Robotic systems for the determination of the composition of solar system materials by means of fireball spectroscopy
© Madiedo; licensee Springer. 2014
Received: 31 December 2013
Accepted: 30 June 2014
Published: 15 July 2014
The operation of the automated CCD spectrographs deployed by the University of Huelva at different observatories along Spain is described. These devices are providing information about the chemical nature of meteoroids ablating in the atmosphere. In this way, relevant physico-chemical data are being obtained from the ground for materials coming from different bodies in the Solar System (mainly asteroids and comets). The spectrographs, which work in a fully autonomous way by means of software developed for this purpose, are being employed to perform a systematic fireball spectroscopic campaign since 2006. Some examples of meteor spectra obtained by these devices are also presented and discussed.
Meteoroids are solid particles with sizes ranging from several tens of microns to around 10 m. These are mostly originated from asteroids and comets, although the analysis of meteorites recovered on Earth show that some meteoroids can also come from other bodies, such as Mars and the Moon. The mechanisms that deliver these particles to Earth provide a unique opportunity to measure from the ground the physico-chemical properties of materials coming from different bodies in the Solar System, and this information could be employed to plan future space missions. These meteoroids are observed as meteors when they impact the Earth's atmosphere. Meteors can also be observed by seismic signals (e.g., Yamada and Mori 2012; Ishihara et al. 2012). Meteors with a luminosity higher stellar magnitude −4 are named fireballs. The light emitted by these phenomena during the ablation in the atmosphere of the progenitor meteoroid allows analyzing these events. Thus, when meteors are simultaneously detected from, at least, two different meteor stations, their atmospheric trajectory and radiant can be easily determined, and the orbit of the progenitor meteoroid in the Solar System can be calculated (Ceplecha 1987). Once the orbital parameters are known, the so-called dissimilarity criteria can be employed to infer which is the potential parent body of these particles (Southworth and Hawkins 1963; Drummond 1981; Jopek 1993; Valsecchi et al. 1999; Jenniskens 2008; Williams 2011; Madiedo et al. 2013a, 2014c).
Geographical coordinates of the meteor observing stations involved in the recording of meteor spectra and devices employed (V: CCD video spectrographs, S: slow-scan CCD spectrographs)
5° 58′ 50″
37° 20′ 46″
V + S
6° 19′ 35″
37° 40′ 19″
V + S
3° 11′ 00″
39° 34′ 06″
6° 56′ 11″
37° 15′ 10″
6° 43′ 58″
37° 06′ 16″
Sierra Nevada (OSN)
3° 23′ 05″
37° 03′ 51″
La Pedriza (OAA)
3° 57′ 12″
37° 24′ 53″
The first spectral devices setup in the framework of the SMART project, which were based on high-sensibility CCD video cameras endowed with holographic diffraction gratings, started operation in 2006 from the meteor observing station located at Sevilla and also at the mobile meteor observing station at Cerro Negro (station nos. 1 and 2 in Table 1, respectively). The first of these stations is performing since that year a systematic spectroscopic campaign. The latter, however, is setup when necessary in a dark countryside environment located at about 60 km north from Sevilla. From 2007 to 2010, four additional meteor-observing stations endowed with automated CCD video spectrographs were deployed (no. 3 to no. 6 in Table 1). The last one (La Pedriza) has been setup in Andalusia (south of Spain) by the end of 2013. The expansion of this network of spectral video devices has been favored by the deployment of new video meteor stations in Spain, which increased from 2 in 2006 to 25 in 2010. From 2011, however, additional efforts were made by setting up new automated spectrographs based on slow-scan high-resolution CCD devices. These also employ 1,000 lines/mm diffraction gratings and operate from station nos. 1 and 2 in Table 1 (Sevilla and Cerro Negro, respectively).
Favorable weather conditions in Spain play a key role in the successful development of the continuous spectroscopic campaign carried out within the SMART project. Thus, as a result of this survey several hundreds of meteor spectra have been recorded so far. These include emission spectra produced not only by sporadic fireballs but also by events associated to major and minor meteoroid streams. In this paper, a description of these systems is given and some relevant results are presented.
