Venus as an Exoplanet(and a host to life?)

Evan Anders

Comparative (Exo)planetology

Astronomical categorization requires large

samples

Our solar system is NOT a large sample

We want to create a “main sequence” of

terrestrial planets Mercurys

s

MarsesEarths

Venuses (Veni?)

Distance from star? →

Pla

net S

ize?

→

We’re in an era of exoplanet discovery

[Image credit: http://www.nasa.gov/content/kepler-multimedia]

(Even if many of them are gas giants…)

http://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html

Some of these planets are terrestrial analogues

● Size ✅● Distance from

host star ✅● Atmosphere ❔

[Image credit: http://www.nasa.gov/content/kepler-multimedia]

Method of atmospheric detection (Transits)

1. Learn about spectral lines in transit

2. Learn about BB spectrum in eclipse

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

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

Things aren’t always hazy

Original: [Deming et al. 2013, ApJ, 774]Figure: [Burrows 2014, Nature, 513]

Water absorption feature

Haze suppressed feature

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?

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.

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]

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]

We still have much to learn from our solar system

Some History:Sir Arthur Eddington, 1928

● Notes that “Mars has every appearanceof being a planet long past its prime”

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]

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.

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]

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]

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”

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]

[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

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.

❔

Looking forward

TESS - Transiting Exoplanet Survey Satellite

K2 - Kepler (phase 2)

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]