Age Determination of Martian Low Shield Volcanoes by

PFG 2012 / 2, 0177–0185 ArticleStuttgart, April 2012

© 2012 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany www.schweizerbart.deDOI: 10.1127/1432-8364/2012/0110 1432-8364/12/0110 $ 2.00

Age Determination of Martian Low Shield Volcanoes byCrater Size-Frequency Measurements

Felix Jagert, Bochum & ernst Hauber, Berlin

Keywords: Volcanism, crater, planetary, remote sensing, Mars

canic resurfacing. Remote sensing images andtopographic data, however, show also sever-al clusters of small, low shield volcanoes inTharsis.

These Martian low shield volcanoes and as-sociated landforms show morphologic char-acteristics that are comparable with basalticvolcanic provinces on Earth (Plescia 1981,Hauber et al. 2009). Plains volcanism of theeastern Snake River Plain in Idaho (USA) canbe considered as possibly the best terrestrialmorphological analogue for this type of vol-canism. It is defined as being an intermediatestyle between flood basalts and the Hawaiianshields (Greeley 1982). The Martian shield

1 Introduction and Related Work

It is known for decades that the planet Marswas volcanically active in its geological past.A major goal in planetary geology is to inves-tigate the times of activity and whether thevolcanoes are now extinct or dormant. Theseresults will be a key to determine the historyof late-stage volcanism of the planet and willhelp to shed light on the endogenic evolutionof Mars.

Much research was done for the big shieldvolcanoes (e.g., Olympus Mons) in the Tharsisregion in the western Martian hemisphere, ahuge topographic bulge with widespread vol-

Summary: Based on high-resolution images of thecurrent NASA-spacecraft Mars ReconnaissanceOrbiter, the technique to determine absolute agesof planetary surfaces by impact crater counts isdemonstrated. The results will be useful to inter-pret geologic processes on the planet’s surface,such as the history of volcanic activity. This paperdescribes the statistical technique for a low shieldvolcano in the Tharsis region and its adjacent plainswith the discussion of the results and problems.Planetary remote sensing data are processed andprepared for the later use in GIS environments andstatistical analysis. The crater counts were done in-dependently with two different cameras, whichprovided comparable results. The derived absoluteages are in good agreement with the results of ear-lier works.

Zusammenfassung: Altersbestimmung von niedri-gen Schildvulkanen auf dem Mars anhand von Kra-tergrößen-Häufigkeitsverteilungen. Auf Grundla-ge der aktuellen, hochauflösenden Bilddaten derNASA-Raumsonde Mars Reconnaissance Orbiterwird die etablierte Methode der absoluten Alters-bestimmung planetarer Oberflächen anhand vonKraterhäufigkeiten demonstriert. Die Ergebnissedienen der wissenschaftlichen Interpretation geo-logischer Prozesse auf dem Mars, wie etwa der Un-tersuchung der vulkanischen Aktivität des Plane-ten. In diesem Beitrag wird diese statistische Me-thode beispielhaft für einen Schildvulkan und des-sen Umgebung in der Tharsis-Region angewandt,Ergebnisse diskutiert und Probleme angesprochen.Dazu wurden die planetaren Fernerkundungsda-ten prozessiert, für die spätere Verarbeitung inGeographischen Informationssystemen vorberei-tet und statistisch ausgewertet. Zum Vergleich derKraterstatistiken wurden die Aufnahmen von zweiverschiedenen Kameras verwendet, die gleichar-tige Ergebnisse lieferten. Die bestimmten Alterstimmen sehr gut mit den Ergebnissen vorherigerAutoren überein.

178 Photogrammetrie • Fernerkundung • Geoinformation 2/2012

2 Age Determination by ImpactCrater Counts

The age determination of planetary surfacesby impact crater counts is an important andestablished tool to interpret geologic surfacesand stratigraphic relationships on other plan-ets and moons. Due to the random accumula-tion of impact craters on a planetary surfaceover a prolonged period, it is possible to as-sess its age. An intense modification by ero-sion, deposition of material or coverage bylava flows is resetting the surface-age to zeroand the crater accumulation is starting again(Hartmann & neukum 2001).

