Geology over Interstellar Distances from Light Curves

[lpetrich]

How much planetary-surface detail might one be able to observe without resolving an exoplanet? Simulations of Light Curves from Earth-like Exoplanets - Planetary Habitability Laboratory @ UPR Arecibo. I have expanded those simulations to include other big Solar-System objects. They were done with the Sun over the equator and the observer looking toward the body from the Sun's direction. Maps were from NASA's Blue Marble site for the Earth, Eclipsophile for the Earth's clouds, and the Celestia Motherlode for all others. I assumed Lambert-law reflection for simplicity.

Venus, Jupiter, Saturn, Uranus, and Neptune are clouded over, and they have very little change, about 1% for Jupiter.

Of the exposed-surface bodies, I find these relative standard deviations. Mercury: 2%, Earth: (red: 12%, green: 11%, blue: 2%), Moon: 16%, Mars: 10%, Io: (red: 5%, green: 7%, blue: 11%), Europa: (red: 5%, green: 8%, blue: 10%), Ganymede: (red: 6%, green: 5%, blue: 8%), Callisto: 10%.

So the Earth, the Moon, Mars, and Jupiter's four big moons all have noticeable asymmetries. For the Earth, the Moon, and Mars at least, these are due to geological activity. The Earth even changes color. For most directions of the Sun and the observer, the Earth is indeed a pale blue dot. But when the Sahara Desert is illuminated and visible, it is big enough, light enough, and yellow enough to make the Earth look grayish.

So if one can directly detect a terrestrial exoplanet, one may be able to tell if it has geological activity. One may also be able to tell if it has an ocean that covers much of its surface but not most or all of it.

This is important, not only in itself, but also for whether there is life on other planets. This is because the most plausible locale to date for the origin of Earth life is in a hydrothermal vent, and hydrothermal vents require geological activity and bodies of water to exist.

[eburacum45]

One factor that could interfere with the detection of land/sea cover is the fact that Earth-like planets would probably have a variable fraction of cloud cover. This makes the land surface tricky to spot through the noise.

Cloud cover might even actively hide the terrain features. If an Earth-like planet has a single, very large (Pangaea-like) continent with a desert in the interior, cloud cover could be reduced in that area; this would reduce the apparent reflectivity of the continental hemisphere and mask the presence of the continent as seen from Earth.

[eburacum45]

The Earth even changes color. For most directions of the Sun and the observer, the Earth is indeed a pale blue dot. But when the Sahara Desert is illuminated and visible, it is big enough, light enough, and yellow enough to make the Earth look grayish.

I think this could help to reduce the masking effect of the clouds on an Earth-like planet. Deserts are yellower, or redder, than clouds, so a large desert would change the colour attributes of the planet, even if the overall reflectivity remained the same.

[lpetrich]

One factor that could interfere with the detection of land/sea cover is the fact that Earth-like planets would probably have a variable fraction of cloud cover. This makes the land surface tricky to spot through the noise.

Earth clouds are always present and present in multiple weather systems, so they are likely to be somewhat averaged out. But they will likely produce some jitter, so one would have to average over several days of observations. For improved calculations, one may also have to do the seasons, to find out how much winter snow.

Cloud cover might even actively hide the terrain features. If an Earth-like planet has a single, very large (Pangaea-like) continent with a desert in the interior, cloud cover could be reduced in that area; this would reduce the apparent reflectivity of the continental hemisphere and mask the presence of the continent as seen from Earth.

Yes, a sort of super-Sahara.

Chris Scotese's PALEOMAP Project has a big collection of continent and climate reconstruction for the last 750 million years. The continents have been very asymmetric over most of that time, and they have often had big deserts in them. Visible Paleo-Earth - Planetary Habitability Laboratory @ UPR Arecibo uses the past Earth to get more comparison data. Habitability of the Paleo-Earth as a Model for Earth-like Exoplanets - Planetary Habitability Laboratory @ UPR Arecibo, Vegetation, Ice and Deserts of the Paleo-Earth - Planetary Habitability Laboratory @ UPR Arecibo, Analysis of the Distribution of Land and Oceans - Planetary Habitability Laboratory @ UPR Arecibo, and also this tidbit: Visible Vegetation Index (VVI) - Planetary Habitability Laboratory @ UPR Arecibo -- from the amount of green light. I think that green's main claim to fame here is that it isn't red or blue -- one could look for an excess of green. But reflecting lots of green light might be some sort of historical accident of Earth vegetation, and ET vegetation may have other colors.

[geonuc]

How much planetary-surface detail might one be able to observe without resolving an exoplanet? Simulations of Light Curves from Earth-like Exoplanets - Planetary Habitability Laboratory @ UPR Arecibo. I have expanded those simulations to include other big Solar-System objects. ...

I'm a bit confused by the title of your link in this statement. Are you just proposing that light curve observation of exoplanets might reveal clues to habitability or have some observations already been made? Your link takes us to a page with light curve simulations for the various solar system bodies you mentioned, not for exoplanets as the title suggests.

In any case, it's a very intriguing idea. Good post.

[lpetrich]

I'm a bit confused by the title of your link in this statement. Are you just proposing that light curve observation of exoplanets might reveal clues to habitability or have some observations already been made?

Proposing it.

Your link takes us to a page with light curve simulations for the various solar system bodies you mentioned, not for exoplanets as the title suggests.

But from that page, the purpose of those simulations is what I'd described: "This is a challenging project in astrobiology, remote sensing, and image visualization and analysis that might provide new insights and approaches for the future characterization of terrestrial extrasolar planets from light curves."

[eburacum45]

I have no doubt that this sort of analysis will become increasingly important as telescopes become more capable of separating out the data from exoplanets. At the least the rotation period should be detectable, and with luck, much more than that.