PTYS 214 – Spring 2011

Homework #8 due today

Homework #9 available for download from the class websiteDue Thursday, Apr. 14

Class website: http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214/

Useful Reading: class website “Reading Material” http://en.wikipedia.org/wiki/Viking_program http://www.cnn.com/2004/TECH/space/08/04/atacama.desert/index.html

Announcements

Extra Credit Presentation

Drew Carlson

The Search for Life on Mars

Viking Mission, 1976: First successful landing of a spacecraft on the surface of another planet, and execution of biology experiments

Two orbiters + two landers

Cryse Basin

Elysium Mons

Hellas

Chryse Planitia

Utopia Planitia

Olympus Mons

Vallis Marineris

Viking Landers

Viking Biology Experiments

1. Pyrolytic Release (PR) experiment

2. Labeled Release (LR) experiment

3. Gas Exchange (GEX) experiment

Gas Chromatograph/Mass Spectrometer (GC/MS) was capable of detecting organics at a level of a few parts per billion (ppb)

Labeled Release Gas ExchangeCarbon Assimilation(Pyrolitic Release)

Single Sample Collector

1. Pyrolytic Release (PR) or Carbon Assimilation Experiment Test for organisms that can use CO and CO2

Martian soil was put in a chamber and exposed to a mixture of CO2 and CO

CO2 and CO were “labeled” with 14C

Hypothesis: “If biota were in the soil it would incorporate some of the CO2 or CO and convert it to organic material”

After some time: Heat the soil break organic material

look for release of 14C

2. Gas exchange (GEX)Look for gases that might be given off by Martian biota

Martian soil was put into a chamber and mixed with plenty of different nutrients (amino acids, glucose, salts, vitamins, etc)

Look for H2, N2, O2, CH4, CO2,and Ar, Kr (for calibration) released from the soil

3. Labeled release (LR)Test for presence of organisms able to assimilate organic compounds from the environment and release back gas to the atmosphere

Martian soil was put into a chamber and mixed with nutrients (glucose and sulfate)

The nutrients were labeled by 14C and 35S

Look for gas release (especially CO2) enriched in 14C and/or 35S

Evolution of radioactivity after nutrient injection from the LR experiment for Viking soil compared to Lunar and naturally sterile Antarctic soil

Control experiments consisted in heating the sample at 160°C for 3 hours prior to injecting the nutrients

Viking Biology Results

How does it look in terms of life on Mars?

ExperimentsResponse of sample

Expected response for sample with

biology

Expected Response for

no biology

Response of heat-sterilized

Control(no biology)

GEX oxygen emitted

oxygen emitted

none

LRlabeled gas

emittedlabeled gas

emittednone

PRcarbon detected

carbon detected

none

Viking Biology Results

What are the control experiments telling us?

ExperimentsResponse of sample

Expected response for sample with

Biology

Expected Response for

no biology

Response of heat-sterilized

Control(no biology)

GEX oxygen emitted

oxygen emitted

noneoxygen emitted

LRlabeled gas

emittedlabeled gas

emittednone none

PRcarbon detected

carbon detected

nonecarbon detected

Gas Chromatograph/Mass Spectrometer Results

(each year, 2.4 x 108 grams of organic carbon is delivered to Mars by asteroids and comets)

With regolith mixing to a depth of 1 km, organics should be present at about 500 ppb

No organics detected above the 10 ppb level

Well below the level expected if there were any active or even dead biota present

Even below the level expected for delivery of organics by asteroids and comets!

Viking ConclusionsImportant: multiple sets of experiments must be

conducted to test for the presence of life

The Martian surface is rich in UV-produced inorganic oxidants at the ppm level, which tends to destroy any organics present and react with water and oxidants to produce CO2

Example? Perchlorate (ClO4-) discovered in the soil by

Phoenix…

This reconciles the apparently contradictory results of the other Viking life experiments

On the other hand . . .

Testing the Hypothesis: Atacama Desert, Chile

The oxidizing soil and hyper-arid conditions in the Atacama Desert are considered an analog for the Martian surface

Atacama desert soil was analyzed with a GC/MS similar to that used by Viking

Navarro-Gonzales et al. (2003) Mars-like soils in the Atacama desert, and the dry limit of microbial life. Science 302, p. 1018

Oldest and most arid desert on Earth

Terrestrial Analogs Results

In the most arid sample, both formic acid and benzene were found when heated at 750ºC

But: temperatures of the Viking experiments did

not exceed 500ºC . . .

Using the temperatures used in the Viking experiments, detection of formic acid was reduced by a factor of 4 and there was no benzene detected at all

benzene

formic acid

One more thing…

Surface soil from the Atacama desert showed no indication of life (no detection of DNA)

Yet, in soil few tens of centimeters below surface living organisms were detected!

