Vision and Voyages for Planetary Science in the - Solar System ...

timescales, including the possible analogies between a possible ancient climate catastrophe on Venus and
the long term future of Earth’s climate system.
Many advances in the last decade for ancient climates have been about Venus, based mostly on
results from the Venus Express spacecraft. Venus Express has found new clues to the mystery of Venus’s
seemingly tortured climatic past by measuring flows of escaping atoms and ions and finding a surprising
altitude-dependence of the deuterium to hydrogen ratio at certain latitudes. Venus’s atmosphere has a
large deuterium/hydrogen ratio compared to Earth and other solar system bodies, and this ratio has been
taken to indicate significant loss of hydrogen (with mass fractionation) from its atmosphere to space.
However, the SPICAV instrument of Venus Express has found that the deuterium/hydrogen ratio is
significantly higher at and above the cloud deck than nearer to the surface. This enrichment could be
caused by some photochemical process (molecular decomposition or planetary escape) or selective
condensation into clouds. 18
Data from the ASPERA instrument on Venus Express suggest provisionally that hydrogen escape
rates are an order of magnitude slower than previously assumed, implying that the hydrogen in Venus’s
atmosphere has an average residence time of some 1 billion years. 19 This result, if confirmed by further
observations during an extended Venus Express mission, has important implications for the history of
water and the current rate of outgassing on Venus. Another significant discovery is that Venus’s
atmosphere is losing unexpectedly large quantities of oxygen to deep space, via non-thermal processes.
This finding calls into question the long-standing assumption that massive escape of hydrogen from
Venus’s atmosphere must have left the atmosphere and surface highly oxidized.
Important Questions
Some important questions concerning the primordial climates on Venus and Mars and the search
for clues into Earth’s early environment include the following:
• Do volatiles on Mercury and the Moon constrain ancient atmospheric origins, sources and
loss processes?
• How similar or diverse were the original states of the atmospheres and the coupled evolution
of interiors and atmospheres on Venus, Earth, and Mars?
• How did early extreme ultraviolet flux and solar wind influence atmospheric escape in the
early solar system?
Future Directions for Investigations and Measurements
To make significant progress towards the goal of understanding the processes controlling climate
on the terrestrial planets requires observations over a significant fraction of a solar cycle in order to derive
a time averaged escape flux for recent epochs and to understand the relative importance of several escape
mechanisms. Several critical areas of investigation are as follows (1) measure and model the abundances
and isotopic ratios of noble gases on Venus to understand how similar its original state was to those of
Earth and Mars, and to understand the similarities and differences between the coupled evolution of
interiors and atmospheres for these planets; (2) characterize ancient climates on the terrestrial planets,
including searching for isotopic or mineral evidence of ancient climates on Venus; and (3) examine the
geology and mineralogy of the tesserae on Venus to search for clues to ancient environments.
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