Detection of a bolide in Jupiter's atmosphere with Juno UVS
Rohini S. Giles, Thomas K. Greathouse, Joshua A. Kammer, G. Randall Gladstone, Bertrand Bonfond, Vincent Hue, Denis C. Grodent, Jean-Claude Gérard, Maarten H. Versteeg, Scott J. Bolton, John E. P. Connerney, Steven M. Levin
mmanuscript submitted to
Geophysical Research Letters
Detection of a bolide in Jupiter’s atmosphere withJuno UVS
Rohini S. Giles , Thomas K. Greathouse , Joshua A. Kammer , G. RandallGladstone , , Bertrand Bonfond , Vincent Hue , Denis C. Grodent ,Jean-Claude Gérard , Maarten H. Versteeg , Scott J. Bolton , John E. P.Connerney , and Steven M. Levin Space Science and Engineering Division, Southwest Research Institute, San Antonio, Texas, USA Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas, USA Laboratoire de Physique Atmosphérique et Planétaire, STAR Institute, Université de Liège, Liège,Belgium Space Research Corporation, Annapolis, Maryland, USA Goddard Space Flight Center, Greenbelt, Maryland, USA Jet Propulsion Laboratory, Pasadena, California, USA
Key Points: • Juno UVS recorded transient blackbody emission from a point source in Jupiter’satmosphere • The emission is consistent with a fireball produced by a 250–5000 kg impactor inJupiter’s upper atmosphere • We estimate an impact flux on Jupiter of 24,000 per year for masses greater than250–5000 kg
Corresponding author: R. S. Giles, [email protected] –1– a r X i v : . [ a s t r o - ph . E P ] F e b anuscript submitted to Geophysical Research Letters
Abstract
The UVS instrument on the Juno mission recorded transient bright emission froma point source in Jupiter’s atmosphere. The spectrum shows that the emission is con-sistent with a 9600-K blackbody located 225 km above the 1-bar level and the durationof the emission was between 17 ms and 150 s. These characteristics are consistent witha bolide in Jupiter’s atmosphere. Based on the energy emitted, we estimate that the im-pactor had a mass of 250–5000 kg, which corresponds to a diameter of 1–4 m. By con-sidering all observations made with Juno UVS over the first 27 perijoves of the mission,we estimate an impact flux rate of 24,000 per year for impactors with masses greater than250–5000 kg.
Plain Language Summary
UVS is an ultraviolet spectrograph on NASA’s Juno spacecraft, which has been inorbit around Jupiter since 2016. In April 2020, UVS observed short-lived bright emis-sion from a point source in Jupiter’s atmosphere. The emission was consistent with ablackbody of temperature 9600 K. We suggest that this was a fireball produced by a 250–5000 kg meteoroid entering Jupiter’s atmosphere.
As the largest and most massive planet in the Solar System, Jupiter undergoes aheavy bombardment of objects, ranging from tiny dust grains to kilometer-sized comets.The largest impacts, such as comet Shoemaker-Levy 9 (Asphaug & Benz, 1996) and the2009 impactor (Sánchez-Lavega et al., 2010), occur rarely but leave scars on the planetthat can persist for several months (Sánchez-Lavega et al., 1998, 2011) and can affectthe distribution of trace species in Jupiter’s upper atmosphere for decades (Lellouch etal., 2006; Cavalié et al., 2013; Benmahi et al., 2020). Impacts from objects in the 5–20m diameter range occur more frequently and produce short bright flashes of light thatcan be observed by amateur astronomers on Earth, but no observable debris (Hueso etal., 2018). Over a period of 8 years (2010–2017), amateur astronomers observed 5 of thesesmaller impacts, leading to an estimated impact rate of 10–65 per year (Hueso et al., 2018),an estimate that was not significantly modified by the observation of a 6th impact in 2019(Sankar et al., 2020). On the smallest scales, the constant influx of dust particles equatesto hundreds of thousands of tons of material per year (Sremčević et al., 2005; Poppe, 2016).While larger (>5 m) impacts can be observed from Earth, observations from or-biting spacecraft have the advantage of being able to detect the fainter flashes that are –2–anuscript submitted to
Geophysical Research Letters produced from the more frequent smaller impacts. One example of this is the small fire-ball observed by the camera on the Voyager 1 spacecraft, which Cook and Duxbury (1981)estimated was caused by an 11 kg (<0.5 m) meteoroid. In this paper, we present obser-vations of another fireball in Jupiter’s atmosphere, this time observed by the UVS in-strument on the Juno spacecraft. In Section 2, we describe the instrument and the methodof observation. In Section 3 we discuss the properties of the bright blackbody-emissionflash observed. In Section 4, we conclude that this flash was caused by a meteoroid ofmass 250–5000 kg (diameter 1–4 m) entering Jupiter’s atmosphere, and we use the sumof all of our observations to estimate an impact flux rate.
