Home
Earthshine
Research Interests
Publications
CV

Research Interests 

bulletTerrestrial Climate
bulletEarth seen as a distant planet
bulletAstrobiology
bulletStellar Oscillations


Terrestrial Climate 

The climate of Earth is a consequence of the redistribution of the Sun's energy over the Earth's surface. Energy coming from the Sun is absorbed, and then redistributed by the atmosphere, the oceans and the Earth's landmass and re-emitted to space. Such distribution is not uniform over the Earth's surface, but depends on latitude -more energy arrives over the equator and less over the poles. This, together with the Earth's rotation, through the Coriolis forces, causes the warm air to rise at the equator, move poleward, then descend at the poles and move equatorward near the surface.

Variations in the incident energy over the Earth's surface over time, or variations on the balance of this energy, will have consequences in the Earth's climate.

One of the observed effects of changes in the amount of energy (or in the solar irradiance) coming from the Sun due to solar activity, comes from studies of global variation of the Earth's temperature. Fluctuations in the Earth's temperature, showing a total global warming of the planet of 0.5 degrees Celsius from 1940 to 1997, have been registered. Nevertheless, from 1940 to 1970 the global effect was a slight decrease in temperature, coinciding with a decrease in the level of solar magnetic activity. Solar activity changes periodically with an eleven-year cycle, and some cycles are more intense than others. Satellite measurements over the period of one solar cycle have shown that the Sun's irradiance is not constant, but it increases with solar activity. This increase of the irradiance (solar total energy output) does not totally explain the observed global warming, without, at least, consideration of some amplification mechanism.

One of these mechanisms may be related to the solar magnetic field. The solar magnetic field, which also varies with the solar cycle, acts as a shield protecting our solar system against cosmic rays. When the solar magnetic field is weaker, this protector shield also becomes weaker, and a higher number of cosmic rays reach the Earth's atmosphere. These cause cascades of ionized and subatomic particles, which are being recorded at ground-based monitors. Over half a century, these records have provided information about variations in the intensity of cosmic rays showing a well-known anti-correlation with solar activity.

Cascades of particles caused by cosmic rays are also thought to be related to cloud formation, although this mechanism still remains unclear. Cloud cover data are being recorded by the International Satellite Cloud Climatology Project (ISCCP). But difficulties with the interpretation of the satellite data in terms of cloud cover over land, and calibration among existing datasets have been found. A direct relationship between low cloud cover and cosmic rays has been proposed, although datasets from longer periods of time are necessary.

 
 

Earth seen as a distant planet 

 

In order to reach our telescope, the moonshine (solar light reflected from the moon) has gone through the local atmosphere on top of the telescope. However, the earthshine (light retroflexed from Earth) suffers absorptions from the same local atmosphere, and also from the global atmosphere of the Earth (the global atmosphere is defined as the portion of the sunlit Earth contributing to the earthshine) allowing the study of the terrestrial atmosphere at a global scale.

 

The study of the earth by means of astrophysical techniques is an ideal complement to Earth sciences, since it allows to improve our knowledge of our own planet from an, until now, unique perspective. It will contribute to answer important social and scientific questions, like the climate change suffered during the past century. But observing the Earth as a distant planet provides also information for planetary physics.

 

Detecting Terrestrial Vegetation Considering Simultaneous Cloud Cover Data:

(from Montañés-Rodríguez, 2006a)

 

A spectacular astronomical revolution is on its way with the development of new scientific projects, such as Kepler (Borucki et al., 2003) and COROT (Borde et al., 2003) observatories, that will detect earth-size planets as they transit their parent stars, and the subsequent SIM (Space Interferometry Mission), TPF (Terrestrial Planet Finder) and Darwin missions that will directly detect the light reflected (or emitted) by these planets (Beichman et al., 2006, Fridlung 2004). Among the most prominent goals of the later missions will be the search for biosignatures indicating the presence of life, but life can exist in a rich variety of forms, if the Earth's past and present is to serve us as a guide.

