The combined effects of escape and magnetic field histories at Mars

Research areas:
Planet. Spa. Sci.
Mars is thought to have hosted large amounts of
water and carbon dioxide at primitive epochs. The
morphological analysis of the surface of Mars shows
that large bodies of water were probably present in
the North hemisphere at late Noachian (3.7~4 Gyr
ago). Was this water solid or liquid? For
maintaining liquid water at this time, when the Sun
was (likely) less bright than now, a CO2 atmosphere
of typically 2 bars is required. Can sputtering,
still presently acting at the top of the Martian
atmosphere, have removed such a dense atmosphere
over the last 3.5~4 Gyr? What was the fate of the
100~200m global equivalent layer of water present at
late Noachian? When did Martian magnetic dynamo
vanish, initiating a long period of intense escape
by sputtering? Because sputtering efficiency is
highly nonlinear with solar EUV flux, with a
logarithmic slope of about 7:F(SPUT) = F(EUV)^7 ,
resulting in enhanced levels of escape at primitive
epochs, when the sun was several times more luminous
than now in the EUV, there is a large uncertainty on
the cumulated amount of volatiles removed to
space. This amount depends primarily on two factors:
(i) the exact value of the non-linearity exponent
(about 7 from existing models, but this value is
rather uncertain), (ii) the exact time when the
dynamo collapsed, activating sputtering at epochs
when intense EUV flux and solar wind activity
prevailed in the solar system. Both parameters are
only crudely known at the present time, due the lack
of direct observation of sputtering from Martian
orbit, and to the incomplete and insufficiently
spatially resolved map of the crustal magnetic
field. Precise timing of the past Martian dynamo can
be investigated through the demagnetisation
signature associated with impact craters. A
designated mission to Mars would help in answering
this crucial question: was water liquid at the
surface of Mars at late Noachian? Such a mission
would consist of a low periapsis (about 100 km)
orbiter, equipped with a boom-mounted magnetometer,
for mapping the magnetic field, as well as adequate
in situ mass and energy spectrometers, for a full
characterization of escape and of its response to
solar activity variations. Surface based
observations of atmospheric noble gas isotopic
ratios, which keep the signatures of past escape
processes, including sputtering for the lightest of
them (Ne, Ar), would bring a key constraint for
escape models extrapolated back to the past.