The Ultraviolet Spectrograph (UVS) is an instrument on the Juno spacecraft, whichhas been in orbit around Jupiter since July 2016 (Bolton et al., 2017). UVS is a photon-counting imaging spectrograph, covering wavelengths of 68–210 nm in the far-ultraviolet(Gladstone, Persyn, et al., 2017). The main purpose of UVS is to study the morphol-ogy, brightness, and spectral characteristics of Jupiter’s auroras, and the instrument’sspectral range covers important H and H auroral emissions (Gladstone, Versteeg, et al.,2017; Bonfond et al., 2017).Juno is a spin-stabilized spacecraft with a rotation period of 30 s; as the spacecraftrotates, the UVS instrument slit sweeps across Jupiter. The wavelength, slit position andprecise time of UV photon detection events are recorded and this information is then usedto build up spatial maps of the ultraviolet radiation (see Figure 1 for an example froma single spin). The UVS instrument slit consists of two wide segments on either side ofa narrow segment. The wide parts of the slit have a width of 0.2 ◦ and a spectral reso-lution of 2.0–3.0 nm, while the narrow part of the slit has a width of ∼ ◦ and a spec-tral resolution of ∼ ◦ . The observations described in this paper were made at the top edge of theupper wide slit, where the spatial resolution is ∼ ◦ . On occasion, the Juno UVS instrument has observed short-lived, localized ultra-violet emission outside of the auroral zone. In Giles et al. (2020), we described a set ofeleven bright transient flashes that were observed with Juno UVS and shared similar char-acteristics; they lasted ∼ emission and the source re- –3–anuscript submitted to Geophysical Research Letters
Figure 1.
A spatial map of Juno UVS measurements during a single spacecraft spin on 10April 2020. Photons with wavelengths 130–160 nm are shown in green and photons with wave-lengths 170–200 nm are shown in red. The inset image magnifies the area around the brightspot. gion was located ∼
260 km above the 1-bar level. Based on these characteristics, we con-cluded that these were likely to be elves, sprites or sprite halos, forms of Transient Lu-minous Events (TLEs) that occur in the upper atmosphere in response to troposphericlightning.In this paper, we focus on another transient bright flash observed by UVS, but thisobservation has very different characteristics than the aforementioned TLEs. The ob-servation was made on 10 April 2020 at 12:57:10 UTC and the bright spot was locatedat a planetocentric latitude of 53 ◦ N and a System III longitude of 200 ◦ W. It was observedat an emission angle of 43 ◦ and a solar incidence angle of 125 ◦ (i.e. on the planet’s night-side). At the time of the observation, the spacecraft was at an altitude of 80,000 km above –4–anuscript submitted to Geophysical Research Letters the 1-bar level of the planet and the sub-spacecraft latitude and longitude were 63 ◦ N and243 ◦ W respectively. This location was not observed by Juno’s infrared instrument (JIRAM,Adriani et al., 2008) or the visible light camera (JunoCam, Hansen et al., 2017) duringthis time period or afterwards. Figure 1 shows the UVS map from the spin in which thebright spot was recorded and includes a magnified image of the spot itself. This mag-nified image has dimensions 3 ◦ × ◦ on the sky, with each pixel being 0.1 ◦ × ◦ . The spotwas only observed during a single spin; it was not seen two spins earlier or three spinslater, which were the closest times when the same latitude and longitude was observed.The colors used in Figure 1 show the number of photon detections made in two dif-ferent spectral regions. Green is used to represent photons with wavelengths 130–160 nmand red is used to represent photons with wavelengths 170–200 nm. The UVS swath in-cludes a segment of Jupiter’s northern auroral oval and the auroral emission appears purelygreen in the image; this is because the main auroral H emission bands are at <170 nm.In contrast, the bright spot appears mostly yellow, indicating that there is significantemission at longer wavelengths as well.Although this bright spot is located close to the northern auroral region, the long-wavelength emission marks it out as unique when compared with auroral features. It isalso unique compared to the transient bright flashes described in Giles et al. (2020), whichwere all