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Reply to Comment

First, I would like to extend thanks to Crismani et al. for their commentary, which highlights an important uncertainty acknowledged in Fries et al. (2016)

Fries, M., Christou, A., Archer, D., Conrad, P., Cooke, W., Eigenbrode, J., ten Kate, I.L., Matney, M., Niles, P., Sykes, M., Steele, A., Treiman, A. (2016) A cometary origin for martian atmospheric methane. Geochemical Perspectives Letters 2, 10–23.

, namely whether the total mass of infall material required to produce the observed methane is on par with that available from meteor showers. I would like to disagree on two points presented in the letter, and then discuss the implications of their findings.

There are two points to discuss from the logical argument presented in the Crismani et al. (2017)

Crismani, M.M.J., Schneider, N.M., Plane, J.M.C. (2017) Comment on “A cometary origin for atmospheric martian methane" by Fries et al., 2016. Geochemical Perspectives Letters 3, doi:10.7185/geochemlet.1715.

letter. The first involves the assertion that meteor shower mass does not add appreciably to the overall meteor flux on Earth. The second involves direct comparison between comet Siding Spring’s encounter with Mars and encounters with the debris trails of dynamically mature comets.

First, the Crismani et al. (2017)

Crismani, M.M.J., Schneider, N.M., Plane, J.M.C. (2017) Comment on “A cometary origin for atmospheric martian methane" by Fries et al., 2016. Geochemical Perspectives Letters 3, doi:10.7185/geochemlet.1715.

letter asserts that meteor showers add a negligible mass to the overall IDP flux. This is based on a reference to Grebowsky et al. (1998)

Grebowsky, J.M., Goldberg, R.A., Pesnell, W.D. (1998) Do meteor showers significantly perturb the ionosphere? Journal of Atmospheric and Solar-Terrestrial Physics 60, 607–615.

, which is based on a total of ~40 data points collected from sounding rockets, of which only 5 occurred “during or near the peak times of a meteor shower” (ibid.). The Grebowsky et al. (1998)

Grebowsky, J.M., Goldberg, R.A., Pesnell, W.D. (1998) Do meteor showers significantly perturb the ionosphere? Journal of Atmospheric and Solar-Terrestrial Physics 60, 607–615.

paper constitutes a minority view on the subject of meteor shower input mass. Using far more voluminous data from eleven separate radar and optical studies, Ceplecha et al. (1998)

Ceplecha, Z., Borovička, J., Elford, W.G., ReVelle, D.O., Hawkes, R.L., Porubčan, V., Šimek, M. (1998) Meteor phenomena and bodies. Space Science Reviews 84, 327–471.

(Table XXIII) reviewed the available literature and show that meteor-shower infall flux varies between 15 to 65 % of the Earth’s meteor flux. This variation arises from changes in meteor shower activity, and in minor part to variations in the steady interplanetary dust particle (IDP) flux. Meteor shower infall accounts for a significant amount of Earth’s infall flux, and the same can be expected for Mars.

Second, the Crismani et al. (2017)

Crismani, M.M.J., Schneider, N.M., Plane, J.M.C. (2017) Comment on “A cometary origin for atmospheric martian methane" by Fries et al., 2016. Geochemical Perspectives Letters 3, doi:10.7185/geochemlet.1715.

letter directly compares Mars’ interaction with comet C/2014 A1 Siding Spring with more typical, annual meteor shower activity. Siding Spring was not discussed in detail in Fries et al. (2016)

Fries, M., Christou, A., Archer, D., Conrad, P., Cooke, W., Eigenbrode, J., ten Kate, I.L., Matney, M., Niles, P., Sykes, M., Steele, A., Treiman, A. (2016) A cometary origin for martian atmospheric methane. Geochemical Perspectives Letters 2, 10–23.

for two reasons. For one, this encounter was a strongly atypical event, and it is not certain that it compares well with a typical meteor shower. Mars directly interacted with the comet’s coma, and infall material was composed largely of fine dust and gas as noted in Schneider et al. (2015)

Schneider, N.M., Deighan, J.I., Stewart, A.I.F., McClintock, W.E., Jain, S.K., Chaffin, M.S., Stiepen, A., Crismani, M., Plane, J.M.C., Carrillo-Sánchez, J.D., Evans, J.S., Stevens, M.H., Yelle, R.V., Clarke, J.T., Holsclaw, G.M., Montmessin, F., Jakosky, B.M. (2015) MAVEN IUVS observations of the aftermath of the Comet Siding Spring meteor shower on Mars. Geophysical Research Letters 42, 4755–4761.