 

For most of its past history, life on Earth existed solely as unicellular microorganisms, which were able to interact with, and transform their environments. In the early Archean period (4.0-2.6 Gyr b.p.) for instance, methanogens were already producing CH4, which also has a small abiotic source, and could have generated a detectable methane-rich atmosphere (Schindler & Kasting 2000). With the advent of cyanobacteria and the subsequent rise in O2 concentration, even larger changes in atmospheric composition took place. Thus, the remote detection of such life forms might be possible by studying the atmospheric composition of a planet (Hitchcock & Lovelock 1967).

 

Suppose that the necessary physical conditions for life were found on a planet, and biological fingerprints detected in its atmosphere (Hitchcock & Lovelock 1967; Selsis et al. 2002; DesMarais et al. 2002; Ford et al. 2001). Could we determine whether this life has evolved further than unicellular organisms into more complex organisms such as plants? The Earth's vegetation has several spectral features, related to chlorophyll, that make detection of plants on Earth an easy task from space when geographical resolution is available. In particular the `red edge', a sharp increase in leaf reflectance around 700 nm (Clark et al., 1993, Kiang et al., 2005), was already detected in 1990 by the Galileo mission (Sagan et al. 1993). Nowadays, vegetation leaf indices and phytoplankton blooms are routinely monitored from space.

 

However, when observing an extrasolar planet, all the reflected starlight and radiated emission from its surface and atmosphere will be integrated into a single spectrum. A similar kind of

globally-integrated planetary spectrum for the Earth can be measured by observing the earthshine ( Montañés-Rodríguez, et al., 2004), the light reflected by the daytime Earth onto the dark portion of the lunar surface.

 

Spectral measurements of the earthshine are one of the few observations one can make of disk-integrated Earth spectra. These earthshine measurements allow us to observe the Earth as it would be seen from a distant planet, i.e., with no spatial resolution. With the recent successes in the detection of exoplanets, the characterization of the spatially averaged spectral reflectance of the Earth, as provided by the earthshine technique, is becoming a keystone in the search for habitable worlds.

 

Several groups have reported in the literature analysis of their observations (Woolf et al. 2002; Arnold et al. 2002; Montañés-Rodríguez et al. 2004; Montañés-Rodríguez et al. 2005; Seager et al. 2005, Hamdani et al., 2006) or modeling (Arnold et al. 2002; Tinetti et al. 2006a) of the earthshine and tried to determine the detectability of the red edge. While some authors claimed to have detected the red edge, others are doubtful and the overall agreement between earthshine modeling and observations has been, so far, limited. The ever-present blocking cloudiness on global scales, with its high albedo, would be expected to obscure the red edge signal.

 

 

In a previous work (Montanes-Rodriguez et al., 2005), we measured with great precision the spectral albedo of the sunlit Earth, p*(l), as reflected from the Moon on 2003 November 19. The earthshine-contributing area during the observations was centered sequentially over Western Africa, the Atlantic Ocean and the Amazonian rainforest. Our analysis allowed us to determine the scale of p*(l), which was comparable to observations of the photometric albedo for the subsequent day, and had an average value of 0.27+-0.01. In Montañés-Rodríguez et al. (2005), the good agreement between the independent photometric and spectroscopic observations of albedo confirmed the validity of applying our photometric data reduction methodology (Qiu et al., 2003; Palle et al., 2003) to spectral data reduction.

 

Our analysis of earthshine data for 2003 November 19 did not show a significant vegetation signal in the Earth's globally-integrated spectrum. We speculated that the lack of a strong red edge was due to cloud obstruction, because the Earth's albedo is dominated by the total cloud amount and optical thickness (Palle et al., 2004). However, we could not compare with the real cloud distribution at the time of observations because they were not yet available. Since then, global satellite observations of clouds from the International Satellite Cloud Climatology Project (ISCCP) have been released covering 2003 November 19 in the latest update, which now covers the period from July 1983 to December 2004. With these data, we have reproduced the Earth's scene for 2003 November 19, such as it would have been viewed from a lunar perspective, and we have generated a synthetic spectral reflectance for that date and time. Finally, we have compared and contrasted our simulations and observations, and quantified the changing slope of the red edge, as Earth rotates and cloud-free vegetated areas appear and disappear from the earthshine.

