Adam Durant: Accretionary Lapilli on Mars?

Small spherules present in many of the images collected over the past few weeks by the Mars rovers have characteristics very similar to accretionary lapilli (particle aggregates; see Table 1). The Mars rover scientific team posted a news release on the JPL website on March 2nd 2004 explaining the origin of these features as concretions, supported by some convincing data. Below are some observations and comparisons to deposits on Earth that illustrate these features share strong similarities to accretionary lapilli.

The significance of accretionary lapilli present on Mars is the following: on Earth, this specific particle aggregate only forms in a plume containing abundant water. This could be the result of a volcanic magma body interacting directly with a body of water producing a “wet” explosive volcanic eruption (e.g. ocean, surficial water, ground water, ice sheet), or where there has been a asteroid impact involving a similar body of water (e.g. accretionary lapilli deposits associated with the Chicxulub crater asteroid impact).

I will first summarize my observations for the following images, and will draw some comparisons to accretionary lapilli from ash layers in the Keanakakoi Ash, Kilauea, Hawaii: this is a good analogy because the dominant volcanism on Mars is basaltic. Following is a table summarizing characteristics of accretionary lapilli:

Table 1: Typical Characteristics of Accretionary Lapilli

CHARACTERISTIC	DETAILS

Diameter	Typically <10 mm; high proportion 1-4 mm; max. 60 mm;
Structure	Spherical, concentrically layered. Thin fine-grained rim (may have multiple layers: layers <1 mm); volumetrically-dominant coarser-grained core; vesicles often present in core; large nucleus particle often present;
Grainsize	Core: mode typically <100 µm; Rim: mode <40 µm; Max size <500 µm;
Density (approx)	~1200 – 1500 kg/m³

Mars Rover Image Archive:

http://www.jpl.nasa.gov/mer2004/

Summary of observations (index refers to image on JPL website):

02-19-04 PIA05324: Tiny Pebbles

It is possible to see individual particles comprising the spheres in this image by zooming in on the original image: there are some larger light and dark particles that appear as specks on the surface of the spheres. This resembles the surface of accretionary lapilli where the majority of the rim is composed of fine particles, with a small proportion of larger ones which protrude from the rim surface.

02-18-04 PIA05319: The Trench Throws a Dirt Clod at Scientists: Figure 1

The image in figure 1 shows an enlargement on image PIA05319 (below left). Here, the trench has dissected some of the spheres revealing a concentrically-layered structure. SEM images of Keanakakoi acc-laps are shown for comparison (below right; scale bar is 1 mm): these illustrate the concentric structure of accretionary lapilli defined by regions of differing grainsize and clearly show a slightly coarser matrix surrounding the aggregates. The structure of the Mars spheres is consistent with a thin, fine-grained rim surrounding a volumetrically dominant core in cross-section. The rim also appears to be slightly more eroded across the surface than the core, which may indicate a change in grainsize between the two regions.

Figure 1

02-17-04 PIA05313: Mark of the Moessbauer

This image demonstrates the cohesiveness of the spheres. The Mossbauer tool imprint showed that the unit is granular and weakly consolidated. Also, where the flat face of the tool had been in contact with the unit, it crushed some of the spheres, leaving lighter patches. This indicated that the spheres are weakly consolidated, and may be composed of a different grainsize particle distribution to the surrounding matrix, manifested as a lighter color when crushed. I have observed similar effects when working on accretionary lapilli deposits.

02-12-04 PIA05273: "Berries" on the Ground: Figure 2

The image in figure 2 shows numerous spheres which have been weathered out, and are sat on what appears to be a wind-blown granular deposit. I have seen similar deposits in Hawaii strong trade-winds blow across an arid region where the Keanakakoi Ash is located. This results in high rates of Aeolian erosion and weathers the accretionary lapilli out from the ash layers. The aggregates are blown along with coarser Aeolian deposits and often sit on the surface of a dune. The similarity between the spheres and Keanakakoi acc-laps is shown below: the morphology of the acc-laps is shown along with cross-sections collected using SEM

Figure 2

02-12-04 http://www.jpl.nasa.gov/mer2004/rover-images/feb-12-2004/2M129819881-med.jpg: Figure 3

The image in figure 3 shows spheres at “squiggle dunes”: these are ~1 mm diameter (below left). The weathering pattern is similar to acc-laps observed on a littoral cone in Nicaragua (below right): here, acc-laps are being eroded by wave action faster than the surrounding matrix and are forming indents at the surface.

Figure 3

02-11-04 PIA05256: Unparallel Lines Give Unparalleled Clues

This image shows a stratified unit with some cross-bedding also evident: accretionary lapilli horizons are often present in close association with surge laminae, which are characteristically cross-bedded.

