NEWS
he discovery of the buried
T
Chicxulub crater in northern
Yucatán, Mexico (Hildebrand
et al. 1991), now accepted as
having played a critical role in the
terminal Cretaceous extinctions,
has stimulated research into its
size and structure. Initial
investigations of the Yucatán
structure had suggested that it
was about 200 km across, but
some aspects of the regional
gravity anomaly patterns had
been interpreted as suggesting a
diameter roughly twice as large
(Sharpton et al. 1993), with the
bulls-eye structure of a multiring
basin. A doubled crater diameter
implies roughly an order of
magnitude greater environmental
perturbation, so the “size
question” had to be addressed.
Recent results presented in the
December issue of Geophysical
Journal International (Connors
et al. 1996) suggest that the crater
is approximately 180 km across.
Subtle surface features, including a
ring of sinkholes known as
“cenotes”, had earlier been shown
to suggest this (Hildebrand et al.
1995) by being collocated with
gravity gradients at the periphery
of the crater. Studies over a larger
geographical region have now
shown no evidence of similar features or gradients at larger radii.
In addition to carrying forward a
previous technique with new data,
the authors of the GJI paper present topographical data portraying
the shallow valley in which it had
been previously noted (Hildebrand
et al. 1995) that the cenotes lie.
Similar results were recently
presented with the suggestion that
there is more than one such
New light on Chicxulub Crater
Martin Connors, Alan R Hildebrand and Mark Pilkington
describe the latest geophysical investigations into the Chicxulub
crater, the relic of the asteroid impact 65 million years ago.
depression and that some complex
buried structure is reflected in the
topography (Pope et al. 1996), but
the new paper, with more closely
spaced topographical data, does
not support this. The simplest
model of northern Yucatán consists
of the crater, near the centre of a
carbonate platform, with a zone
of block faulting to its east. The
tectonic setting has led to some
confusion about the size of the
crater, but has also favoured its
preservation. We are thus fortunate
that this large and important
crater, although buried, is
apparently intact.
The consistent picture, arising
from several data sources, that the
90°W
Horizontal gradient of
the Bouguer gravity
anomaly in northern
Yucatán. Chicxulub crater
is centred on the wellorganized concentric
gradient structures. The
rim structures are marked
by rings of sinkholes
(shown as white dots).
The strong east coast
gravity gradients are
also associated with
sinkholes, but organized
gradients or associations
between gradients and
sinkholes are absent
elsewhere, and in
particular do not mark
structures greater than
180 km diameter.
(Picture: Geological
Society of Canada.)
4
88°W
87°W
20°N
0 1 2 3 4 5
Adrian James, Project Manager for the Mars-96 FONEMA instrument,
reports on consequences of the failed launch last November.
cosmodrome in Kazahkstan. This
was the largest, and arguably the
most important scientifically, of
three missions to the Red Planet
launched this year, the others
being the US Mars Global
Surveyor and Pathfinder.
Unfortunately, after just over an
hour in orbit things began to go
wrong: the second firing of the
fourth stage of the Proton booster
failed to burn for the correct time.
This manoeuvre would have
89°W
Connors M et al. 1996 Geophysical Journal
International 127 F11–F14.
Hildebrand A R et al. 1991 Geology 19 867–871.
Hildebrand et al. 1995 Nature 376 415–417.
Pope K O et al. 1996 Geology 24 527–530.
Sharpton V L et al. 1993 Science 261
1564–1567.
21°N
What went wrong with Mars-96?
n 16 November last year, the
O
Russian Mars-96 spacecraft
was launched from the Baikonur
GJI paper attempts to create
should be improved by the
addition of data from recent
investigations. This past autumn
has seen a multifaceted attack on
the crater, including very detailed
gravity surveys in the rim area
and offshore regions previously
unsampled. Global Positioning
System measurements and the
merging of these with previous
datasets will allow better
geodetic control and may reveal
subtle surface expressions of
buried structure.
Yet more promising are detailed
seismic soundings, both on land
and at sea. Shipborne reflection
(British Institutions Reflection
Profiling Syndicate) and ocean bottom seismometer refraction studies
(University of Texas at Austin) profile four radii of the crater at sea;
on land deployment of geophones
supports wide angle reflection and
refraction studies (Imperial College
of Science, Technology and
Medicine). The data reduction has
begun, with first results expected
soon, and many ongoing projects
will be able to work with the
tremendous volume of data. If the
seismic data, in particular, support
our contention about the size and
structure of the crater, the basis for
theorizing about the terminal
Cretaceous events will be in place.
placed the Mars-96 spacecraft into
a highly elliptical orbit from which
its own propulsion system should
have sent it to Mars. Instead, the
spacecraft burnt up on re-entry
into the Earth’s atmosphere and
its remains crashed into the
Pacific Ocean.
Originally, Mars-96 was to be
one of a number of Russian missions to Mars (Zakharov 1994).
Although political upheaval and
consequent budgetary problems
meant the two-year delay of this
mission and cancellation of the
others, this spacecraft was still
6
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Gravity gradient (g.u./km)
the largest planetary probe ever
launched. The orbiter was
equipped with 24 scientific instruments to investigate all aspects of
the Martian environment. Also on
board were two soft landers and
two penetrators to study seismic
activity, surface composition and
meteorology. The scientific return
from this mission would have been
unprecedented. A number of UK
groups were involved with many of
the instruments, with the Mullard
Space Science Laboratory of
University College London as the
lead institute for the FONEMA
instrument, so UK scientists were
looking forward to having access
to much of the data returned from
the spacecraft. As well as losing
the instruments, UK scientists
have lost the opportunity of major
involvement in the study of
Mars and the whole field of
solar system studies.
The losses of the Phobos spacecraft in 1989, Mars Observer in
1993 and now Mars-96 have been
a series of devastating blows to
planetary science. But Mars is
important and the two US spacecraft are even now on their long
journey to the Red Planet. With
more US spacecraft planned for
1998, 2001 and 2003 and hopefully European and Russian missions
too, there is still much to do in the
exploration of Mars. I am confident that UK scientists will be
there to take up the challenge.
Zakharov A V 1994 Phil Trans R. Soc
Lond. A 349 295–307.
February/March 1997 Vol 38 Issue 1