Earth Week x Astrobites 2022: “Comparative Planetology as a Catalyst for Climate Conversation” Recap

Hosted by Dr. David Grinspoon

Dr. Grinspoon is an astrobiologist. He is currently the Senior Scientist at the Planetary Science Institute and was the former inaugural Chair in Astrobiology at the Library of Congress. He is also a member of Astronomers for Planet Earth (A4E). 

Our second event for our Earth Week x Astrobites series was hosted by Dr. David Grinspoon who spoke about “Comparative Planetology as a Catalyst for Climate Conversation.” His research is in planetary science and astrobiology—specifically climate evolution of Earth-like planets. He focuses on questions regarding habitability: What does it mean to be habitable? How do planets gain and lose their “habitability”? 

More details about Dr. Grinspoon’s background and unique trajectory can be found in the introductory post for Earth Week and his interview. The recording of his presentation here (and accompanied closed captioning text). This event was the most technical of the events (though still geared to the general public) and due to the length of the article, the Q+A session is summarized here.  

A meta talk 

This workshop was unique in that it was not just a public talk that Dr. Grinspoon has given many times before (called “Climate Catastrophes in the Solar System”). He spoke to us with annotations while “commenting on the giving of [the presentation].” His goal for this workshop was to not just talk about the science behind climate and planetary science, but to explain to us his choices for each slide, why he chose to include it, and how to effectively engage with the different audiences. In his words, a “meta” talk, in hopes that we are able to adapt it to reach the general public. 

Why comparative planetology? 

Dr. Grinspoon’s experience in a whole suite of work environments ranging from purely academic roles to working at the Library of Congress has allowed him to interact with a variety of groups. He found that astronomy and planetary science “strangely equipped” him to talk about climate change for people who are not exactly concerned with our Earth’s future. Coming at it from a space-based perspective provides a common entry point that can be a useful way to frame climate change for people who do not necessarily have a scientific background. Thus as astronomers and planetary scientists, we have an important perspective and duty to help the general public understand the gravity of our situation in ways that they can grasp. 

What is a climate model?

Dr. Grinspoon starts off by explaining what a “climate model” is. They are tools that we use to make predictions of our future. The models take in several inputs, or external factors called forcings, that then are subject to physics (i.e. fundamental laws of conservation, theoretical radiative transfer). The models eventually produce outputs including temperatures and greenhouse gas concentrations in our atmosphere. We can change around the inputs as we make different assumptions, such as how our CO2 emissions will change over time, to test different possibilities and evaluate the resulting outcomes. These predictions range from the “best” and “worst” case scenarios, depending on the predicted average temperature of our planet. There are really scary futures where we could see an average temperature increase of ~5°C. 

In these scenarios, the biggest assumptions are about human behavior. “The biggest source of uncertainty in our climate projections is not our ability to physically model the climate,” although there are still things we have to improve, such as modeling clouds. Our assumptions of human behavior collectively, such as how much we carbon keep emitting, are why there is such a disparity between the “best” and “worst” case scenarios. It is not because we do not know the physical principles. 

“However, there is something encouraging in this knowledge because it is empowering. That is, one realizes that what we do collectively as a species is the biggest difference between the bad and less bad scenarios.”

Our future is in our hands.  

What is comparative planetology?

Comparative planetology is a relatively new branch of space science where we can explore and compare different phenomena and their effects across multiple celestial objects. Within the last generation, we have been able to look at and explore terrestrial planets in a way that had not been done before. Dr. Grinspoon is most interested in comparing Venus, Earth, and Mars, and focuses on these planets in his research. We will see how the current (and future missions) to Venus and Mars have changed our understanding of Earth’s history as well. 

Comparative planetology through pictures

Figure 1: Slide from Dr. Grinspoon’s talk illustrating the striking similarity of delta formation that we can recognize on Venus (left), Earth (middle), and Mars (right). 

Figure 1 illustrates comparative planetology’s power and beauty. We can recognize similar delta formations, but the details in how these formed are different for each of these planets. Though there is a commonality in the phenomena the Universe enacts, the formation pathways and how these features arose can tell us the history of these worlds and provide clues to planet formation writ large. 

On the left in Figure 1 is a Venusian river delta. In the case of Venus, there was no liquid water at the time the river-like feature formed, so it is likely it was formed from a runny lava that flowed over the surface. On Earth, we see the same form but it is shaped by something we think at the moment is unique to Earth: biology. Life coupled with water interacts with and shapes our environment. On the right, Mars, we see a similar river delta, but we can tell it is ancient because of the superimposition of an impact crater that is highly eroded. This is telling of a formation that was likely a river delta but has dried up for billions of years. These differences tell us a story about climate change. Planets that underwent different activity compared to Earth all now have visually Earth-like features. 

