Determination of exposure age

The stony meteorites (Fig. 1) were identified on the basis of their metallic iron content. Although they were discovered serendipitously dispersed across almost 10 km, their chemical and mineralogical composition is virtually identical7,8,9. Their chemistry is most similar to the HED group of meteorites albeit with an elevated (Fe, Ni) metal content7,8. They thus resemble the silicate component of mesosiderites, which are a group of stony-iron meteorites7.

Figure 1: Stony meteorites at Meridiani Planum. False colour Pancam images of the candidate stony meteorites (a) Barberton (acquired on sol 123 with sequence P2535), scale bar is 5 cm; (b) Santa Catarina (sol 1,055, sequence P2564), scale bar is 6 cm; (c) Santorini (sol 1,748, sequence P2597), scale bar is 4 cm; and (d) Kasos (sol 1,884, sequence P2574), scale bar is 4 cm (credit: NASA/JPL/Cornell). Full size image

The four meteorites are found on a diversity of terrains that make up the area traversed by the Opportunity rover17. Santa Catarina and similar cobbles7,18 appear strewn across the smooth, sandy annulus around the Victoria impact crater (Fig. 2). Barberton is found on the eroded rim of Endurance crater on smooth terrain with small ripples about 6.2 km north of Victoria crater. Santorini and Kasos are ∼1.5 and ∼3.3 km south of Victoria crater, respectively, in large rippled terrain. Because the meteorites appear paired based on their chemistry8, they may be part of the same fall. One possibility is that they are fragments of the impactor that formed Victoria crater as suggested by their similar distribution to fragments around terrestrial impact craters8,19. This may be difficult to reconcile, however, with their occurrence on top of the annulus at Victoria that was produced by erosion20. An alternate possibility is that their arrival times post date the age of the terrains on which they are found, which is suggested by their occurrence on all the different sand and outcrop surfaces, and their relatively unweathered appearance compared with iron meteorites found in the same region10. Barberton likely arrived at Endurance after the terrains formed and after the crater was eroded into its present form21. The meteorite is on the rim of Endurance crater whose overturned flap and ejecta have been eroded and covered with sand to make the smooth surface with small ripples. Furthermore, Endurance has experienced considerably less total erosion than Victoria20,21 and is therefore younger given their proximity and similarity in target materials. Moreover, Barberton is not on a pedestal like the iron meteorite Block Island10 and adjacent surfaces are not significantly scoured as might be expected given the ease with which surrounding rocks are eroded17,20. Nevertheless, these two hypotheses constrain other intermediate possibilities (that is, the meteorites fell after some of the Meridiani terrains formed), and we will consider both hypotheses to constrain the maximum and minimum meteorite exposure age on the surface.

Figure 2: Meteorite accumulation at Victoria Crater. False colour, seam corrected Pancam mosaic showing the Santa Catarina cobble field at the rim of Victoria crater, which is visible on the right hand edge of the mosaic. The images were acquired on sol 1,049 with sequence IDs P2555 and P2556 (credit: NASA/JPL/Cornell). Full size image

If the meteorites are fragments of the impactor that formed Victoria crater, the age of the crater would be their surface exposure age. Given that Victoria crater impacted into Burns formation sulfate sandstones20, which form the light-toned bedrock of Meridiani Planum, the crater retention age of the bedrock (∼70 Myr; ref. 17) would provide an upper limit. The smooth sandy surface of the Victoria crater annulus formed after ejected blocks of sulfate sandstone were eroded down about 1 m until granule-sized blueberry concretions that had weathered out of the bedrock inhibited further erosion20. Using the same methods for determining the crater age of other Meridiani terrains17, we find a 40 Myr model age for the smooth inner annulus around Victoria. We estimate the annulus took about 10 Myr to form by applying the eolian erosion rate for Meridiani over the past 10 Myr (0.1 m per Myr)17 to erode 1 m of sulfate sandstone. This yields an impact age of around 50 Myr ago for Victoria crater, which is less than the maximum age from the age of the bedrock (70 Myr ago), as would be expected given the moderately modified form of the crater20.

If the meteorites fell after the terrains on which they are found had formed, we can use the age of the terrains as their maximum exposure age. The smooth terrain is about 23 Myr old and the large ripple terrain is about 18 Myr old (ref. 17), so the maximum exposure age is around 20 Myr. If the meteorites fell well after the terrain age of 20 Myr, it would be helpful to estimate the minimum exposure age. We use two methods to constrain the minimum exposure age. First, because Barberton likely postdates the erosion of Endurance crater into its current form, the inferred age of 2–4 Myr for the crater based on its morphology and crater counts17 sets an upper limit to its exposure age. Second, stereo MI images indicate that of order 1 cm of the lowermost portion of Santa Catarina has been eroded away (Fig. 3). These surface modifications could have occurred as the result of eolian abrasion alone or as a combination of abrasion and aqueous alteration as indicated by the ferric iron. They demonstrate that the rock probably had an extended residence time on the Martian surface. Figure 3 also shows the highly brecciated structure of the rock, which likely contributes to differential mass removal by providing strength variations throughout the rock volume. The rate of abrasion to form ventifacts in hard rocks at the Pathfinder landing site was estimated to be on the order of 10−4–10−5 m per Myr (ref. 22). Because the rate of abrasion at a site is dependent on both the sand supply and the frequency of winds strong enough to saltate sand, we adjust the rate of abrasion by the difference between the long-term erosion rates at Mars Pathfinder and Meridiani (about 2 orders of magnitude), which is dependent on the same two factors20,22. This would suggest abrasion rates of hard rocks at Meridiani of order 10−2–10−3 m per Myr, which would correspond to 1–10 Myr to erode the base of Santa Catarina. As a result, the 2–4 Myr and 1–10 Myr ages for Barberton and Santa Catarina, respectively, would represent the minimum exposure age for the stony meteorites on Mars.