It is possible to map a geologic unit and toestablish relative and absolute ages by cra-ter size-frequency distributions, a techniquedescribed in detail by Hartmann & neukum

(2001) and ivanov (2001), neukum & Wise

(1976) and references therein. In the past, lu-nar crater frequencies could be inverted to ab-solute ages by comparison with Apollo rocksamples that were dated by radiometric crys-tallization ages. A chronology function couldbe approximated which is describing a near-ly constant cratering rate during the last 3 Gaand an exponentially decreasing rate before3 Ga (early intense bombardment). The cra-tering chronology model of the Moon was ad-justed for Mars taking into account several

clusters are overlapping each other and forma spatially extended volcanic unit with hun-dreds of vents. Single shield edifices have av-erage diameters of several tens of kilometers.Although a widespread eolian dust mantle ismasking the spectral signature in large partsof Tharsis, the observable geomorphologicfeatures show that the Martian low shield vol-canoes are most likely consisting of basalticlavas with very low viscosity (Hauber et al.2009, Hauber et al. 2011).

To reconstruct the timeline of volcanic ac-tivity of the low shields clusters, Hauber et al.(2011) recently produced a Tharsis-wide chro-nology by crater counts and also estimatedlava rheologies. Most small volcanoes haveages of <100 million years (Ma) and two clus-ters are older with ages of a few hundred mil-lion years up to ages of 0.3–2.9 billion years(Ga) (Hauber et al. 2011). The results showthat plains volcanism was active very recentlyin the last few percent of Mars’ geologic his-tory. An ongoing investigation of the volcanicvents in this area could probably detect late-stage eruptions that were even more recent. Inconclusion, it cannot be excluded that Mars isvolcanologically only dormant, and might beactive in the near future.

The geological mapping and investigationof small surfaces on Mars is possible sincecameras on planetary space probes are trans-mitting high-resolution data with resolutionsof meters per pixel. In our study the high-res-olution data of the Mars Reconnaissance Or-biter (MRO) have been used for geologicalmapping and the preparation of crater countsto estimate the absolute age of a low shieldvolcano and the surrounding plains in NoctisLabyrinthus, in the east part of Tharsis. In ad-dition a laser altimetry track, also acquired bythe Mars Global Surveyor (MGS), has beenused to construct a profile for morphometricanalysis.

In the past, crater counts were done us-ing a stereo comparator with analog images.Nowadays, the complete workflow can be per-formed digitally with Geographic InformationSystems (GIS). The aims of this study are todemonstrate the age determination of plane-tary surfaces by impact crater counts and theintegration of planetary remote sensing datainto GIS.

Fig. 1: Mars chronology function by Hartmann& neukum (2001).

F. Jagert & E. Hauber, Age Determination of Martian Low Shield Volcanoes 179

Hartmann & neukum (2001) calculatedthat crater counts provide dates with an uncer-tainty within a factor of 2 for ages <3.5 Ga and±100 Ma for ages >3.5 Ga, caused mainly byflux uncertainties. This implies that all esti-mated absolute ages can be either too young ortoo old. The observation of the recent Martianimpact-rate by repeated remote sensing imag-es over a seven-year period actually showedthat the predicted rate is in close agreementwith the current observed impact-rate (Hart-mann 2007, Werner 2009).

3 Investigation Area

As an example to demonstrate the techniqueof crater size-frequency measurements, weselected a low shield volcano in the north

parameters like different orbital mechanics,crater scaling and impact flux (neukum et al.2010) (Fig. 1). Important for the adaption ofthe lunar chronology to the Martian chronol-ogy was the estimation of the cratering rate ofMars relative to the Moon (production func-tion). With this function the production-rate ofcraters of any given diameter on a given sur-face is assumed to be known (Hartmann &neukum 2001, ivanov 2001).

These established methods enable to deter-mine relative and absolute ages of a given sur-face by cumulative crater size-frequency dis-tributions (CSFD). The idea is to plot the cra-ter counts as CSFD normalized to a unit areaand to fit the production function to the datapoints. With a correct fit the chronology func-tion can calculate absolute surface model ages(micHael & neukum 2010).

Fig. 2: Low shield volcano in the north of Noctis Labyrinthus, centered at ‑3.65°N/253.27°E (CTX‑image B02_010226_1765_XI_03S106W; taken in October 2008). The marked profile a - a’ isshown in Fig. 3. The shield shows several summit craters and elongated vents and is mainly bor‑dered by faults. The subdued circular depression in the NW of the main summit is not an impactcrater but rather a collapse structure induced by tectonics.