Now, comparative planetology is not just limited to our Solar System. The field of exoplanets has recently confirmed over 5,000 extrasolar planets. As we discover more planetary systems, we find that the Solar System—which once served as “ground truth” for planetary formation—is more of an exception. Characterizing exoplanets allows us to contextualize how Earth formed and gained its habitability for life as we know it. 

What is habitability?

Dr. Grinspoon then moves onto explaining the classical study of what makes a planet well-suited for life as we know it: the habitable zone. The Earth gets enough radiation from the Sun to maintain liquid water, which has not necessarily always been true for Venus and Mars, and most exoplanets. But as the Sun evolves, so too does the habitable zone. Thus our climate, and the other planets’, will evolve with our Sun. 

What is the “energy budget”? 

A common buzzword as climate change became more prevalent in the media in the 1990s was “energy budget.” In the simplest sense, a certain amount of energy comes into the planet and a certain amount of energy is radiated back into space. The balance determines the surface temperature to high accuracy. As he talks about the energy budget, Dr. Grinspoon suggests bringing up terms such as albedo, but to use common analogies so the public can have an intuitive understanding. He personally likes the campfire example where you get warmer as you move closer to the fire to explain radiation. 

What is the greenhouse gas effect?

He then moves to explain the greenhouse gas effect, and why CO2 plays an important role. Essentially, our atmosphere allows solar radiation (visible light) to warm our planet’s surface, but is prevented from reradiating back into space. As the surface radiates the energy back as heat in the infrared, greenhouse gases (e.g. water, H2O; carbon dioxide, CO2; and methane, CH4) absorb the heat. However, these greenhouse gases do not make up the majority of our atmosphere. Earth’s atmospheric composition is mostly nitrogen (78%), oxygen (21%), and trace amounts of gases like carbon dioxide, argon, ozone, and water vapor. 

Why do the trace gases absorb the infrared radiation, but not the main atmospheric molecules (N2 and O2)? Well molecular nitrogen and oxygen are diatomic, meaning they are molecules made of two atoms. In these cases, the two atoms are the same, and thus they are symmetric, nonpolar, and do not have many vibrational modes. The bigger molecules respond to radiation differently because structurally they have more ways to “bend” and “vibrate”—it is those states that respond to infrared light. 

How are the Venusian, Terrestrial and Martian environments different?

Venus Earth Mars
Major Gases [%] CO2 (96.5); N2 (3.5) N2 (78); O2 (21), H2O (1);  Ar (0.93) CO2 (95.3); N2 (2.7); Ar (1.6)
Minor Gases [ppm] H2O (30); SO2 (180); Ar (70); CO (23) CO2 (> 416); Ne (18); He (5.2); Kr (1.1); CH4 (1.5) O2 (1,300); CO (700); H2O (100); Ne (2.5)
Surface Pressure [bar]  92 1.013 0.0065
Surface Temperature  735 °K, 863.3 °F 288 °K; 58.7 °F 218 °K, -67.3 °F
Why are the atmospheres so different? Has suffered loss of water; limited volatile recycling from its interior due to lack of tectonic activity Longevity of oceans and geological activity; evolution of O2 in the atmosphere due to life Has suffered atmospheric loss due to its small size and weaker gravity; limited volatile recycling from its interior due to lack of tectonic activity
Table 1: Summary of major properties of Venus, Earth, and Mars. Property data can be found here

Table 1 summarizes some of the key planetary properties along with a brief description of what likely created our Earth and its neighbors. Presenting this information in tabular form is helpful, but can turn out to be information overload. Dr. Grinspoon explains that it is important to succinctly and directly summarize the key things to take away:

  1. Venus and Mars are carbon-dioxide dominated; though CO2 is in very trace amounts on Earth, it has increased by a dramatic percentage (> 415 ppm now) in the last few decades, which has huge implications for life as we will discuss 
  2. The surface pressure of Venus is almost 100 times Earth’s; Mars’ surface pressure is just a fraction of Earth’s 
  3. The surface temperatures (affected directly by surface pressure) span orders of magnitude from a very cold -67 °F on Mars to 863 °F on Venus 

Why are the Venusian, Terrestrial, and Martian environments different?