; material that tends to be dynamically removed from comet debris streams. This resulted in deposition of mass higher in the atmosphere than typical (ibid.), while the only asset capable of searching for methane at the time was the Mars Science Laboratory (MSL) rover, which is located on the planet’s surface a full ~100 km below the maximum mass deposition altitude (ibid.), and—unfortunately—almost perfectly antipodal to the site of maximum mass input (Moores et al., 2014

Moores, J.E., McConnochie, T.H., Ming, D.W., Archer, P.D., Schuerger, A.C. (2014) The Siding Spring cometary encounter with Mars: A natural experiment for the Martian atmosphere? Geophysical Research Letters 41, 4109–4117.

). As such, it is not surprising that MSL has not reported an increase in methane even if the Siding Spring encounter produced the gas. This is the second, and more important, reason why Siding Spring did not feature in Fries et al. (2016)

Fries, M., Christou, A., Archer, D., Conrad, P., Cooke, W., Eigenbrode, J., ten Kate, I.L., Matney, M., Niles, P., Sykes, M., Steele, A., Treiman, A. (2016) A cometary origin for martian atmospheric methane. Geochemical Perspectives Letters 2, 10–23.

—without a robust measurement for methane to test whether the gas is produced, the encounter is interesting as a study of an anomalous encounter, but of very limited utility in testing the hypothesis of a meteor shower origin for martian methane plumes (hereafter “the hypothesis”).

Aside from these two items, the Crismani et al. (2017)

Crismani, M.M.J., Schneider, N.M., Plane, J.M.C. (2017) Comment on “A cometary origin for atmospheric martian methane" by Fries et al., 2016. Geochemical Perspectives Letters 3, doi:10.7185/geochemlet.1715.

letter makes an important point about the total infall mass of meteor showers versus the calculated mass necessary to account for the larger methane plumes. The letter states that meteor flux onto Earth lies in the 10³–10⁵ kg/day range, and this is in agreement with other sources that estimate a ~10⁴ kg/day flux (Mathews et al., 2001

Mathews, J.D., Janches, D., Meisel, D.D., Zhou, Q.H. (2001) The micrometeoroid mass flux into the upper atmosphere: Arecibo results and a comparison with prior estimates. Geophysical Research Letters 28, 1929–1932.

). Thus, a meteor shower flux onto Mars of the order of 10⁸ kg required to generate the Mumma et al. (2009)

Mumma, M.J., Villanueva, G.L., Novak, R.E., Hewagama, T., Bonev, B.P., DiSanti, M.A., Mandell, A.M., Smith, M.D. (2009) Strong release of methane on Mars in northern summer 2003. Science 323, 1041–1045.

methane plume would have to be an extraordinary event. Even if the Mumma et al. (2009)

Mumma, M.J., Villanueva, G.L., Novak, R.E., Hewagama, T., Bonev, B.P., DiSanti, M.A., Mandell, A.M., Smith, M.D. (2009) Strong release of methane on Mars in northern summer 2003. Science 323, 1041–1045.

is in error by an order of magnitude as proposed by Zahnle (2011)

Zahnle, K., Freedman, R.S., Catling, D.C. (2011) Is there methane on Mars? Icarus 212, 493–503.

, other measurements in the 10s of ppbv of methane exist (see Table 1 of Fries et al., 2016

Fries, M., Christou, A., Archer, D., Conrad, P., Cooke, W., Eigenbrode, J., ten Kate, I.L., Matney, M., Niles, P., Sykes, M., Steele, A., Treiman, A. (2016) A cometary origin for martian atmospheric methane. Geochemical Perspectives Letters 2, 10–23.

