Abstract. We present here a new solution for the astronomical computation of the insolation quantities on Earth spanning from −250 Myr to 250 Myr. This solution has been improved with respect to La93 (Laskar et al. 1993) by using a direct integration of the gravitational equations for the orbital motion, and by improving the dissipative contributions, in particular in the evolution of the Earth-Moon System. The orbital solution has been used for the calibration of the Neogene period (Lourens et al. 2004), and is expected to be used for age calibrations of paleoclimatic data over 40 to 50 Myr, eventually over the full Palaeogene period (65 Myr) with caution. Beyond this time span, the chaotic evolution of the orbits prevents a precise determination of the Earth's motion. However, the most regular components of the orbital solution could still be used over a much longer time span, which is why we provide here the solution over 250 Myr. Over this time interval, the most striking feature of the obliquity solution, apart from a secular global increase due to tidal dissipation, is a strong decrease of about 0.38 degree in the next few millions of years, due to the crossing of the s 6 + g 5 − g 6 resonance (Laskar et al. 1993). For the calibration of the Mesozoic time scale (about 65 to 250 Myr), we propose to use the term of largest amplitude in the eccentricity, related to g 2 − g 5 , with a fixed frequency of 3.200 /yr, corresponding to a period of 405 000 yr. The uncertainty of this time scale over 100 Myr should be about 0.1%, and 0.2% over the full Mesozoic era.

As the obliquity of Mars is strongly chaotic, it is not possible to give a solution for its evolution over more than a few million years. Using the most recent data for the rotational state of Mars, and a new numerical integration of the Solar System, we provide here a precise solution for the evolution of Mars' spin over 10 to 20 Myr. Over 250 Myr, we present a statistical study of its possible evolution, when considering the uncertainties in the present rotational state. Over much longer time span, reaching 5 Gyr, chaotic diffusion prevails, and we have performed an extensive statistical analysis of the orbital and rotational evolution of Mars, relying on Laskar's secular solution of the Solar System, based on more than 600 orbital and 200 000 obliquity solutions over 5 Gyr. The density functions of the eccentricity and obliquity are specified with simple analytical formulas. We found an averaged eccentricity of Mars over 5 Gyr of 0.0690 with standard deviation 0.0299, while the averaged value of the obliquity is 37.62 • with a standard deviation of 13.82 • , and a maximal value of 82.035 •. We find that the probability for Mars' obliquity to have reached more than 60 • in the past 1 Gyr is 63.0%, and 89.3% in 3 Gyr. Over 4 Gyr, the position of Mars' axis is given by a uniform distribution on a spherical cap limited by the obliquity 58.62 • , with the addition of a random noise allowing a slow diffusion beyond this limit. We can also define a standard model of Mars' insolation parameters over 4 Gyr with the most probable values 0.068 for the eccentricity and 41.80 • for the obliquity.

Context. Thanks to remarkable progress, radial velocity surveys are now able to detect terrestrial planets at habitable distance from low-mass stars. Recently, two planets with minimum masses below 10 M ⊕ have been reported in a triple system around the M-type star Gliese 581. These planets are found at orbital distances comparable to the location of the boundaries of the habitable zone of their star. Aims. In this study, we assess the habitability of planets Gl 581c and Gl 581d (assuming that their actual masses are close to their minimum masses) by estimating the locations of the habitable-zone boundaries of the star and discussing the uncertainties affecting their determination. An additional purpose of this paper is to provide simplified formulae for estimating the edges of the habitable zone. These may be used to evaluate the astrobiological potential of terrestrial exoplanets that will hopefully be discovered in the near future. Methods. Using results from radiative-convective atmospheric models and constraints from the evolution of Venus and Mars, we derive theoretical and empirical habitable distances for stars of F, G, K, and M spectral types. Results. Planets Gl 581c and Gl 581d are near to, but outside, what can be considered as the conservative habitable zone. Planet "c" receives 30% more energy from its star than Venus from the Sun, with an increased radiative forcing caused by the spectral energy distribution of Gl 581. This planet is thus unlikely to host liquid water, although its habitability cannot be positively ruled out by theoretical models due to uncertainties affecting cloud properties and cloud cover. Highly reflective clouds covering at least 75% of the day side of the planet could indeed prevent the water reservoir from being entirely vaporized. Irradiation conditions of planet "d" are comparable to those of early Mars, which is known to have hosted surface liquid water. Thanks to the greenhouse effect of CO 2 -ice clouds, also invoked to explain the early Martian climate, planet "d" might be a better candidate for the first exoplanet known to be potentially habitable. A mixture of several greenhouse gases could also maintain habitable conditions on this planet, although the geochemical processes that could stabilize such a super-greenhouse atmosphere are still unknown.Key words. astrobiology -atmospheric effects -stars: planetary systems IntroductionThe M-type star Gl 581 hosts at least 3 planets, which were detected using radial velocity measurements by Bonfils et al. (2005) (planet "b") and Udry et al. (2007) (planets "c" and "d"). The properties of this star and its planets are given in Table 1. Before this discovery, only two exoplanets were known to have a minimum mass below 10 M ⊕ , which is usually considered as a boundary between terrestrial and giant planets, the latter having a significant fraction of their mass in an H 2 -He envelope. The first one was GJ 876d, a very hot planet (P ≤ 2 days) with a minimum mass of 5.9 M ⊕ (Rivera et al. 2005). The other one is OGLE-0...

