Venus as an Exoplanet (and a host to life?)lasp.colorado.edu/~espoclass/ASTR_5835_2015... · Venus...
Transcript of Venus as an Exoplanet (and a host to life?)lasp.colorado.edu/~espoclass/ASTR_5835_2015... · Venus...
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Venus as an Exoplanet(and a host to life?)
Evan Anders
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Comparative (Exo)planetology
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Astronomical categorization requires large
samples
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Our solar system is NOT a large sample
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We want to create a “main sequence” of
terrestrial planets Mercurys
s
MarsesEarths
Venuses (Veni?)
Distance from star? →
Pla
net S
ize?
→
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We’re in an era of exoplanet discovery
[Image credit: http://www.nasa.gov/content/kepler-multimedia]
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(Even if many of them are gas giants…)
http://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html
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Some of these planets are terrestrial analogues
● Size ✅● Distance from
host star ✅● Atmosphere ❔
[Image credit: http://www.nasa.gov/content/kepler-multimedia]
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Method of atmospheric detection (Transits)
1. Learn about spectral lines in transit
2. Learn about BB spectrum in eclipse
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Method of atmospheric detection (Transits)
1. Learn about spectral lines in transit
2. Learn about BB spectrum in eclipse
[Borucki et al, 2009, Science, 325]
Transit
Eclipse
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Exoplanetary atmospheres are detectable
Definitive detections of sodium and potassium in the optical
Line depths are less than expected:● Hazes? (favored hypothesis)● Subsolar metallicities?
[Pont et al, 2013, MNRAS, 432]
HazesHot Jupiter HD 189733B
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Things aren’t always hazy
Original: [Deming et al. 2013, ApJ, 774]Figure: [Burrows 2014, Nature, 513]
Water absorption feature
Haze suppressed feature
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Transit Depths of Terrestrial Planets
Detecting a transiting Earth/Venus-like planet requires photometric precision of about 0.01% (1 part in 104 change in flux)
For atmospheric detection? What change in flux would we need to be able to measure?
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6.25 x 10-4 %Our detector needs to be sensitive to a flux change of about one part in a million
across multiple wavelengths to detect an “ideal” Venusian atmosphere.
That’s two orders of magnitude smaller than the overall change in flux due to the planet’s radius.
Ouch.
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So what can we definitively say about terrestrial worlds?
● Whether the planet has a thick or thin atmosphere
● How close the planet is to its host star / the temperature of the planet
● The planetary size
[Ricci et al. 2009, ApJ, 690]
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Habitable Zones
Modern estimates of our system’s habitable zone:--Inner edge: 0.5 - .99 AU--Outer edge: 1.02-1.688 AU
Discrepancies arise due to model parameters (greenhouse effect, atmospheric compositions, etc.)
["Kepler-22b System Diagram" by NASA/Ames/JPL-Caltech][Vladilo et al., 2013, ApJ, 767; Kopparapu
2013, ApJL, 767; Zsom et al., 2013, ApJ, 778]
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We still have much to learn from our solar system
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Some History:Sir Arthur Eddington, 1928
● Notes that “Mars has every appearanceof being a planet long past its prime”
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Some History:Carl Sagan, 1969
● Venera 4 and Mariner 5 results imply surface T = 750 K and surface P = 90 atm.
● Even earlier results from Mariner 2 showed no surface water
● Venus fell out of favor as a potential harbor of life
[Sagan, 1969, Astrophys. and Space Phys., 1]
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Our Solar System’s Three Big Terrestrial Planets
● Earth: wet and alive.
● Mars: once wet and maybe alive? Certainly dry now.
● Venus: once wet, now very dry.○ What about clouds?
● Two of three worlds that once had water are now dry○ desiccation is probably common.
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Life might exist in Venusian clouds
1. The clouds are an aqueous environment
2. The cloud region is roughly at STP. Temperatures of ~300-350 K and a pressure of 1 bar.
3. The clouds are large, continuous, and very stable compared to clouds on Earth.
[Grinspoon et al 1993, Planet. Space Sci., 41]
4. The atmosphere is in chemical disequilibrium (H2, O2, H2S, SO2 mixing)
[Grinspoon & Bullock, EVTP] [Taylor & Grinspoon 2009, JGR, 114]
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Life might exist in Venusian clouds
[Grinspoon & Bullock, EVTP]
5. There are non-spherical unknown “mode 3” particles in the lower cloud deck which are comparable in size to microbes on Earth [Grinspoon et al 1993, Planet. Space Sci., 41]
6. The superrotation of the atmosphere makes photosynthetic life more plausible (with days on the order of 4-6 Earth days)
7. The mysterious unknown UV absorber has properties in common with a photosynthetic pigment.
[Khatuntsev et al 2013, Icarus, 226]
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Life might exist in Venusian clouds
[Grinspoon & Bullock, EVTP]
5. There are non-spherical unknown “mode 3” particles in the lower cloud deck which are comparable in size to microbes on Earth
[Grinspoon et al 1993, Planet. Space Sci., 41]
6. The superrotation of the atmosphere makes photosynthetic life more plausible (with days on the order of 4-6 Earth days)
7. The mysterious unknown UV absorber has properties in common with a photosynthetic pigment.
If Venus has life, we need to
re-define “habitable”
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Understanding H2O Evolution is Key
● Water controls surface environments
● Water controls surface-atmosphere interactions & atmospheric evolution
● Water leads to life (as we know it)
[Grinspoon & Bullock, EVTP]
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[Grinspoon & Bullock, EVTP]
An understanding of the water & climate evolution on Venus
will help us constrain exoplanet observations (and
vice versa)
● Water controls surface environments
● Water controls surface-atmosphere interactions & atmospheric evolution
● Water leads to life (as we know it)
Understanding H2O Evolution is Key
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The Takeaway:
❔ ✅ ✅(Alone and unloved)
We need to better understand Venus in order to constrain terrestrial planet evolution.→ (The inverse is true as well)VEX is an important step towards understanding the evolution of Venus.
❔
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Looking forward
TESS - Transiting Exoplanet Survey Satellite
K2 - Kepler (phase 2)
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Carl Sagan, 1961
● Upper atmospheric temperatures of ~200-300 K measured
● Brightness temperature measurements range from 350-600 K○ Possible explanation: Ionosphere is ~600 K
and surface is ~ 350 K
[Sagan, 1961, Science, 133]