02-09-04 PIA05235: Stone Mountain: Figure 4

The image in figure 4 shows a stratified clastic unit with a weathered appearance similar to ash layers of the Keanakakoi Ash, Kilauea, Hawaii. Here, frequent sustained winds have sand-blasted the surface of these layers, resulting in finer-grained layers/laminae standing proud of the surface relative to coarser-layers/laminae. Spheres can be seen weathering out of individual layers. The appearance is also consistent with the stratified unit being fine grained, a requirement for accretionary lapilli. The upper surface of the stratified unit has cracks, which may be indicative of desiccation. Accretionary lapilli ash layers in the Keanakakoi Ash also have desiccation cracks on the upper surface indicating that they were wet when deposited.

Figure 4

02-09-04 PIA05237: Mars Rock Formation Poses Mystery: Figure 5

The image in figure 5 shows a close shot of the strata containing the spheres which are weathering out. The texture of some of the layers resembles a texture I observed in very fine-grained ash layers of the Los Chocoyos Ash, Guatemala, in a unit abundant in accretionary lapilli. This unit was very wet when deposited.

Figure 5

22nd March: Discussion

Hematite signature measured by Opportunity:

Here's some interesting articles on iron oxide and its origin on Mars:

http://www.psrd.hawaii.edu/Mar03/Meridiani.html

...quoting researchers from this page, "the hematite may have formed by a later secondary mechanism in preexisting ash beds. In this case, Hynek and his colleagues favor precipitation of the hematite [as opposed to primary formation] when iron-rich fluids circulated within the layered volcanic ash. This kind of secondary formation of the gray hematite, they say, is most consistent with their regional geologic, topographic, and spectral observations."

T. D. Glotch et al., Hematite at Meridiani Planum: Detailed Spectroscopic Observations and Testable Hypotheses, Lunar and Planetary Science, 2004

It is completely plausible that volcanic tephra layers could have first been deposited, and then subsequent fluid circulation through these horizons could have formed secondary hematite-rich deposits after deposition. The Mossbauer spectrometer on Opportunity identified the spherules as having peak occurrences of hematite. My only explanation (as an alternate to the concretion hypothesis) would be related to the porosity of accretionary lapilli. If you look at figure 2 showing the cross-sections through the Hawaii acc-laps, you may notice that they have an abundance of circular voids, or vesicles, present within the core: this is typical of accretionary lapilli that I have looked at. These vesicles may act as preferential sites for hematite precipitation from groundwater, producing the elevated hematite signal. This may, however, not be a suitable explanation for some of the spherules, such as the ones in this image taken on Feb 19th:

http://www.jpl.nasa.gov/mer2004/rover-images/feb-19-2004/captions/image-1.html

Presence of sulphate in outcrops investigated by Opportunity:

Quoting NASA from the news release on March 2nd, 2004 (http://www.jpl.nasa.gov/releases/2004/74.cfm), "Jarosite may point to the rock's wet history having been in an acidic lake or an acidic hot springs environment."

If these spherules (or at least some of them) are accretionary lapilli, the process that formed them required direct interaction between magma and water. Therefore, the presence of Jarosite provides evidence that there was a body of water present on (or in the upper surface) of Mars at some point in time. Also, accretionary lapilli are usually deposited quite close to source, as their large size (relative to individual particles) causes them to fall out of volcanic plumes fast. The close association between the spherules and Jarosite (indicating wet conditions) may provide some more evidence that there was water nearby to the vent.

Spherules as concretions:

This image from Opportunity shows a spherule "triplet":

http://www.jpl.nasa.gov/mer2004/rover-images/mar-18-2004/captions/image-18.html

Accretionary lapilli are found as isolated spherules in deposits; I have never seen any with this triplet morphology. Concretions, on the other hand, can take this form. Therefore, I believe that that (at least) some of the spherules in the Mars rover images are concretions.

Take a look at the full size image following this link:

http://marsrovers.jpl.nasa.gov/gallery/all/1/m/041/1M131832380EFF0574P2952M2M1.HTML

The spherule in the lower left of the image has a non-distinct lineation running across it that may be a continuation of the stratification to the right: I think that the origin of this is best explained by the concretion hypothesis.

26th March: Discussion

Spectra taken by Opportunity's Moessbauer spectrometer at various spots in "Eagle Crater":

http://photojournal.jpl.nasa.gov/catalog/PIA05640

Ripples:

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/059/1P133430467EFF0830P2557L7M1.JPG

http://marsrovers.jpl.nasa.gov/gallery/all/1/n/059/1N133430407EFF0830P1952L0M1.JPG

http://marsrovers.jpl.nasa.gov/gallery/all/1/p/062/1P133698684EFF08A6P2568L7M1.JPG

Spherules:

http://photojournal.jpl.nasa.gov/catalog/PIA05634

http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20040326a/18-BE-08-MIslide-A082R1_br2.jpg

30th March: Discussion of Ripples

I'm not an expert on dunes and ripples. However, I think that unless there was water on the surface until very recently (months), aeolian redistribution will mask any surface evidence of fluvial / marine bedforms.

Here's some terrestrial aeolian anaologies I found online:

Ripples on a dune in Eureka Valley, California

LARGE RIPPLES ON EARTH AND MARS.

This is not current, but refers to larger features taken by the Viking spacecraft, and it highlights the aeolian origin of dunes investigated during that effort.

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