We present the consistent evolution of short-period exoplanets coupling the tidal and gravothermal evolution of the planet. Contrarily to previous similar studies, our calculations are based on the complete tidal evolution equations of the Hut (1981) model, valid at any order in eccentricity, obliquity and spin. We demonstrate both analytically and numerically that except if the system was formed with a nearly circular orbit (e < ∼ 0.2), consistently solving the complete tidal equations is mandatory to derive correct tidal evolution histories. We show that calculations based on tidal models truncated at 2nd order in eccentricity, as done in all previous studies, lead to quantitatively and sometimes even qualitatively erroneous tidal evolutions. As a consequence, tidal energy dissipation rates are severely underestimated in all these calculations and the characteristic timescales for the various orbital parameters evolutions can be wrong by up to three orders of magnitude. These discrepancies can by no means be justified by invoking the uncertainty in the tidal quality factors. Based on these complete, consistent calculations, we revisit the viability of the tidal heating hypothesis to explain the anomalously large radius of transiting giant planets. We show that even though tidal dissipation does provide a substantial contribution to the planet's heat budget and can explain some of the moderately bloated hot-Jupiters, this mechanism can not explain alone the properties of the most inflated objects, including HD 209 458 b. Indeed, solving the complete tidal equations shows that enhanced tidal dissipation and thus orbit circularization occur too early during the planet's evolution to provide enough extra energy at the present epoch. In that case either a third, so far undetected, low-mass companion must be present to keep exciting the eccentricity of the giant planet, or other mechanisms -stellar irradiation induced surface winds dissipating in the planet's tidal bulges and thus reaching the convective layers, inefficient flux transport by convection in the planet's interior -must be invoked, together with tidal dissipation, to provide all the pieces of the abnormally large exoplanet puzzle.

Since the first images of polar regions on Mars revealed alternating bright and dark layers, there has been speculation that their formation might be tied to the planet's orbital climate forcing. But uncertainties in the deposition timescale exceed two orders of magnitude: estimates based on assumptions of dust deposition, ice formation and sublimation, and their variations with orbital forcing suggest a deposition rate of 10(-3) to 10(-2) cm yr(-1) (refs 5, 6), whereas estimates based on cratering rate result in values as high as 0.1 to 0.2 cm yr(-1) (ref. 7). Here we use a combination of high-resolution images of the polar layered terrains, high-resolution topography and revised calculations of the orbital and rotational parameters of Mars to show that a correlation exists between ice-layer radiance as a function of depth (obtained from photometric data of the images of the layered terrains) and the insolation variations in summer at the martian north pole, similar to what has been shown for palaeoclimate studies of the Earth. For the best fit between the radiance profile and the simulated insolation parameters, we obtain an average deposition rate of 0.05 cm yr(-1) for the top 250 m of deposits on the ice cap of the north pole of Mars.

Surface conditions on Mars are currently cold and dry, with water ice unstable on the surface except near the poles. However, geologically recent glacierlike landforms have been identified in the tropics and the midlatitudes of Mars. The ice has been proposed to originate from either a subsurface reservoir or the atmosphere. We present high-resolution climate simulations performed with a model designed to simulate the present-day Mars water cycle but assuming a 45 degrees obliquity as experienced by Mars a few million years ago. The model predicts ice accumulation in regions where glacier landforms are observed, on the western flanks of the great volcanoes and in the eastern Hellas region. This agreement points to an atmospheric origin for the ice and reveals how precipitation could have formed glaciers on Mars.

Recent geological observations in the northern mid-latitudes of Mars show evidence for past glacial activity during the late Amazonian, similar to the integrated glacial landsystems in the Dry Valleys of Antarctica. The large accumulation of ice (many hundreds of meters) required to create the observed glacial deposits points to significant atmospheric precipitation, snow and ice accumulation, and glacial flow. In order to understand the climate scenario required for these conditions, we used the LMD (Laboratoire de Météorologie Dynamique) Mars GCM (General Circulation Model), which is able to re

[1] We present a time-marching model which simulates the exchange of water ice between the Martian northern cap, the tropics, and a high-latitude surface reservoir. Net annual exchange rates of water and their sensitivity to variations in orbital/rotational parameters are examined using the Martian water cycle modeled by the LMD three-dimensional Global Climate Model. These rates are propagated over the last 10 Myr to follow the thickness of the reservoirs. The effect of a sublimation dust lag is taken account to test simple models of layer formation. Periods of high mean polar summer insolation ($5-10 Ma ago) lead to a rapid exhaustion of a northern polar cap and a prolonged formation of tropical glaciers. The formation of a northern cap and of a high-latitude icy mantle may have started 4 Ma ago with the average decrease of polar insolation. Tropical ice may have disappeared around 2.7 Ma ago, but small glaciers could have formed during the last peaks of polar summer insolation. Over the last 4 Myr, most of the present cap may have formed at the expense of tropical and high-latitude reservoirs forming distinct layers at almost each $51-kyr/120-kyr insolation cycle. Layers thickness ranges from 10 to 80 m, variations being produced by the modulation of the obliquity with $2.4 and 1.3 Myr periods. Because only $30 insolation cycles have occurred since 4 Ma ago, we found an inconsistency between the recent astronomical forcing, the observed number of layers, and simple astronomically based scenarios of layers formation.