(HiRISE) have a swath-width of about ≥3 kmat 200 km altitude with resulting resolutionsof about 0.25–0.3 m/pixel (malin et al. 2007,mceWen et al. 2007). The satellite acquiredsimultaneous observations with the two cam-eras of this site in October 2008, which havebeen selected for crater counts. These two ob-servations made it possible to date the volcanoby independent crater counts at different rang-es of spatial resolutions to verify the results ofthe CSFDs.

In addition, data of the Mars Orbiter La-ser Altimeter (MOLA) of the Mars GlobalSurveyor (MGS) Mission have been used fortopographic analysis. This instrument mea-sured topographic elevations of the entireplanet with orbital nadir-tracks with an along-track shot spacing of ~300 m (vertical accu-racy ~1 m) (smitH et al. 2001). A single trackof this dataset was generated directly over themain summit, which was used to draw a pro-file of the shield construct (Fig. 3).

Planetary remote sensing data are accessi-ble through the Orbital Data Explorer (ODE).The ODE is a web-portal which is distributingthe data of several planetary missions fromNASA and ESA for Mars, Moon and Mercuryat no cost (WanG et al. 2011).

4.2 Pre-Processing

The software ArcGIS 9.3/10 from Esri wasused for mapping and crater counts. Mostplanetary data cannot be directly importedinto commercial GIS environments. There-fore, several processing steps are essential.

The planetary images were pre-processedwith the Integrated Software for Imagers andSpectrometers (ISIS 3) for LINUX, a soft-ware developed by the USGS (U.S. Geologi-cal Survey) and NASA, with multi-missioncapabilities for handling data acquired by sev-eral planetary space probes. The camera mod-els, spacecraft and planetary ephemerides andthe definition of planetary bodies (in our case:Mars) is completely included in this softwarepackage (anderson et al. 2004, eliason 1997,Gaddis et al. 1997).

With ISIS, the CTX data have been radio-metrically calibrated (including the removalof systematic noise) and geometrically recti-

of Noctis Labyrinthus in the eastern part ofTharsis. The study site is a transitional zonebetween the volcanic plains in Tharsis and aregion strongly modified by tectonics (Fig. 2).The fault trends in this area are mainly ori-ented NNW/SSE, N/S and ENE/WSW. Theregion shows much more craters than the lavaplains and is covered by an eolian dust layer,which is also visible as dunes. This low shieldwas chosen because it is clearly distinguish-able from the surroundings. The eruption tookplace at several summit craters and elongatedvents, which are aligned along a tectonic trendoriented NNW/SSE. The topographic profile(Fig. 3) shows the morphometry of the mainsummit, which has a height of only a few tensof meters with a base sloping downwards tonorth. The general slope of the terrain forcedthe lava to flow northwards, where it drainedinto grabens in ENE/WSW direction or wasstopped by an old crater rim. The observedmorphometry and gentle slopes of the volca-nic construct emphasize the results of Hauber

et al. (2011) that this type of volcanism is con-sisting of basaltic lavas with low viscosity.

4 Data and Methods

4.1 Remote Sensing Data

We used images of the Mars ReconnaissanceOrbiter, a planetary space probe which is or-biting Mars since 2006. There are two cam-eras onboard: The panchromatic images of theContext Camera (CTX) are ideal to cover anentire shield construct with a swath-width of~30 km and a spatial resolution of ~5.5 m/pix-el. The panchromatic and multispectral imag-es of the High Resolution Science Experiment

Fig. 3: Topographic MOLA-profile, crossingthe low shield volcano from a–a’. The volcanohas a height of only a few tens of meters and issloping to north.


distortion area of this cartographic projectionwithout the need of reprojection. These geo-registered JPEG2000 images are supported byArcGIS and can be added instantly.

The topographic MOLA data are accessiblethrough a query mask from the ODE. The sin-gle point-measurements can be downloadedin common file formats, e.g. as ASCII files or3D-shapefiles for the use in ArcMap or Arc-Scene (ArcGIS). A resulting profile is shownin Fig. 3.