Figure 3: Physical weathering of Santa Catarina. Microscopic Imager mosaic of Santa Catarina merged with Pancam colour images (acquired on Sol 1,055 with sequence p2564). Visible are signs of deep incision, differential erosion, and scalloping, producing serration along rock margins. Note the absence of any residual fusion crust or obvious regmaglypts. Note also the highly brecciated structure of the rock. Figure modified from ref. 8. Full size image

Chemical weathering rate and comparison to Earth

We used the ferric iron content (Table 1; as published by Schröder and co-workers8) obtained with Opportunity’s Mössbauer spectrometer23 in these meteorites (see Methods) as a measure of alteration since their fall. It is possible that a fraction of this Fe(III) stems from the formation of a fusion crust24. However, it is unclear whether passage through Mars’s CO 2 atmosphere would have led to the formation of an oxidized fusion crust at all, and there is no obvious evidence for a fusion crust8 or regmaglypts (Fig. 3). Some Fe(III) might have been removed by wind abrasion, on the other hand, a process suggested to have affected Santa Catarina and the iron meteorites at Meridiani10. We take the average amount of Fe(III) measured in the stony meteorite fragments and derive a chemical weathering rate of 9±4% Fe(III) formation per 50 Myr or per 1–20 Myr if they are either part of the impactor that created Victoria crater or if they fell after the terrains on which they are found had formed, respectively. That translates into a rate of 0.18% per Myr Fe(III) formation (for 50 Myr) to 9% per Myr (for 1 Myr) with an average of 0.45% per Myr (for 20 Myr) if a linear rate is assumed. Note, however, that studies of weathering in meteorites in terrestrial settings (see below) show that Fe oxidation does not progress in a linear fashion where rapid initial oxidation is followed by passivation12,13,14.

Table 1 Distribution of iron between mineral phases and oxidation states* †. Full size table

To put this number into perspective we need to compare it to studies of similar groups of meteorites collected from various weathering environments on Earth. Bland and co-workers have done several such studies using ordinary chondrites12,13,14. Ordinary chondrites are divided into three geochemically and mineralogically distinct groups: H chondrites have a high Fe content with a high metallic Fe versus FeO in silicates ratio; L and LL chondrites have a lower Fe content with less Fe metal and more FeO in silicates. The stony meteorites at Meridiani are not ordinary chondrites but when we compare them to a Mössbauer spectroscopy survey of chondrites25 (see Methods), the distribution of iron between mineral phases is the same as that of L and LL chondrites (Fig. 4). In Fig. 4, L and LL chondrites are nicely separated from H chondrites. Two L chondrites plot within the field of H chondrites, which is most likely a result of more severe chemical weathering that not only oxidized metallic iron but also some ferrous iron in olivine. The stony meteorites discovered at Meridiani plot within the field of the L and LL chondrites. One of these stony meteorites, Kasos, has less iron in olivine and instead more iron in pyroxene than the others (Table 1). Because there is no difference in chemical composition, this may simply reflect some heterogeneity in mineral grain distribution on the scale of the field of view of the Mössbauer instrument (∼1.4 cm).

Figure 4: Distribution of Fe in ordinary chondrites and stony meteorite finds on Mars. The plot shows the amount of ferrous iron in olivine (percentage of total iron) as a function of metallic iron plus ferric iron (percentage of total iron) in ordinary chondrites returned from the Larkman Nunatak region in Antarctica (filled black squares: LL chondrites; filled red circles: L chondrites; filled blue triangles: H chondrites) and compares it to the stony meteorite finds on Mars (open red stars). The error bars represent the general uncertainty of ±2% absolute except for the data point representing Kasos where the uncertainty is ±4% (compare Table 1). Full size image

In Fig. 5 we plot the fraction of ferric iron formed as a function of time for H chondrites and L chondrites investigated by Bland and co-workers12,13,14, and the stony meteorites at Meridiani. Because ferric iron was determined from surface measurements in the Meridiani meteorites and from subsurface measurements in the terrestrial finds (see Methods), and because weathering proceeds along surfaces and follows fractures into a rock, the MER measurements may represent an overestimation compared with the terrestrial measurements. We distinguish between meteorites recovered from hot desert regions on Earth (Sahara, Australia, SW, USA) and cold desert regions (Antarctica) in Fig. 5. Both areas represent slow terrestrial chemical weathering rates. The plot shows that H chondrites weather faster in all environments than L chondrites because they contain more metallic iron (for example, Fig. 4) than L chondrites. Metallic iron is the most sensitive phase towards chemical weathering. Weathering rates in Antarctica are significantly slower than those in hot desert regions on Earth. The Antarctic rate arguably represents the slowest chemical weathering rate anywhere on Earth. If we apply the shortest possible residence time (1 Myr) for the stony meteorites at Meridiani and compare at 9% oxidation, the Martian rate approaches the Antarctic rate. The Martian rate becomes significantly slower the longer the true residence time is. If we apply the time of formation of Victoria Crater (50 Myr ago), the Martian rate would be two orders of magnitude slower than the Antarctic rate. The weathering rates diverge as oxidation progresses with time. At 50% oxidation, a level where ordinary chondrites start to disintegrate (see below), Martian weathering rates could be up to 4 orders of magnitude slower than the Antarctic rate.