Publications

 Click here for my Google Scholar Citations page

Yan J., Ding, F., Ramirez, R.M., et al., 2017. Abrupt climate transition of icy worlds from snowball to moist or runaway greenhouse. Nat. Geosc., doi:10.1038/ngeo2994

As we had shown in Ramirez and Kaltenegger (2016), icy exomoons (and planets) could deglaciate and may become habitable during the red giant phase of stellar evolution. Here we show that runaway greenhouse states are immediately triggered on such icy exomoons orbiting F- and G-stars. However, icy moons orbiting later (K and M) stars, with a carbonate-silicate cycle, or with relatively dirty ice, may remain habitable after deglaciation.

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Ramirez, R.M., 2017. A warmer and wetter solution for early Mars and the challenges with transient warming. Icarus 297, 71 – 82

I show that the ice albedo problem makes it much harder to transiently warm a glaciated early Mars, requiring surface pressures that are ~10 – 60% higher than is the case for a warm relatively non-glaciated planet. Mineralogy constraints suggest that temperatures well exceeding ~300 K are needed to produce the distribution of surface clays, requiring surface pressures (usually > 5 – 10 bar) that exceed available constraints under such episodic scenarios. Based on the evidence, a warm (possibly semi-arid) early climate is probably the most likely scenario.

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Ramirez,  R.M., Kaltenegger, L., 2017. A volcanic hydrogen habitable zone. The Astrophysical Journal Letters, 837, 1

We propose that volcanically-outgassed hydrogen can widen the traditional N2-CO2-H2O habitable zone. We show that such volcanically-outgassed hydrogen can extend the outer edge in our solar system from 1.67 to 2.4 AU. The outer edge extends out similarly for other star types. Planets with sustained volcanic hydrogen outgassing can stay habitable for relatively longer timescales (0.5 – 1 Gyr or even longer) than from the primordial hydrogen accretion mechanism of Pierrehumbert and Gaidos (2011)

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Ramirez, R.M., Kasting, J.F., 2017. Could cirrus clouds have warmed early Mars? , Icarus 281, 248 – 261

Urata and Toon  (2013) had argued that cirrus clouds composed of particles ~10 microns and larger could have provided enough of a greenhouse effect to warm early Mars. We show that although possible, this mechanism is very unlikely, because unrealistically high cirrus cloud fractions and carefully chosen parameters are needed to produce warm conditions.

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Ramirez, R.M., 2016.  Atmosphere’s solar shock. Nature Geoscience,                                          doi: 10.1038/ngeo2728

I give my summary of Airapetian et al, which argues that regular solar storms from  a much more active younger Sun  (3.8 billion years ago) could have initiated atmospheric reactions that led  to the production of the powerful greenhouse gas, N2O, and HCN, a compound that is vital to life. If their study is correct, I suggest that it may have implications for both early Mars paleoclimate and very active young stars.

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Ramirez, R.M., Kaltenegger, L., 2016. Habitable Zones of Post-Main Sequence Stars. The Astrophysical Journal, 823, 6, 14pp

We compute habitable zone boundaries for A5 – M1 stars as they evolve through the post-main-sequence (RGB – AGB).  We assess how  stellar winds and mass loss affect both 1) the retention of planetary atmospheres and how 2) planetary orbits move outward during the entire stellar evolution. The maximum time that a planet can stay within the post-main-sequence habitable zone can be a couple hundred million and last up to several billion years. Such planets are located in the outer reaches of their  stellar systems and can be detected with current direct imaging techniques.

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Sloan, G. C., Goes, C., Ramirez, R. M., Kraemer, K. E., & Engelke, C. W., 2015. Infrared Spectral Properties of M Giants. The Astrophysical Journal811, 1, 45.

We observed 20 sample M giants with the infrared spectrometer on Spitzer. There is significant scatter in SiO band strength within a spectral class. All stars show significant OH band absorption. We also find that V-K color may be a better indicator of molecular band strength than spectral class. I computed the strength of water vapor absorption lines at different stellar temperatures and pressures.

