Chris Colose Profile picture
22 Feb, 29 tweets, 10 min read
While #Mars2020 #CountdownToMars retains a big news and #scicomm presence, I wanted to offer this tangential thread on some of the current science around ancient Mars and habitability studies. Gear up!
Today Mars is lifeless with an ultra-thin atmosphere (even if you could get over the cold and lack of oxygen, your organs would rupture, outgas, and cause a rapid death). But it also leaves behind an imprint of fascinating geology from a potentially habitable past.
One of the grand astrobiological & climate gifts that Mars presents us with is the robust evidence for substantial liquid water on the ancient surface- an extensive record of fluvial deposits and eroded terrains when liquid water availability and surface runoff were high.
e.g., (ref 1, I'll numbers refs for bottom of thread)

In the deep past, Mars was endowed w very large-scale, integrated/branching river valley systems, tributaries that begin near topographic divides, and craters filled with sedimentary deposits. Google image a lot of examples.
The main valley network-forming era occurred over 3.5 billion years ago, ending in the late Noachian or early Hesperian, as reflected in this review schematic (2)
Mars is notable for its hemispheric asymmetry in topography. The valley networks are predominantly in the SH. These were first described from Mariner 9 images, and can extend thousands of km and likely formed on many hundreds of thousands to tens of million year timescales. (2,3)
The Jezero crater open-basin lake (where Perseverance is, labeled in my tweet image above) contains spectacular exposed fluvial sedimentary deposits and was likely active near the Noachian–Hesperian boundary. Anyway...onto the past.
In planetary climate work, there’s a problem explaining liquid Water on early Mars. To some extent, it’s a problem on Earth too, an issue called the “Faint Young Sun Paradox” although it’s less of a paradox now and more of an issue of just testing plausible hypotheses.
The basic issue is that the early Sun was less luminous than today because hydrogen fusion changes the molar mass of the core over time, causing it to contract and heat up. At ~4 billion years ago, the Sun is ~25% less luminous. This is fundamental in climate evolution.
Earth would freeze over with a 25% less luminous Sun, but it hasn’t been hard to come up with theories for a greenhouse-heavier atmosphere that can offset this. With Mars, the further distance from the sun makes the “paradox” more difficult. In fact, you can't overcome it w CO2
Most people have probably heard of the “Habitable Zone,” shown in schematics like this (this from wiki, but Sonny Harman, the creator, a colleague/former officemate :-)). It's just a range of stellar fluxes where you can have liquid water stable on the planet’s surface.
Early Mars is not in the HZ. The basic problem is this: the lore is that for an “Earth-like” (tectonically active) planet, CO2 abundance is set on very long timescales by a balance of production by volcanism & loss by weathering of minerals & precipitation of carbonates.
There’s a negative feedback here bc weathering is temp/precip dependent, so a colder planet will have less weathering and more CO2 buildup (Earth may have escaped “snowball” episodes through this mechanism). Actually, if you want a super hot Earth, just make a snowball & wait!!
So near the outer edge of the HZ, one might expect a planet with an operational weathering thermostat to have higher CO2 playing tug of war with lower starlight. Perhaps many bars of CO2 (Earth’s *total* atmos pressure is 1 bar of which CO2 is a trace gas)
However, there’s a limit imposed by two factors 1) many bars of CO2 buildup also increases the planet’s albedo (even faster than its greenhouse effect) 2) CO2 eventually condenses at mild temperatures w high pressures. CO2 forms clouds like water on Earth limiting buildup.
The CO2 condensation also inhibits the greenhouse effect by changing the tropospheric temperature structure(just as water vapor condensation does on Earth, called the “lapse rate feedback”). The point is you can’t just ask CO2 to be arbitrarily high. (4)
In fact, in traditional HZ theories and models, this CO2 Rayleigh scattering/condensation limit, when CO2 buildup can no longer offset low sunlight and keep the planet above freezing, has *defined* the outer edge of the HZ. It’s not perfect, but a start.
Mars also had only so much carbon. Earth and Venus have like 90 bars of it; on Earth it’s all in rocks, Venus in the atmosphere. If Mars formed w similar carbon per unit planet mass (and pressure goes as mass times a lower gravity), then Mars could only of had ~10 bar CO2.
There’s also some data constraints on ancient Mars atmospheric pressure (up to maybe 2 bar), primarily from size–frequency statistics of ancient craters. In a sense, Mars reveals its ancient secrets better than Earth, because the surface isn’t always being recycled by geology.
If a CO2 greenhouse doesn’t work to get Mars warm enough, then what? The latest thinking is that reduced gases, namely H2 and CH4, in a CO2 atmosphere might do the trick. If you guessed there’s a lot of wrinkles here too, you’d be right!
First, CH4 actually isn’t a great GHG. On top of that, it absorbs near-infrared radiation which causes a warming of the upper atmosphere, and sometimes can *cool* the surface. There’s been some suggestions that collision-induced absorption between CO2/CH4 can overcome this (5)
There’s interesting implications and debate here about a transient vs. more sustained “warm and wet” Mars. People argue about this.