dominated by H emission, like the aurora. Despite these different spectral char-acteristics, the bright spot does clearly originate from within Jupiter’s atmosphere; Fig-ure 1 shows that it is located far from the limb of the planet so there is no possibilityof confusion with a star close to Jupiter, and the bright spot spectrum shows CH ab-sorption from Jupiter’s atmosphere (see Section 3.2) so it cannot originate from an ob-ject located between the spacecraft and the planet.We conducted a search for other such events, by filtering for occasions when therewas a sharp increase and then decrease in the number of long-wavelength photon countsrecorded, but no similar events were found. The spot is analyzed further in Section 3 andthe results are discussed in Section 4. Because of the way in which the UVS maps are built up as the spacecraft spins,Figure 1 contains information about both the spatial extent and the duration of the brightspot. By fitting a two-dimensional Gaussian to the total photon counts, we find that thespot has a FWHM of 0.29 ◦ in the along slit direction and 0.23 ◦ in the across slit direc- –5–anuscript submitted to Geophysical Research Letters tion. This is consistent with the shape that we would expect for a point source that hasa constant brightness on the 17-ms timescale it takes the slit to pass over the source (Greathouseet al., 2013). Based on the distance of the spacecraft at the time of the observations (80,000km), the upper limit for the spot diameter is 400 km.Because the shape of the bright spot is consistent with the FWHM of a point source,the bright spot must also be approximately stationary in latitude and longitude over the17-ms duration of the observation. In order to observe motion, the spot would have hadto move >0.1 ◦ in 17 ms, equating to a horizontal velocity of >8000 kms -1 . Since we donot observe any elongation, any horizontal movement must be below this speed.While the brightness appears approximately constant on the timescale it takes theslit to pass over the source, the bright spot was only observed during a single spin, soit must be transient on slightly longer timescales. UVS data was recorded from the samelatitude and longitude two spins (60 s) earlier and three spins (90 s) later, and the brightspot was not present in either of these maps. This places an upper bound of 150 s onthe duration of the bright spot, giving a duration of between 17 ms and 150 s.Because the bright spot was observed on the night side of the planet, it is not pos-sible to use UVS data to search for any local atmospheric changes in its aftermath, suchas changes in the aerosol abundance. Any such changes could therefore have lasted longerthan 150 s. The bright spot spectrum was calculated by summing all photon detections withina 1 ◦ × ◦ box centered on the bright spot. The spectrum is shown by the black data pointsin Figure 2 and is presented in terms of spectral irradiance measured at the spacecraft.As suggested earlier by the color of the bright spot in Figure 1, the bright spot emissionhas an unusual spectral shape compared to both the auroral emission and the TLE emis-sion described in Giles et al. (2020). The auroral and TLE spectra are dominated by H emission, and the H Lyman band can be recognized by a double peak at 160 nm. Be-yond 170 nm, the H emission drops to low levels. In contrast, the spectrum shown inblack in Figure 2 increases smoothly between 150 and 190 nm, suggesting blackbody emis-sion. The rapid drop-off at wavelengths shorter than 140 nm suggests strong atmosphericabsorption by CH .In order to model a blackbody spectrum attenuated by atmospheric absorption,we used the atmospheric composition Model C from Moses et al. (2005). In this modelatmosphere, the CH homopause is located at a pressure of 2 . × − mbar, or 360 km –6–anuscript submitted to Geophysical Research Letters
140 160 180 20010 −15 −14 −13 (a) Varying blackbody temperature
140 160 180 200Wavelength (nm)10 −15 −14 −13 I rr ad i an c e ( W / c m / n m ) (b) Varying emission altitude
140 160 180 200Wavelength (nm)10 −15 −14 −13 I rr ad i an c e ( W / c m / n m )
400 km300 km250 km200 km150 km (c) Best fits
140 160 180 200Wavelength (nm)10 −15 −14 −13 I rr ad i an c e ( W / c m / n m ) CH and C H linked via modelC H varyingindependently Figure 2.
