Sciences and Exploration Directorate

Brief Bio

Dr. Alexander ("Xander") Thelen is a NASA Astrobiology Postdoctoral Program fellow associated with the NASA Goddard Space Flight Center Planetary Systems Laboratory. Dr. Thelen's research pertains to the chemical composition and dynamics of planetary atmospheres; in particular, Dr. Thelen's work is focused on Titan and other outer solar system moons. Using high spatial and spectral resolution data from the Atacama Large Millimeter/submillimeter Array (ALMA), Dr. Thelen has begun investigating seasonal variations in Titan's atmospheric composition to continue the legacy of the Cassini-Huygens mission. Additionally, Dr. Thelen is interested in the study of other solar system objects using ALMA, the development of interplanetary CubeSats, and the investigation of Ocean World analogues.

Research Interests

Chemical Composition and Dynamics of Titan's Atmosphere

Solar System: Moons

The study of Titan's atmospheric composition through spatially resolved observations using the ALMA interferometer. Submillimeter spectra are modeled using the Non-linear optiMal Estimator for Multi-variatE spectral analySIS (NEMESIS) radiative transfer code to determine the vertical and latitudinal distribution of Titan's trace atmospheric constituents - many of which are hydrocarbon (CxHy) and nitrile (CxHy[CN]z) species. These results will allow us to understand how Titan's atmosphere responds to seasonal changes during the moon's northern summer, after the end of the Cassini-Huygens mission. Additionally, the distribution of trace atmospheric species, and the search for previously undetected species, may be important for astrobiology studies of Titan.

Submillimeter Investigation of Planetary Atmospheres

Solar System: Atmospheres

The investigation of other planetary atmospheres, including Mars, Neptune, and Jupiter, using techniques developed for studying Titan with ALMA. Through the wide range of chemical species observable in the submillimeter – including CO, hydrocarbons, and nitriles – measurements of chemical abundance, temperature, and atmospheric winds can be derived. With the newest generation of ground-based facilities such as ALMA, observations can be made with high spatial resolution allowing for unprecedented studies of atmospheric dynamics and composition from the ground.  

Europa CubeSats

Solar System: Moons

Previous work involved a concept study of a Europa CubeSat to ride along with the Europa Clipper spacecraft. The Deployable Atmospheric Reconnaissance CubeSat with Sputtering Ion Detector at Europa (DARCSIDE) mission concept was developed at New Mexico State University in cooperation with JPL. The spacecraft would utilize an accelerometer and solid state particle telescope to monitor the atmospheric drag imparted upon the CubeSat from Europa's tenuous atmosphere, and determine the heavy ion flux at the moon from Jupiter's magnetosphere. These results could help to elucidate the connection between the origin and composition of Europa's atmosphere, and the harsh Jovian radiation environment.


Postdoctoral Scholar

California Institute of Technology - Pasadena, CA

October 2022 - Present

NPP Astrobiology Fellow

NAI/USRA - NASA Goddard Space Flight Center, Greenbelt, MD

December 2019 - October 2022

Research Associate

Catholic University of America/NASA GSFC - Washington, DC

October 2018 - December 2019

NASA ASTAR/JGFP Graduate Fellow

New Mexico State University - Las Cruces, NM

2015 - 2018


Ph.D., Astronomy

New Mexico State University


Dissertation: "The Chemical Composition and Dynamics of Titan's Atmosphere as Revealed by ALMA"

Advisors: Dr. Nancy Chanover, Dr. Conor Nixon

B.S., Physics (Astrophysics)

University of California, Santa Cruz


Thesis: "The Kinematics of Andromeda's DIG Disk"

Advisor: Dr. Puragra Guha Thakurta

Professional Societies

American Astronomical Society

2012 - Present

AAS/Division for Planetary Sciences

2014 - Present

American Geophysical Union

2016 - Present

Professional Service

AGU Fall Meeting Planning Committee: 2021–2022

Primary Convener: 'Phosphine in the Venusian Atmosphere: Observations and Implications', AGU 2020 Fall Meeting; 'Outer Planets and Beyond', AGU 2021 Fall Meeting; 'Outer Planets and Beyond', AGU 2022 Fall Meeting

Convener: 'Asteroids, Meteoroids, and Moons: Inner Solar System Processes', AGU 2021 Fall Meeting