4.3 Mapping and Crater Counts

After the steps in section 4.2, the planetary re-mote sensing images can be used for furtherwork in ArcGIS. The geological mappingsand crater counts were done with the ArcGIS-

fied from raw-instrument data to a map-pro-jected image. To minimize distortion, a sinu-soidal-projection was used with the centralmeridian crossing the center of the image.

To import the CTX data into ArcGIS,the images were exported to PNG files withworld files for georegistering. In ArcGIS thedatum of Mars 2000 defined by the Interna-tional Astronomical Union (IAU) was added(semimajor axis: 3396.19 km, semiminor axis:3376.20 km), which is compatible to the da-tum used in ISIS.The HiRISE data are accessible via ODE

already completely radiometrically correct-ed and map-projected in the georegisteredJPEG2000 file format. They are delivered inan equirectangular-projection and can be re-projected to avoid distortion. In our case thelocation is centered near the equator, a low-

Fig. 4: Mapped stratigraphic units and counted craters (circles) superimposed onto the imagestrips. a) CTX image with volcano (red) and the adjacent plains (blue). The surrounding plains canprobably reflect the buried, older age under the volcano. b) HiRISE image with the mapped vol‑cano (red). This subset represents a statistical sample of the volcanic unit.


rim and with lava flowing radial outwards, itis most likely not an impact crater but rather avolcanic vent.

For complete statistics we counted everyimpact crater (without building classes) withits centre in the counting area. A pre-selectionof craters can generate a subjective result.

Two surfaces are dated at the investigationsite (Fig. 4). The geological unit of interest isthe low shield volcano in the CTX and HiRISEimages. In addition, it is of interest to estimatethe age of the surrounding plains, which ismost likely the age of the surface buried un-der the volcano. Therefore, homogeneous sur-faces adjacent to the volcano, bounded by tec-tonic faults, are mapped as well. In the SW, theplains are probably contaminated by second-ary craters, hence they are not used for dating.

For the volcanic unit in the CTX image wemapped 133 craters with diameters rangingfrom 21–191 m. The surrounding plains con-tain 249 craters (diameters: 22–743 m) and areprobably showing a weak contamination bysome huge secondary craters in the westernpart. The HiRISE observation shows smallercraters due to the increased spatial resolution.The HiRISE image of the volcano contains 93craters (diameters: 14–193 m).

4.4 Crater Size-FrequencyDistributions

After the mapping of geologic units and cratercounts, the recorded diameters of the cratersand the size of the counting area can be im-ported into the software Craterstats developedat FUB. With this software it is possible to plotthe CSFD of the crater counts and to fit it tothe production function with a least squares

extension CraterTools developed at the FreieUniversität Berlin (FUB) (kneissl et al. 2011).With this tool it is possible to map the geologi-cal unit and to measure the crater-diameters(with a circle) inside the counting area. Afterthis step it is possible to export the results forfurther analysis. A newer version of this toolis calculating the crater diameter internallywithout map distortion, independent of theprojection used in ArcGIS.

The dating of a planetary surface makesonly sense for homogeneous geological or geo-morphological units. Different units should bemapped separately. Otherwise the crater pop-ulations of two different stratigraphic ages aremerged and the CSFD would show a mixedage. Surfaces with steep slopes at scarps arenot usable for crater counts because the im-pact craters are distorted or eroded by masswasting processes. There is no need to mapthe complete geological unit. It is also suffi-cient to map and count an area which is rep-resenting a statistical sample of the unit (e.g.,HiRISE image in Fig. 4).

Areas with secondary crater clusters shouldbe excluded from the crater count becausethese types of craters are not included in theMartian crater production function (section2). Secondary craters are impact craters whichare generated by the debris (ejecta) of a largeimpact. Not every circular feature is an im-pact crater. Normal impact craters displaynearly perfect circular shapes with a craterrim (Fig. 5). Inside of these craters are depres-sions, sometimes filled with sediments. Volca-nic vents, calderas or other collapse structurescan be mistaken for impact craters. The localcontext will help in most instances. When apit crater is not circularly shaped, without a

Fig. 5: Examples of impact craters in the counting areas. From left to right: Crater with diameter of269 m (CTX), crater with diameter of 191 m (CTX), crater with diameter of ~105 m (HiRISE), heavi-ly degraded crater with diameter of ~87 m (HiRISE).