). Therefore, a meteor shower delivering three or four orders of mass more than the typical flux is needed to generate methane in the tens of ppbv as Crismani et al. (2017)

Crismani, M.M.J., Schneider, N.M., Plane, J.M.C. (2017) Comment on “A cometary origin for atmospheric martian methane" by Fries et al., 2016. Geochemical Perspectives Letters 3, doi:10.7185/geochemlet.1715.

state. One interpretation of this is that the 10⁸ kg infall mass is unlikely to the point that it disproves the hypothesis. However, while meteor showers of this magnitude are uncommon, they are not out of the question. Strong outbursts of meteor activity, called “meteor storms”, can achieve flux in excess of four orders of magnitude greater than the typical background (Beech et al., 1995

Beech, M., Brown, P., Jones, J. (1995) The potential danger to space platforms from meteor storm activity. Quarterly Journal of the Royal Astronomical Society 36, 127.

). In fact, the Mariner IV spacecraft was damaged in the vicinity of Mars by meteoroid strikes after it had completed its primary mission. Mariner IV’s carried a dust detector which measured a flux four orders of magnitude greater than the background at the time, damaging the thermal protection for the spacecraft and imparting an attitude change in the spacecraft. On Earth, storms of this magnitude are estimated to occur about once per century (Beech et al., 1995

Beech, M., Brown, P., Jones, J. (1995) The potential danger to space platforms from meteor storm activity. Quarterly Journal of the Royal Astronomical Society 36, 127.

), which would seem to degrade this rationale as a source of martian methane plumes, given that the literature includes four methane plumes of ~10 ppbv and two in the tens of ppbv range since 1988 (see Table 1, Fries et al., 2016

Fries, M., Christou, A., Archer, D., Conrad, P., Cooke, W., Eigenbrode, J., ten Kate, I.L., Matney, M., Niles, P., Sykes, M., Steele, A., Treiman, A. (2016) A cometary origin for martian atmospheric methane. Geochemical Perspectives Letters 2, 10–23.

). However, many more comets interact with Mars than with Earth. Treiman and Treiman (2000)

Treiman, A.H., Treiman, J.S. (2000) Cometary dust streams at Mars: Preliminary predictions from meteor streams at Earth and from periodic comets. Journal of Geophysical Research: Planets (1991–2012),105(E10), 24571–24581.

find that four times more short-period comets, and about twice as many long-period comets interact with Mars than interact with Earth. Therefore, the number of methane plumes in the ≥10 ppbv range may compare favorably with the relative number of cometary interactions. Since no direct measurements of meteor flux exist for Mars, we are left with this statistical analysis which does not provide a clear answer to the problem. Few spacecraft sent to Mars have included a capability to detect meteor activity, such as Mariner IV in 1967 and an attempt to count meteors with a MER rover (Domokos et al., 2007

Domokos, A., Bell, J.F., Brown, P., Lemmon, M.T., Suggs, R., Vaubaillon, J., Cooke, W. (2007) Measurement of the meteoroid flux at Mars. Icarus 191, 141–150.

). The high meteor flux problem noted in the Crismani et al. (2017)

Crismani, M.M.J., Schneider, N.M., Plane, J.M.C. (2017) Comment on “A cometary origin for atmospheric martian methane" by Fries et al., 2016. Geochemical Perspectives Letters 3, doi:10.7185/geochemlet.1715.

letter is important as a test of the hypothesis, but there is sufficient uncertainty in the available data on martian meteor flux and historical meteor shower intensity that we cannot currently prove or disprove the hypothesis on this issue.

The most definitive test of the hypothesis is a direct test for methane evolution correlated with the time and location of a statistically significant suite of proven, significant meteor showers. This can be done with existing instrumentation and expertise, and within the science operations plans of current missions. NASA’s MAVEN mission can detect martian meteor showers via observation of Mg+ in the high martian atmosphere (Benna et al., 2015

Benna, M., Mahaffy, P.R., Grebowsky, J.M., Plane, J.M.C., Yelle, R.V., Jakosky, B.M. (2015) Metallic ions in the upper atmosphere of Mars from the passage of comet C/2013 A1 (Siding Spring). Geophysical Research Letters 42, 4670–4675.

). ESA’s Trace Gas Orbiter can perform very sensitive methane detection that is resolved in time, area, and altitude. And the interaction dates for Mars and the debris streams of a large number of comets is known. These spacecraft are capable of verifying the occurrence of meteor showers and then watching for a correlated appearance of methane, offering a solid test of the hypothesis.

Editor: Eric Oelkers

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