Session Chair: DPS 2020, AGU Fall Meeting 2021

Proposal Panelist/External Reviewer: NASA ROSES, CDAP, FINESST, Keck TAC

Proposal Executive Secretary: NASA CDAP

Manuscript Referee: Astrophysical Journal Letters, Planetary Science Journal, Acta Astronautica, Astronomy & Astrophysics


ALMA Student Observing Support Award

2020 ALMA Ambassador

2021 ALMA Ambassador


Submillimeter Observations of Titan's Atmosphere: Implications for Astrobiology

NASA Astrobiology Postdoctoral Program (NPP) Fellowship - NASA/NAI - Awarded: 2019-03-01

Dates: 2020-01-02  - 2022-01-02

Global Measurements of the Atmospheric Temperatures on Mars Using ALMA, and the Comparison to Climate Models and Spacecraft Datasets

FLaRe - NASA GSFC - Awarded: 2019-06-07

Dates: 2019-06-07  - 2019-12-31

Coverage: 0.25

The Chemical History and Evolution of Titan's Atmosphere as Revealed by ALMA

NASA ASTAR/JGFP Fellowship - NASA/NMSU - Awarded: 2015-09-01

Dates: 2015-09-01  - 2018-09-01

Amount 165,000

Other Professional Information

Articles and Press Releases:

  • "How Scientists Are Searching for Life on Titan" - July 15th, 2019

NASA Astrobiology feature story.

  • "How to Plan a Space Mission" - July 12th, 2020

New Yorker article on the 2019 Planetary Science Summer Seminar:

  • "NASA Scientists Discover ‘Weird’ Molecule in Titan’s Atmosphere" - October 27th, 2020

NASA article on the detection of c-C3H2 in the atmosphere of Titan using ALMA

Special Experience

Accepted Observing Proposals:

Apache Point 3.5 m

  • Observations of Jupiter's Atmosphere at Visible Wavelengths [DIS] (PI: A. E. Thelen) - 2012, 2013

Atacama Large Millimeter/submillimeter Array

  • Elucidating Titan's High-Altitude Nitrogen Chemistry [Main Array] (PI: M. A. Cordiner) - Cycle 7 2019
  • Mapping Trace Gases in Jupiter's Atmosphere [ACA, TP] (PI: A. E. Thelen) - Cycle 7 Supplemental Call 2020
  • An Investigation of Latitudinal Variations in Titan's Atmospheric CH4 [Main Array] (PI: A. E. Thelen) - Cycle 8 2021
  • Unlocking Titan's Atmospheric Chemistry with ALMA [Main Array] (PI: A. E. Thelen) - Cycle 8 2021
  • Blowing in the Wind: Temporal Variability of Titan's High-Altitude Circulation System [Main Array] (PI: M. A. Cordiner) - Cycle 8 2021
  • Titan's Atmospheric Asymmetry [Main Array] (PI: N. A. Teanby) - Cycle 8 2021

Other Facilities:

  • Titan's stratospheric winds from disk-resolved occultation imaging - Gemini South 2022
  • Co-I: Gemini North, Keck

International Astrobiology Summer School  - 2016

Astrobiology Graduate Conference Proposal Writing Workshop - 2017

JPL Planetary Science Summer School 

Development of BRIDGE mission concept to interstellar objects or long-period comets - 2019

GSFC Planetary Science Winter School

Participation in flight instrument design study - 2020

Field Experience

Measurements and sample collection in Yellowstone National Park hot springs - 2019



Camarca, M., K. de Kleer, B. Butler, et al. A. B. Akins, A. Thelen, I. de Pater, M. A. Gurwell, and A. Moullet. 2023. Thermal Properties of the Leading Hemisphere of Callisto Inferred from ALMA Observations The Planetary Science Journal 4 (8): 142 [10.3847/psj/aceb68]

Cordiner, M. A., G. L. Villanueva, H. Wiesemeyer, et al. S. N. Milam, I. de Pater, A. Moullet, R. Aladro, C. A. Nixon, A. E. Thelen, S. B. Charnley, J. Stutzki, V. Kofman, S. Faggi, G. Liuzzi, R. Cosentino, and B. A. McGuire. 2022. Phosphine in the Venusian Atmosphere: A Strict Upper Limit From SOFIA GREAT Observations Geophysical Research Letters 49 (22): [10.1029/2022gl101055]