 

 

 

Astrobiology

Biosignatures and the search for life:
Since Earth is the only known planet able to sustain life at a global scale, first efforts of determining the presence of life in other planets are based on biological experiments dependent on common features of life on Earth. Living organisms on Earth, and perhaps in some extrasolar planets, can be divided in unicellular life, multicelular life and intelligent life.

Unicellular life:
Methanogens, the more simple chemosynthetic bacteria, were probably the first organisms that appeared on Earth. They are responsible of the initial rise of atmospheric methane (Lovelock, 1988) that also raised the temperature of the planet, warmed then by a fainter sun, and contributed to the development of more complex bacteria.

These simple unicellular organisms are being searched in other planets in the solar system and have also been grown on a simulation of Martian soil in laboratory (Kral et al. 2004). Three independent teams have recently detected methane in located equatorial regions of the Martian atmosphere (Kransnopolsky et al. 2004, Formisano et al. 2004, Mumma et al. 2004). Although this unexpected methane concentrations may turn out to have a geological origin (see Lyons et al. 2005), a living source remains as a plausible hypothesis.

Another proposed candidate for habitability in the solar system is Saturn’s satellite, Titan. On the contrary of all other solar system satellites Titan has an atmosphere, and this many similarities to the abiotic (pre-life) earth’s reducing atmosphere, which was composed of CH4, N2, NH3, H2, and H2O (Schaefer and Fegley, 2005). Titan’s atmosphere is a natural laboratory for organic chemistry. However, the satellite is extremely cold, doesn’t have liquid water and has very little sunlight, three conditions considered essential for life.

Present earth's biosphere is basically formed by organic matter generated, through photosynthesis, from carbon dioxide and water, with the input of solar energy. Early photosynthetic bacteria, such as cyanobacteria (blue algae), are believed to be responsible for the initial rise of oxygen in Earth 2300 million year ago (Holland, 1994 and Farquhar et al. 2000).

Complex Life:
Complex life is constituted by highly specialized systems of cells, some of them capable of carrying out complicated functions.

Photosynthesis of multicellular organisms is closely linked to several harvest molecules, being chlorophyll-a (Chl-a) the dominant on Earth.  Chl-a behaves like an antenna, serving plants to collect solar energy.
All healthy vegetation is chemically similar and shows green and infrared reflectance enhancement. The spectral inflexion point or red edge of the enhancement has been used to describe the variation in leaf and canopy chlorophyll concentration, since it may change depending on plant health conditions, specie and incident light.

At Earth, vegetation may extend over large areas of the planetary surface allowing the direct detection of its pigments spectral signal from space. The detection of this signal represents an unquestionable indication of habitation. This red edge has been detected when observing a rather clear-sky green Earth’s region with spatial resolution from the space (Sagan et al., 1993), although its detection in globally integrated observations is controversial (Woolft et al., 2002, Montanes-Rodriguez et al., 2005).

If we compare Earth’s present atmosphere with that of an abiotic Earth we conclude that the biosphere is continuously regenerating gasses, which are in thermodynamic disequilibrium with the geochemistry of the rest of the planet.

Therefore, any by-product of biological processes in non-chemical balance with other species in the atmosphere (as suggested by Lovelock (1965, 1975) and Hitchcock and Lovelock (1967), constitutes an atmospheric biomarkers. They are indirect consequence of biological activity and have being recently summarized in Gaidos and Selsis (2006),

Intelligent life:
At radio wavelength, the signals of our television and radio sets dominate the electromagnetic spectra of the earth and are larger than the natural emissions from the Earth or the Sun. This gives this region of the spectra a very strong potential for the detection of such artificial signals in exoplanets. The SETI project is dedicated to this mission of listening to the stars.