Instrumentation and data reduction techniques
The video spectrographs setup by the University of Huelva work in a fully autonomous way by means of the MetControl software, which is described below. This application has been developed in the framework of the SMART project. These spectrographs are slitless systems based on the same low-lux CCD video cameras employed at meteor stations in Table 1 to monitor meteor and fireball activity (Madiedo and Trigo-Rodríguez 2008). Thus, two different Watec cameras (902H2 and 902H2 Ultimate, manufactured by Watec Corporation, Tsuruoka-shi, Japan) are employed. A diffraction grating (500 or 1,000 lines/mm, depending on the device) is attached to the optics of each CCD video camera. These devices, which can image meteor spectra for events with a luminosity higher than mag. −3/−4, generate interlaced video imagery in AVI format by following the PAL video standard. Thus, the images are recorded with a resolution of 720 × 576 pixels at a rate of 25 frames per second (fps). So, their time resolution is of about 0.02 s (once the images are deinterlaced). This makes possible the analysis of the evolution with time of meteor spectra. Their typical spectral resolution is of about 2.5 nm/pixel. Each video spectrograph is connected to a PC computer by means of a video acquisition card (model DC60, manufactured by EasyCap Capture, Shenzhen, China). In this way, the images are stored on a hard disk, and no compression is employed in order to preserve image quality. The computers are synchronized by means of a GPS antenna (35-HVS, manufactured by Garmin, Schaffhausen, Switzerland) to keep a precise timing of meteor and fireball events. Since these video spectrographs are based on 8-bits (i.e., 256 gray levels) devices, the most intense emission lines can saturate the CCD sensor for very bright events. In fact, this is one of the major drawbacks of these devices. Despite the grating, these video spectrographs are sensitive enough to image mag. +2/+3 and brighter stars that can be employed for an astrometric analysis. Thus, atmospheric trajectories can also be calculated for multi-station events by following the planes intersection method (Ceplecha 1987) and, so, the evolution of the conditions in meteor plasmas as a function of height (and not just as a function of time) can be analyzed. These spectra are reduced by means of the CHIMET software, which was also developed in the framework of the SMART project and is described in (Madiedo et al. 2011).
On the other hand, the higher-resolution spectroscopes consist of five slitless slow-scan high-sensitivity CCD devices that employ 1,000 lines/mm diffraction gratings. Two of these are manufactured by ATIK (models ATIK 4000LE and ATIK 16HR, Norwich, England), and the other three are manufactured by SBIG (one ST10 and two ST8 CCD cameras, Santa Barbara, CA, USA). These operate since August 2011 from station nos. 1 and 2 in Table 1 (Sevilla and Cerro Negro, respectively), where an array of high-sensitivity CCD video cameras is also used for the monitoring of meteor activity. These spectrographs generate imagery in FITS files which are sent to GPS synchronized computers. The exposition time is adjusted according to the conditions of the night sky (typically, 30-s expositions are employed). These systems are currently covering an extension of about 50° × 50° in the night sky. Their typical spectral resolution is of about 0.8 nm/pixel. Dead time between images, which is of about 10 s, is one disadvantage with respect to the operation of CCD video spectrographs that can work continuously during the whole night. On the other hand, since no rotary shutter or any similar device is employed, these spectrographs are not useful to obtain meteor velocity data. Thus, they cannot be employed to obtain the orbit in the Solar System of the meteoroids producing the emission spectra. Nevertheless, their imagery can be combined with the recordings obtained by the CCD video devices to obtain such information.
The MetControl software
Atmospheric trajectory and radiant data for the fireball discussed in the text (J2000)
94.2 ± 0.5
67.5 ± 0.5
115.8 ± 0.2
31.5 ± 0.2
36.9 ± 0.3
35.1 ± 0.3
33.5 ± 0.3
SPMN101212 N. χ-Orionid
95.1 ± 0.5
71.2 ± 0.5
81.1 ± 0.3
21.1 ± 0.2
26.6 ± 0.3
24.5 ± 0.3
37.9 ± 0.3
99.7 ± 0.5
79.1 ± 0.5
300.7 ± 0.2
−9.9 ± 0.1
24.0 ± 0.3
21.5 ± 0.3
37.5 ± 0.3
Orbital parameters (J2000) for the fireballs discussed in the text
1.31 ± 0.02
0.907 ± 0.003
24.9 ± 0.6
327.5 ± 0.3
262.5421 ± 10−4
SPMN101212 N. χ-Orionid
2.4 ± 0.1
0.79 ± 0.01
1.7 ± 0.2
95.44 ± 0.2
78.33956 ± 10−4
2.6 ± 0.1
0.76 ± 0.01
7.1 ± 0.1
262.8 ± 0.4
124.3210 ± 10−4
The spectrum of a Geminid fireball
The parent body of the Geminid meteoroid stream is asteroid 3200 Phaeton. This produces the annual Geminid meteor shower, which is active from about November 27 to December 18 and peaks around December 14 (Jenniskens 2006). The perihelion distance of this stream is of about 0.14 AU, and this relatively small value has been correlated to the observed depletion of volatiles in Geminid meteoroids (Kasuga et al. 2006).