Thelen, A. E., C. A. Nixon, R. G. Cosentino, et al. M. A. Cordiner, N. A. Teanby, C. E. Newman, P. G. Irwin, and S. B. Charnley. 2022. Variability in Titan’s Mesospheric HCN and Temperature Structure as Observed by ALMA The Planetary Science Journal 3 (6): 146 [10.3847/psj/ac7050]

Villanueva, G. L., M. Cordiner, P. G. Irwin, et al. I. de Pater, B. Butler, M. Gurwell, S. N. Milam, C. A. Nixon, S. H. Luszcz-Cook, C. F. Wilson, V. Kofman, G. Liuzzi, S. Faggi, T. J. Fauchez, M. Lippi, R. Cosentino, A. E. Thelen, A. Moullet, P. Hartogh, E. M. Molter, S. Charnley, G. N. Arney, A. M. Mandell, N. Biver, A. C. Vandaele, K. R. de Kleer, and R. Kopparapu. 2021. No evidence of phosphine in the atmosphere of Venus from independent analyses Nature Astronomy 5 (7): 631-635 [10.1038/s41550-021-01422-z]

MacKenzie, S. M., S. P. Birch, S. Hörst, et al. C. Sotin, E. Barth, J. M. Lora, M. G. Trainer, P. Corlies, M. J. Malaska, E. Sciamma-O’Brien, A. E. Thelen, E. Turtle, J. Radebaugh, J. Hanley, A. Solomonidou, C. Newman, L. Regoli, S. Rodriguez, B. Seignovert, A. G. Hayes, B. Journaux, J. Steckloff, D. Nna-Mvondo, T. Cornet, M. Y. Palmer, R. M. Lopes, S. Vinatier, R. Lorenz, C. Nixon, E. Czaplinski, J. W. Barnes, E. Sittler, and A. Coates. 2021. Titan: Earth-like on the Outside, Ocean World on the Inside The Planetary Science Journal 2 (3): 112 [10.3847/psj/abf7c9]

Parks, M. C., C. A. Nixon, G. L. Villanueva, et al. M. D. Smith, A. S. Khayat, A. E. Thelen, E. Villard, S. B. Charnley, and P. G. Irwin. 2021. Observations of Mars with ALMA: potential for future constraints of global circulation models Journal of Astronomical Telescopes, Instruments, and Systems 7 (02): [10.1117/1.jatis.7.2.025001]

Moore, K., S. Courville, S. Ferguson, et al. A. Schoenfeld, K. Llera, R. Agrawal, P. Buhler, D. Brack, K. Connour, E. Czaplinski, M. DeLuca, A. Deutsch, N. Hammond, D. Kuettel, A. Marusiak, S. Nerozzi, J. Stuart, J. Tarnas, A. Thelen, J. Castillo-Rogez, W. Smythe, D. Landau, K. Mitchell, and C. Budney. 2021. Bridge to the stars: A mission concept to an interstellar object Planetary and Space Science 197 105137 [10.1016/j.pss.2020.105137]

Cordiner, M. A., E. Garcia-Berrios, R. G. Cosentino, et al. N. A. Teanby, C. E. Newman, C. A. Nixon, A. E. Thelen, and S. B. Charnley. 2020. Detection of Dynamical Instability in Titan’s Thermospheric Jet The Astrophysical Journal 904 (1): L12 [10.3847/2041-8213/abc688]

Thelen, A. E., M. A. Cordiner, C. A. Nixon, et al. V. Vuitton, Z. Kisiel, S. B. Charnley, M. Y. Palmer, N. A. Teanby, and P. G. Irwin. 2020. Detection of CH3C3N in Titan’s Atmosphere The Astrophysical Journal 903 (1): L22 [10.3847/2041-8213/abc1e1]

Nixon, C. A., A. E. Thelen, M. A. Cordiner, et al. Z. Kisiel, S. B. Charnley, E. M. Molter, J. Serigano, P. G. Irwin, N. A. Teanby, and Y.-J. Kuan. 2020. Detection of Cyclopropenylidene on Titan with ALMA The Astronomical Journal 160 205 [10.3847/1538-3881/abb679]

