Thread by @CColose

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.

essoar.org/doi/abs/10.100…

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!

(1)nature.com/articles/s4156…
(2)link.springer.com/article/10.100…
(3)agupubs.onlinelibrary.wiley.com/doi/full/10.10…
(4) nature.com/articles/ngeo2…
(5)agupubs.onlinelibrary.wiley.com/doi/full/10.10…
(6)sciencedirect.com/science/articl…
(7)sciencedirect.com/science/articl…
(8)agupubs.onlinelibrary.wiley.com/doi/10.1029/20…
(9)essoar.org/doi/abs/10.100…