The bright spot spectrum (black) compared with various model spectra. (a) Thecolored lines show the shape of blackbodies with different temperatures. There is no atmosphericabsorption. (b) The colored lines show the spectral shape of a 9000 K blackbody placed at dif-ferent altitudes and therefore with different amounts of atmospheric absorption. (c) The coloredlines show two different best-fit model spectra. –7–anuscript submitted to
Geophysical Research Letters above the 1 bar level. For a given altitude and emission angle, we used the atmosphericcomposition model to calculate the column density for each atmospheric gas. The threestratospheric gases that contribute significantly to absorption in the 125–210 nm rangeare CH , C H and C H . Absorption cross-sections for these three gases were obtainedfrom Chen and Wu (2004), Lee et al. (2001), Smith et al. (1991) and Bénilan et al. (2000).The atmospheric transmission was then calculated using the column density for each gasand the absorption cross-sections, and this was multiplied by a blackbody spectrum toobtain a modeled top-of-atmosphere spectrum. The colored lines in Figures 2(a) and 2(b)show these modeled spectra for a range of blackbody temperatures and emission alti-tudes.Figure 2(a) shows modeled spectra for blackbodies with five different temperatures.For these spectra, no atmospheric absorption is included and the spectra have been scaledto match the average irradiance of the bright spot spectrum. Within the 125–210 nmspectral range, increasing the temperature from 7000 K to 12,000 K flattens the spec-trum. At shorter wavelengths, the data diverges sharply from all of the blackbody spec-tra due to the lack of atmospheric absorption. We note that the data also diverges fromthe blackbody spectra at ∼
195 nm. As demonstrated by the larger error bars, the UVSsensitivity is much lower at these longer wavelengths than at shorter wavelengths; by 195nm, the instrument’s effective area is an order of magnitude smaller than at 160 nm (Hueet al., 2018). We therefore attribute this apparent small dip in the irradiance to limi-tations in the radiometric calibration.Figure 2(b) shows modeled spectra obtained using different emission altitudes (i.e.different amounts of atmospheric absorption). Each spectrum uses the same 9000 K black-body as the source emission and the altitude is defined as the height above the 1-bar level.The red lines in Figures 2(a) and 2(b) are identical, as there is negligible absorption abovean altitude of 400 km. As the source is moved deeper in the atmosphere, the amount ofabsorption increases. By 250 km there is significant CH absorption shortwards of 140nm. By 200 km, there is also a significant C H feature at 152 nm. By 150 km, the at-mosphere is opaque shortwards of 155 nm due to the combination of C H , C H andCH . When the blackbody temperature, emission altitude and a scaling factor were allallowed to vary together, the best fit was obtained with a temperature of 9600 ±
600 Kand an altitude of 225 ± H absorption feature at 152 nm is clearly too strong; an altitude of 225 km pro- –8–anuscript submitted to Geophysical Research Letters vides a good fit for the CH and C H column densities, but the C H column density(2 . × cm -2 ) is too high. Instead, if we allow the C H column density to vary in-dependently of the other gases, we retrieve a value of 1 . × cm -2 , approximatelyhalf of the value at 225 km. The spectrum obtained with this value is shown by the blueline in Figure 2(c). Compared to the Moses et al. (2005) model atmosphere, this sug-gests that the atmosphere near the bright spot has lower C H volume mixing ratio. Thisis unsurprising as the Moses et al. (2005) model describes equatorial latitudes, and Nixonet al. (2007) used Cassini CIRS observations to show that the C H volume mixing ra-tio decreases at higher latitudes. The blue line in Figure 2(c) is therefore consistent withan altitude of 225 km at a latitude of 53 ◦ N. The bright spot described in Section 3 has the following properties:(i) Its duration is longer than 17 ms and less than 150 s.