5 Results and Discussion

The results of the dating are shown in Fig. 6.The ages show the last time when the geolog-ic activity ended. The last activity of the lowshield volcano was 45.1 Ma (CTX) or 42.4 Ma(HiRISE) before today. Therefore, the cratercount of the volcano based on the CTX im-age is verified by the counts on the HiRISEimage, which has a completely different reso-lution and is mapped independently. The es-timated ages are very comparable with onlya small difference and with overlapping errorbars, although the HiRISE observation is onlya subset of the unit. The uniform distributionof craters and the lack of huge craters are indi-cating a short emplacement time without longperiods of inactivity.

The adjacent plains around the volcanoshow an age of 2 Ga with a small “bump” atlarge craters, which is steepening the plot andhas been excluded. These craters have simi-lar sizes and are located only in the western

method. The crater count results are plot-ted as reverse-cumulative histogram with logNcum vs. log D. For this work we used the es-tablished Martian chronology model of Hart-mann & neukum (2001) and the Martian pro-duction function of ivanov (2001). The datingby CSFDs is discussed in detail by micHael &neukum (2010) and references therein.

For the volcanic surface a minimum diame-ter of >60 m was used for best fitting (adjacentplains: >250 m). Ncum shows a characteristicfall-off at smaller craters (Fig. 6). A technicalreason for this trend is that it becomes moredifficult to count small craters at the resolutionlimit. With complex textures it is likely thatsome craters are lost. A second reason (espe-cially for the plains adjacent to the volcano) isthat the population of smaller crater diametersbelow 60 m is disturbed, probably caused bycontinuous erosion and dust deposition. Thereare some outliers with diameters in the rangeof 450–700 m. These potentially secondarycraters have been excluded from the fit.

Fig. 6: Cumulative crater frequency plots with the fitted production function. The crater list isbinned using pseudo-log bins. Numbers of craters used for the best fit are shown on top of theplots. We use the production function (PF) by ivanov (2001) and the chronology function (CF) byHartmann & neukum (2001). Left: CTX data plot for the volcanic surface (red), fitted to crater dia-meters >60 m, model age: 45.1 Ma. The second plot shows the surrounding plains (blue), fitted tocrater diameters >250 m, model age: 2 Ga. Right: HiRISE data plot for the volcanic surface (sub‑set), fitted to crater diameters >60 m, model age: 42.4 Ma.


Acknowledgements

We thank tHomas kneissl and GreGory mi-cHael (Freie Universität Berlin) for makingavailable their software tools CraterTools andCraterstats. In addition, we thank Petr Brož(Academy of Sciences of the Czech Republic)and Piotr Jodłowski (Freie Universität Berlin)for their support of this study.

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6 Conclusion

The age determination by crater size-frequen-cy distributions is an established tool in plan-etary science to reconstruct the chronologyof planetary surfaces. In this study we dem-onstrated a consistent relationship of the ob-servable geologic stratigraphy and the derivedages by crater counts. This technique can pro-duce reliable results for most surfaces. Excep-tions are areas with continuous modificationor intense contamination by secondary cra-ters, which would be observable in the result-ing CSFDs.

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The high-resolution images of the two cam-eras of the Mars Reconnaissance Orbiter areideally suited for the preparation of cratercounts on small areas. This was not possiblewith mid- or low-resolution data for severaldecades in the past. The data of MRO madeit attractive to re-investigate numerous smallsurfaces on Mars. It has been shown that withincreasing spatial resolution more small cra-ters can be detected, with a certain lower limitdue to the effects of erosion, complex texturesor burial by dust deposition.


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Addresses of the Authors:

M.Sc. Geogr. felix JaGert, Geographisches Insti-tut, Ruhr-Universität Bochum, Universitätsstr. 150,44780 Bochum, Germany, Tel.: +49-234-9507096,e-mail: [email protected]

Dipl.-Geol. ernst Hauber, Institut für Planeten-forschung, Deutsches Zentrum für Luft- u. Raum-fahrt e.V. (DLR), Rutherfordstr. 2, 12489 Berlin,Germany, Tel.: +49-30-67055-325, e-mail: [email protected]

Manuskript eingereicht: Dezember 2011Angenommen: Januar 2012

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Age Determination of Martian Low Shield Volcanoes by