However, there's been updated spectroscopy of the collision absorption that has poo-poo'd on CH4 (6,7) so the past CIA radiative transfer was wrong
H2 remains a candidate.A lot of intro textbooks tell you that diatomic gases can’t be GHGs, since they don’t have a permanent electric dipole moment (no direct vibrational or rotational absorption bands), but they can gain an induced dipole moment via collisions w other molecules
Some of the “updated” radiative transfer calculations show that you can warm Mars above freezing with modest amounts of H2 (5% ish) in a <2 bar atmosphere, in line with the paleopressure constraints. (8)
The H2 question becomes a challenge of keeping it there. It’s so light that it escapes to space, esp from a small planet. Volcanic outgassing of H2 from a more-reduced-than-Earth early martian mantle and “slow enough” atmospheric escape? Maybe. This is still researched.
Modeling this stuff in 3-D is also at the frontier, e.g. if you want to simulate the precipitation relative to where the river valley networks are. There’s a lot of unknown inputs like obliquity, that varies much more than on Earth due to a lack of a moon & proximity to Jupiter.
e.g., this paper by @SGuzewich et al. (9) is one attempt to map out some of the parameter space, test hypotheses, and offer guidance on how to get the valleys from a climate standpoint.…
Anyway, that's it. TLDR- Mars is quite interesting (I mean, still no Venus) and people are still trying to figure out how to give it ancient liquid water from a climate and planetary evolution standpoint. But, it was there. Hopefully @NASAPersevere teaches us some!

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More from @CColose

14 Sep 20
Thread on Venus—a bit on phosphine detection, but for broader context

Venus is a very hot (~900 Fahrenheit) planet with a thick atmosphere (~90 x the pressure on Earth, like being a kilometer deep into the ocean).

But it may have been the first habitable planet.
The starting point for habitability discussions—be it on ancient Mars or distant exoplanets— is usually having a climate in which liquid water is stable in the liquid form at the surface. That doesn’t need to be the only starting point, but it is compelled by Earth requirements.
Planet formation models don’t suggest Earth &Venus should have started with vastly different water amounts, while the Pioneer Venus mass spectrometer measured very high Deuterium-to-Hydrogen ratios(~150x terrestrial water, 1), often interpreted as an ancient ocean that was lost.
Read 25 tweets
30 Jun 20
This is a terrible article. There are some errors, but the main issue is to take the wildest claims from activists that never came from the clim sci community, and in fact have been called out ("12 years", "Amazon is lungs of planet"), and apologize that the activists were wrong.
Naturally, the nonsense that came out of the activists are not "the IPCC." This has been robust across multiple assessment reports.
This doesn't mean climate change isn't the biggest environmental problem. It almost certainly is. A 3 degree world is one never encountered by humans, and the scope of the problem and its solution set is extraordinarily difficult to communicate.
Read 4 tweets
31 Jan 19
Thread time (mildly technical) for some broader context in relation to #PolarVortex

Because it’s popular, let’s dig in, and also is this thing related to global warming? Let’s start with the basics. We’ll get to polar vortex toward the end.
It turns out when you take a “fluid,” like an atmosphere, or a big water tank in a lab, and subject it to temperature gradients (e.g., driven by differential solar heating) and then make the fluid rotate, interesting things happen.
Digression (a): In reality, when you take (under)graduate atmospheric science classes, you bond not over how you will become rich promoting global warming, but rather hours spent writing down equations that describe those interesting things.
Read 24 tweets
29 Jan 19
Intro to seasons:

The Earth is tilted bigly. Venus wishes it had our tilt. Lying Venus says it has a tilt of 180 degrees. They just say that because it spins backwards. Lying Venus. Venus doesn't have the guts to have seasons! I watched her on FOX news and agree she is hell.
Then in the summer we tilt and get more sunshine. A LOT MORE. I know all about sunshine, believe me, you wouldn't believe the amount of sunshine we get.
Don’t know how the poll numbers are so good, but it looks like the sunshine is up 1009%? The fake news won't report this.
Read 5 tweets
15 Dec 18
After a week of cool science at AGU, these give me headaches, but here's a short history of Venus.
Once upon a time, there was a planet, pretty comparable to Earth in size. It could of had a nice climate for a bit...depending on things like topography and rotation rate. A slow spin exposes its dayside to the sun for almost two months at a time, which sounds bad but...
...if you tell a General Circulation model about that rotation state, you can make the sun-lit part of the planet overcast for many months, shading it, so you can retain temperate conditions for some billions of years. It wouldn't work for Earth rotation. Depends on other things.
Read 12 tweets
7 Aug 18
Because it’s in the news, this is a thread on #HothouseEarth.
@bobkopp has one too

But I want to say some other things. First bit a summary, then my thoughts.
This cartoon in their paper is an anchor point for the thread, which the authors (Steffen et al.) present to frame their discussion of climate stability and tipping points
Stefan et al. envision Earth’s climate as resting in a stable state in the various valleys in the cartoon, or at least oscillate between but remain bounded by two states (e.g., glacial-interglacial limit cycles).
Read 24 tweets

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