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Batalha, N., Goldman, Shawn-Domagal, Ramirez, R.M., Kasting, J.F., 2015. Testing the Early Mars H2-CO2 hypothesis with a 1-D photochemical model. Icarus, 258, 337 – 349

The H2-CO2 greenhouse cocktail proposed by Ramirez et al. (2014) to warm early Mars  is put to the test.  We find that at least a couple percent H2 is achievable, with higher concentrations requiring additional H2 sources or a slowing of the hydrogen escape rate below the diffusion limit.

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Ramirez, R.M., Kaltenegger, L., 2014. Habitable Zones of Pre-Main-Sequence Stars. The Astrophysical Journal Letters, 797, 2, L25

Habitable zone boundaries for pre-main-sequence F – M stars are derived. We show habitable zone boundaries around these stars are farther out and wider relative to the main-sequence boundaries, allowing next-generation telescopes to resolve potentially habitable environments even around faint, cool M-stars.

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Ramirez, R.M., Kopparapu, R., Lindner, V., Kasting, J.F., 2014. Can increased atmospheric CO2 levels trigger a runaway greenhouse? Astrobiology, 14, 8, doi:10.1089/ast.2014.1153

In Kopparapu et al.(2013) we originally showed that the inner edge boundary (0.99 AU) was extremely close to Earth’s orbit. When we apply a more realistic relative humidity profile, we find that Earth is not as close to the inner edge as had been thought, making it less susceptible to either a moist or runaway greenhouse. We argue that further improvements to this problem require it to be revisited in 3-D.

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Ramirez, R.M., Kopparapu, R., Zugger, M., Robinson, T.D.,Freedman, R., Kasting, J.F., 2014. Warming early Mars with CO2 and H2. Nat. Geosc., 7, 59 – 63

In this paper, we show how a CO2-H2 greenhouse for early Mars could have raised mean surface temperatures above the freezing point of water, generating a long-lived greenhouse state. The associated warm temperatures would produce enough rainfall to form the ancient valleys.

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Kopparapu, R. K.,Ramirez, R.M., Kasting, James, Eymet, V; et al, 2014. Habitable zones around main-sequence stars: dependence on planetary mass ApJ Letters , 787, L29, doi:10.1088/2041-8205/787/2/L29

We demonstrate that changing the planetary mass has a modest affect on the location of the inner edge of the HZ. The impact on the outer edge is trivial. We also rederive new inner edge limits based on recent 3-D results.
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Kasting, J.F., Kopparapu, R., Ramirez, R.M., Harman, C., 2013. Remote life detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late-K stars.PNAS , doi: 10.1073/pnas.1309107110

We argue that the traditional liquid water habitable zone (HZ) (Kasting et al., 1993; Kopparapu and Ramirez et al., 2013) should be the standard definition used for habitable exoplanet detection. We also explain why astronomers should work in stellar fluxes rather than effective temperatures. I had made the relative humiditiy comparison between our model and that of Zsom et al. (2013) as well as helped derive the relative humidity parameterization we used in that paper.

 

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Kopparapu, R., Ramirez, R.M.(co-primary author), Kasting, J., et al., 2013. Habitable zones around main-sequence stars: New Estimates. ApJ, 765, 2, 131

We completely update Kasting et al. (1993) with revised habitable zone (HZ) boundaries. Major differences to the previous work are the HITEMP database and new water continuum absorption for the inner edge. For the outer edge, longer line shapes are used for CO2. Parameterizations are derived so that astronomers can more easily calculate the HZ for main-sequence stars.
Kopparapu and I contributed equally to this work, with me in charge of the climate model research and development.

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Greeley et al., 2008. Mars Aeolian Features seen from the ground and orbit. . J. Geophys. Res., 113, E06S06, doi:10.1029/2007JE002971

This was my research assistantship at ASU. I assisted in data collection and analysis. We measured lengths of wind features, created Rose diagrams, and inferred prevailing wind directions on Mars. This work confirmed that the prevailing wind direction in Columbia Hills was from the north-northwest, although the topography can influence the wind regime into other directions.

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