Viking only used surface soil…

1. Limited pyrolysis temperatures

2. Not possible to do ‘follow-up’ experiments

3. Soil samples limited to the surface

4. All three Viking’s experiments assumed that we would be able to culture potentially present Martian organisms

even on Earth only 1 in 100 organisms can be cultured at best

Viking results do not rule out the possibility of life in the martian soil

Is there another way to discover martian life?

Limitations of Viking Experiments

ALH84001 has become famous because it appeared to contain structures that were considered to be fossilized remains of bacteria-like life forms

Evidence for Life in Martian Meteorite(s)

History of ALH84001 Crystallization Age: ~4.5 Gyr old

Carbonate globules formed ~3.9 Gyr old

Rock remained on the surface of Mars until 16 Myr ago when it was ejected

It fell into Antarctica 13,000 years ago

Covered with snow and ice until 700 years ago

Recovered in 1984

Ovoid structures (20-100 nm)

Carbonate globules (50-250 m)

Magnetite crystals(Fe3O4)

Polycyclic Aromatic Hydrocarbons

Arguments in favor of “life on Mars” from ALH84001

1. Polycyclic aromatic hydrocarbons (PAHs) can form as decay products of microorganisms

2. Magnetite crystals have structures similar to crystals produced by some terrestrial bacteria

3. Ovoid structures in carbonate globules are similar to terrestrial microbes

McKay et al. (1996) Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH 84001. Science 273, p. 924

ALH84001:Martian PAHs?

1. Contamination Problem: Most of the organic molecules (maybe even up to 80%!) could be contamination, including PAHs

Some Martian organic carbon is present in the carbonate globules (which were formed on Mars)

2. PAHs can be produced abiotically when impact generated gases (CO, CO2, H2) cool

ALH84001:Ovoid structures

1. The size of these structures is 20-100 nanometers, considered to be too small to contain even a single ribosome

On Earth, the smallest terrestrial bacteria (deep sea hydrothermal vent) is ~150 nm - viruses can be 20-400 nm but they are not independent organisms

2. These structures could have an non-biologic origin, maybe artifacts of sample preparation

We need more than just shape to characterize “fossils” of ancient living organisms

ALH84001:Magnetite Crystals

On Earth microorganisms called magnetotactic bacteria (like MV-1) produce chains of tiny magnetic minerals

But, similar grains can be made inorganically

Bell (2007)Thomas-Keprta et al. (2000)

Example: an impact event…

Inconclusive!

Summary of ALH84001

The morphological fossils (“ovoid” structures) could be artifacts of sample preparation (more evidence is needed)

PAHs could have been produced by non-biological processes; there is strong evidence of terrestrial contamination for organic molecules in the meteorite

The magnetite grains can be made abiotically, such as during the impact even that ejected the rock from the surface of Mars!

McKay et al. found fossil like structures in other Martian meteorites (Nakhla 1.3 Gyr and Shergotty 165 Myr)

Can Martian Biota “Hide” Below the Surface?

Primitive life is very resilient

On Earth we found that

– Some bacteria can grow under -15°C (and lower)

– Some bacteria have tolerance to extreme desiccation for long periods of time

– Some bacteria live in rocks at substantial depth (>1 mile) and do not need light or O2

Methane in the Martian Atmosphere

Mumma et al. (2009) Science 323, p. 1041

What Does It Mean?

Martian atmosphere is strongly oxidizing: CO2, N2, Ar, CO, O2, traces of H2O

CH4 production by atmospheric chemistry is negligible

Normally, CH4 in the atmosphere would be removed in less than 300 years; results suggest much faster removal (interaction with the soil)!

Methane in the Martian atmosphere… …must have been released RECENTLY and

from SUB-SURFACE RESERVOIRS

Sources of Methane

On Earth:

− 90% of atmospheric CH4 is produced by living systems

− Non-biological sources of CH4 are related to CO2 combining with H2O at high pressures and temperatures (like in the carbonate-silicate cycle), which requires volcanism or active plate tectonics

On Mars: There is no plate tectonics nor indication of volcanism

today!

Stay tuned…

~10 km

Like Grand Canyon, Nanedi Vallis may have required millions of years to form

Nanedi Vallis

(Mars Global Surveyor)

Grand Canyon

Stability of the Martian Environment

Could a CO2/H2O atmosphere have warmed early Mars above

freezing? (after all Mars experienced major volcanic activity early on…)

No plate tectonics without plate tectonics, the carbonate-silicate feedback breaks down, increasing CO2 in the atmosphere

Increase of atmospheric CO2 cause condensation and cloud formation

CO2 clouds decrease the world’s albedo

Less solar radiation reaches the surface, warming the planet

A 30% CO2 atmosphere would start to condense at 200K (-73ºC)

Problems…

CO2 alone does not work!

Alternate Possibilities for an Early Warm Mars

Additional greenhouse gas: CH4

- hard to justify high levels of CH4 on Mars

Liquid water occurs on Mars surface right after large impacts

- some features required millions of years to form and warming effects from impacts do not last that long

The mystery of a warm and wet early Mars remains unresolved …

Quiz Time !