(ii) The shape is consistent with a point source, which equates to a maximum diam-eter of 400 km.(iii) Its spectral shape is consistent with a blackbody of temperature 9600 K locatedat an altitude of 225 km above the 1-bar level (a pressure of 0.04 mbar).In Giles et al. (2020), we presented several instances of transient bright flashes ob-served in the Juno UVS data and we concluded that these were consistent with TLEsknown as elves, sprites or sprite halos, upper atmospheric responses to tropospheric light-ning. The bright spot that we present in this paper differs from the previous bright flashesin two main ways, highlighted in Figure 3. Firstly, it has a significantly longer duration.The TLEs had a duration of ∼ ∼
17 ms. This difference in duration is shown in Figures 3(a) and(b); the Gaussian shape in (a) is governed by the width of the slit, while the rapid ex-ponential decay of (b) takes place on a much shorter timescale. Secondly, the spectraof the TLEs were dominated by H Lyman band emission, giving them a very similarspectral shape to auroral emission. In contrast, this bright spot has a blackbody-like emis-sion spectrum. The spectra are compared in Figures 3(c) and (d). As discussed in Sec-tion 3, the shape of (c) is consistent with blackbody emission, while the double peak at160 nm seen in (d) is indicative of H emission. –9–anuscript submitted to Geophysical Research Letters (a) Bolide time series −40 −20 0 20 40Time (ms)050100150200 C oun t s / m s (b) TLE time series −40 −20 0 20 40Time (ms)050100150200 C oun t s / m s (c) Bolide spectrum
140 160 180 200Wavelength (nm)10 −14 −13 I rr ad i an c e ( W / c m / n m ) (d) TLE spectrum
140 160 180 200Wavelength (nm)10 −14 −13 I rr ad i an c e ( W / c m / n m ) Figure 3.
A comparison of the time series and spectra of the bolide (this paper) and theTLEs (Giles et al., 2020). (a) and (b) show the number of observed UV photons as a functionof time, relative to the peak event time. The time series shown in (b) is for the TLE observedon April 10 2020 at 17:24:35 UTC. (c) shows the same spectrum presented in Figure 2 and (d)shows the averaged TLE spectrum from Giles et al. (2020).–10–anuscript submitted to
Geophysical Research Letters
Because of both the duration and the spectral shape, the bright spot described inthis paper is unlikely to be a TLE. Lightning and associated TLEs do not have black-body spectra; on earth, they have emission spectra that are dominated by nitrogen andoxygen lines (Walker & Christian, 2017; Rodger, 1999) and on Jupiter their emission spec-tra are dominated by hydrogen (Borucki et al., 1996; Yair et al., 2009). Elves have sub-millisecond timescales, and while sprites can last for several tens of milliseconds, the brighterones are often shorter (Lyons, 2006). There are other longer-lasting TLEs, such as bluejets, but these would still be H dominated and they emerge directly from the thunder-storm anvils (Rodger, 1999), so we would expect them to be deeper in the atmosphere.The shape of the spectrum also rules out the possibility that this is a transient auroralevent, as auroral emission is H dominated.Given the spectrum and the duration, we suggest that the bright spot we observemay be a bolide/fireball in Jupiter’s atmosphere. Borovička and Spurn`y (1996) studiedthe lightcurves and spectra of two of the brightest bolides observed in the Earth’s at-mosphere and found that the spectra were a mixture of