These rocky worlds are so close to each other, and astrophysically speaking, were formed from the same material. Why is it that we see this diverse set of evolutionary paths?  The main reasons are planetary size, the position relative to the sun, and … life! 

Dr. Grinspoon suggests that though we can go in depth into why these properties changed each planet individually, it is through the comparison of the planets that we can provide an important basis through which the general public can understand climate change. 

Size: We know that CO2 is a major driver of climate change, and the warming of Venus is due to the massive CO2 envelope. But interestingly, Mars, despite having a CO2-rich atmosphere, has relatively little greenhouse effect, because it lost much of its atmosphere. Size, arguably, is what makes Mars very different from the other two; even though it is farther than the Sun, and thus is colder, it is because Mars is small that its atmosphere was lost. Atmospheric loss can be a result of several processes, but the simplest cause is that the escape velocity is much, much lower, making Mars much more vulnerable to loss of its atmosphere since an atom does not have to be very fast to escape its atmosphere. Additionally, Mars’ size prevents it from sustaining volcanism, and thus, Mars doesn’t maintain its atmosphere the way Earth and likely Venus do: because of ongoing geological activity. 

Location: The most intuitive reason for the differences in the atmosphere is each planet’s position in the Solar System, since as distance increases from the Sun, an object gets colder.

Life: Generally we say Earth is different because its conditions were conducive to sustaining life. But the converse is true, too: it is also that same life that evolved that has affected the climate and environment.

 “Life has profoundly changed, not just the atmosphere, but the geology, the surface rocks and its mineralogy of our planet. Even Earth’s interior composition and structure is a result of billions of years of life affecting it.” 

Venus: Earth’s twin 

Venus, in particular, is well-suited to understanding where evolutionary paths of Earth and Venus diverged, making one perfect to harbor life. It is remarkable that Venus and Earth are almost the same size and have similar densities, and thus, escape velocities. Dr. Grinspoon uses the analogy that if Earth was a regulation basketball, Venus is a regulation soccer ball. We  do not know of any other planetary system where neighboring planets are as close and similar in size as Earth and Venus are. The fact that Venus is so similar to Earth in terms of bulk properties lends itself to questions asking why they are different and what resulted in these two completely different words. Part of the answer definitely comes from their positions; Venus gets about twice as much as sunlight due to the inverse square law. However, we are still unraveling these mysteries and two missions to understand Venus’ atmosphere (DAVINCI) and surface (VERITAS) will be able to reveal gaps in our knowledge. 

The Faint Young Sun (FYS) Problem

We know that the young Sun was much cooler, and thus let off less radiation. When we calculate the surface temperature of Earth at these early stages, and if we assume the greenhouse effect was about the same, Earth should have been frozen over for much of its early history.  But we know it was not due to geological and biological events, as evidenced by sedimentary rocks that point to there being a hydrological cycle on Earth much earlier than simple radiation calculations suggest. 

Solutions to FYS Problem 

Based on our neighbors, maybe CO2-rich atmospheres are actually the norm! Something had to have been different causing another greenhouse effect to prevent the Earth from freezing over. At this point, we do not know much, but we think it had to do with the fact that there was a lot more CO2 early on. However, CO2 alone could not have been enough. Other ideas to reconcile the FYS and Earth’s warm climate include Earth having a methane greenhouse early on which could have been produced biologically (see: biogenic methane). This is an open problem, and maybe it could be that there is a gap in our modeling not accounting for some processes that could have compounded to generate a warmer Earth. 


Feedbacks, or processes that amplify (positive) or diminish (negative) the effect of climate change are extremely important to understand both our climate and other planets’. They rarely occur in isolation, and thus their effects can compound in different ways depending on initial conditions.  

Positive feedbacks are destabilizing, leading to “runaway” effects (e.g. runaway greenhouse gas effect), whereas negative feedbacks are stabilizing. Figure 2 shows the positive and negative feedbacks that affect climate change. The positive feedbacks (water vapor, ice/albedo) occur when different parts of the overall system reinforce each other and push along a path that becomes very unstable. A few times in Earth’s history, positive feedbacks have occurred. For example, the “ice ages” when our entire planet froze over. One of the negative feedbacks, the IR flux, has to do with the fact that hot surfaces radiate more infrared heat. As the temperature increases, the emission of infrared radiation back also increases which increases the amount of outgoing infrared radiation as Earth warms. As you heat something up, it can more effectively radiate and cool off; though this is a very simple negative feedback loop, there are many other factors (e.g. absorption of infrared radiation by greenhouse gases) that can break down the feedback, as we see in our current climate. 