Traditionally, this is considered the best range of the electromagnetic spectrum to search for and detect `artificial' signatures emitted by an intelligent civilization. That might be an anthropogenic prejudice due to the fact that humans have chosen this specific technology for telecommunication processes, but there are certain advantages to the use of radio waves for interstellar communications, if such are ever to occur. For example there is almost nothing in outer space able to block or absorb radio frequencies, and it is easy to pin point the origin of such waves with accuracy. It is also quite easy to encrypt information within.

Rotational modulation of the radio emission of a Earth-like exoplanet will inform about the spatial distribution of the emitters and additionally about the distribution of the continents (Sullivan et al. 1978).


 

 

Stellar Oscillations

The following is a summary of my thesis, A theoretical study of pulsations in extreme-helium stars, that I completed at Armagh Observatory as a PhD student with Simon Jeffery and Phillip Dufton.

Extreme helium stars (EHes) are early-type supergiants which show an unusually high surface helium abundance pointing to an advanced stage of evolution. Different theories have been proposed to explain their origin, with the late thermal pulse model and the binary white dwarf merger model being the most recent. Resolving which theory is correct demands detailed observational and theoretical studies of many different properties of these stars.

The existence of radial pulsations in EHes provides a tool to determine their masses and radii. In some cases, contraction rates can also be measured from changes in their pulsation periods. The nature of these pulsations has been examined in this thesis. Non-linear pulsation models of EHes have been computed using a hydrodynamic code including OPAL opacity data computed for the specific compositions of the EHes. These models were compared with recent high-resolution spectroscopic observations of two low-luminosity EHes, V652Her and BXCir, in which kappa-mechanism pulsations are driven by high iron-group element opacities. These observations provided high-precision measurements of their radial velocity curves and their radii. Non-linear pulsation models were computed in order to obtain additional information about the masses, which remain less well determined empirically.

Before making these pulsation models, a study of the projection factor which relates the apparent (observed) expansion velocity and the true expansion radial velocity in a pulsating star's inertial frame was made. This factor depends on the relative limb darkening in continuum and spectral lines and may therefore be a function of composition. The projection factor is well-known for normal hydrogen-rich stars, as Cepheids stars.However its variation with helium abundance (and other parameters such as effective temperature) had not previously been studied. The behaviour of the projection factor and of the apparent line profile for radially pulsating stars was investigated over a range of temperatures, gravities and hydrogen/helium ratios. Effective temperature was found to be the most important factor.

The results found from the non-linear analysis of low-luminosity helium stars, impose additional constraints on their dimensions, including mass. A model for V652Her which best reproduces the observed velocity and luminosity curves has a mass and effective temperature of 0.7 and 23400. For BX Cir, the mass must lie between 0.50 and 0.38 if the temperature is in the range 22400 - 24000 . However, the luminosity of both models was smaller than those measured directly by a factor of two.

A study of the pulsations in high-luminosity helium stars was also carried out. These stars, with probably a different evolutionary origin to V652Her, show an additional enhancement in the surface carbon abundance.

Models were calculated and compared with recent observations of three objects, PVTel, FQAqr and V2244Oph. In these stars, semi-periodic light variations have been observed. The proposed excitation mechanism is the same as for the RCrB variables - ``strange mode'' pulsations. The nature of the strange modes mechanism has been investigated for the first time in a non-linear analysis. Two density inversion regions are associated with the iron-group element ionization and the HeII ionization zones. The cavities associated with these regions were found to be responsible for driving the pulsations. The hydrodynamic pulsation models for these high-luminosity helium stars show semi-periodic light and velocity curves with irregular amplitudes. These amplitudes were greater than those observed by a factor of 4. Periods were similar to those observed, at least in the case of PVTel.
 

 

 

 

[Home] [Earthshine] [Research Interests] [Publications] [CV]

For comments or questions regarding this web please contact pmr@bbso.njit.edu.
Last updated: 07/11/06.