blackbody continuum and emis-sion lines. The emission lines come from ablated meteoroid material and the blackbodycontinuum comes from thermal bremsstrahlung (free-free) emission. In the beginning phaseof the bolide, Borovička and Spurn`y (1996) found that the line emission dominated, butduring the brightest part of the lightcurve, the blackbody continuum dominated. A tem-perature of 9600 K is approximately consistent with the Planck temperatures measuredin Earth bolides. Borovička (1994) found that fireball spectra in the Earth’s atmosphereexhibit two distinct characteristic temperatures: a primary component with a brightnesstemperature of 4000 K and a secondary component with a brightness temperature of 10,000K. The lower temperature is thought to be from the thermal radiation of the meteoroiditself, while the second part is associated with the shock wave. The greater the speedof the meteoroid, the more dominant this second part is. We do not observe any move-ment of the bright spot during our 17-ms observation timescale, and this is consistentwith a typical impact velocity of tens of kms -1 (Crawford et al., 1994); a horizontal ve-locity of >8000 kms -1 would be required to be detectable by UVS.Our observations are somewhat similar to the observations made by the UVS in-strument on the Galileo spacecraft when comet Shoemaker-Levy 9 collided with Jupiterin 1994 (Hord et al., 1995). They observed a brightening during one spatial scan only(no brightening in scans obtained 5 s beforehand or 5 s afterwards) and when combinedwith simultaneous observations from the Photopolarimeter Radiometer, they concludedthat the brightness temperature was 7800 +500 − K. Our observations are also similar toseveral observations made by amateur observers of bolides on Jupiter (Hueso et al., 2018). –11–anuscript submitted to
Geophysical Research Letters
These observations were recorded in the visible and the observed bright flashes lasted1–2 s in each case. One of these events was simultaneously recorded in two different fil-ters, and fitting a blackbody to these two measurements gave a temperature of 6500–8500 K (Hueso et al., 2013).Hueso et al. (2010) used the Earth-based observation of a Jovian bolide to estimatethe size of the impacting object and we follow their analysis approach here. If we cor-rect for atmospheric absorption, extend the blackbody shape to all wavelengths and in-tegrate, the total irradiance observed at the spacecraft would be 1 . − . × − Wcm -2 (taking into account the 600 K error on the brightness temperature). Assuming isotropicradiation, the total power emitted is 1 . − . × W. As we only observe the brightflash for a very short period of time as the spacecraft spins, we do not know the totalduration of the emission and this adds significant uncertainty to the calculation. Here,we assume a duration of 1–2 s, based on Hueso et al. (2018). There is additional uncer-tainty from the fact that we do not know which stage of the light curve we are observ-ing - it could be the peak of the emission, or the peak could occur shortly before/afterour observations. Including a 50% uncertainty for this factor, we obtain a total opticalenergy of 5 × – 6 × J.Brown et al. (2002) found that the efficiency, µ , with which the kinetic energy ofan impactor is converted into optical energy can be empirically described by µ = 0 . × E . (1)where E is the optical energy in terms of kilotons of TNT (1 kton = 4 . × J). For our observed total optical energy range, the efficiency is 7–10% and the kineticenergy of the impactor is therefore 7 × – 6 × J. Assuming an impactor veloc-ity of 60 kms -1 , the velocity at which fragments of Shoemaker-Levy 9 collided with Jupiter (Crawfordet al., 1994), and a velocity error of 20% leads to a