Figure 2: A consolidation of figures from Dr. Grinspoon’s slides summarizing the different feedbacks occurring on Earth. Left: Positive (destabilizing) feedbacks. Right: Negative (stabilizing) feedbacks.

Well, what has kept Earth’s climate stable for so long? 

The carbonate-silicate cycle acts as a climate thermostat. It is an important negative feedback loop on Earth. Essentially, volcanoes are always putting out gases (CO2) into the air; they are carbon sources. But we can remove CO2 from the air (carbon sinks). The most important one is due to rainfall and weathering. Liquid water rain dissolves CO2 in the atmosphere, forming carbonic acid that runs over the land and interacts with silicate rocks. This dissolves the rocks, leading to calcium and carbonate ions in the ocean which then get redeposited as carbonate rocks on the ocean floor. However, Earth’s plate tectonics recycle the ocean floor; the carbonate rocks get subducted or sucked into the interior of the Earth, subjecting them to heat and pressure, causing them to come back out through volcanoes. This is the long-term carbon cycle that humans have nothing to do with. 

The above describes just the cycle, not the thermostat. The Earth maintained its stability because the weathering reactions (rainfall and chemical reactions) are highly temperature-dependent. This weathering is really efficient when it is hot, and the Earth is really good at naturally sucking out CO2 from the atmosphere. The source (volcanoes) is essentially independent of climate whereas the sink is dependent on the environment—leading to a thermostat effect. The hotter the Earth is, the more CO2 it will pull out of the atmosphere, having a powerful, long-term thermostat effect on the Earth.

So why do we need to worry? Earth is able to self-regulate the carbon dioxide imbalances in its atmosphere. Well, yes, it will. But not for 500,000 years, which we do not have

The broken thermostats

Currently, we believe that this same climate thermostat probably at some point operated on both Venus and Mars, because we believe that Venus, and have very strong evidence that Mars, had surface water when they were young. But then it broke. And both thermostats broke in different ways. 

On Venus, the water was lost, stopping weathering which is dependent on dissolved CO2 in water that can then react with rocks. If the system is dry there is no carbon sink anymore; volcanoes keep going and you now have a CO2-rich atmosphere.

On Mars, the problem was that the volcanic activity, or the carbon source, stopped. Volcanism died out early because Mars, being a small planet, cooled off quickly. Over time, the atmosphere was lost into space. 

When the Earth was frozen over

Another important and unique part of Earth’s evolution is that the feedbacks have worked to destabilize Earth drastically. Though Earth is remarkably stable in its climate, there have been dramatic exceptions such as “snowball Earth”: when Earth was fully frozen over. The biggest episode happened about 2.1 billion years ago. 

The Paleoproterozoic snowball Earth coincides with the rise of oxygen on Earth, when life was able to perfect photosynthesis and use carbon dioxide, water and sunlight to make sugar and oxygen. The increased oxygen in the atmosphere caused not only a mass extinction, since most organisms at this time could not handle all the oxygen chemically, but also crashed the climate causing global glaciation. 

The current understanding is that during this time period, the Earth was kept warm, partially by methane (CH4). At this time, cyanobacteria evolved and released oxygen through photosynthesis, and oxygen rapidly destroys methane. If Earth was warmed by a methane greenhouse (recall one of the solutions to the FYS problem), then oxygen’s destruction of methane on relatively short timescales would have led to an ice runaway. 

General circulation models 

General circulation models (GCM) are climate models that model the circulation of the atmosphere and oceans and can be used to predict climate change on Earth. Dr. Grinspoon and his colleagues seek to modify GCMs to see if tweaking the input parameters can effectively predict the climates of other planets. If so, this serves as a validation that we understand the physics well. However, when GCM predictions fail to reproduce observational evidence, gaps of our physical knowledge are revealed—another way that studying these worlds helps us better understand Earth’s climate variables. Right now, one of the largest sources of uncertainties in GCMs is due to the difficulty of modeling clouds. However, a paper (co-authored by Dr. Grinspoon) showed using GCM modeling that with a slow rotating planet like Venus, clouds can play a big role in stabilizing surface oceans. If Venus had an Earth-like rotation rate, it would have lost its oceans much more quickly. The interesting implications of this result are that:

  1. We might have to rethink the habitable zone. The classic understanding of the habitable zone is the range of distances from a host star where liquid water can exist. However, this result shows that the habitable zone can also be dependent on the planet’s rotation rate.
  2. If Venus did have oceans for over 2 billion years, which we now think is possible, then that means through much of our Solar System history, there were not one, but two, rocky planets with surface water oceans, and they were neighbors! This means that they potentially could have exchanged material—even life!  