Elemental abundances relative to Fe derived for the fireballs analyzed in the text
4400 ± 200
SPMN101212 N. χ-Orionid
4000 ± 200
3600 ± 200
Spectrum of a Northern χ-Orionid fireball
The Northern χ-Orionid meteoroid stream produces an annual display of meteors from about December 1 to January 5, peaking around December 11 (Jenniskens 2006). The suggested progenitor body of this stream is the potentially hazardous asteroid (PHA) 2008XM1 (Madiedo et al. 2013a).
Emission spectrum of an α-Capricornid bolide
According to the measured relative intensities of lines corresponding to the Na I-1, Mg I-2, and Fe I-15 multiplets, the Na/Mg and Fe/Mg intensity ratios yield 1.50 and 0.40, respectively. Once again, the Na/Mg intensity ratio fits the expected value for chondritic materials (figure five in Borovička et al. (2005)). However, the triangular diagram in Figure 6 shows that the meteoroid was Fe-poor, since the position corresponding to this particle falls well below the solid curve describing chondritic meteoroids at a velocity of around 24 km s−1. This Fe depletion is confirmed by the relative abundances inferred from the emission spectrum and listed in Table 4. This table shows also that the meteoroid was Mg-rich, since the elemental abundance for this element is higher by a factor of about 2.5 with respect to the chondritic value. This is in agreement with the result obtained from the α-Capricornid spectrum analyzed by Borovička and Weber (1996). It is worth noting that a higher than chondritic Mg/Fe ratio was also found by Abe et al. (2005) when analyzing the emission spectrum of a bright Leonid fireball, produced by a meteoroid associated with comet 55P/Temple-Tuttle. One possibility for the Mg enrichment obtained from the analysis of this α-Capricornid spectrum could be associated to a high content in the parent comet of this stream (and so also in α-Capricornid meteoroids) of Mg-rich pyroxene, as has also been reported for comet Hale Bopp (Wooden et al. 1999). However, lower Mg abundances have been found for other for α-Capricornid fireballs, which imply inhomogeneities in the composition of the parent comet at the cm-level (Madiedo et al. 2014b). Again, the low Ca abundance would be a consequence of the incomplete evaporation of this element during ablation.
A software package (MetControl) has been developed to achieve an automated operation of these spectrographs. The images obtained by these systems can be used to obtain an insight into the chemical nature of meteoroids producing meteor events with a brightness above mag. −3/−4. In this way, information about the composition of different Solar System materials can be obtained. This chemical information could be employed to plan future space missions.
The spectra recorded by the spectrographs contain contributions from Fe, Ca, Mg, and Na. These spectra can be employed to infer the likely chondritic composition of meteoroids and their parent bodies from the analysis of the relative intensity of emission lines produced by Mg I-2, Na I-1, and Fe I-15 multiplets. Besides, abundances with respect to Fe of Mg, Na, and Ca can be calculated. Atmospheric nitrogen and oxygen contributions also appear in these spectra.
Meteoroids produced by 3200 Phaeton and 2008XM1 have abundances of Na, Mg, and Ca compatible with the expected for chondritic materials. The lower-than-expected Ca abundances can be explained on the basis of the incomplete evaporation of this element during the ablation of these meteoroids in the atmosphere.
The analysis of the spectrum produced by an α-Capricornid fireball indicates that progenitor meteoroid was Fe-poor with respect to the chondritic value. In addition, this particle exhibited an overabundance of Mg with respect to the expected value for chondritic materials. This could be associated to a likely high content of Mg-rich pyroxene in comet 169P/NEAT, as has also been reported for comet Hale Bopp.
The SMART project has been funded by the author of this paper and partially supported by the Spanish Ministry of Science and Innovation (project AYA2009-13227). I also thank AstroHita Foundation for its support in the establishment and operation of the automated meteor observing station located at La Hita Astronomical Observatory (La Puebla de Almoradiel, Toledo, Spain).
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