Kisiel, Z., C. A. Nixon, M. A. Cordiner, A. E. Thelen, and S. B. Charnley. 2020. Propionitrile in the two lowest excited vibrational states in the laboratory and on Titan Journal of Molecular Spectroscopy 372 111324 [10.1016/j.jms.2020.111324]

Cordiner, M. A., N. A. Teanby, C. A. Nixon, et al. V. Vuitton, A. E. Thelen, and S. B. Charnley. 2019. ALMA Spectral Imaging of Titan Contemporaneous with Cassini's Grand Finale The Astronomical Journal 158 (2): 76 [10.3847/1538-3881/ab2d20]

Thelen, A. E., C. A. Nixon, M. A. Cordiner, et al. S. B. Charnley, P. G. Irwin, and Z. Kisiel. 2019. Measurement of CH3D on Titan at Submillimeter Wavelengths The Astronomical Journal 157 (6): 219 [10.3847/1538-3881/ab19bb]

Thelen, A. E., C. Nixon, N. Chanover, et al. M. Cordiner, E. Molter, N. Teanby, P. Irwin, J. Serigano, and S. Charnley. 2019. Abundance measurements of Titan’s stratospheric HCN, HC3N, C3H4, and CH3CN from ALMA observations Icarus 319 417-432 [10.1016/j.icarus.2018.09.023]

Thelen, A. E., C. Nixon, N. Chanover, et al. E. Molter, M. Cordiner, R. Achterberg, J. Serigano, P. Irwin, N. Teanby, and S. Charnley. 2018. Spatial variations in Titan’s atmospheric temperature: ALMA and Cassini comparisons from 2012 to 2015 Icarus 307 380-390 [10.1016/j.icarus.2017.10.042]

Teanby, N. A., M. A. Cordiner, C. A. Nixon, et al. P. G. Irwin, S. M. Hörst, M. Sylvestre, J. Serigano, A. E. Thelen, A. M. Richards, and S. B. Charnley. 2018. The Origin of Titan’s External Oxygen: Further Constraints from ALMA Upper Limits on CS and CH2NH The Astronomical Journal 155 (6): 251 [10.3847/1538-3881/aac172]

Thelen, A. E., N. J. Chanover, J. Murphy, K. Rankin, and S. Stochaj. 2017. A Europa CubeSat Concept Study for Measuring Atmospheric Density and Heavy Ion Flux Journal of Small Satellites 6 (2): 591-607

Talks, Presentations and Posters


Investigating Titan's Atmospheric Chemistry with ALMA

July 16, 2022

American Astronomical Society 240th/Laboratory Astrophysics Division Meeting

Exploring Titan's Atmospheric Chemistry and Dynamics with ALMA


Kapteyn Astronomical Institute Colloquium, University of Groningen

The Search for New Molecular Species in the Atmosphere of Titan

2, 2020

Astrochemistry Discussions online talk series.

ALMA Observations of Titan's Dynamic Atmosphere into the Post-Cassini Era

August 15, 2020

Japanese Geophysical Union - American Geophysical Union Joint Virtual Meeting 2020


Titan's Variable Mesospheric Thermal Structure as Measured by ALMA


American Geophysical Union Fall Meeting

Detection of CH3C3N in Titan's Atmosphere

November 2020

AAS/Division of Planetary Sciences 52nd Meeting

The Temporal Variability of Titan’s Atmosphere During Northern Summer


American Geophysical Union Fall Meeting

Submillimeter Detection of CH3D on Titan

October 2019

AAS/Division of Planetary Sciences + EPSC Joint Meeting

Titan's Atmospheric CH3D Measured in the Submillimeter with ALMA

June 2019

New Horizons in Planetary Systems

Titan's Atmospheric Dynamics Revealed Through Temperature and Chemical Abundance Measurements from 2012-2015 ALMA Observations

November 2018

AAS/Division for Planetary Sciences 50th Meeting

ALMA and Cassini Comparisons of Titan's Stratospheric Temperature from 2012-2015

November 2017

AAS/Division for Planetary Sciences 49th Meeting

Measuring Spatial and Temporal Variations in Titan's Atmosphere with ALMA

May 2017

Titan Through Time 4

Spatial Variations of Chemical Abundances in Titan's Atmosphere as Revealed by ALMA

November 2016

AAS/Division for Planetary Sciences 48th Meeting