Geoscientist Online

In the 1960s they took Apollo astronauts to see the Ries Crater, Germany. Maybe they were a generation too early, writes Ted Nield from MetSoc 2009.

Geoscientist Online 16 July 2009

In the 1960s they took the Apollo astronauts to the German town of Nördlingen, which sits in the middle of the Nördlinger Ries Impact Structure - a 24km diameter crater formed about 14.3 million years ago in a sedimentary pile straddling the Permian to Miocene, overlying a crystalline Palaeozoic basement. The idea was to teach them about impact structures, and how they form. However the 1960s analogy with the Moon may have been overplayed, scientists now believe. The lesson of Ries might be better learned by future voyagers to Mars.

Horton Newsom (University of New Mexico) told meteoriticists in Nancy today that before 2004 impacts on Mars were all assumed to take place on basalt substrates. Only one impact structure on Earth is known to have formed on basalt substrate, namely the Lomar Crater (India) which formed in Deccan Trap volcanics about 52 (plus or minus 6) thousand years ago.

Lomar was formed by a chondritic stony meteorite 1.8km in diameter, and the belief in the basaltic nature of Martian surface resulted in intensive research there - much of it conducted by Horton Newsom himself. However, since we have learned so much more about the nature of the Martian surface, we now know that impacts there take place on a bewildering variety of surfaces - including sulphates and carbonates, and may do so, moreover, in the presence of abundant water.

For this reason, impacts on the Earth probably tell us more about Martian impacts than they do about lunar ones, which are indeed mostly dry and mostly basaltic. Dr Axel Wittman (Lunar and Planetary Institute, Houston) described some painstaking work he has carried out on the sedimentary structure of the basal Ries ejecta blanket, thrown out mostly to the southwest.

This basal layer, known as the Bunte Breccia (map) was initially thought (by analogy with the Moon) to have formed essentially ballistically, the impact generating a ground-hugging debris-laden base surge. However the presence of groundwater in the sediment pile makes it much more likely that the Bunte Breccia formed in a similar way to the aprons of debris that surround the so-called “rampart” craters on Mars.

Rampart craters, so far unique to the Red Planet, lie at the origin of some very distinctive “splat” features, which are thought to form as the heat of impact melts or boils water trapped in the subsurface, producing fluidised ejecta and a distinctively petal-like pattern. Wittmann and his co-worker Dr Thomas Kenkmann (Museum für Naturkunde, Humboldt University, Berlin) investigated the Bunte Breccia for sedimentological evidence of fluidised emplacement.

What they found confirmed their hypothesis fully. They believe that shortly after the impact the surficial sediments became thixotropic, and that a gliding surge of ejecta (which would become the Bunte Breccia) spread outward from the impact site. Loss of momentum caused the basal portions of the flow to “freeze”, which explains their chaotic nature and the lack of any directional indicators. Solidified basal portions then became decoupled from the still-fluid upper portions of the flow, creating sub-horizontal glide planes. As momentum was progressively lost from the bottom up, many such glideplanes formed as successive ejecta layers stacked one on top of the other.


Above the basal Bunte Breccia lies the Ries Suevite. This deposit is cross-cut by “venting pipes”, which the scientists interpret as steam escape structures emanating from the quenching zone, where water expelled from the solidified Bunte, lying on top of the now solidified pile, was vapourised by the suevite emplacement.

Wittmann and Kenkmann suggest that their work provides a sedimentological model against which future geo-astronauts visiting Mars might one day compare the “rampart” sediments that encircle some of Mars’s most interesting impact structures.

Ries is not alone

The Ries impactor was probably a chondritic stony meteorite, which interestingly might well have been accompanied in its final plunge by a small iron moon.

Ries is not the only crater in this part of SW Germany. It is widely believed that the 3.8 km Steinheim Basin impact structure formed at the same time. This crater is hosted by Triassic to Jurassic rocks, and samples of its suevite ejecta have recently yielded metallic spherules. Dr Elmar Buchner (University of Stuttgart) told the Meteoritical Society today that the presence of such spherules strongly indicated that Steinheim resulted from the impact of an “iron” meteorite.

Buchner believes there is a strong possibility that the Ries impactor, which would have been much larger, was actually one half of a binary pair, an asteroid with a small orbiting “moon” (comparable to the asteroid 243 Ida – picture). Clearwater Lakes, Canada, are also thought to have been created by such an impact of a binary asteroid.

This possibility, he said struck him as more likely than the alternative hypothesis – namely, that two meteorites should strike Bavaria independently at about the same time. Unfortunately, the date of the Steinheim impact is not as well constrained as that of Nördlinger-Ries, so currently two different dating methods are being used to rectify this deficiency. Results are expected later this year.