Origin and evolution of planetary and satellite atmospheres, and the formation of
giant planets, is one of the main areas of research in the Planetary Science Laboratory.
To achieve this ambitious goal, studies of physical, chemical, meteorological, and
astrobiological processes of planets and satellites with atmospheres are being carried
out. The focus of our current research is especially on the giant planets, Titan,
Mars, Venus, and the extrasolar planets. Numerical modeling, and analysis and interpretation
of data from observations with spacecraft, satellites and from the ground are being
done. Ready access to critical observational data sets has been possible due to
the participation of several PSL members on the teams of current, past, and future
planetary missions.
Present spacecraft projects include
Cassini-Huygens Gas Chromatograph Mass Spectrometer
(GCMS) and the Aerosol Collector Pyrolyzer (ACP) investigations (Atreya), Planetary
Fourier Spectrometer (PFS) on the
Mars Express and
Venus Express missions (Atreya), Sample
Analysis at Mars with GCMS and Tunable Laser Spectrometer on the
Mars Science Laboratory (Atreya), Meteorology
packages on MSL and
Phoenix Mars Polar Lander (Renno), and
Juno-Jupiter Polar Orbiter (Atreya). Some
highlights of our findings are given below.
Publications: Please click on the websites of PSL scientists under
PEOPLE to find their publications.
TITAN - A new world! (Cassini-Huygens)
With the
GCMS on the Cassini-Huygens entry probe
we discovered that in Titan's meteorology, methane plays a role similar to hydrological
cycle on Earth. Titan's "methalogical" cycle also appears to be seasonal. The complex photochemistry
of the atmosphere ensures, however, that methane is destroyed irreversibly in about
ten million years. In the absence of the warming due to methane-derived hydrocarbon
hazes and the collision-induced opacity, nitrogen would gradually condense, leading
to a substantially reduced atmosphere. The GCMS findings on radiogenicargon (
40Ar)
and carbon isotopes argue for a hydrogeochemical process-serpentinization-in Titan's
interior to replenish the methane lost by photolysis. Moreover, lack of noble gases
implies that nitrogen was brought to Titan originally in the form of nitrogen compounds,
most likely ammonia, but not molecular nitrogen. We expect the analysis and interpretation
of the Cassini-Huygens data to continue for many years.
Click to see
movie from Huygens descent. Movie and image courtesy of University
of Arizona/ESA/NASA.
MARS - Methane, Organics, Life, Habitability (Mars Science Laboratory - MSL,
Mars Express)
The detection of methane on Mars in 2004 by the
Planetary Fourier Spectrometer on Mars Express
and by two ground-based telescopes has provided renewed impetus to the search for extinct
or extant life on Mars, as methane is a potential biomarker. Although chemolithotrophic
methanogens in the subsurface is one tantalizing possibility, present data sets
cannot discriminate between this and other potential sources such as hydrogeochemical, volcanic,
or cometary. However, observations are continuing to determinine the spatial and
temporal variation of methane that could provide some clues to the source of methane
on Mars.
On the other hand, the
SAM-Suite on the Mars Science Laboratory
(
MSL)
is being developed to address, amongst other things, the question of existence of
life on Mars, now or in the past. In addition to trace species, biomarkers and organics,
the SAM-Suite, which includes a GCMS and a Tunable Laser Spectrometer, is expected
to make precise measurements of relevant isotopes in samples collected from the atmosphere,
surface, subsurface and the rocks. MSL is scheduled to be launched in 2011, with arrival at
Mars in 2012. The image is courtesty of NASA/JPL.
The lack of detection of organic material (except methane
in gas phase) on Mars is puzzling. Our research hints at a new possibility - oxidants
produced in chemistry triggered by electrostatic fields generated in Martian dust devils
and storms. Since aeolian processes are ubiquitous and expected to have been around
throughout the planet's geologic history, oxidizers such as hydrogen peroxide could
have found their way into the Martian regolith, destroying any organic material present
on the surface. They could also shorten the lifetime of methane, perhaps requiring
a larger source of the gas. Our electro-photochemical research is continuing together with
laboratory simulation experiments. The above image is of hydrogen peroxide mixing
ratios (x 10
-8) in the Martian atmosphere, under normal, i.e. non-dust
event conditions (TEXES observations, with Th. Encrenaz, et al., 2004).
Potential sources and sinks of methane on Mars. S. K. Atreya, Int'l Mars Conference,
Ischia, Italy, September 2004. Also, Atreya, Mahaffy and Wong, Planetary Space Science,
2006.
The Giant Planets (Cassini-Huygens,Juno,Galileo)
Our
focus of research here is on the formation of the gas and icy giant planets and
their atmospheres. Our approach is to determine and compare the heavy element composition
and meteorology in well-mixed atmospheres, asc omparative planetology of well-mixed
atmospheres of the outer planets is key to the origin and evolution of the solar
system, and, by extension, of extrasolar systems. Abundance of the heavy elements
is essential for constraining the planetary formation models. This means probing
to levels well-below the clouds, at least in the case of gas giants.
Galileo Probe was the only probe that ever sampled
the atmosphere of a giant planet in situ. The findings of the Galileo Probe Mass
Spectrometer were surprising, in that the abundance of heavy elements, C, N, S,
Ar, Kr, and Xe relative to hydrogen, on Jupiter were found to be three times their
respective solar values. However, the Probe failed to measure the abundance of water
in well-mixed atmosphere, hence the oxygen elemental ratio, since it entered a 5
micron hot spot - the Sahara Desert of Jupiter. The determination of water abundance
in well-mixed atmosphere is critical, as water was presumably the original carrier
of heavy elements to Jupiter (and most likely, to all outer planets). The image
is from S. K. Atreya, et al., Planet.Space Sci., 51, 105, 2003. Updated from T.
C. Owen, et al., Nature, 402, 269, 1999.
We expect to settle the question of the abundance of water in Jupiter's well-mixed
atmosphere with microwave radiometer measurements on
Juno - Jupiter Polar Orbitar. Juno is scheduled to
be launched in 2011 with arrival at Jupiter in 2016. In the meantime we continue to further
develop models of giant planet formation and the origin of their atmospheres, by
analyzing the latest measurements of elemental and isotopic abundances in all outer
planets from Earth and space, and then constraining the models with the results.
Future
The
answers to big questions of the formation of solar systems and of the atmospheres
within them would require probing the well-mixed atmospheres of all four outer planets
in our solar system. The technological challenges of entry, data transmission and
extreme thermal and pressure environments are daunting. We are developing concepts
of future probe and microwave radiometer missions to the outer planets, so that
enabling technologies could be identified and made available in the near future.
