved from laboratory studies of simulated planetary
atmosphere of different pressure and temperature under the effects
of radiation sources, used as proxies of different bands of the
stellar emission.
This part of the programme is organized in two complementary research
activities: optical characterization of simulated atmospheres, and
effects of the stellar radiation on the planet atmosphere and
spectral biomarkers or biosignatures induced mainly by UV irradiation.
These two activities will contribute to build a unique data base
for the future interpretation of the exoplanetary atmosphere spectra.
A. Measurements of weak absorption in dense planetary atmospheres
We plan to simulate a planetary atmosphere in the laboratory with
chemical composition, temperature and variable density and measure
the optical characteristics with a really sensitive technique able
to measure absorption coefficient up to about 10-8cm-1 in the
spectral range of 1-12 µm. With this technique, known as Cavity
Ring Down (CRD), is possible to reproduce a optical path of some
tens of km into a cell of 50 cm of length. In order to reproduce
the different condition of a real atmosphere, it is possible to
vary the temperature of the cell in both directions and insert gases
with pressure in the range between 0 and 50 bar. The CRD cell will
be mounted inside a vacuum chamber (already available at INAF -
IAPS) and it will operate with cooling or warming system and
illuminated by a tunable laser with appropriate optics and detectors.
Preliminary experiments performed with the collaboration of CNR-ISAC
(exploiting a tunable laser at 1.18 µm) show that it is possible
to detect 50 ppm of water vapour in a CO2 atmosphere at 40 bar of
pressure. With these experiments, the continuum absorption effects
have been observed in a very dense atmosphere. This situation is
typical for atmospheric condition at the surface of Venus.
We consider that the investigation of the optical properties of a
CO2 atmosphere with traces of other gases like water vapour, carbon
monoxide, oxygen and other gases could be a necessary starting
point. The best way to study the properties of that atmosphere in
term of observables is to exploit that part of the spectrum where
the CO2 absorption is very week (spectral windows). These spectral
windows allow us to penetrate and explore very deeply in the
atmosphere where on the other hand the absorptions is very difficult
to measure in the lab. The more important spectral windows are at
1.09, 1.18, 1,27 and 2.3 µm. To investigate the absorption of the
gases present in those spectral windows the CRD technique is unique.
For each wavelength corresponding to the spectral windows DFB lasers
are available. Furthermore a tunable laser in the range between
1.26 and 1.36 µm is also potentially available.
B. Radiation induced photochemical modifications of planetary atmospheres and organic
compounds ( resp. A. Ciaravella )
The experimental investigation of the radiation induced modifications
of the planet atmosphere requires the design of dedicated atmospheric
cells in which the gas or mixture of gases can be confined in order
to be irradiated and analysed.
Expected results at the end of first year:
The final objective of this work will be the construction of a
database of simulated spectra for an identified set of exoplanets.
Moreover, a sensitivity analysis on the observable spectra will
drive and constraint the range of chemical-physical variability
important to infer the conditions, dynamics, meteorology and finally
the potential habitability of the examined exoplanet.
Even if the statistical analysis of data (radiances) instead of
derived physical quantities (temperatures, mixing ratios) is of
lesser immediate interpretation, it benefits of results unbiased
by retrieval algorithms too costly in terms of manpower and
computational times to be implemented in this project. Conversely,
it can be applied directly to exoplanets data to assess a likely
degree of variability inside the recorded observable.
The cell will be integrated inside the UHV (Ultra High Vacuum)
chamber of the LIFE laboratory at the OAPA, where both irradiation
and diagnostics of the processed gases can be performed. The LIFE
chamber is in fact equipped with IR and Mass spectrometers which
will provide in situ monitoring of the gas evolution. The integration
of the cell inside the UHV chamber has the advantage of eliminating
the IR contamination of the ambient background, and the possibility
of using, aside for Visible and UV radiation sources, EUV and X-rays
sources very important to simulate the stellar activity. In this
context it will possible to explore also the effects of irradiation
on organic compounds and their evolution.
The main technical requests for the design of the cell are:
No such a cell or similar products are available on the market and
therefore we need to assemble a purposely built cell. Italian
companies have been identified that could contribute in design and
building the cell.
Te design of the cell and planning of the experimental program
will be realized in the first six months of the programme, while
its construction and tests will take other six months. The run of
the experimental programme will be performed in the following years.
The programme will last at least five years to conduct the
experimental programme to cover a complete grid of parameters.