mass estimate of 250–5000 kg. Fordensities of of 250–2000 kgm -3 , this equates to a diameter of 1–4 m.By considering the total power emitted and the blackbody temperature, we can usethe Stefan-Boltzmann law to calculate the area of the emitting region. Assuming an emis-sivity of 1, the effective diameter of the emitting region is 9 m. This is consistent withan impactor diameter of 1–4 m; Borovička and Spurn`y (1996) found that the maximumdiameter of the radiating region is on the order of 10 times larger than the initial diam-eter of the body. –12–anuscript submitted to Geophysical Research Letters
A mass estimate of 250–5000 kg seems broadly consistent with the altitude at whichwe observe the bright flash: 225 km above the 1-bar level, or a pressure of 0.04 mbar.Numerical impact simulations have been used to estimate the altitudes of bolides in Jupiter’satmosphere, and have found that impactors with masses of ∼ × kg reach theirpeak brightness at 60–150 km above the 1 bar level (Hueso et al., 2013; Sankar et al.,2020). Smaller impactors, such as our observation, would not penetrate as deeply intothe atmosphere. Borovička and Spurn`y (1996) studied the entry of a 5000-kg impactorin the Earth’s atmosphere, equivalent to the upper end of our mass estimate. They foundthat the peak brightness occurred at an altitude of 67 km, which corresponds to a pres-sure of 0.07 mbar (ISO, 1975). This is slightly deeper in the atmosphere than our ob-served bright flash pressure level of 0.04 mbar, which is consistent with 5000 kg beingthe upper bound of our mass estimate.By considering all observations made with Juno UVS over the first 27 perijoves ofthe mission, we find that UVS obtained a total effective coverage of 8 . × km s.In that time, there was a single bolide observation. There are clear limitations in cal-culating an impact flux rate from a single observation, but a maximum likelihood esti-mation calculation leads to an impact rate of ∼ Juno UVS observations recorded transient blackbody emission from a point sourcein Jupiter’s atmosphere. Spectral modelling showed that the emission is consistent witha 9600 K source located 225 km above the 1-bar level, and the emission lasted between17 ms and 150 s. The blackbody nature of the spectrum, the temperature and the du-ration of the emission are all consistent with a bolide in Jupiter’s atmosphere. Based onthe energy emitted, we estimate that the impactor had a mass of 250–5000 kg, which wouldcorrespond to a diameter of 1–4 m. This impactor size is larger than the small fireballobserved by Cook and Duxbury (1981) and smaller than the superbolides described by(Hueso et al., 2018). We estimate an impact flux rate of 24,000 per year, for masses of –13–anuscript submitted to
Geophysical Research Letters
Acknowledgements
We are grateful to NASA and contributing institutions, which have made the Junomission possible. This work was funded by NASA’s New Frontiers Program for Juno viacontract with the Southwest Research Institute. B.B. is a Research Associate of the Fondsde la Recherche Scientifique - FNRS.
Data Availability Statement
The Juno UVS data used in this paper are archived in NASA’s Planetary Data Sys-tem Atmospheres Node: https://pds-atmospheres.nmsu.edu/PDS/data/jnouvs_3001 (Trantham,2014). The data used to produce the figures in this paper are available in Giles (2021).
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Supporting Information for “Detection of a bolide inJupiter’s atmosphere with Juno UVS”
Contents of this file
1. Text S1: Impact rate2. Text S2: Comparison of impact rate with previous studies
Introduction
This supporting information contains details about the impact flux rate calcula-tion and the comparison to previous impact flux rate estimates.