Comparative moon-ology: What can Titan tell us? 

Comparative planetology techniques are not just for other planets. Titan, one of Saturn’s moons, has some important similarities to Earth. They both have mostly nitrogen atmospheres and Titan’s surface pressure is 1.5 bars (~50% higher than Earth’s). Titan is the only moon in our Solar System with a thick atmosphere—even thicker than that of Earth’s. Additionally, Titan has had methane in all 3 phases (analogous to water on Earth).

The interesting parallel in a comparative climate sense is that Titan’s feedback mechanisms are similar to Earth’s. Methane is the key to maintaining its warm temperature, though 90 K does not seem very warm. Titan is much, much colder than Earth, meaning that if there was no heating mechanism, methane would freeze-out and condense onto solids. However, the gaseous atmosphere suggests there are other physical phenomena at play. Additionally, photochemistry and interaction with high-energy electrons from Saturn’s magnetosphere suggest that the methane in the atmosphere should be destroyed on very short timescales. However, the methane interacting with the nitrogen makes for rich organic chemistry in the atmosphere, which has implications for prebiotic chemistry and makes Titan an interesting case astrobiologically. As we know from Earth, life can have a large impact on climate. 

Titan also has a very thick haze in its upper atmosphere, which leads to a relatively strong anti-greenhouse effect. This is when aerosols of the right size distribution lets infrared radiation through but blocks visible light, which then cools the surface. We see this effect here on Earth too. For example, during volcanic eruptions, which put aerosols into the stratosphere (e.g. Mount Pinatubo), the Earth can be cooled. 

Concluding remarks

Modeling complex planetary climates requires several assumptions. However, application of our Earth models to other planets (and moons) show that they work. We understand the physics (sans clouds) relatively well—our major uncertainty comes from human behavior. 

What is new here on Earth is premeditated climate change. We understand what will happen. We see it coming. This gives us a certain responsibility. And we need all the help we can get, including data from other planets to help us see what we are missing in our models.

Planetary observations and modeling allow us to check our assumptions and provide “indispensable perspective on the uniqueness, the deep past and long-term fate of Earth.”

We ended the event with a Q+A session, which is documented separately due to this very long summary. We also have live-tweet coverage for the event by Yoni Brande. Thank you to Dr. Grinspoon for being part of our first Earth Week series here at Astrobites, and for being part of not one, but two events! He was also part of the third event (our panel on Astro/physicists working in climate/environmental/policy-adjacent spaces). 

Feel free to reach out to Dr. Grinspoon!
Contact information: 

Dr. David Grinspoon ([email protected], @DrFunkySpoon)

Edited by Ishan Mishra and Yoni Brande

Featured Image Credit: Presentation screencapture – Dr. David Grinspoon, Venera Missions,  NASA;  Logo – Suchitra Narayanan 

This article was written as a part of our Climate Change Series. We’d love to hear what you would like to see from this initiative – if you have ideas, please let us know in this google form.

About Suchitra Narayanan

Aloha! I am a fourth-year PhD candidate, P.E.O. Scholar, and NSF Fellow at the Institute of Astronomy (UH Mānoa) jointly working at the Center for Astrophysics | Harvard & Smithsonian studying the “Organosulfur Chemistry in the Birthplaces of Planets.” I characterize the fundamental properties and formation pathways of complex sulfur organics through ultra-high vacuum chamber experiments. I am also part of the eDisk ALMA Large Program to understand the structure, dynamics, and chemistry of embedded disks (the earliest stages of stellar and planet formation). I focus on combining laboratory experiments, telescope observations, and theoretical modeling (both at the micro (computational chemistry) and macro (2D thermo-chemical models) scales) to reconcile what is known as the “missing sulfur” problem. I'm broadly interested in astrochemistry, with a focus in its role in planetary formation and eventual evolution of resulting (exo)planetary atmospheres. I am also part of the AAS Sustainability Committee, and am passionate about using our knowledge as astronomers to better our life here on the only planet we can call home. I originally am from Coimbatore but have spent most of my life in the Bay Area. I studied both chemical engineering and astrophysics at University of California, Berkeley. When I’m not in the laboratory, you can find me at the piano (I’ve been classically trained since I was 4!) or in the ocean (I’ve been a competitive swimmer/water polo player, and open water lifeguard for East Bay Regional Park District). Please reach out if you're interested in astro+climate work!

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