ADAM SHOWMAN
NASA Ames Research Center

Talk:   Fri., 16 February
        Noon
        375 LeConte

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"Topics in Giant Planet Atmospheres:
  1. Jupiter's downdrafts, and
  2. Atmospheric circulation of short-period
     extrasolar giant planets"

ABSTRACT

In situ measurements of Jupiter by the Galileo Probe have
raised major questions about Jupiter's atmosphere.  According
to Probe measurements, the abundances of ammonia, hydrogen
sulfide, and water -- which condense and form clouds near
0.5, 2, and 5 bars, respectively -- were low at the condensation
altitudes and only rose toward expected values 100 kilometers deeper.
The water abundance at 20 bars was only one-tenth that expected
from formation models and meteorological studies.  To explain
this puzzling dryness, several authors have suggested that the probe
entered a local downdraft that advects dry air from above the
clouds down to 10 bars or deeper.  This hypothesis is consistent
with the fact that the probe entered an anomalously cloud-free
feature called a 5-micron "hot spot."  Until recently, however,
this downdraft hypothesis remained untested.   I have performed
simulations to test the hypothesis.  My simulations explain the
local dryness and large-scale behavior of hot spots and, while
preliminary, suggest that Jupiter may be "wet" after all.

Next I will switch gears and discuss a new project concerning
the evolution and atmospheric circulation of short-period
extrasolar giant planets.  These "hot Jupiters" (so-named because
of their small orbital distances and high effective temperatures)
have a high probability of transiting their stars.  These
transits allow estimation of the planets' radii.  One planet,
HD209458b, has already been detected in transit, and more
detections are sure to follow.  The radii of these planets
depend on their cooling history.  I will argue that to explain
these planets' radii, the cooling models (which currently ignore
atmospheric dynamics) must take account of the atmospheric
circulation.  I will present preliminary simulations of the
circulation, discuss implications for the evolution, and describe
future directions.