Text S1: Impact rate
By multiplying the time spent observing Jupiter by the footprint of the UVS sliton the planet, we find that over 27 perijoves, UVS obtained coverage of 9 . × km s.However, a given point on the planet is only observed for 17 ms as the instrument slitpasses over it, and the bright flash caused by a meteor typically lasts 1–2 s. Therefore,the effective coverage considered in the search for bright flashes must be 1.5/0.017 = 88times larger, giving a total effective coverage of 8 . × km s. Using the surface areaof the planet, this is the equivalent of observing the entirety of Jupiter for 1300 seconds(although we note that our observations are not evenly distributed across the planet).During that time period, we observed one bolide. It is not possible to calculate anaccurate estimate of the average occurrence rate from a single event. However, we canuse maximum likelihood estimation (Eliason, 1993) to find the occurrence rate that ismost likely to produce a single bolide observation in our observation time period. If bolidesoccur independently of each other, their occurrence can be described by a Poisson dis-tribution. In this case, the probability of a single event occurring in time T is given by( rT ) e − rT , where r is the occurrence rate. This probability is a maximum when r = 1 /T .In this case, T is 1300 seconds, so r is 1 per 1300 seconds, or ∼ –19–anuscript submitted to Geophysical Research Letters
Text S2: Comparison of impact rate with previous studies
Bland and Artemieva (2006) compiled a range of observational studies in order toproduce a graph showing the rate of impact of meteoroids in the Earth’s upper atmo-sphere as a function of meteoroid size. There is a log-log relationship between impactrate and size. For meteoroids in the mass range 3–10 kg, the relationship islog N = − .
926 log m + 4 .
739 (2)where N is the number of impacts per year of mass m or greater. Using this equation,the Earth undergoes ∼
330 impacts of size >250 kg and ∼
20 impacts of size >5000 kgeach year. However, given the difference in orbit, mass and area, we clearly expect fluxrates to be different on Jupiter. There have been two previous estimates of small impactorflux rates on Jupiter, one based on even smaller impactors (Cook & Duxbury, 1981), andone based on slightly larger impactors (Hueso et al., 2018). If we assume that the dis-tribution of meteoroid sizes is the same for Jupiter as it is for the Earth, we can use Equa-tion 2 to compare our flux rate with these previous studies.Cook and Duxbury (1981) used the camera on Voyager 1 to search for fireballs anddetected one such event. Based on this, they estimated an impact rate of 1 . × − s -1 km -2 (3 × impacts per year) for impactors of mass >2.8 kg. Using Equation 2,we would expect >250 kg impactors to be 64 times less frequent than >2.8 kg impactors,i.e. the results of Cook and Duxbury (1981) suggest a rate of 5 × impacts per yearfor impactors >250 kg. Similarly, we would expect >5000 kg impactors to be 1000 timesless frequent than >2.8 kg impactors, i.e. the results of Cook and Duxbury (1981) sug-gest a rate of 3 × impacts per year for impactors >5000 kg. Our estimated flux rateof 24,000 impacts per year is approximately 10–200 times lower than these values.Hueso et al. (2018) combine the results of several amateur observations over mul-tiple years to estimate a impact rate for Jupiter for impactors in the 1 × − × kg range. Amateur observers have observed five fireballs in Jupiter’s atmosphere in 2010–2017, and by estimating the effective time during which the amateur observing commu-nity takes measurements, Hueso et al. (2018) estimate an impact rate of 10–65 eventsper year. Following the mass distribution given in Equation 2, this corresponds to a rateof 2,600–17,000 impacts per year for masses >250 kg and a rate of 160–1,000 impactsper year for masses >5000 kg. Our estimated flux rate of 24,000 impacts per year is higherthan both of these values, but our lower mass estimate of 250 kg produces an impact ratethat is significantly more consistent with Hueso et al. (2018).The previous two paragraphs compare three impact rate estimates: 3 × peryear for impactors >2.8 kg (Cook & Duxbury, 1981), 24,000 per year for impactors >250– –20–anuscript submitted to Geophysical Research Letters × kg (Hueso et al., 2018).If the mass distribution of impactors follows Equation 2, then our measured rate is lowerthan the value expected from Cook and Duxbury (1981) and higher than the value ex-pected from Hueso et al. (2018). However, we note that all three rates have a significantamount of uncertainty as they are based on a very small number of impact observations:one each in the case of this paper and Cook and Duxbury (1981) and five in the case ofHueso et al. (2018). While Hueso et al. (2018) has a larger number of impacts, the ef-fective duration of the observations is uncertain. References
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